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BACKGROUND INFORMATION FOR WESTINGHOUSE OWNERS GROUP ABNORMAL RESPONSE GUIDELINE ARG-1 LOSS OF RHR WHILE OPERATING AT MIO-LOOP CONDITIONS Revision 0 March 15, 1990 i
9202210454 920116 PDR ADOCK 05000424 S
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TABLE OF CONTENTS SECTION PAGE 1.-
INTRODUCTION 1
2.
DESCRIPTION 2
2.1 RHR System Description 3
2.2 Operation of RHR Pumps With Air Entrainment 3
2.3 Fluid System Evaluation-7 2.4 RCS Level Description 8
2.5 Transient Analysis Description 11 2.5.1 Analysis for Case A 19 2.5.2 Analysis for Case B 31 2.5.3 Analysis for Case C 47 2.5.4 Analysis for Case D 67 2.5.5 Analysis for Case E 78 2.5.6 Analysis for Case F 82 2.5.7 Analysis for Case G 104 3.
RECOVERY / RESTORATION TECHNIQUE 104 3.1 High Levsl Action Summary 104 3.2 Key Utility Decision Points 109 4.
DETAILED DESCRIPTION OF GUIDELINE 111 4.1 Detailed Description of Steps, 111 Notes, And Cautions with Appendix 4.2 Step Sequence Requirements 147 5.
FREQUENT QUESTIONS 149 6.
REFERENCES 150 FIGURES 2.1-1 RESIDUAL HEAT REMOVAL SYSTEM 4
2.4-1 REACTOR COOLANT SYSTEM LEVEL MEASUREMENT SYSTEM 10 2.5.1-1 RCS PRESSURE FOR CASE A 25 2.5.1-2 CORE AN9 DOWNCOMER TEMPERATURES FOR CASE A 26 2.5.1-3 CORE AND DOWNCOMER MIXTURE LEVELS FOR CASE A 27 2.5.1-4 HOT AND COLD LEG INDICATED LEVELS FOR CASE A 28 2.5.1-C TEMPERATURE IN ACTIVE SGs FOR CASE A 29 2.5.1-6 PRESSURIZER MIXTURE LEVEL FOR CASE A 30 ARG-1
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r TABLE OF CONTENTS SECTION PAGE FIGURES 2.5.2-la RCS TOTAL, SG INLET STEAM PRESSURE FOR CASE B1 35 2.5.2-lb RCS TOTAL, SG INLET STEAM PRESSURE FOR CASE B2 41 2.5.2-2a CORE AND DOWNCOMMER TEMPERATURES FOR CASE B1 36 2.5.2 2b CORE AND 00WNCOMMER TEMPERATURES FOR CASE B2 42 2.5.2-3a CORE AND DOWNCOMER MIXTURE LEVELS FOR CASE B1 37 2.5.2-3b CORE AND DOWNCOMER MIXTURE LEVELS FOR CASE B2 43 2.5.2 4a HOT AND COLD LEG INDICATED LEVELS FOR CASE B1 38 2.5.2-4b HOT AND COLD LEG INDICATED LEVELS FOR CASE B2 44 2.5.2-Sa TEMPERATURE IN ACTIVE SG FOR CASE B1 39 2.5.2-Sb TEMPERATURE IN ACTIVE SG FOR CASE B2 45 2.5.2-6a SG NO. 1 INLET STEAM FLOW FOR CASE B1 40 2.5.2-6b SG NO. 1 INLET STEAM FLOW FOR CASE B2 46
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2.5.3-la RCS PRESSURE FOR CASE Cl 54 2.5.3-lb RCS PRESSURE FOR CASE C2 60 2.5.3-2a CORE AND DOWNCOMER TEMPERATURES FOR CASE Cl 55 2.5.3-2b CORE AND DOWNCOMER TEMPERATURES FOR CASE C2 61 2.5.3-3a CORE AND COLD LEG MIXTURE LEVELS FOR CASE C1 56 2.5.3-3b CORE AND COLD SIDE MIXTURE LEVELS FOR CASE C2 62 2.5.3-4a HOT AND COLD LEG INDICATED LEVELS FOR CASE C1 57 2.5.3-4b HOT AND COLD SIDE INDICATED LEVELS FOR CASE C2 63 2.5.3-5a CORE INLET, COLD LEG SPILL FLOWS FOR CASE C1 58 2.5.3-5b CORE INLET, COLD LEG SPILL FLOWS FOR CASE C2 64 2.5.3-6a COLD LEG SPILL, CHARGING, HL S1 FLOWS FOR CASE Cl 59 2.5.3-6b COLD LEG SPILL, CHARGING, HL SI FLOWS FOR CASE C2 65 ARG-1 Rev. O
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I!RLE OF CONTENTS SECTION PAGE FIGURES 2.5.3-7 REFILL LEVELS vs OPENING SIZE FOR VARIOUS HOT LEG SI 66 FLOWS 2.5.4 1 RCS PRESSURE FOR CASE D 73 2.5.4 2 CORE AND 00WNCOMER TEMPERATURES FOR CASE D 74 2.5.4-3 CORE AND DOWNCOMER MIXTURE LEVELS FOR CASE D 75 2.5.4-4 HOT AND COLD LEG INDICATED LEVELS FOR CASE D 76 2.5.4 5 TOTAL RHR RETURN FLOW FOR CASE D 77 2.5.6-1 RCS PRESSURE FOR CASE F 88 2.5.6 2 CORE AND DOWNCOMER TEMPERATURES FOR CASE F 89 2.5.6-3 CORE AND DOWNCOMER MIXTURE LEVELS FOR CASE F 90 2.5.6-4 HOT AND COLD LEG INDICATED LEVELS FOR CASE F 91 2.5.6-5 PRESSUR12ER MIXTURE LEVEL FOR CASE F 92 2.5.6-6 RCP SEAL LEAK FLOW FOR CASE F 93 2.5,7-1 RCS PRESSURE FOR CASE G 99 2.5.7-2 CORE AND 00WNCOMER TEMPERATURE FOR CASE G 100 2.5.7-3 CORE AND DOWNCOMER MIXTURE LEVELS FOR CASE G 101 2.5,7-4 HOT AND' COLD LEG INDICATED LEVELS FOR CASE G 102 2'.5.7-5 TEMPERATURE IN ACTIVE SGs FOR CASE G 103 4.1-1 VENT PATH FOR RCS PRESSURE LESS THAN 2 PSIG 145
'4.1-2 VENT PATH AREA FOR RCS PRESSURE EQUAL TO 20 PSIG 146 TABLES 2.5-1
SUMMARY
OF CASES FOR PHASE 3 HID-LOOP VAllDATION 17 2.5-2
SUMMARY
OF RELEVANT INPUT PARAMETERS USED IN THE 18 PHASE 3 MID-LOOP ANALYSIS 2.5.1-1 TIME TABLE OF EVENTS FOR CASE A 24 ARG-1
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r TABLE OF CONTENTS SECTION PAGE IABLES 2.5.3-1 TIME TABLE OF EVENTS FOR CASE C 53 2.5.4-1 TIME TABLE OF EVENTS FOR CASE D 71/72 2.5.5-1
SUMMARY
OF STEADY STATE BLEED AND FEED RECOVERY 81 CONDITIONS 2.5.6 11ME TABLE OF EVENTS FOR CASE F 87 2.5.7-1 TIME TABLE OF EVENTS FOR CASE G 98 l'
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.lNTR000CT10N Guideline ARG 1, LOSS OF RHR WHILE OPERATING Ai MID LOOP CONDITIONS, provides the actions necessary to maintain core cooling and to protect the reactor core in the event that Residual Heat Removal (RHR) cooling is lost during mid-loop operations.
Entry to this guideline will occur if the RHR pumps have tripped or when the operator recognizes that the RHR pumps are cavitating based on motor current oscillations, erratic flow oscillations, or excessive pump noise.
A low RHR flow alarm and/or other symptoms requiring the tripping of the RHR pumps can also be used as entry conditions for this guideline.
If RHR flow is rapidly restored, the operator can terminate ARG-1 and return to the appropriate procedure for the existing plant condition (plant draindown, maintenance activities, etc.).
If RHR flow cannot be rapidly reItored: the operator starts trending core exit thermocouple temperatures and initiates contingency recovery actions while trying to return RHR to service.
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DESC91PTIE I
Operation of the Residual Heat Removal (RHR)- System with the Reactor Coolant Systen partially drained has been an industry concern for many years since-this delicate mode of operation places the plant in a condition highly susceptible tc the loss of the RHR function. The USNRC, prompted by the April 10, 1987 Diablo Canyon loss of RHR event, issued Generic Letter 87 12, LOSS OF RESIDUAL HEAT REMOVAL (RHR) WHILE THE REACTOR COOLANT SYSTEM (RCS) IS PARTIALLY FILLED, and requested that all utilities with PWRs respond to their concerns pursuant to 10 CFR 50.54(f). After reviewing the resnonses, the NRC stated that none of the responses were fully satisfactory fron, any licensee and for some licensees the responses were unsatisfactory in each of the 12 categories evaluated. Added to the NRC's displeasure is the fact that loss of RHR accidents continued to occur at an unacceptably high rate and the identification that core damage can potent' ally occur in a shorter time than previously believed.
During the June 23, 1988 NRC Staff presentation to WOG representatives, a list of serious deficiencies was outlined by the N.^.
The NRC list of serious deficiencies included the following two ite"': 1) Mitigation planning to prevent core damage was often poor, and 2) Planning to prevent a release shoulo core damage occur was often nonexistent.
The NRC continued by
- discussing corrective actions which included the development of operating procedures for mid-loop conditions and emergency procedures to cover the mitigation of the loss of RHR function from mid-loop conditions.
- Further, the NRC issued Generic letter No. 88-17, LOSS OF DECAY HEAT REMOVAL, on October 17, 1988, recommended expeditious actions and programmed enhancements for operation of the NSSS during-shutdown cooling or during conditions where such cooling would be provided. These recommendations would apply whenever there is irradiated fuel in the reactor vessel. Again, all utilities were requested to respond to these recommendations pursuant to 10 CFR 50.54(f).
In order to address the NRC's concerns in the procedural area, the Westinghouse Owners Group decided to develop this guideline to support mid-loop operations.
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3 2.1 Residu&1 Heat Removal System Description The Residual Heat Removal (RHR) system removes heat from the Reactor Coolant System (RCS) during plant shutdown operations at low reactor coolant system pressures.
The RHR system for the reference plant design consists of two RHR pumps (that also function as low head SI-(LHS!) pumps) and two RHR heat exchangers. The RHR system provides normal shutdown heat removal when RCS 0
pressure and temperature are reduced to approximately 400 psig and 350 F.
During normal shutdown heat removal operations, reactor coolant flows from the RCS hot legs to the RHR pumps, through the tube side of the RHR heat exchangers, and back to all of the RCS cold legs through the Safety injection System (SIS) cold leg injection header.
Heat is transferred from the RCS to the Component Cooling Vater (CCW), which is circulating through the shell side of the RHR heat exchangers.
Figure 2.1-1 is a simplified flow diagram of the reference plant RHR system.
2.2 ODeration Of RHR Pumos With Air Entrainment The residual heat removal (RHR) pumps draw suction from the reactor coolant system hot legs while the plant is operating at mid loop conditions.
It has-been identified that vortexing can occur in the hot leg, especially when the RHR pump is operating at high flow rates, This vortexing can result in air entrainment in the reactor coolant, thus the RHR pumps could operate with air entrained in the fluid entering the pump suction.
Pump operation under these conditions must be evaluated.
The operation _of_RHR pumps with air entrainment has been addressed by NUREG/CR-2792, An Assessment of Residual Heat Removal and Containment Spray Pump Performance linder Air and Debris Ingesting Conditions (Reference 1).
It was identified in Reference 1-that RHR pump operation with volumetric air quantities less than 2 to 3 percent will result in negligible degradation of the pump performance, increasing volumetric air quantities will result in deterioration of pump performance.
The rate of deterioration is a function of the pump design, operating point and the fluid mixture.
Reference 1 also identified that air volumetric fractions between 7 percent and 15 percent can i
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result in the loss of prime in the pump.
It was conclude ~ that most centrifugal pumps are fully degraded with greater than 15 percent air entrainment.
Failure mechanisms due to air entrainment are dependent on the quantity of air ingested and the operating point of the pump. Operation at less than 2 to 3 percent does not normally result in pump degradation, however, these small quantities of ingested air will increase the NPSH requirements of the pump. The possibility of pump hydraulic degradation occurring is increased if the pump has a low NPSH margin.
Additionally, cavitation can occur in the pump if the NPSH margin is inadequate to handle the increased NPSH requirements.
Cavitation can result in increased pump vibration levels which will lead to accelerated wear and aging of the pump seals and bearings. Cavitation can also lead to erosion of the pump impeller, diffu',er and wear rings.
These are basically long term f ailure mechanisms. Minor cavitation on a short term operational basis should not cause significant pump damage. Minor cavitation is cavitation which does not result in performance degradation and does not result in increased pump vibration levels.
Operation with increasing quantities of ingested air, up to the point of loss of prime, will result in the development of relatively short-term failure mechanisms.
These short-term failure modes include:
1.
Excessive rotor vibration due to non-uniform density of fluid 2.
Bearing or mechanical seal failure due to excessive vibration 3.
Wear ring rub due to reduced hydrodynamic bearing effects 4.
Shaft failure due to wear ring seizure 5.
Vibration fatigue of components 6.
Mechanical seal leakage due to lack of seal face lubrication.
7.
Motor failure from water spray from seal leakage.
The severity of these failure mechanisms is highly dependent on the pump design and fluid mixture.
For this reason, the time which the RHR pump may operate in these conditions cannot be precisely predicted, although it is ARG-1 Rev. 0
o expected that pump damage or failure could occur in minutes. Thus it is important that the operator be able to identify the occurrence of air ingestion and take action before pump damage occurs.
In the event that the quantity of air ingestion exceeds the critical limit at which the pump loses prime, damage to or fail,ee of the pump due to the previously identified mechanisms can occur in a matter of seconds.
The detection of air ingestion into the RHR pump can be identified by pump degradation in the form of reduced developed head or oscillating developed J
head caused by small quantities of entrained air in the fluid mixture reaching the pump suction.
These small quantities of entrained air can best be detected through monitoring of the pump suction and discharge pressures and the motor amperage. The loss of head is associated with reduced or oscillating brake horsepower. As the quantity of entrained air increases, the effects en developed head and horsepower become more pronounced.
Additionally, oscillations in pump flow may be detected and pump assembly vibration level; will increase. The plant operator must interpret these pump symptoms as air ingestion related and take the appropriate corrective
- actions, in the event that a pump is shutdown due to suspected cavitation or operation with ingested air, some precautionary steps are recommended prior to the start of any pump. The pump casing and the surrounding piping should be vented to ensure that there are no gas pockets. Upon startup, the vibration levels, motor amperage, differential pressures and pump flow should be closely monitored. Additionally, the operating pump should be observed for seal leakage or unusual noise.
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2.3.
Fluid System Evaluation A fluid system evaluation was perforned to provide analytical information related to air ingestion into the RHFt system resulting from vortex formation. The primary input was from scale model testing and was supplemented by previous testing results and evaluations, plant operating experience, and in-plant testing. The fluid system evaluation resulted in the development of correlations for RCS hot leg levels and RHR flowrates to be used as guidelines for operations with the RC5 partially filled.
These correlations are based on limiting air entrainment to maintain acceptable RHR pump operation.
The conclusions and recommendations resulting from the fluid systems evaluation are as follows:
(1)
A specific relationship between air ingestion into the RHR system and hot leg level exists. The relationship -is a function of RHR flowrate, with higher flowrates requiring higher hot leg levels to limit air ingestion.
The relationship can be defined by a correlation of critical submergence depth (S ) relative to the c
Froude number (F )-
r (2) Operating at low RHR system intake flowrates during partial loop operations greatly reduces the risk of air entrainment.
l (3) Once the suction line vortex is formed and significant air is ingested into the RHR system, the RCS level must be increased and L
the RHR system must be vented in order to restore sufficient RHR l-system performance.
If adequate time is not available to vent the j
entire system at the high-point vent locations, the fastest way to l
_restere RHR flow is to recover level and sweep entrained air from l-the system by operating the system at a relatively high flow rate.
l However, this method has the potential of causi, pump damage or a water-hammer event.
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-(4)
Plant test data suggest that the first symptom of air entrainment is noise at the RHR pump.
This is followed by a drop in suction pressure and, finally, by oscillations in suction pressure, discharge flowrate, and/or motor current.
(5) Differences exist in RCS levels between active cold legs, inactive cold legs, active hot legs, and inactive hot legs during partial loop operations. An active leg is defined as a loop from which an RHR pump either takes suction or into which it discharges.
The magnitude of the level difference is signif_icant (on the order of between 1 and 2 inches, depending on the RHR system flowrate).
These variations are described in detail in WCAP-12111, RCS LEVEL-GRADIENTS.
2.4 RCS level Descriotion As with the Emergency Response Guidelines (EF.Gs), the LOSS OF RHR WHILE OPERATING AT MID-LOOP CONDITIONS Guideline was developed utilizing the generic reference plant configuration to maximize the applicability of the technical' guidance, in particular, the reference plant is the high pressure (HP) plant and is described in the ERG Executive volume.
As noted in Section 2.5, a Diablo Canyon Unit 2 model is u:ed for the transient analysis simulations.
The differences between the high-pressure and low-pressure (LP) reference plant designs have minimal impact on this guideline and for that reason they are discussed in the background document step description tables for the applicable steps. The reference plant description outlines the systems and instrumentation assumed for the reference plant during the development of the ERGS. Since the accidents covered in the ERGS are assumed to be initiated from a hot and pressurized reactor coolant system, no mention
.was made of any additienal instrumentation that is designed for use in shutdown modes. This section describes the assumed level measurement _ system used for determining water level in the reactor coolant loops when the reactor coolant system is depressurized and partially drained.
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The reference plant measures RCS level in two locations, the first being an RCS hot' leg level measurement and the second being a cold leg measurement.
A simplified drawing' of the RCS level measurement system is shown on Figure 2.4-1.
Both level measurements utilize differential pressure transmitters which measure the pressure differ'ences between the bottom of the loop and either the reactor vessel head or the pressurizer. The resultant level is a collapsed liquid level measurement f rom the bottom tap location to the top location that is compensated for any pressure changes in the reactor coolant system.
The hot leg level is representative of both level in the reactor vessel core region and the hot leg RHR inlet level.
The cold leg level is representative of reactor vessel downcomer level.
With this level system arrangement, variations in actual level are expected at different locations in the RCS piping.
These variations have been described in WCAP-12111, RCS LEVEL GRADIENTS.
It is important to-recognize these variations when determining plant specific RCS level values for use in procedures developed from this guideline.
Caution should also be exercised when using RCS cold leg level since it does not necessarily reflect what the I
water level is in the core region or the hot leg.
In addition, due t'o the reactor coolant pump weir effect, if seal injection is in service it is possible to reduce level in the hot leg without observing a level decrease in j
the cold leg.
The reactor coolant pump weir effect is described in detail in WCAP-12111, i-i i
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2.5 Transient Analysis Descriotion A number of thermal hydraulic analyses for 2, 3, and 4-loop plants are presented in Sectica 3 of WCAP-11916. These Analyses describe the RCS and secondary response for various loss of RHR cooling scenarios during mid loop operation.
A number of possible RCS configurations are evaluated, including configurations with cold and hot side openings, SG nozzle dams in place, and SGs in dry and wet layup conditions. A detailed procedures' validation is not included in WCAP-11916, however, a number of high level recovery strategies are evaluated. These include providing RCS makeup requirements with charging flow, RWST gravity feed, and high-pressure 51.
Alternate modes of decay heat removal are also described, e.g., via SG condensation if secondary inventory is available as a heat sink.
Bleed and feed using one pressurizer PORV and one high pressure 51 pump is also considered for cases in which SG condensation would not be an effective means of decay heat removal.
The Phase ! " Interim Guidance for Loss of RHR at Mid-loop Conditions" (transmitted November 7, 1988 via WOG-88-156) was developed using results from the WCAP 11916 analyses, information from the US NRC Generic Letter 88-17, and utility feedback from th'e September 1988 WOG Mid-Loop Workshop.
Improvements to the interim guidance were then made, including restructuring the guidance into the familiar two column ERG format.
A Phase 2 version
" Loss of RHR While Operating at Mid-loop Conditions" was subsequently issued (transmitted May 31, 1989 via WOG-89-ll8). The Phase 2 guideline was the starting point for the validation analyses presented in the following sections of this background document. Changes to the Phase 2 guidance were then made in an iterative fashion until the current more streamlined Phase 3 version was complete.
t for this background document, a number of new thermal hydraulic analysis cases hr.ve been been performed to validate the recovery guideline for loss o#
RHR during mid-loop operation.
For the most part, these cases ("A" through "G") simulate the recovery actions taken for the current Phase 3 version of the guideline. T4ble 2.5-1 summarizes the seven cases presented.
The cases investigated represent a fairly diverse set so as to accomplish the task of exercising most of the steps in the guideline for a variety of possible configurations.
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All transient analysis cases for the Phase 3 mid loop validstion were simulated with the non condensible gas version of the TREAT computer program (TREAT NC) using the 4-loop Diablo Canyon Unit 2 model.
For determination of r
the decay heat power, the actual core power level (3411 MWt) was assumed instead of the "uprated" [ower level (3700 MWt) previously assumed in WCAP-11916. A number of other plant parameters of interest assumed are listed in Table 2.5-2.
Additional details on the analysis mocel and TREAT NC can be found in WCAP-11916 in Section 3.2.3 and Appendix C, respectively.
For purposes of guideline validation, results from the Phase 3 analyses are expected to be representative for most Westinghouse plants.
Certain plant parameters will, of course, influence the results mcre than others.
Potential plant-specific differences that may be importait will be made more apparent in the analysis descriptions which follow.
One of the paramoters in Table 2.5-2, the " low charging flow" makeup rate, will be discussed now since reference to this flow is made in a number of analysis cases.
As noted in Table 2.5-2, the assumed value for this parameter,108 gpm, is (quivalent to the flow from one positive-displacement charging pump.
For most fjigh-pressure and low pressure 4 loop plants, this is the approximate capacity of each of the PD pump (s).
For the 2-loop low pressure plants, the flow from each PD pump is approximately 50-60 gpm, i.e., about half that assumed for the 4-loop plant.
For most 3-loop low pressure plants, the PD pump capacity lie' somewhere between the 2 and 4 loop values.
In the analyses for the 4-loop plant described in the following sections, makeup flows are sometimes compared to the capacities of one or two PD pumps.
Based on scaling with power level and plant size, is reasonable to simply extend these results to the 2 and 3-ioop plants since the makeup flows for these plants would be proportionally lower.
In many cases, the recommended flows are within the capacity of one PD pump.
In most of the remainder of the cases (except Case C), the flow recommended is within the capability of two PD pumps (or one centrifugal charging pump).
For
- Ca:;e C, hot leg injection it necessary since charging at a very high flow rate (434 gpm) will n-t rec, up with the RCS inventory loss.
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i in the transient plots provided for the various cases, the indicated cold and hot side levels (L and t, respectively) are defined as follows:
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He 26.14 L
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Lh*(Hh 26.14) / (62 vh) where H and Hh are the cold and hot side mixture levels as e
referenced from the bottom of the downcomer and vh is the specific volume in the core / upper plenum mixture 3
region (units of ft /lbm).
The levels defined above are given in feet.
Referring to Table 2,5 2, the bottom of the hot legs is at H 26.14 ft. Thus, the level indications assumed above represent the collapsed levels above the bottom of the hot legs.
For most of the cases of interest, the cold leg and downcomer 3
densities are close to 62 lbm/ft ; therefore, no density correction is l
needed for L.
c In performing the TREAT NC simulations, it is assumed that the operator will control RCS inventory to near mid loop (within several inches either way) based on some indicated level in the hot leg, in particular, it is assumed that an indication of the collapsed level above the bottom of the hot leg is 3
available, calibrated to a reference censity of'62 lbm/ft.
This corresponds to th as defined above (the utility shcald review their level indication systems to see how closely they match those assumed here and make adjustmentsaccordingly). At typical initial conditions assumed (core exit / hot leg temperature of 120cf and cold leg /downcomer temperature of 0
80 F), the diff+rence between hot and cold side levels (due to density differences) is act large approximately 1.0 inch.
Other phenomena also l~
contribute to the RCS level gradients (reftr to WCAP 12111), however, densi' differences tend to have one of the more pronounced effects. After less of RHR coon;, the hot side level swells and cold side level drops in such a j
manner i as to maintain a delta.p (gravitational head) balance between the 0
cold ano hot sides. When the core exit temperature reatnes 200 F, the hot ARG-1 13 -
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and cold side levels differ by about 4 to 5 inches, in view of this difference, the operator using a cold side level indication to control RCS inventory would initiate charging whereas the operator using a hot side level indication would not.
After the RCS reaches saturation, the hot and cold side levels can differ by a few feet, depending upon the core and upper t
plenum void fractions.
Thus, the distinction between cold versus hot side levels becomes even more significant after boiling starts to occur.
As noted in Appendix E of WCAP 11916, a length averaged core / upper plenum void fraction consistent with the Yeh correlation is modeled.
It is assumed
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that the void fraction at the core exit, upper plenum, and hot legs are identical. Although this treatment is somewhat simplistic, it proves to be sufficiently accurate for the intended purpose of guideline validation.
These assurrptions are further supported by the FLECHT SEASET test described in Section 5 of WCAp 10415 (NUREG/CR 3654 or EPRI NP 3497).
In that portion of the FLECHT SEASET test in which two phase natural circulation flow is evident, the upper plenum void fraction is higher than it is at the core exit because the two phase mixture, upon exiting the heated region, froths up as it is carried over into the SG tube region.
The mixture subsequently condenses in the down side of the SG tubes.
As the test progresses and after the loop flows decrease essentially to zero (i.e., more representative of mid-loop operation), the core exit end upper plenum void fractions become approximately the same (about 70%, corresponding to a core average value of approximately 45*.).
This Ft.ECHT SEASET test represents a high decay heat (approximately 2%) which is about five times hightr than that expected for mid loop operation early after shutdown (for mid-loop operation, the decay heat, per the ANS S.1 1979 standard would be between 0.48% and 0.33% at 2 and 5 days after shutdown, respectively).
For these ranges of decay heat, the core exit void fraction (per the Yeh correlation) increases roughly as the square rovt of the decay heat.
Consequently, the void fractions in the FLECHT SEASET test are about twice that expected for mid loop operation early after. shutdown (e.g., 30% core exit, 20% core average for mid loop operation early after shutdown at low RCS pressures < 50 psia).
For the late in shutdown post-refueling cases (0.1% decay heat), the void fracticns are further reduced by a factor of two.
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I 004 final note about the TREAT.NC model that should be pointed out is the simplifying assumption of a single global system pressure in the RCS.
l TRf AT.NC (as well as the condensible version 1REAT) evaluates the pressure differentials between regions (i.e., nodes) for the solution to the momentum equation.
For the energy equation, however, steam table properties are evaluated at a single global pressure. Although the precise location of this glubal pressure is not important, the pressure most likely represents the pressure in the larger nodes of the RCS, e.g., in the core / upper plenum or the upper head regions. At high RCS pressures, this global pressure 4
i approximation is reasonable since the localized pressure differences between regions are small by comparison.
For the lower pressure mid loop transients.
l the global pressure simplificativ.i is again usually not a significant drawback since most of the regions in the RCS are in fairly close communication with each other.
[ven the cold and hot sides of the RCS are linked together via the downcomer / upper head spray nozzles and tne annular gaps between the hot leg nezzles and the core barrel.
After the RCS heats up to saturation, swells, and mixture " plugs" the conner. tion between the hot leg and the surge line, there will be some inaccuracies due to the global pressure assumption.
This is because the comparatively large pressurizer vapor volume becomes isolated from the rest o the RCS at this time.
f As noted from Table 2.5 2, the bottom of the pressurizer is approximately 8 feet above mid loop in the Diablo Canyon Unit ? model.
Thus, a pressure difference on the order of several psi should be sufficient to force water or two phase mixture up the surge line to the bottom of the pressur1rer.
A i
slightly higher RCS pressurization would of course be required to accomplish this since the pressurizer pressure is also adjusting to a higher pressure during this process. Although the pressurizer level behavior follows these expected trends as the RCS pressure increases, the response sometimes appears more erratic than cxpected, for example, in Case A, the levei in the pressurizer fills to the top of the surge line and then stops during a 10 minute time period during which RCS pressure increases from about 7 to 9 psig.
Pressurizer level then rather abrupt'y increases and upper plenum level decreases.
In a more accurate fully compressible model, it is expected that pressurizer level would have continued to slowly increase af ter the surge line initially fills and the corresponding rapid rite later would have been more gradual. With the possible exception of Step 8, the operator does 15 -
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t not take action based on the pressurizer level response.
in Step 8, the operator simply allows hot leg injection to refill the RCS to a high level arbitrarily selected as midspan in the pressurizer (refer to results for Case C in Section 2.5.3).
Since a precise value is not required, it does not appear that the global pressure limitation in TREAT poses a drawback for this study.
This is not to imply that the pressurizer level should be ignored during a mid loop transient.
Havirg fluid in the pressurizer is beneficial during the period when the RCS is being cooled.
When the RCS is cooled by the RHR system, the voids colltpre and the fluid level shrinks.
This can be a problem if the fluid level at the RHR intake (s) drops to less than that required to prevent vortex formation.
Should this occur, it wil' be necessary to shut off the RHR pumps (again) and reenter the guideline at the beginning.
To prevent this from occurring, it may be necessary to increase charging or start a safety injection pump to increase the RCS inventory prior to or during the RCS cooldown.
If there is water in the pressurizer, it will drain out as the RCS cools, the pressure decreases, and the fluid shrinks.
1his will help keep the hot leg level at a level sufficient for RHR pump operation.
In Case A, the fluid in the pressurizer prevents the loss of level during cooldown from becoming a problem.
However, as discussed lat(r, Case A does not model a situation where the cooldown shrink is extreme, in Cases D, F, and G, it is necessary to maintain or increase charging when RHR cooling restored.
For these <:ase!.. the pressurizer inventory dras not drain enough (or fast enough) to help recover hot side level.
ARG.I 16 -
Rev. 0
Table 2.5 1 Summary of Cases for Phase 3 Mid loop Validation Method of Special (ng Conditions of RCS/SGs
[Ltcovery Kev Features Remarks F
A RCS Intact and Vented Charging Demonstrate p
to the PRT at 0.5 psig Vent and Guideline Use g~
- S Days After Shu.down Restore RHR for Typical Water in 2 of 4 SGs Refueling Case k
Small Leak Causes Loss of RHR Cooling B
One Cold Side SG Charging to Investigate Two Cases Manway Removed Match / Exceed
" Flooding" Indicate Water in All SGs Boil off Concern Flooding No SG Nozzle Dams (if flooding is Not a Two Cases: 2 and S Limit not Concern Days After Shutdown Reached)
C
- High Decay Heat -
Hot leg
- lest Guideline Two Cases 2 Days After Shutdown injection Effectiveness C1 and C2 Small Cold Leg Opening With One for " Marginal" (S! Pump (4" Valve Removed) 51 Pump Adverse RCS Cycled in r
Three Przr Safety Configuration Case C1, Valves Removed Case Left On SG Nozzle Dams in in C2)
Place in All SGs 0
- Same as "C* Except No Charging to Demonstrate Cold leg Opening Match / Exceed Guideline Use Boil off for Large Hot Vent and Side Vent Restore RHR
- Investigate RHR Restart for High Decay Heat Case E
Late in Shutdown.
- Charging to
- Demonstrate Evaluated 1/3 fresh fuel (30 days)
Match / Exceed Feed and Bleed Based on
- One Przr PORV Open, Boil-off for Low Decay Case F and PRT Rupture Otsk Off Heat Case WCAP 11916 SGs Dry (or with (Case GB)
Nozzle Dams i.e P1 ace)
Results F
Late in Shutdown
- Charging
- Investigate RCS
- Two Przr PORVs Open, Vent and Pressurization PRT Rupture Disk Off Restore RHR and Seal Leak
- RCP Seal Repair with PORV Vents (Variable Opening)
Investigate RHR
- SG Nozzle Dams in Restart for low Place in All SGs Decay Heat Case G
- Same as "F" Except
- Same as "F" Same as "F" S Days After Shutdown, Except SGs Wil Water in 2 of 4 SGs Help Reduce (No SG Nozzle Dams)
Pressure ARu 1 Rev. O
!'~'
Table 2.5 2 Summary of Relevant Input Parameters Used in the Phase 3 Mid loop Analysis i
Olablo Canyon Unit 2
"' ant Geometry Modeled (four Loop) 3411 Hut Licensed Core Power Decay Heat Powers Assumed:
16.4 MWt (0.48%)
,2 Days After Shutdown 11,3 MWt (0,33%)
5 Days After Shutdown 3.41 MWt (0,10%)
30 Days With 1/3 Fresh fuel Approximate Steady State Initial Conditions Used in Analyses:
RHR Flow Tcold Thot 0
0 Il9 F 81 F 2900 gpm 2 Days After Shutdown 0
0 Il9 F 81 F 2000 gpm 5 Days After Shutdown 0
0 105 F 82 F 1000 gpm 30 Days After Shutdown level Elevations in the RCS:
0.0 ft Bottom of the Downcomer 22.13 ft Top of Active Fuel 26.14 ft Bottom of Hot Leg 26.20 ft Bottom of Coid Leg 27.35 ft Mid loop, Mid Point of Surge Line Contact Bottom of Pressurizer (8 ft Above Mid Loop) 35.35 ft 41.30 ft Top of Vessei
~~- -
Makeup Flow Rates:
Low Charging Flow (Also, Flow From One PD Purr?)
108 gpm (15 lbm/sec) 434 gpm (60 lbm/sec)
Maximu'a Charging Flow (from One CCP) 615 gpm (85 lbm/sec)
Flow From One High Fressure S1 Pump Vent Path flow Areas:
2 0.011 ft One Pressurizer PORY 2
One Pressurizer Safety Valve (Removed) 0.147{t 2.1 ft One SG Manway Removed (19" Diameter) 0 100 F Temperature of SGs, Intermediate Loop 0
70 F Temperature of RWST l
ARG 1 Rev. 0
1
- 2. 5.1 6.n.3 v s i s f or Qu_A Case A represents a situation where the plant is preparing for refueling operations and the RCS develops a relatively small, 36 gpm, Icak.
The plant has been shutdown for 5 days, and the decay heat is about 11.3 MWt.
The RCS is at mid-loop levels with the RHR system maintaining the average temperature at about 100*f (core exit temperature is about 120 f, and RHR discharge into the cold sides is about 80*f).
The pressurizer PORVs are open to the Pressure Relief Tank (PRT).
The PRT is maintaining the RCS at about 0.5 psig with an overpressure of nitrogen.
Two of the four steam generators are filled on the secondary side, and the other two steam generators are drained.
Except for the leak and the PC,~<vs. there are no RCS openings.
The situation presented here is similar to the loss of RHR event at the Diablo Canyon plant in April, 1987 In this case, however, only two steam generators (instead of all four) are available as a secondary heat sink.
Thus, this case is more limiting in terms of RCS pressurization than the Diablo Canyon event.
Table 2.5.1 1 is a time table of the significant events for the Case A simulation.
The transient starts with a 36 gpm leak in the crossover leg of RCS loop 1 at five minutes, The leak causes a loss of level in the loops sufficient to cause the loss of the RHR pump 33 minutes into the simulation.
At this point the plant operator enters the guideline, " Loss of RHR While Operating at Mid toop Conditions," at Step 1.
Step 1 of the guideline leads to Step 2, where the operators are dirteted to isolate all known RCS drain paths.
It is assumed that at this time no known drain paths exist.
In Step 3 the time to boiling is to be determined, in figure 1 of the guideline this time is estimated to be 12 minutes.
This is less than that needed to initiate containment closure.
The next steps to be taken are Step 4, the evacuation of nonessential personnel in containment.
Step 5, the initiation of containment closure, and Step 6, the starting of the containment fan coolers.
TREAT-NC does not explicitly model these steps, however the operators would perform them in an actual event. At Step 7 it is determined that the RCS level is less than mid loop, but still above the bottom of the hot legs.
The operator is then directed t( Step 10.
ARG 1 Rev. 0 l
1
d Step 10 instructs the operator to increase the RCS inventory until the indicated hot side level is above mid loop. This is done with a relatively 3
small charging flow of 109 gpm 35 minutes into the simulation.
The charging
]
pump is left on until the RCS level is abcut 4 inches above the mid loop level.
This level is reached 20 minutes later, at 55 minutes.
At Step 11 the operator initiates actions to find the RCS leak, it is assumed the leak is found and isolated at 65 minutes simulation time.
Because the charging pump is satisfactorily raising the RCS level, RWST gravity feed will not be necessary and the note before Step 12 is ignored.
At Step 12 the operator confirms two steam generators are available as a heat sink, opens the steam generator PORVs and proceeds on to Step 13.
This step directs the operator to vent the RHR system, in the simulation this action is completed two hours into the transient. After the ACS level is confirmed to be adequate for the restarting of a RHR pump, and the venting is complete.
RHR flow is restored.
In the guideline these actions are performed in Steps 14 and 15.
After the RHR pumps are running, the RCS is cooled to less than 140*F, and the simulation is terminated.
Graphs of the RCS Pressure, RCS Temperature Mixture Levels, and Indicated Levels for this transient are shown in Figures 2.5.1-1 through 2.5.1-4 The figures show the important RCS parameters stay within reasonable levels.
Figure 2.5.1 1 shows the RCS pressure transient. As would be expected, the pressure begins to increase once saturation is reached, at approximately 54 minutes. The pressure increases rather rapidly at first, then more slowly as condensation in the steam generators becomes effective.
Heating of the secondary water then controls the steam partial pressure in the steam generator tubes. The RCS pressure, that is, the total steam and air (or nitrogen) pressure, continues to rise until RHR cooling is restored.
The rapid depressurization when RHR cooling is restored is evident at about 125 minutes.
The RCS pressure does not return to its original value of 0.5 psig because the transien_t causes the steam generators to heat up, as shown in Figure 2.5.1 5.
Since no forced cooling of the steam generators is assumed, the steam partial pressure in the steam generator tubes will remain high.
There are no vents-in the RCS to reduce pressure to that of the atmosphere, so the total RCS pressure stabilizes approximately at the saturation pressure of the secondary water temperature, lhis is why the RCS ARG-1 20 -
Rev. 0
pressure remains above 10 psig at the end of this transient.
Since the RC5 is vented to the PRT the operator may be aware of this pressure on the PRT pressure guage.
4 For plants with different steam generator volumes or a different number of steam generators filled with water in their secondary side, this type of transient is likely to produce a different pressure response.
A more complete study of the effects of condensation in the steam generators can be found in Section 3.8 of WCAP 11916.
Figure 2.5.1 2 shows the hot side and cold side temperatures during the 0
The maximum temperature reached is approximately 260 F.
The temperature increases rapidly until it reaches a point where sufficient steam is produced to allow significant heat transfer to the steam generators secondaty water. Once that occurs the temperature increase continues, but at a slower rate.
The hot side temperature drops rapidly once the RHR pumps are restarted.
The slight increase in the cold side temperature at about 125 minutes is caused by the transfer of the hot side coolant to the downcomer when the RHR pumps are restarted.
The rapid collapse of the voids in the hot side when RHR flow is restored causes the pressurizer to drain.
The accompanying flow reversal in the core causes the downcomer to heat up as shown in the figure.
After RHR cooling is established, the hot and cold side temperatures decrease to their original values.
The rate for the initial cooldown shown in Figure 2.5.1 2 may be too large to be desirable in most plants, in the simulation, the RHR pump flow was started and increased to 2000 gpm within five minutes, with all the flow going through the RHR heat exchanger.
It may be more desirable to restore RHR flow at a lower flow rate and allow more flow to bypass the heat
-exchanger when the RHR pump is restarted. This would not only reduce the rate of change in the temperature, but also minimize the drop in the fluid level at the RHR intake.
The loss of level, and possible loss of RHR intake flow, will occur as the voids are collapsed in the RCS as it cools, for this case, the loss of level was not dramatic, since the pressurizer became filled during the transient. As the RCS cooled, the fluid in the pressurizer drained back into the RCS.
This helped maintain the fluid level in the hot sides.
In most cases, it is recommended that the operator start the RHR pump ARG 1 Rev. 0
.,s m.
. ~.
I at a low flow rate, wit'n flow bypassed around the heat exchanger.
The drop in the indicated level, as shown in Figure 2.5.13, cauld be corrected with charging flow if necessary.
I The hot and cold side mixture levels are shown in figure 2.5.1-3.
The indicated levels are shown in Figure 2.5.1 4.
The indicated level is the collapsed level referenced from the bottom of the hot sides.
Hid-loop level for the mixture levels is at 27.35 feet.
This is translated to a value of 1.21 feet in the indicated level graph scale.
The rapid rise in the hot side level, and the accompanying loss of cold side level, can be seen after the RCS temperature reaches saturation. As discussed earlier, there is a drop in level when the pressurizer begins to fill. When the pressure increases to a point sufficient to force water into the pressurizer, the coolant flows from the hot side into the pressurizer.
When the RHR pumps are restarted, the hot side level drops rapidly as the voids are collapsed.
The initial increase in the hot side fluid shown at about 130 minutes occurs when the RCS pressure reduction allows the pressurizer to beginning draining.
(This can be seen in Figure 2.5.1 6, Pressurizer Mixture Level.) As the RHR further cools the RCS, the decrease of the hot side level because of the fluid shrink is evident. As discussed earlier, the draining of the pressurizer prevented the loss of level because of the void collapse from becoming too large.
The amount of fluid in the pressurizer, as well as the hot side level, should be considered when trying to decide if addittoral charging flow might be necessary to prevent a loss of level at the RHR intake once the RHR pump is started.
Figure 2.5.1 5 shows the temperature increase of the steam generator secondary water during the transient. Once sufficient steam is produced in the primary side of the steam generator, the heat transfer rate is sufficient to allow a significant heating of the secondary side. Since there is no secondary cooling assumed in the simulation, the steam generators do not cool significantly after the RHR pump flow is restored.
As discussed earlier the heating of the secondary water limits the RCS pressure decrease after the RHR flow is restored if the RCS is not vented to the containment atmosphere.
ARG 1 Rev. O
n The pressurizer level through the transient is shown in figure 2.5.1 6.
Once the RCS pressure was high enough to support the column of water in the surge line, and the hot side level had risen to the bottom of the surge line, it filled with water.
This can be seen at about 55 minutes. As discussed in s
previous sections, it is likely that the filling would be less abrupt than shown in the figure, but the calculation methods used in TREAT NC produced a discontinuous rate of filling, The rapid manner in which the pressurizer fills once the appropriate RCS pressure and level is achieved is evident in the figure.
The pressurizer empties almost as rapidly once the RHR pumps are restarted and the RCS pressure is reduced, at 125 minutes.
The rapid emptying of the RCS fluid into the hot side helps to maintain level in the hot side as the fluid shrinks due to the RHR cooling.
ARG 1 23 -
Rev 0
Table 2.5.1 1 Time Table of Events for Case A Event Time (minutes)
RCS Intact with 2 Pzr PORVs open to PRT, Mid loop Operation at 2000 gpm RHR Flow, 0-5 RCS Pressure 0.5 psig Unidentified Leak Occurs in Crossover 5
Leg of Loop 1 (36 gpm leak rate)
RCS Levels Approximately 3" Below Mid-loop, 33.4
' RHR Pumps Stepped (Guideline Entry)
Charging Flow Started, increased to 109 gpm 35.4 (Step 10)
RCS Reaches Saturation at 213'F; Time to boil was 21 minutes.
54.3 i
Charging Flow Stopped after Level increased To Approximately 4" Above Mid Loop 55 (Based on Hot Side Level Indication)
Leak Identified and Isolated 65 (Step 11)
RHR Vent Operations Complete, RHR Flow 125 - 130 Started and Increased to 2000 gpm (Step 15)
RCS Temperature (T,) Less than 140 F, 149 Exit Guideline End of Transient Modeled 167 ARG-1 Rev. O
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2.5.2 6.nJLivsis for Case B In this analysis, it is assumed that a vent path is provided by removal of a k
SG manway on the cold side of one SG.
The hot side SG manyays are still in place. This configuration is similar to Case E3 presented in WCAP 11916.
In contrast to the WCAP 11916 analysis, however, it is postulated that the SGs have water in the secondary side.
Case B has been included to address a NRC concern that, with water in the secondary side of the SG, the steam flow into the SG tubes may be blocked by condensation at the entrance to the SG.
A typical case (Case Bl. five days after shutdown) and a more bounding case (Case B2, two days after shutdown) are included to address this " flooding" concern.
1he configuration modeled for Case B is conservative since the vent and condensation processes are limited to one of four SGs.
This maximizes the amount of decay energy and steam flow to be removed by a single SG.
Initial conditions for Case B1 (five days after shutdown) and Case 82 (two days after shutdown) are summarized in Table 2.5.2.
For both cases, Thot 119 F and Tcold 81 F (initial RHR flows are 2000 and 2900 gpm, respectively). To initiate both transients, RHR cooling was assumed lost at 5 minutes. The RCS reaches saturation (Thot 212 F) at 24 and 19 minutes, respectively.
The two cases vere run until 40 and 25 minutes, respectively.
At that time, the steam partial pressure in the lower region of the active SG reaches 14.7 psia (this region represents the lower half of the straight tube length, approximately 16 ft long). These pressure transients are shown in figures 2.5.2-la and 2.5.2 lb.
Had the transients been extended, the steam pressures in the remaining portions of the active SG would also nave approached atmospheric pressure of 14.7 psia.
It was not necessary to extend the analyses further, however, since the steam flows into the SG tubes had approached steady state boil off conditions. At this point, it is demonstrated that flooding would not be a concern.
ARG 1 Rev. O i
0 The RCS temperature on the hot side reaches 212 f and remains constant; 0
the downcomer temperature remains close to the initial value of 81 f throughout the transient (Figures 2.5.2 2a and 2.5.2-2b).
The core / upper plenum region becomes highly voided (core / upper plenum average void fraction exceeds 207.). This causes a fairly large (6 ft) scparation between the h0t and cold side mixture levels (Figures 2.5.2 3a and 2.5.2 3b) and several feet difference between the hot and cold sido indicated levels (Figures 2.5.2 4a and 2.5.2 4b). These levels are as p*eviously described in an earlier part of Section 2.5 As the RCS boils, sieam enters the tube side of the active SG. During the period of the transient studied, most of this steam produced conuenses on the SG tLbet ena returns to the RCS; a much smtller fraction is predicted to pass through the SG tubes and out the cold side manway vent (initially, the manway vent relieves only air wnich has been displaced by the incoming steam).
Thus, reflux condensation in the active SG will be effective and limit the RCS inventory losa for this case until the temperature of the secondary water approaches saturation.
Figures 2.5.2-5a and 2.5.2 5b illustrate the heatup of the secondary water in the active SG.
It should be noted that the initial 0
temperature of the SG water (i.e., approximately 100 f) may be somewhat optimistic, particularly for the higher decay heat Case B2 (two days af ter shutoown would also be an optialistic time to assume for manway removal).
However, the secondary volume assumed in the tube and separator region 3
(approximately 2100 ft ) is conservatively low equivalent to just on span in the narrow range (more appropriately, covering the top of the tubes on wide range).
The over all response could therefore be typical.
It should also be noted that the heat capacity of the secondary water in one SG is comparable to tne heat capacity for the RCS heatup volume in the core, upper plenum, and hot leg regions. Thus, by opening the cold side manways on one (or preferably more) of the SGs, inventory losses due to boil-off will be effectively delayed until the temperatures in the active SGs approach saturation.
ARG-1 Rev. O
Figures 2.5.2 6a and 2.5.2 6b show the steam flow into the SG tubes of the active SG. As noted, the two cases have been run until the steam flows Based approach the steady state boil off flow 4/hrg, q being the decay heat.
on hrg C70 BTV/lbm at atmospheric pressure, and the decay heat rates of 11.3 and 16.4 HWt (see Table 2.5 2), the boil of f steam flows are 11.0 and 16.0 lbm/sec, respectively, at five and two days after shutdown.
To determine whether or not the steam and counter current liqu'd flows are high enough to cause a " flooding" or liquid entrainment concern, the " flooding" correlations must be checked. TREAT uses a floodina correlation extracted from NOTRUMP (see Appendix W of WCAP 10079 P A). The limiting flow area used for 2
this check,is the 29" diameter hot leg flow area (4.59 f t ); this is slightly conservative since the elbow for the SG inlet nozzle expands to 31" diametcr 2
(C.24 ft }.
Based or, the above steady state boil off rates and a steam density of 0.038 lbm/ft3 (at 15 psia), the limiting steam welocities are 63 ft/sec for Case 81 (fhe days after shutdowh) and 92 ft/sec for Case B2 (two days after shutdown).
Based on the TREAT and NOTRUMP correlations, the steam selocities at the flooding limit are considers,bly higher.
Based on Wallis, " Numerous competing correlations exist for predicting the onset of droplet entrainment..." Therefnre, it is important to cross check the above results against some of these other correlations to confirm that flooding is not a concern.
Based on Steen's correlation for the onset of entrainment (limited to cases in which the liquid fraction is small and fluid viscous forces are also small), the steam velo:ity at stmospheric pressure conditions should be less than 154 ft/sec. The steady stete steam velocity for Case B1 is 41% of this entrainment threshold value; the steady state steam velocity for Case B2 is 60% of this value.
Wallis has extended the Steen's correlation to allow for varying amounts of liquid entrainment. Again assuming atmospheric pressure conditions, in the Wallis correlation, there is no liquid entrainment at steam velocities below ARG 1 33 -
Rev. O
75 ft/sec. At a steam velocity of 125 ft/sec, there will be a small amount of liquid entrainment (approximately 5%). This confirms that there is no flooding concern for Case Bl.
Per the Wallis correlation, a small amount of liquid entrainment (less than 2%) is predicted for Case B2.
The resulting small additional inventory loss would have an insignificant impact on the time to core uncovery, so this small amount of liquid entrainment is judged to be acceptable, in view of the conservative assumptions for this case, it is reasonable to argue that the flooding limit would not even be exceedad for Case B2.
To summarize the results for Case B, it ic found that a large cold side SG manway vent path behaves like a large hot side vent path.
This assumes that the path for boil off steam through the associated SG tubes is not blocked by installation of a hot side SG nozzle dam or by closure of the hot leg loop isolation valve.
For typical (5 days after shutdown) and bounding (2 days after shutdown) mid loop conditions, the bail off steam will not be flooding limited due to reflux condensation in the SG tubes.
In fact, the SG secondary inventory in the active SG(s) provides an effective heat sink that will delay RCS inventory losses due to boil off until the water in the active SG(s) approaches saturation. Because of this and also due to the absence of a j
possible " spill penalty" (see WCAP-11916, Section 3.4), the cold side SG manway vent path may be preferable to the hot side SG manway vent path.
Either vent path configuration is " good" and this provides flexibility to the utility in planning their outage and SG maintenance work.
The simulations for Cases B1 and B2 are run only until the air in the inlet tube sections is forced to the upper and down side regions of the SG and subsequently out the manway vent. At the time the transients are stnpped, the steam flows into the SG tube region of the active SG are approximately the same as the steady state boil-off steam flows. The recovery actions for the large hot side vent modeled in Case 0 would be similar to those of Case B, so there is no need to extend the Case B analysis further for purposes of guideline validation.
Case D instead of Case B proves to be the preferable one to extend for a long acovery transient since the TREAT NC models can be run faster.
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2.5.3 Analysis for Case C The analyses for Cases C.1, C.2, and C.3 in Section 3.5 of WCAP-11916 I
investigate the impact of having large cold side openings with the hot side isolated by SG nozzles dams in one or both loops of a 2-loop plant (see Figure 2.4-1 for location of SG nozzle dams).
If the loop with the cold side
=-
opening is isolated by a SG nozzle dam (or closed loop isolation valve) on the hot side, it was found that prolonged core uncovery could occur, starting only a few minutes after the RCS reaches saturation.
To recover from an event of this type, it would have been necessary to provide a large hot side vent capable of maintaining RCS pressure less than approximately 2 psig or to use hot leg injection with at least one high pressure SI pump at a rate sufficient to limit or suppress boiling.
Potential " adverse RCS" configurations have been described in more detail in WCAP 11916, the WOG "early warning" letter (0G-88-21), and in the NRC Generic Letter 88-17.
It should be noted that any configuration with SG nozzle dams in place could become " adverse" if a hot side vent path sufficient to prevent SG nozzle dam failure is not provided and a cold side SG nozzle dams fails first upon RCS pressurization beyond the failure pressure. Nozzle dams are normally tested to about 28 psig, i.e., about twice the opera. ting pressure when the refueling cavity is flooded. Typical liciting yield point and failure pressures for the nozzle dams are on the order of 50 psig and higher.
in the Phase 3 analysis for Case C, a smaller (4" Schedule 160) cold leg opening at mid-loop is considered.
All SG nozzle dams are assumed to be in place.
A pressurizce-vent path is also provided via removal of the three pressurizer safety valves (6" Schedule 160 openings). Although this vent path could be sufficient to limit the RCS pressurization at about two weeks (or 336 hours0.00389 days <br />0.0933 hours <br />5.555556e-4 weeks <br />1.27848e-4 months <br />) after shutdown (see Appendix to Section 4.1, figure 4.1-1), it is not adequate for the higher decay heat case assumed here for Case C.
For Case C, the initial temperatures and decay heat correspond to those summarized in Table 2.5-1 fer two days after shutdown (That " II9 I' 0
Tcold - 81 F, and decay heat = 16.4 MWt). Again, the decay heat assumed should be conservative since plant personnel would have to hurry and also work around some f airly hot metal at two days af ter shutdown. Nevertheless, such a configuration may be possible under forced outage conditions in three or four days, so the initial conditions are not overly conservative.
ARG-1 Rev. O
.umium-.
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Full charging flow capability exceeding 430 gpm is assumed available from one CCP.
For the cold side opening postulated, however, charging cannot keep up I
with the flow through the opening after the RCS starts to boll.
Consequently, hot leg injection is also needed. Hot leg injection from one high-head SI pump is assumed available at a rate of 615 gpm (85 lbm/sec).
Based on the assumed RWST temperature (70 F), the high-head 51 flow will 0
almost suppress boiling: mc T - 85 x 1.0 x (212-70) 12,070 BTU /sec =
p 12.7 MWt (decay heat - 16.4 MWt, less than 25% higher).
Considering the above information, this case is ' marginal" in several respects.
First, the vent path alone is not adequate to limit the RCS pressurization to less than 2 psig.
Based on the decay heat boil-off and vent path resistance assumed (K/A2 17.7 ft-4), the RCS would pressurize to more than 8 psig at steady state boil-off conditions.
Second, the inventory loss through the cold side opening after the RCS starts to boil is just beyond the capability of normal charging. Although charging would prolong the time to core uncovery (possibly for 15 minutes after the RCS starts to boil), hot leg injection is eventually needed to replenish the RCS inventory lost from the opening.
Einally, the hot leg S1 flow assumed is not sufficient to completely suppress boiling. With the charging flow and/or vent path provided, however, the SI shortfall is found to be acceptable.
This case is fairly useful since it illustrates the impact of these parameters plus some other features of interest.
The time table of events for Case C is given in Table 2.5.3-1.
The transient is initiated by' the loss of RHR cooling at 5 minutes. As in Case A, this simulation first exercises Steps 1 through 7 of the guideline (again, no actions can be modeled by TREAT-NC for Steps 4 through 6). On the first pass through the guideline, the operator could go from Step 7 to 11 (if level is still near mid-loop, as it would be prior to the start of boiling), check for RCS leakage at Step 11, bypass Step 12 (installation of SG nozzle dams prevents the operator from establishing a secondary heat sink), and assume a delay for completion of the actions in Steps 13 through 15 (vent and restore He would then transition back to Step 4 for another loop through the RHR).
guideline. Rev. 0 ARG-1
After the RCS reaches saturation (19 minutes transient time), the hot side starts to pressurize faster than the cold side and this forces inventory out the cold side opening.
Fer the check in Step 7 and as directed in Step 10, the operator initiates charging about five minutes later, after the indicated hot side level is below mid-loop. Charging flow is then increased to maximum flow in an attempt to restore level. When indicated hot side level goes off-span low (i.e., below the bottom of the-hot legs),
hot leg injection using one SI pump is then initiated per Step 8.
Hot leg injection is found to be adequate for RCS makeup and decay heat removal for the mitigation phase of this transier', so the operator could stop charging.
Longer term, the operator would continue attempts ta vent and restore the RHR system.
After charging is stopped, the analysis is continued in two ways.
In Case Cl, the SI pump is stopped after the indicated hot side level increases above mid-loop (39 minutes transient time). As expected, the RCS reaches saturation again and the hot side level drops to mid-loop about 5 minutes later. With the exception of the downcomer and cold leg temperature in the loop with the opening, most of the parameters of interest appear to be cyclic, having a period of 20 minutes (the cold side temperatures slowly heat up due to the addition of hotter water from the core region). Out of each 20 minute time period, it is expected that the 51 runp and possibly charging pump would be operated roughly 5 to 10 minutes if this mode of recovery were allowed to continue. Cycling the pumps this often would likely cause damage to the pump motors; thus, the pumps would more likely have to be operated on mini-flow (and isolation valves cycled) during the time periods when makeup flow is not required.
As an alternative to SI pump cycling, Case C2 considers the situation in which the SI pump is simply left on. The RCS then refills until the flow out
- the cold side opening approaches the hot leg SI flow. The transient is stopped at 75 minutes when the cold side level is 3.6 ft above mid-loop.
After several hours, it is expected that the cold side level would increase
~
to-5.5 ft above mid-loop.
At that point, the 51 flow (615 gpm - 85 lbm/sec) would match the flow through the opening (81.4 lbm/sec) plus the small residual boil-off flow (3.6 lbm/sec) assumed vented from the pressurizer.
I ARG-1 Rev. 0
The core / upper plenum region is slightly voided longer term in Case C2 l
(approximately 6% region average void fraction).
This causes the level in the upper plenum or hot side region to be approximately 2 feet higher than it is in the I
cold side or downcomer region.
The RCS pressure transients for Cases C1 and C2 are shown in Figures 2.5.3-la and 2.5.3-lb, respectively.
Pressure is limited to less than 8 psig (by the pressurizer vent path) prior to initiation of hot leg SI at 33 minutes.
Boiling is then suppressed after hot leg injection is initiated until about 39 minutes. The cycle then repeats for Case C1.
For Case C2, a slight pressurization to about 0.5 psig is This necessary since the S1 flow does not completely suppress boiling longer term.
Since the SI " shortfall" pressurization is predictable by a simple energy balance.
is about 25% of the decay heat and the pressurizatior, increases as the square of the boil-off (or decay heat) for subtritical flow through the vent, the long term pressure for Case C2 should be approximately (0.25)2(8 psig) = 0.5 psig (the steady state boil-off pressure without forced makeup is about 8 psig).
The hot and cold side temperatures are provided in Figures 2.5.3-2a and 2.5.3 2b.
0 For Case C1, the hot side (core exit / upper plenum) temperature reaches 230 F and then decreases to 210 F with hot leg SI. The downcomer temperature slowly heats 0
0 For Case C2, the hot side temperature remains at saturation long term (213 F) up.
and the downtomer temperature slowly increases due to the addition of hotter water from the core region.
Mixture levels and indicated levels (collapsed levels above the bottom of the hot leg) are provided in Figures 2.5.3-3a, 2.5.3-3b, 2.5.3-4a, and 2.5.4b.
Apart from the initial swell when the RCS reaches saturation, the hot side levels decrease The level during the time periods'during which hot leg SI is not provided.
transients for Case C1 are clearly cyclic.
For Case C2, the levels fill at a decreasing rate as the flow through the opening become more equal to the SI flow.
The transient is stopped at 75 minutes when the hot side (upper plenum) mixture level reaches the upper support plate. The corresponding cold side level in the downcomer is 3.6 feet above mid-loop or 2.3 feet below the top of the downcomer.
After a few hours, the cold side level will reach 5.5 feet above mid-loop, very close to the top of the downtomer.
Levels in the upper plenum may fill the control rod guide tubes and possibly spill over into the upper head and downcomer.
l
' Rev. 0 ARG-1
~.-.---
,1-j
- Figures 2.5.3-Sa ~and 2.5.3 5b show the core flow and the flow through the
- cold leg opening for Cases C1 and C2, respectively. The core flow is-initially-400 lbm/sec (2900 spm), the same as the RHR flow. The core flow I
then drops to near zero following the loss of RHR and remains low during the initial heatup to saturation. After the RCS reaches saturation (at 19 minuter), Lthe core-inlet flow becomes negative (this flow " feeds" the cold side opening)._. The magnitude of this reverse core flow is reduced when charging is started (at roughly 25 minutes). However, the charging flow is not hig enough to reverse the direction of the core inlet flow and refill the hot side. After hot leg SI is started (at 33 minutes), the flow through the_ opening is-reduced due to the depressurization and the core flow becomes positive for a period of time. At 39 minutes, the SI pump is stopped (Case
- Cl) and the RCS reaches saturation. The core flow then goes negative, the flow through the opening increases, and the cycle repeats itself.
In Case C2 (51 pump left on), the core reverse flow becomes nearly the same as the break flow. The two curves in Figure 2.5.3-5b therefore appear as mirror images.
Figures 2.5.3-6a and 2.5.3-6b illustrate the flowrates from the cold side
- opening, charging, and hot leg Si for Cases C1 and C2, respectively.
As can be noted, charging does not quite " keep up" with the flow through the.4" valve opening. This illustrates why hot leg injection is required for recovery. Had the opening size been reduced to about 3" diameter or less, charging:would have been able to provide adequate makeup and hot leg injection would not_have been needed.
The 4" Schedule 160 opening modeled for Casa C2 is equivalent to an opening of. 3.44" diameter located at mid-loop. Apart from the small ' amount of boil-off, most of the hot leg SI flow is used to refill the RCS to a level at
- which the flow through the opening _ balances the injection flow._ For case C2, i~
this balance would be achieved when the level is 5.5 feet above mid-loop (or 5.5' feet above the opening). Had it not been for the 0.5 psig pressurization due to the small amount of boil-off, level would equilibrate another one foot higher-(i.e., at'6.5 feet.above mid-loop). Again, it would take several hours for'the cold side opening spill flow to match the-hot leg SI flow.
However, most of the predicted level increase occurs within an hour after hot leg SI is initiated.
1 ARG-1 Rev. 0 4
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It is possible to extend the results of Case C2 to consider other opening sizes and a wider range of hot leg SI flows.
Figure 2.5.3-7 shows, as a function of opening size, the level in the RCS at which the spill flow from
)
the opening balances the hot leg SI flow. The RCS level is referenced from mid-loop, the assumed location of the opening.
Curves are provided for hot leg injection flows of 400, 600, and 1000 gpm.
In developing these curves, it is assumed that the hot leg SI flow is sufficient to completely suppress boiling. The flow through the opening is therefore proportional to the area of the opening and varies as the square-root of the refill level.
Accounting for the pressurization due to boil-off, the level for Case C2 is shown at 6.5 feet on the 600 gpm curve. This would be the approximate position for the 4" Schedule 160 opening several days after shutdown (or longer) when the hot leg Si flow is sufficient to completely suppress boiling.
For openings larger than approximately 8 inches in diameter, the level will stabilize at the same approximate elevation as the opening.
If hot leg SI is required, one would expect the operator to simply., tart the SI pump and leave it operating. There should be no concern with cycling the SI pump for these large opening cases.
For openings approximately 3 inches diameter and smaller, charging flow would be sufficient for makeup. The symptom-based guideline would not direct the operator to establish hot leg injection unless charging is not available.
To avoid cycling the SI pump for the 3" to 8" diameter break size, a high refill level is selected for Step 8.
Bssed on the features of Figure 2.5.3-7, this level is somewhat arbitrarily selected to be mid-span on the pressurizer.
For the Diablo Canyon plant, this level is more than 30 feet higher than mid-loop.
If hot leg SI is required, the pump will be started and left running or the frequency of operation of the SI pump will be low enough so as not to present an equipment operability problem.
ARG-1 Rev. O
q.;
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Table 2.5.3 1 Time' Table of Events for Case C-hui Time (Minutes) i
.RCS Vented (3' Pzr Safety Valves Removed),
Mid loop _ Operation _at-2900 gpm RHR Flow.
0-5 Two Days After Shutdown,EHigh Decay Heat _(16.4 MWt),
!All SG-Nozzle Dams Installed, 4" Cold Leg Opening in_ Loop 1
- Lossf 0f RHR Cooling Postulated (Guideline Entry) 5 0
19
~RCS Reaches Saturation 1(212 F) _
Indicated Level _ Approximately 2" Below Mid loop, Charging Flow Started at 43_gpm (Step 10) 24 Charging FlowLIncreased to Maximum Flow (430 gpm) 24.5 26.5 6
and Maintained at This Value-(Step 10, Continued)
Indicated Level Reaches the Bottom of the Hot Legs 30.8
[
, (Checked-in_ Step-7)-
- Hot legfinjection with One HHS1 Pump (Step 8)'
33 i
. Hot Leg Injection Is Effective so Operator Stops Charging Flow In -an Attempt to ~ Reduce Inventory Loss 35 Case C1 Case.32 Level Is Restored.Above Mid-Loop, Hot Leg 51 is Terminated 39
!L.
. Celd Side, Level ReachesLTop of Cold Legs-54 o
75 3 Cold Side Level 3.6 ft Above Mid-Loop End:of Transient Modeled 43 75 For Case C1 - Response Appears to be Periodic For Case C2 2. Cold-Side Level WillcEventually.
Reach:5.5 ft Above Mid-loop
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2.5.4 Analysis for Case D The initial conditions for this case are identical to those of Case C except i
there are no cold side openings. The RCS is at mid-loop and three pressurizer safety valves are removed. The decay heat is 16.4 MWt, as is assumed two days after shutdown. All of the steam generator nozzle dams are installed.
The initial temperature at the core exit is 119'F, and the temperature of the cold legs and downcomer is 81'F.
These are maintained with a RHR system flow of 2900 gpm.
The decay heat used in this case is probably a little high since it would be difficult for plant personnel to get the nozzle dams installed so soon after plant shutdown. The intent with this case was to provide a bounding situation with steam generator nozzle dams in place, no cold side openings, and a hot side vent path which is adequate for this configuration. As anticipated, charging flow is sufficient to match the mass lost as vapor through the safety valves and is adequate to maintain stable conditions in the RCS until the RHR flow can be restored. This is in contrast to Case C where hot leg safety injection is required to maintain and increase RCS inventory. Case D also features the reestablishment of RHR flow when the RCS is highly voided.
Increased charging is needed to accommodate the rapio void collapse when RHR flow is restored.
Table 2.5.4-1 lists the significant events for this case.
The RHR pumps are postulated to be stopped at five minutes as before. The operators enter the
" Loss of kHR While Operating at Mid-loop Conditions" guideline at Step 1.
There are no known drain paths assumed, so Step 2 is completed. As directed in Str.p 3 the time to boiling is determined to be less than 10 minutes from Figure 1 of the guideline.
Steps 4, 5, and 6 are not modeled in TREAT-NC, but it is assumed the operator initiates evacuation of the containment building, begins containment closure actions, and starts the containment fan coolers.
Step 7 requires a check of the RCS level.
It is found to be near mid-loop, and the guideline directs the operators to Step 11.
ARG-1 Rev. O
At Step 11, the operators again check for any leaks in the RCS.
There are none assumed in this case. Step 12 is bypassed since the steam generator nozzle dams prevent the operator from establishing a secondary sink.
Steps 13 and 14 require the venting of the RHR system and preparation for starting the RHR pumps. A charging pump is run prior to and following the establishment of RHR flow to increase the RCS inventory.
In this case it is assumed the RHR pumps are restarted at 80 minutes.
The simulation ends when the RHR flow cools the RCS to the initial conditions.
Figure 2.5.4-1 shows the RCS pressure transient.
The RCS reaches saturation at 19 minutes, and starts to pressurize. The high decay heat produces a relatively large core boil-off of about 15 lbm/s or 109 gpm. The vent path through the safety valves limits the steady state pressure to just over 8 psig.
When the hot side indicated level decreases below mid-loop, a charging pump is started at 65 minutes. With a flow rate of 109 gpm the charging flow can just keep up with the l
boil-off, and there is a slight pressure reduction.
To increase the level faster and prepare for the restart of a RHR pump, the charging flow is increased to 218 gpm at 70 minutes. This is about twice that of the toil-off flow.
This increases l
the depressurization.
Finally, the RHR flow is reestablished. The flow is started at half of the initial flow rate,1450 gpm, and all of this flow bypasses
)
L the RHR heat exchanger. The heat exchanger and total RHR flow are increased latet l
in steps to maintain a cooldown rate of less than 100 F/ hour.
These actions are also taken to minimize the level loss due to shrink and void collapse as the RHR flow cools the RCS.
The amount of voiding in the core is high enough that a rapid depressurization (and level loss) occurs as soon as RHR flow is restored.
This is evident in the figure at 80 minutes.
l Figure 2.5.4-2 shows the temperatures of the core exit and downtomer during the transient. The hot side temperature begins to decrease with the initiation of charging flow at 65 minutes. The increase in the charging flow causes a further i
decrease in the core exit temperature. The reestablishment of P.HR flow is I
apparent by the rapid decrease of the hot side temperature at 80 minutes. When the RHR flow is reestablished, it is first started with the heat exchanger bypassed. This causes the cold leg and downcomer temperatures to increase. As the RHR and heat exchanger flows are inc, eased the hot and cold side temperatures decrease to almost their initial values, where the simulation is terminated.
ARG-1 Rev. 0 l
l r
I
The core / upper plenum and downcomer mixture levels are shown in Figure 2.5.4 3.
A similar graph is shown in Figure 2.5.4 4, the core /upoer plenum and downcomer indicated levels. As the hot side heats up, and the density of the hot side fluid decreases, the mixture level increases. The cold side level decreases to maintain a pressure equilibrium.
The hot side level begins to decrease when the loss of inventory from the pressurizer (through the open safety valves) becomes significant. At 65 minutes when the charging flow is started, the decrease in the level is reduced.
Since the charging flow is not sufficient to keep up with the combined effects of boil off and shrink of the ho* side fluid (the relatively cold charging flow collapses some voidth the initial charging flow only slows the loss of hot sidt level.
To increase the level faster the charging flow is increased at 70 minutes to about twice the rate of core boil off, 218 gpm.
This significantly increases the hot side level, and causes a further increase in 1
the cold side level.
l The hot side level is allowed to reach about three inches 6bove mid loop btfore the RHR flow is restarted at 80 minutes.
The charging flow is i
continued at 218 gpm to reduce the expected loss of level due to the vnid l
collapse and shrink.
As discussed earlier, the RHR flow is established at 4 i
low rate with all of the flow bypassing the RHR beat exchanger.
Even with these precautions, the h:t side indicated level drops 0.6 feet (the mixture level drops 1.0 foot) when the RHR flow is restarted "harging flow at 218 Spm is left on until the hot side indicated levs' 4t about four inches above mid loop. This occurs at 98 minutes.
The h st
-tel then cat exchanger o seases slowly, but remains above mid-loop as tht as a flows are increased. When the simulation is terminat hot and cold side lovelt are about at mid loop.
Figure 2.5.4 5 shows the total RHR flow returned to the cold legs.
Initially this flow is 2900 gpm. The loss of flow occurs at five minutes, When the flow is reestablished at 80 minutes, it is kept at 1450 gpm for 20 minutes.
During this time the flow through the RHR heat exchanger is increased, and the bypass flew is decreased. To keep the hot $1de level loss to a minimum F
and control the cooldown, the flow is increased gradually until the original flow of 2900 gpm is reached.
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To summarize thete results. Cise D f.atures a loss of RHR recovery scenario for a high decay heat case (two days after shutdown) with a large hot side vent (three pressurizer safety valves removed). Although the relief path modeled is not adequate to limit the RCS pressure for the case where there are cold side openings (see Case C), the vent path is still large enough at
)
this early time after shutdown to maintain RCS pressure significantly less than 20 psig.
The actual pressure reached is about 8 psig.
Since this is significantly less than a typical SG nozzle dam test r essure (i.e..
28 psig), there is no concern w.th the potential f ailure of a cold side SG nozzle dam and the resulting rapid loss of RCS inventory and core uncovery.
Thus, for the configuration modeled in Case 0, the hot side vent is large enough to limit the rate of RCS inventory loss to simple boll off.
After approximately one hour, enough RCS inventory is boiled off to require makeup (hot side indicated level below mid loop).
Boil off at this early time after shutdown is within the capacity of one F0 pump (108 gpm).
To refill level faster, charging is increased to twice this value (to 218 gpm) and maintained at this flow rate while RHR cooling is restored.
l Due to the high decay heat and low RCS pressure, the core / upper plenum i4 average void fraction reaches approximately 20% prior to RHR restart.
In addition, there is essentially no water in the pressurizer to help refill the upper plenum during the void collapse which occurs immediately following RHR restart.
For this comparatively high void fraction case, the mixture levels in the upper plenum and hot leg regions drop approximately 1.0 ft (0.6 0.7 ft for indicated level) when RHR flow is restarted (at 1450 gpm bypassing the RHR heat exchanger). To reduce the amount of void collapse when the RCS is boiling, it is recommended that RHR flow be restored slowly, it is also recommended that high charging flow or high pressure S1 be operated prior to RHR restart to reduce the void fraction and increase inventory. Otherwise, a rapid loss of level due to void collapse will occur and the RHR pumps could loose suction pressure.
After RHR flow is restored. Case D also demonstrates that it is possible to control the cooldown rate to less than IG0 F/hr by slowly increasing the 0
flow through the RHR heat exchanger and slowly decreasing the flow through the bypass line.
ARG 1 Rev. O
l l
l Table 2.5.4 1 Time Table of Everts for Case D
[1tni Time (Hinultu RCS Vented (3 Pzr Safety Valves Removed),
Mid loop Operation at 2900 gpm RHR Flow, 05 Two Days After Shutdown. High Decay Heat (16 A MWt),
All SG Nozzle Dams Installed (No Cold Side Openings)
Lots of RHR Cooling Postulated (Guideline Entry) 5 RCS Reaches Saturation (212*F) 19 Indicated Hot Level Swells to 3.2 ft 12 ft Above Mid loop).
RCS Conditions Hear Steady State Boil off Conditions 26 (8 psig,15 lbm/s boil off, 20% Core Avg Void Fraction).
Indicated Hot Side Level at Mid Loep. Operator Establishes a Minimum Charging Flow (43 gpm) Prior to RHR Restart.
65 (Step 14)
Charging Flow increased to 109 gpm (15 lbm/s) 66 70 Indicated Hot Side Level Shrinks to 1" Below Mid loop.
Charging Flow increased to 218 gpm (About Twice Boll of t, 70 - 71 Indicated Hot Side Level 3.3" Above Mid ioop.
Charging Maintained at 218 gpm as RHR Cooiing is Re established 80 RHR Flow Established at 1450 gpm, All Bypaes Flow B0 82 Indicated Hot Side Level Orops 0.6 ft (1.0 ft Actual)
RHR Hx and Bypass Flow Control Valves Adjusted to Maintain 85 - 86 1450 gpm RHR Flow at a Cold Leg Return Temperature of 14~~'
ARG-1 Rev. O t
Table 2.5.4 1 (contirtued) f Time Table of Events for Case O LY.tal liteJMinutfil i
RHR Return flow to cold legs Decreased to 125'F 90 91 (Core Exit, Ocwncomer Temperatures Stable at 204'F.131'F)
Charging Flow Stopped (Indicated Hot Level 4.3 Alwie Mid loop) 98 i
RHR Flow Increased to 2175 gpm, Return Temperature - llo'F 100 101 i
RHR Flow increased to 2500 gpm, Return Temperature = 95'T 115 116 RHR Flow increased to 2900 gpm, Return Temperature = 80'F 130 131 End of Transient Modeled RCS Conditions Similar to the 150 Initial Conditions J
1 i
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4, 2.5.5 Analysis for Case E for the evaluation of Case E, the possibility of bleed and feed recovery is
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investigated for some low decay heat cases. Before describing the cases, it'is instructive to briefly summarize the results for a high decay heat case of 17.8 MWt which was previously analyzed.
Bleed and feed recovery for the high decay heat case was investigated in Case G.8 of WCAP 11916.
For that scenario one pressurizer PORV was assumed to be open. A 60 lbm/sec (434 gpm) cold leg injection flow from a charging /S! or high head $1 pump was used to increase the RCS inventory. At this charging (or SI) flow, the RCS pressure stabilized at approximately 225 psia (210 psig) and the pressurizer and most of the upper plenum filled with water.
The mixture inside the pressurizer was about 15'F subcooled prior to its discharge through the PORV. The temperature corresponding to this subcooling is about 375'F.
Although this is greater than the normal RHR cut in temperature of 350 F, it is less than -the usual RHR design temperature of 400'F.
0 Therefore, restarting RHR at this temperature would be acceptaple.
As a last resort this mid loop application of the bleed and feed mode of
)
recovery was considered acceptable for cases where the steam generators were dry and steam generator nozzle dams were not installed.
Note that this bleed and feed recovery is similar to the bleed and feed recovery used in FR H.1 except a reduced set of equipment (one PORV and one charging or SI pumo) can be used for this application.
Case G.8 of WCAP-11916 also studied bleed and feed at a lower cold leg injection flow of 30 lbm/sec (217 gpm).
For this case, RCS pressure stabilized at approximately 500 psig and the mixture at the core exit and in the pressurizer was a two-phase mixture.
The corresponding temperature exceeds the typical RHR design temperature of 400*F. Thus, efforts to increase the.
Injection flow to lower the temperature would have to be considered before the RHR system could be restored to operation.
Extending the results of Cast G.8 to a lower decay heat of 9.0 MWt it is estimated that at 60 lbm/sec (434 gpm) the core exit temperature can be maintained below 212 F if the RWST temperature is less than 70*F.
i ARG 1 Rev. 0
For the 4 loop. 3411 HWt plant this would be the approximate decay heat nine days (216 hours0.0025 days <br />0.06 hours <br />3.571429e-4 weeks <br />8.2188e-5 months <br />) after shutdown.
The RCS pressure for this case would be detern.ined by the pressure drop across the pressurizer PORV(s).
For one PORV open this pressure drop is estimated to be 74 psi. This makes the RCS pressurein the loops about 100 psig after including the pressure due to the height of water in the pressurizer. With two PORVs open, the pressure drop across the PORVs is significantly reduced to about 19 psi.
The pressure in the loops would then be about 45 psig.
This pressure still exceeds the routine test pressure of the steam generator nozzle dams, 28 psig, but is comparable to the typical minimum yield strength of the steam generator nozzle dams, approximately 50 psig, for the post refueling (even lower decay heat power, 3.4 MWt) an injection flow is selected which maintains the core exit temperature less than 212tf for an RWST temperature of 70*F.
This flow is 22.8 lbm/sec (165 gpm) and would be well within the capacity of one centrifugal charging pump or two positive displacement charging pumps. With one PORV open, the pressure drop across the PORV would be approximately 11 psi and the corresponding pressure in the RCS loops would be about 36 psig. With two PORVs open, the pressure drop would be less than three psi, making the RCS pressure in the loops approximately 28 psig.
Thus, for the case with two POP.Vs open, the RCS pressure would be comparable to the usual test pressure of the steam generator nozzle dams, if the injection flow is further reduced to 15 lbm/sec (109 gpm), the steady state flow through one pressurizer PORV is expected to reach two phase critical flow conditions at a pressure of 35 psia (20 psig) inside the pressurizer (i
- 259'f).
The pressure in the loops will be approximately 45 m
psig.
The final case investigated here applies to those plants with the electrically operated pressurizer PORVs which cannot open unless the RCS pressure is at least 100 psig. Without any vent paths in the RCS it is expected that it would take at least several hours following the loss of RHR cooling to reat.h this pressure. This is based on an extrapolation of the pressurization curves from Section 3.3 of WCAP 11916. Once the PORV opens, a very low charging flow would ARG-1 Rev. 0
be needed since at the higher pressure all of the decay energy would be relieved as boil off through the pressurizer PORV.
In one possible situation RCS pressure is allowed to increase to 120 psig.
At that point the PORV is opened and saturated critical flow of 3.0 lbm/sec will go through the pressurizer PORV.
The corresponding makeup requirement is only about 22 gpm.
A further increase (decrease) in makeup flow will result in a decrease (increase) in RCS pressure. Although a stable bleed and feed solution can be predicted for this case, it would take a number of hours to develop.
Before attempting this relatively high pressure bleed and feed operation, it would be preferable to delay pressurization by charging at a higher flow, possibly at a rate on the order of 100 to 200 gpm. This flow should be sufficient to suppress bolling in the core.
Table 2.5.5 1 summarizes all of the cases discussed in this section.
ARG 1 80 -
Rev. 0
.~
lable 2.5.51 Summ3ry of Steady State Bleed and Feed Recovery Conditions Decay Approx. Time RCS Pressure Core Exit Cold Leg inj. No. PORVs Heat After Shutdown in loops Temperature Flow open (MWtl (days)*
(osial
,fD (1bm/seci (c el 17.8 2
210 375 60 434 1
17.8 2
500 470 30 217 1
9.0 9
100
<212 60 434 1
4 9.0 9
45
<212 60 434 2
3.41 30 36
<212' 23 165 1
i 3.41 30 28
<212 23 165 2
3.41 30 45 2b9 15 109 1
3.41 30 120 350 3
22 1
Notes:
For a typical 4 'eoop plant with licensed core power of 3411 MWt
- Flow area is 0.011 ftr per valve with a flow loss coefficient, K 1.4
Rev. O k-
i 2.5.6 Analysis for Case E This case represents a situation where the plant is performing maintenance on the Reactor Coolant Pumps (RCPs) in two of the loops.
The RCp seal packages amd Number 1 runners are already remov6d. All steam generator nozzle dams are in place, and the pressuriter PORVs are opened to the pressurizer Relief Tank (PRT).
The PRT rupture disk has been removed, so the pressurizer is essentially vented to the containment atmosphere. The plant has been shut down for about 30 days, and one third of the fuel has been replaced in the refueling operations.
The decay heat is relatively low, 3.4 MWt, and the RHR system flow is 1000 gpm.
This is saintaining temperatures of about 82'F in the cold sides and 105'F in the hot sides.
As in the other cases, the RHR cooling is postulated to be lost at five minutes. The guidt:line, " Loss of RHR While Operating at Mid loop Conditions," is entered at this time.
Table 2.5.6 1 shows the important l
events in this transient.
It is assumed the RHR pump is not restored for approximately 5.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />, so the table shows the time in hours as well as minutes.
1 The guideline is entered at Step 1, and the operator performs the actions through Step 6.
Step 7 directs the optrator to Step 11, as the RCS hot side indicated level is still near mid loop.
At Step 11, the operator is to isolate any leaks in the RCS.
In Step 12 refilling the steam generators is considered.- The nozzle dams cannot be removed easily so the operators decide not to establish a secondary heat sink.
Step 13, with the preceding notes, is next. The time to boiling is relatively long because the decay heat is small.
Vent operations for this case are assumed to take several hours to complete, and the operators are delayed in performing Steps 14 and 15.
The operators monitor the RCS and ensure the fluid remains at a level appropriate for RHR restart as instructed in Step 14.
Charging flow is later established and increased when-the RHR system is restored. The operators establish the RHR flow slowly and adjust the RHR heat exchanger and bypass flow control valves to limit the cooldown to 100'f.
Charging flow is stopped when the RCS level is seen to be increasing, as directed by Step 16.
1 ARG-1 Rev. 0
figures 2.5.61 through 2.5.6 4 si.ow the RCS pre:sure, temperatures, and levels, respectively for this case.
The time of this transient is long, so the scale in these figures is in hours.
This tends to exaggerate some features on the graphs when compared to the other cases.
The RCS prJssure increases rather slowly once the saturation temperature is reached at 90 minutes.
The loss of the steam generator expansion volume and availability for steam condensation because of the steam generator nozzle dam installations is somewhat countered by the low decay heat and the vent path through the PORVs. The pressure rise is changed slightly at 40 psig when the RCP shafts lift from their backseats.
The pressure continues to increase until a charging pump is turned on at 4.1 hours1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />.
The initial 44 gpm flow of cold water from the charging pump is sufficient to condense some of the steam in the RCS, causing a slight reduction in the pressure and temperature.
These parameters are shown in Figure 2.5.6 1 and Figure 2.5.6 2, respectively.
The charging flow is subsequently increa:ad to 218 gpm over 20 minutes. As more cold water is pumped into the RCS, more steam is condensed, further reducing the RCS pressure and temperature.
When the RHR pump is finally restarted at 4.4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, the pressure and temperature reduction is rapid. To minimize the loss of fluid level at the RHR intake as the hot sides are cooled down, the RHR system is started with a minimum flow of 500 gpm, all bypassing the heat exchangers.
In effect, the RHR system pumps the hot side fluid into the cold sides, and the hot side water is replaced with colder water from the vessel downcomer.
This condenses much of the steam, but causes heating of the cold legs.
This effect is shown in Figure 2.4.6 2.
When the heat exchanger flow valve is finally upened, it is opened slowly.
The charging pump is turned off shortly before the heat exchanger flow is initiated. The termination of the charging flow causes a change in the rate of pressure and temperature decrease, but the addition of the heat exchanger flow allows the cooldown to continue.
The RHR flow and the flow through the heat exchanger are increased slowly over 105 minutes.
The simulation is terminated at 6.1 hours1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> when it is apparent the RCS temperatures will eventually reach the initial temperatures of 82'F in the cold sides and 105'F in the hot sides.
L l
L ARG 1 83 -
Rev. O L
~_
The graphs of the hot and cold side levels, figure 2.5.6 3 and Figure 2.5.6 4, show the effect of starting the charging pump in anticipation of the shrink and void collapse of the RCS fluid, and the associated loss of level.
As before, the indicated leve, is the collapsed level above the bottom of the hot legs.
Early in the transient, the cold side level drops in response to the density change of the the hot side fluid and the loss of mass through the RCP seals and PORVs. This has a ' benefit" in that the level at the suction drops to the level of the RCP backseat.
This reduces the inventory loss at the seal.
The pressurizer level increases in response to the increase in the RCS pressure and fluid volume, as shown in Figure 2.5.6 5.
The hot side level remains relatively stable near mid loop for most of the transient, until the charging pump is started.
Starting the charging pump increases the level in the downtomer and hot side.
Charging also effectively condenses some of the steam in the RCS by reducing the steam flow from the core region.
The action to increase the charging pump flow to 218 gpm is done more in anticipation of the loss of level when the RHR is started (because of the void collapse and shrink of the coolant). This rate is well within the capability of the centrifugal charging pumps '(for high pressure plants), and i
equivalent to two positive displacement pumps (for low pressure plants).
(See Section 2.5 for a discussion on the relevance of these numbers for plants which are smaller than the reference plant.) The pressurizer fills as the RCS heats up, but the temperature and inventory losses in this case are high enough to cause concern about the loss of level at the RHR intake, even with the fluid which would be added by the draining of the pressurizer.
The rapid decrease of the hot side level at 4.4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> when the RHR system is started shows why charging flow is needed before RHR flow is restored.
The decrease in level in the hot side is almost six inches, even with the fluid added from the pressurizer as it drains, if the hot side level was lef t at mid loop, it is apparent the RHR pump would have been stopped shortly after being started.
It-should be noted that the charging pump may have been left on longer than was necessary, as the pressurizer began to refill once the charging flow was increased at 4.4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. This action helped ensure the fluid level at the RHR intake remained high enough to prevent cavitation or vortex formation.
ARG-1 84 -
Rev. 0 e.
The flow rate through the RCP seal area is dependent upon the RCS pressure w
and the RCP model. The leak rates assumed here are comparatively high, but still typical for most pump medels.
For the RCPs postulated for this analysis the flow rate is limited to less than 5 gpm until the RCS pressure reaches 40 psig.
At 40 psig, the RCP shaft lifts from its backseat, and the flow rate increases rapidly to about 45 gpm.
If the flow were all liquid, it is likely the leak would have a flow rate closer to 75~gpm.
Figure 2.5.6 6 shows the flow rate through the RCP seal.
Up to the time when the RCS reaches 40 psig the flow rate is not very l'arge.
This is due to the fact that the cold side levels drop to the level of the backseat in the pump suction piping.
The increase in the flow rate when the RCS pressure reaches 40 psig is apparent. The increase in flow causes a short lived, small decrease in the pressure, but when the pressure again increases, the flow rate rises abruptly. This occurs at 4.1 hours1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />.
This leak rate would continue to increase dramatically as the RCS pressure increases, if that is allowed to happen.
Fortunately, it takes almost four hours to attain 40 psjg in this case. Also, with the low decay heat the charging flew causes a pronounced reduction in pressure. This stops the large leak rate shortly after it starts.
For similar situations where the decay heat is larger, it is likely the RCS pressure would reach the pressure needed to lift the RCP shaft more quickly than shown in this case.
For Case F, however, the maximum pressure the RCS would reach is about 50 psig " h both PORVs open and both of the RCP shafts lifted.
This maximum pressu...s a function of the critical flow through the PORVs and the RCP seal area, and will vary from plant to plant. At low decay heat, as in this case, the maximum pressure will be comparable to this value.
Another important consideration is the impact of the pressurization on the nozzio dams.
Although some dams have been tested and found to withstand about 50 psi or more at yield, they are usually only routinely tested to a pressure about half of this value, i.e., about twice the operating pressure when the refueling cavity is flooded.
The installation of no.ule dams reduces the expansion volume available for the steam produced in the RCS.
It also removes the ability of the steam generators to condense steam and maintain RCS pressure and temperature at more desirable levels.
In the situation presented in this case, the decay heat is sufficiently low that such pressures require a relatively long time to attain.
If the decay heat is higher, the pressure and temperatures would increase more rapidly once the ARG 1 Rev. 0 E
_ _. ~ _ _ _ _ _... _ _ _. _ _
I RHR flow was lost.
Should a nozzle dam fail in the cold side before one fails in the hot side. RCS inventory would be rapidly lost.
The most probable recovery action then is hot side injection to prevent core
- uncovery, if faced with this type of situation the operators may want to consider steps to either slow or prevent a rise in temperature and pressure through the use of charging or safety injection flow (or other means) until the RHR system can be resto-ed.
These actions may prevent the failure of a nozzle dam.
~
Although the scenerlo presented in this case with steam generator nozzle dams appears acceptable for the low decay heat case, special precautions would be necessary with a higher decay heat. These additional precautions include providing a large hot side vent and/or the capability for hot side injection with at least one high pressure safety injection pump.
4' f
ARG 1 86 -
Rev. 0
Yable 2.5.6 1 Time Table of Events for Case F Event Time (minutes)
Time thour,tj RCS Vented with 2 PORVs Open to PRT, 05 0
0.08 PRT Rupture Disk Removed, Mid loop Operation with RHR Flow at 1000 gpm, RCP Seal Repair Operations in Loops 1 and 2 (seal packages removed)
Loss of RHR Cooling, 5
0.08 Entry into guideline RCS Reaches Saturation 90 1.5 RCS Pressure Reaches 40 psig, RCP 207 3.4 Seal Leak Rates increase Charging Flow Started 245 4.1 Charging Flow increased and RkR Flow 266 4.4 Restarted Charging Stopped When Level Appears 280 4.7 to Stabilize Initial Warmup of RHR and RCS Complete.
294 4.9 Operators Start 100'F/hr cooldown End of Transient modeled 402 6.7 ARG 1 87 -
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'2.5.6 Analysis for Case G This case is very similar to the previous case, Case F, in that the plant is shutdown with keactor Coolant Pump (RCP) seal maintenance operations in progress in two of the loops.
(The RCP seal packages are removed.)
The presturizer PORVs are opened to the Pressurizer Relief Tank (PRT), and the rupture disk has been removed from the PRT. This case is assumed to occur only five days after shutdown and has a higher decay heat, (11.3 MWt instead of 3.4 MWt) than Case F.
Consequently the initial RHR flow is higher at 2000 At before the RHR system is maintaining RCS temperatures of about g;s 818F in the cold sides and 119'T in the hot sides. An important difference between this case and Case F is that there is water in the secondary side of two steam generators.
The average temperature of the secondary water is assumed to be 100'F. Given these initial conditions, there is also a similarity between this case and Case A.
The time table of significant events for Case G is shown in Table 2.5.6 1.
Again, the RHR pump is assumed to be stopped at five minutes.
The operator then enters the " Loss of RHR While Operating at Mid loop Conditions
guideline at Step 1.
At Step 2 all drain paths are assumed to be isolated.
The graph in Figure 1 of the guideline-indicates the time to boiling is rather short, so the evacuation and cicture of containment is started, as directed in Steps 4 and 5.
The containment fan coolers are started according to Step 6.
At the Step 7 level check, the RCS level is still near the mid-loop level, so the next step performed is Step 11.
It is assumed there are no kr.own RCS leakage paths.
In Step 12 the operators confirm a secondary hett sink is available in two steam generators and open the steam generator PORVs.
As shown in Table 2.5.7-1 the RHR pump is assumed to be inoperable until 185 minutes. Until then, the guideline requires the maintenance of RCS level and the preparation of the RHR system for operation (Steps 13 and 14).
A charging flow of 109 gpm for five minutes is used to restore level at 145 minutes, as the indicated level had dropped to about six inches below I
mid loop.
s ARG 1 94 -
Rev. O L
f Onc( the RHR pump is started, the operators start a charging pump to maintain the hot side level as the fluid shrinks 'during the cooldown. This is Step 15c of the guideline.
The charging flow is kept at 218 gpm for 10 j
,(
minutes, then stopped. A simulation was done without the charging flow on j
i during the cooldown.
The hot side level dropped below an indicated level of 0.75 feet.
The guideline directs the operators to maintain the level at mid loop to prevent vortex formation and/or cavitttion at tne RHR pumps.
With such a low level, it is possible this would have occurred, even with a low RHR flow.
The simulation is terminated after the RCS hot side temperature reaches 140'F, and the operator exits the guideline.
Figure 2.5.7 1 shews the RCS pressure transient.
The effect of the water in the seenndary side of the steam generators is evident if this graph is compared to the similar graph in Figure E.5.6 1 (Case F).
The maximum pressure is only about 23 psig after three hcurs, even with the higher decay heat. The slight pressure drop at 105 minutes is due to an increase in the primary to secondary heat transfer modeled af ter the active steam generators reach saturation.
In a real situation this " dip" would not be expected, as the change in heat transfer rate would not be abrupt. The charging pump is started at 145 minutes, and the introduction of the relatively cold water produces another decrtase in the RCS pressure.
This is augmented by a surge of fluid out of the pressurizer. When the charging pump is stopped five minutes later, the pressure again increases until the RHR pump is restarted at 185 minutes, The seenndary side temperature in the two active steam generators reaches approximately 230'F, as shown in Figure 2.5.7-5.
The effective boiling heat transfer in the secondary side of the steam generators and the reflux condensation in the priury side prevents the rapid ten:perature and pressure increases seen in Case F, even with the higher heat input to the RCS frora the core.
ARG 1 95 -
Rev. 0 E
The hot and cold side temperatures are shown in Figure 2.5.7 2.
The hot side temperature also shows the effect of the boiling heat transfer and the charging flow at 105 minutes and 145 minutes, respectively. The downcemer I
temperature increases when the charging flow is started.
This is due to a core flow reversal as the pressurizer drairs The RCS could have been cooled at a slower rate by starting the RHR flow at a minimum and controlling the RHR heat exchanger bypass flow to keep the hot side temperature decrease rate down.
This would have reduced the rate of charging flow needed to maintain level in the hot side, as the shrink and void collapse would have been slowed.
The hot and cold side levels are shown in Figure 2.5.7-3 (mixture levels) and Figure 2.3.7 4 (indicated level). The cold side level drops in response to the decrease in the density of the hot side fluid.
The level at the RCP also drops. This further reduce > the already very low leakage from the RCP seal areas.
(As discussed in Section 2.5.6, Case F, any flow through the RCP seal area was very small until the RCP shaft lifted from its backseat at about 40 psig.) When the hot side level drops to about six inches below the mid loop level.(mid loop - 27.35 feet) a charging pump is started to restore the level at 145 minutes.
A flow rate of 109 gpm is maintained for five minutes until the level again reaches mid-loop. The increase in t.ie cold side level is apparent in the graphs.
Once the RHR pump is restarted at 185 minutes, the rapid cooldown used in this simulation causes the pressurizer to drain quickly.
But, even the added fluid from the pressurizer is not able to stop the loss of level in the hot side due to the void collapse and fluid shrink, in an attempt to compensate and maintain sufficient level at the RHR intake, 218 gpm of charging flow was started at 192 minutes. This stopped the hot side level decrease before it went below mid-loop. Dependir) upon the RHR system design, it is possible that some cavitation would have been seen in the RHR pump, so the initiation of charging or safety injection flow to increase the hot side level prior to the restart of the RHR pump is considered prudent in most situations where the RHR flow has been lost for a prolonged period of time.
ARG-1 Rev. O
- _. ~ - _
Figure 2.5.7-5 shows the temperature of the secondary water in the filled steam generators.
The models used in this analysis do not include any secondary flow or cooling other than through heat transfer with tiie primary l
side. With the steam generator PORVs open, and the secondary side boiling, some feed water will eventually be needed, but the steam generators could boil for a number of hours before any secondary inventory would have to Le replaced.
The addition of secondary side makeup will also hcip reduce the secondary side temperature, if the RCS temperature stayed about the same, the larger temperature difference the primary and secondary sides would result in higher heat transfer to the steam generators. However, if the secondary side makeup flow is restrictod such that the secondary side fluid temperature does not decrease below 212*t, (boiling is allowed to continue in the secondary fluid) the much larger heat transfer resulting will enhance the heat transfer from the RCS, further restricting tt,e heating of the RCS.
5 ARG-1 Rev. O
6 Table 2.5.7-1 Time Table of Events for Case G Event Time (minutes)
)
RCS Vented with 2 Pzr PORVs open to PRT, PRT-Rupture Disk Removed, Mid-Loop Operation at 2000 gpm RHR Flow, 0-5 RCP Seal Repair Operations in Loops 1 and 2 L:ss Of RHR Cooling Postulated to Occur (Guideline Entry) 5 RCS Reaches Saturation (212'F) 26 Temperatures in Active SGs Reach Saturation (SG PORVs Opened per Step 12 to 105 Limit SG Pressure Rise) x Indicated Level Approximately 6" Below Mid-loop, I
109 gpm Charging Flow Used-to Restore-Level (Step 14) 145 - 150 RHR Vent Operations Complete, RHR Flow 185 - 190 Started and increased to 2000 gpn.
(Step 15)
Charging Flow Used at 218 gpm to Restore Level 192 - 202 During Cooldown Shrink (Sten 15)_
0 RCS Temperature (Tg) Less than 140 F, 205 Exit Guideline End of Transient modried 220 1
L ARG-1 Rev. 0 l
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- 103 -
Rev. O
3.
RECOVERY / RESTORATION TECHNI.Q11 The primary objective of the recovery / restoration technique incorporated into y
guideline ARG-1 is to restore RHR flow, in addition, other means of core cooling are established and protective measures are taken (e.g., containment closure and evacuation) while performing actions to reestablish RHR flow.
The following subsections provide a summary of the major categories of operator actions and the key utility decision points for guideline ARG-1, LOSS.0F RHR WHILE OPERATING AT HID LOOP CONDITIONS.
3.1 Hioh level Action Summary A high level action summary of the actions performed in ARG-1 is given on the following page.
These are discussed below in more detail.
o Check if RHR pumos should Be Stooped 1
If air is ingested into the suction of the RHR pumps, the pumps are likely to start cavitating.
The symptoms of' cavitation are typically erratic behavior of pump current (oscillations between high amperage values and low amperagu values), erratic behavior of pump flow and excessive pump noise.
Pump suction pressure (typically a local measurement) will also oscillate.
If any plant personnel are in the immediate vicinity of the pump, loud audible noises can typically be heard when air ingestion starts.
If no actions are taken, damage to the pump may occur due to vibration and the pump may eventually seize and stop running.
As soon as the operator.is aware that air ingestion or cavitation is occurring, actions should be initiated to protect the RHR pump.
If the air ingestion started during an RHR flow increase evolution, then a reduction of flow to the pre-air ingestion value may stop the air ingestion and the pump cavitation.
Inadequate level in the RCS hot leg will also result in air ingestion and pump cavitation. The operator should also check RCS level to ensure _ adequate level exists in the hot leg for continued pump operation.
If the RHR pump continues to cavitate or RCS level is not adequate to-allow continued pump operation, then the operator should stop the RHR pump.
ARG-1
- 104 -
Rev. 0
,.. s:
MAJOR ACT10ri CATEGORIES IN ARG-1 1-o Check If RHR Pumps Should Be Stopped o Address Containment Related Concerns o Establish Alternate Means of Decay Heat Removal o Establish Support Conditions And Restore RHR Cooling t
ARG-1
- 105 -
Rev. O L
o Address Contair. ment Related Concerns When the RHR system is lost, any nonessential personnel in the containment building should be evacuated. As an additional precautionary measure, those essential personnel entering the containment building to perform recovery or contingency actions may need to use respirators and don protective clothing.
Since RHR cooling has been lost, actions must be initiated to address the possibility of core uncovery and subsequent fuel damage due to RCS inventory loss.
Since two of the three fission product barriers may be open (the RCS and the Containment), actions must now be taken to close the containment and provide a barrier to the release of radioattive material should the event proceed to a core damage situation.
In addition to initiating containment closure, it is desirable to maintain containment habitability while recovery actions are being performed and to control the release of radioactivity should a core damage situation develop.
Area radiation should be monitored to determine if additional actions are necessary to control the release of radioactivity. Available fan coolers shoulv started to reduce containment heat load.
j o
Establish Alternate Means of Decay Heat Removal If the steam generators contain water and are not isolated from the RCS via nozzle dams or loop isolation valves, they can function as a heat sink for the RCS when the RCS starts to heat up. This, in turn, will significantly reduce the RCS pressure rise if the vent paths from the RCS are not adequate to prevent pressurization.
This also permits more options for feeding the RCS to maintain adequate inventory.
To establish an adequate heat sink, at least half of the SGs must be filled and the SG 1evel must be maintained above the top of the U-tubes. Half of the SGs correspond.; to one LG for a 2-loop plant, two SGs for a 3-loop plant and 2 SGs for a 4-loop plant. The steam generator condensation or reflux process is most effective near the bottom of the tubes on the inlet side of the steam generators.
If faced with a trade-off situation in which a fixed amount of flow is available, it is preferable to feed more steam generators at the slower rate per SG. The SG atmospheric relief valve (SG PORV) for the active SGs should be opened before the secondary water reaches saturation, i
ARG-1
- 106 -
Rev. 0 j
~ _.
e l
o Establish Suonort Conditions And Restore RHR Coolina Before attempting to restart the RHR pumps, the operator must ensure that the RCS inventory is adequate to support operation of an RHR pump. The minimum RCS level necessary for running 'he RHR pump is a function of RHR flow and the operator should ensure thic tr, level will be adequate for the anticipated RHR flow when the pump is restarted.
Refilling of the reactor coolant system can be accomplished in several ways.
First, a charging pump aligned through the normal charging flowpath can be used. Alternately, with the RCS depressurized, it is usually possible to makeup to the RCS by gravity feed from the RWST depending on the elevation difference between the RWST and the RCS.
The RCS can also be fed by the VCT since pressure in the VCT is normally high enough to transfer water into the RCS. However, to use the VCT, makeup capability through the blender must be available to ensure that adequate borated water can be added to the RCS and that dilution of the RCS will not occur.
If the RCS level is on scale, but less than the mid-loop elevation, the operator can monitor the changes in RCS level as he feeds the RCS.
Since the operator can monitor level, normal charging flow can be controlled to restore level to the mid-loop elevation.
If the indicated RCS level is offscale low on the mid-loop instrumentation system, then it is difficult to accurately estimate the inveatory remaining prior to core uncovery.
In this case, RCS makeup is initiated at a comparatively high rate using hot leg injection.
Once air has been ingested into the RHR system, it is necessary to purge the air from the lines in order to reestablish normal system operation.
The air entrainment is the result of vortexing at the RHR suction connection to the hot leg. Depending on plant layout, a significant amount of air may be trapped in the RHR inlet piping.
If there is a'long horizontal piping run, it could take over one hour for the air to bubble back into the RCS.
If the ARG-1
- 107 -
Rev 0 L.1
RHR pumps are needed before complete venting is accomplished, the RCS level should be increased to the top of the RCS hot legs and one RHR pump should be started at high flow rates.
Once' adequate level has been restored to the RCS and the RHR pumps have been vented, the operator can restart one RHR pump to restore cooling to the RCS water.
Prior to the actual start of the pump, the operator should be aware of the RCS level vs. RHR flow limitations to prevent the reoccurrence of vortexing and air entrainment. Also, starting an RHR pump may result in an RCS level decrease due to shrink or void collapse.
It may be necessary to add makeup to the RCS to sustain RHR pump operation. To restart one RHR pump, the operator would typically close the RHR heat exchanger flow control valves and start the pump on miniflow and then gradually iticrease the RHR flow until the desired flowrate is achieved.
If the operator is successful in restoring RHR flow, then ARG-1 can be terminated and the operator can return to his normal operating procedures for the condition that the plant is in (plant draindown, mid-loop operation, etc.).
If the operator is not successful in restoring RHR flow, then he should start trending core exit thermocouple temperatures and implement any 3
alternate actions for cooling the RCS (e.g., bleed and feed, feed and spill, spent fuel pit cooling) while continuing efforts to return RHR to service.
l l
l ARG-1
- 108 -
Rev. 0 l
3.2 Key Utility Decision pointt The only key utility decision point in this guideline in which tne utility
(
must determine the appropriate course of action is when RHR cooling cannot be restored and alternate means of core cooling should be established. Options are discussed as follows:
The desired RCS fluid inventory can vary depending on whether the RCS is opened or intact (no openings),
if the RCS is intact and the loops are not isolated with SG noule dams or loop isolation valves, a secondary heat sink using half or more SGs will be an effective alternate mode of decay heat removal that will last for several hours or longer.
Since there would be no significant fluid inventory losses for this case, makeup requirements can easily be met with a minimum amount of charging flow or possibly RWST (or VCT) gravity feed if iriitiated early enough.
For this situation, it should also be possible to refill and pressurize the RCS and then operate the RL s to sweep the noncondensibles from the loops and thereby improve the primary-to-secondary heat transfer. This latter mode of recovery, although not studied in the original spectrum of analyses presented in WCAP-11916, is a reason &ble extension to the SG condensation studies of that report.
If the RCS remains intact but the SGs cannot be made available for cooling, a bleed and feed mode of recovery similar to that used in FR H.1 can be used.
Studies indicate that a reduced set of equipment (one pressurizer PORV and one high-pressure SI pump) will be adequate for removing decay heat and maintaining RCS inventory.
For a high decay heat case, RCS level would stabilize well above mid-loop and RCS pressure would stabilize between 100 and 400 psig. Thus, RCS l
conditions would permit RHR cooling to be reestablished once other support conditions are achieved.
Some other bleed and feed scenarios for lower decay heat conditions are evaluated in Section 2.5.5.
If the reactor coolant system is not intact (for example, manways or valves removed), then RCS level should be restored to at least the i
mid-loop level required to support RHR pump operation as described L
earlier.
Consideration could also be given to initiating a feed and spill operation to prevent the RCS from reaching the bolling point.
ARG 1
- 109 -
Rev. O
Otherwise, a smaller amount of makeup (two to three times boil-off) is recommended to remove decay heat and increase level.
If there is a large cold side opening and the loop is not isolated with SG nozzle dams or closed loop isolation volves, it is recommended that makeup be provided to one of the intact loop cold legs to avoid potential spill from the opening.
Since most plants have a charging and an equivalent alternate charging line, it should be possible to inject into an intact loop provided there is only one loop with a cold side opening.
Other plant specific alternate sources for cooling the RCS, such as Spent Fuel Pool cooling, should also be explored.
Some plants have the capability to line up the spent fuel cooling system and cool the RCS.
i ARG-1
- 110 -
Rev. O
4.
DETAILED DESER!pTION OF GUIDELINI-This section provides a very detailed discussiori of the generic guideline ARG-1 to facilitate utility procedure writing and training efforts.
By presenting guideline background information in greater detail through the use of a :,tructured format (i.e., step descriution tables and step sequence t-
.s), plant specific applicability can be more easily determined.
The separate and unique subsections containing this information follow.
4,1 D.q. tailed Description of Steos. Notes. and Cautions This section contains a one page (or more) step description table for each separate guideline step, note, and caution. Notes and cautions are always presented relative to the step they precede.
Refer to the Users Guide in the WOG ERG Executive Volume for a discussion on the use of the step description
- tables, The Step Description Tables for the steps of guideline ARG-1 are presented on the following pages.
ARG 1
- III -
Rev. 0 f
l
11EP DESCRIPTION TABLE FOR ARG-1 STEP 1 CAUTION CAUTION:
Changes in-RCS pressure could result in inaccuracies in RCS level readings.
PURPOSE:
To alert the operator that it may be necessary to compensate RCS level readings due to changes in RCS pressure.
EA311:
Depending on the type of instrumentation used, the location of the instrument taps, and the high side tap vent path, the indicated RCS level may not be accurate and compensation may be necessary to determine the actual RCS level.
For example, a tygon tube vented to containment will show level higher than actual level if the RCS starts to pressurize resulting in additional water being forced into the tygon tube.
KNOWLEDGE:
N/A PLANT SPECIFIC Th' FORMATION:
N/A-4 ARG-1
- 112 -
Rev. 0
STEP DESCRIPTION TABLE FOR ARG 1 STEP 1 11LP:
Check If RHR Pumps Should Be Stopped e
PURPOSE:
To stop the RHR pumps if RCS level is not adequate to support pump operation or if the pumps are cavitating.
BASIS:
If air is ingested into the suction of the RHR pumps, the pumps are likely to start cavitating. The symptoms of cavitation are typically erratic behavior of pump current (oscillations between high amperage values and low amperage values), erratic behavior of pump flow and excessive pump noise.
Pump suction pressure (typically a local measurement) will also oscillate, if any plant personnel are in the immediate vicinity-of the pumps, loud audible noises may be heard when air ingestion starts.
If no actions are taken, damage to the pumps may occur due to vibration and the the pumps may eventually seize and stop running. As soon as the operator is aware that air ingestion or cavitation is occurring, actions should be initiated to protect the RHR pumps.
If the air ingestion started during an RHR flow increase evolution, then a reduction of flow to below the pre-air ingestion value may stop the air ingestion and the pump cavitation.
Inadequate level in the RCS hot leg will also result in pump cavitation and air ingestion.
If level is greater than (1), the operator should reduce flow to (2) which is an acceptable level vs. flow combination.
If the RHR pumps continue to cavitate or kCS level is not cdeouate to allow coris inued pump operation, then the operator should stop the RHR pumps until '.he proper operating conditions are reestabi,.:.;d.
KNOWLEDGE:
N/A PLANT SPECIFIC INFORMATION:
o (1)
Enter plant specific RCS level corresponding to the lowest allowable RHR flow from RCS' level vs. flow curves including allowances for normal channel accuracies.
o (2)
Enter plant specific lowest allowable RHR flow for mid-loon operations including allowances for normal channel'accurat es.
-ARG-1
- 113 -
Rev. 0 4
STEP DESCRIPTION TABLE FOR ARG-1 STEP 2 11[P:
Isolate Letdown And Known Drain Paths 1
PURPOSE:
To isolate letdown and all known drain paths to prevent further loss of RCS inventory.
BAS.IS:
A possible cause for the loss of RHR cooling could be attributed to a reduction in RCS inventory from excessive draining of the RCS. To prevent further reduction in RCS inventory, the operator should close all known RCS letdown and drainage paths which can be operated from the control room.
KNOWlrDGE:
N/A PLANT SPECIF.JC INFORMATION:
N/A
-1 l
l
-ARG-1
- 114 -
Rev. O
_..____._.___m STEP DESCRIPTION TABLE FOR ARG l STEP _3_.
111f:
Determine The Time _To Boiling Based On Existing conditions s',
PURPOSE:
To calculate how much time is available before containment closure actions must be initiated, j
f The time _to__ reach the boiling point in the RCS is determined using Figure 1
- and is. compared to the time which the utility has committed in their response to-Generic Letter 88 17 for containment closure actions.
If sufficient time (i.e. the time necessary-to perform Steps 7 through 14 before returning to Step 4) is not available, then steps must be initiated to evacuate containment and: establish containment closure.
In-the April lo,1987 event at Diablo Canyon Unit 2, the RCS reached saturation in 30 to 45 minutes following the loss of Decay lleat Removal (DHR). More importantly..this boiling caused RCS pressurization, an
--unanticipated condition. A different RCS configuration, such as blocked hot legs and an opening in the cold legs, could have quickly led to core uncovery
'following initiation of boiling. -Further,-the loss of DHR at Diablo Canyon occurred at a low initial RCS temperature and with a decay heat generation rate-less than half of that which could occur during loss of DHR accidents, For;certain RCS configurations, core uncovery can occur soon after boiling
. starts. Severe core damage can follow as soon as adiabatic heatup of.the core reaches the point of rapid chemical reaction. There are two important conclusions:
(1)
The time available for operators to respond to a loss of DHR can be far less-than previously believed. Actions are necessary-to reasonably assure an adequate operator response during such conditions.
(2)- This situation constitutes a previously not analyzed plant condition that can realistically be encoantered.
KNOWLEDGE:
r.
L Understanding of methodology for determining time to boiling (Figure 1)..
L Section-3.10 of WCAP 11916 explains several options available for performing J
these calculations.-
PLANT SPECIFIC INFORMATION:
o-(3)'
Enter plant specific time for containment closure actions as determined by response to Generic Letter 88-17.
Plant specific curve showing the time to reach saturation following a o
-loss of RHR at mid-loop in units consistent with expected outage durations.
l ARG-1
- 115 -
Rev. O p
_. _... _. _ _ _. _. -. _ _ _ _ ~.
STEP DESCRIPTION TABLE FOR ARG-1 STEP 4
11EP:
Initiate Actions To Protect Personnel. Working in Containment p
PURPOSE:
To ensure personnel-inside containme'nt'are protected from a.
_ potential adverse radiological' environment.
BASIS:
As the' core reaches boiling, steam may be released into-the containment through vents.in the-RCS.. The release of steam into the. containment can create a habitability (temperature) concern for operators who may be in the area to perform recovery actions. The potential also exists for release of radioactive materials from the RCS into containment causing-a contamination problem.
When the hHR system is lost, any nonessential personnel in the containment building should-be-evacuated.
As an additional precautionary measure, those essential personnel entering the containment building to perform recovery or contingency actions may need.to use respirators and don protective clothing.
Containment radiation should he periodically monitored and at some-point i*
may also1be-necessary to evacuate essential personnel from containment._
~
KNOWLEDGE:
p Plant specific guidelines.and procedures for radiological protection of personnel.
l-L PLANT SPECIFIC INFORMATI.QH:
N/A p,
l -
lC l-l F
i
~
j l
1 L
L
(:
i
-ARG-1
- 116 -
-Rev. O
STEP DESCRIPTION TABLE FOR ARG 1 STEP 5
112:
Initiate Actions To Establish Containment Closure PURPOSE:
To establish a boundary to prevent the release of fission products.
h,ASJji:
A If RHR cooling has been lost, and initial attempts to restore cooling have been unsuccessful, t.ctions must be initiated to address the possibility of core uncovery and subsequent fuel damage due to RCS inventory loss.
Since two of the three fission product barriers may be open (the RCS and the Containment), actions must now be taken to close the containment and provide a barrier to the release of radioactive material should the event proceed to a core damage situation.
The time available between the loss of RHR cooling, core uncovery, and core damage varies depending on plant ccnditions at the time of event initiation.
For adverse RCS configurations (i.e., a large cold leg opening with the hot icgs isolated), the time to core uncovery could be as short as 10 to 15 minutes (i.e., within miiutes after the RCS reaches saturation). Core damage can then start to occur at approximately 30 minutes if the core remains uncovered.
For more favorable RCS configurations (i.e., one with a large hot side vent), the time to core damage would exceed 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />, even for short
~
times after shutdown.
Longer times after shutdown, and/or especially favorable RCS configurations (i.e., water in the secondary side of at least half of the SGs), may further increase the available core uncovery or core damage time.
In any event, containment closure must be accomplished before potential core damage.
Therefore, the containment must always be in a condition such that it can be closed within the available time.
Once containment clnsure is initiated, closure should continue until controlled and stable decay heat removal has been restored (either RHR cooling or heat removal via the secondary plant) and the RCS has returned to a controlled and stable condition.
KNOWLEDGE:
lieans. to initiate containment closure.
PLANT SPECIFIC INFORMATION:
Plant specific guidelines for establishing containment closure during typical shutdown conditions.
Containment closure as discussed in this guideline includes establishing the desired position of available containment isolation valves to minimize releases outside containment.
ARG-1
- 117 -
Rev. O b
STEP DESCRIPTION TABLE FOR ARG 1 STEP 6
EIIE:
Start Available Containment Fan Coolers EVRPOSE:
To ensure that all available fan coolers are running.
BASIS:
This step instructs the operator to start all available containment fan coolers.
The intent of this step is to provide containment heat removal capability and therefore reduce containment pressure. With the possibility of temporary containment closure in effect, the containment pressure limitation may be substantially less than design pressure.
In the reference plant design, the containment fan coolers provide an alternate heat removal means to the containment spray pumps which are not available under these particular circumstances.
For those plants without containment fan coolers, other available containment pressure suppression equipment should be placed in service as appropriate.
KNOVLEDGE:
Other containment pressure suppression equipment available to be placed in service.
ELANT SPECIFIC INF0RMATION:
j N/A ARG - 118 -
Rev. 0
ff ~
STEP DESCRIPTION TABLE-FOR ARG-1 STEP 7 CAUTION 1 CAVTION:
Personnel working in containment should be warned before refilling the RCS to avoid inadvertent contamination of personnel working near RCS openings, PURPOSE:
To alert the operator of the need to warn personnel of the potential radiological * :ard near RCS openings.
BASIS:
A warning should be provided to those personnel working on or near RCS openings prior to refilling the RCS.
It may be necessary for some of these personnel to assist in the refilling effort and any directions provided should consider proper balancing of reactor and personnel safety.
[NOWLEDGE:
Location of RCS openings and potential radiological hazard.
PLANT SPECIFIC INFORMATION:
N/A ARG-1
- 119 -
Rev. O L
STEP DESCRIPTION TABLE FOR ARG-1 STEP 7 CAUTION 2---
CAUTION:
Only borated water should be added to the RCS in maintain adequate shutdown margin, PURPOSI:
To inform the operator that inadvertent dilution of the RCS without RHR flow may result in a loss of core shutdown margin.
BASIS:
With the RCS in a reduced inventory condition and the RHR pumps stopped, any dilution may result in a loss of core shutdown margin faster than expected.
Without RHR flow to mix the RCS, it is also possible to form pockets of diluted water that might not be detected by sampling or nuclear instrumentation until RHR flow is restored. Only borated water should be added to the RCS to maintain adequate shutdown margin.
For example, when using-the VCT for filling the RCS, makeup through the blender must have a high enough boron concentration to assure that a loss of adequate shutdown margin will not occur.
[NOWLEDGE:
N/A I
PLAN' SPECIFIC INFORMATION:
N/A ARG-1
- 120 -
Rev. O J
STEP DESCRIPTION TABLE FOR ARG-1 STEP 7 11EP:
Check RCS Level b
PURPOSE:
To determine if RCS level is sufficient to support RHR pump operation and if not to determine how the RCS should be filled.
SASIS:
If the indicated RCS level is offscale low on the mid-loop instrumentation system, then it is difficult to accurately estimate the inventory remaining prior to core uncovery. The operator should go to Step 8 and initiate makeup to the RCS at a comparatively high rate using hot leg injection.
If the RCS level is on scale, but less than the mid-loop elevation, the
- operator can monitor the change in RCS level as he feeds the RCS.
The RCS is refilled as directed in Step lo, If the RCS level is at or greater than the mid loop level, then preparations-for starting an RHR pump and restoring RHR flow are initiated beginning with Step 11, KNOWLEDGE:
N/A PLANT SPECIFIC INFORMATION:
o (4)
Enter the plant specific RCS level corresponding to mid-loop elevation in the RCS hot legs including allowances for normal channel accuracies, o
.(5)
Enter the plant specific RCS level corresponding to just on scale elevation in the RCS hot legs including allowances for normal channel accuracies.
ARG-1
- 121 -
Rev. O b
JJEP DESCRIP?l0N TABLE FOR fRG-1 STEP S._.
$12:
Refill The RCS As Follows PURPOSE:
To provide an RCS injection path that will result in refilling the RCS, BASIS:
Since RCS level is offstale icw when entering this step, it is difficult to determine the effectiveness of refilling the PCS by the normal charging or safety injection cold leg injection paths. The operators are therefore instructed to initiate hot leg (or upper plenum) injection with at least one high-head SI pump.
Since charging flow will generally keep up with 100 gpm boll off (expected boil-off several days after shutdown for a 4-loop plant), it is not expected that hot leg injection will be required unless the rate of RCS inventory loss significantly exceeds boil-off. Thigsituationcouldoccurifthereisa or larger (i.e., beyond the expected cold sioe opening approximately 1 in capacity of maximum charging) and a large hot side vent is not provided to relieve boil off steam.
If RCS pressure cannot be maintained less than approximately 2 psig and there is a cold side opening, the RCS level can rapidly decrease in the hot leg / upper plenum after the RCS reaches sd uration. The resulting loss in RCS inventory could lead to core uncovery.
An undesirable situation occurs when there is a large cold side opening and the loop with the opening is isolated on the hot side. The cold sioe opening could be caused by removal of a SG manway for SG tube inspection or maintenance, removal of a colo 169 check valve, failure of an SG nozzle dam, or removal of an loop isolation valve for repair or inspection. The loop would be considered isolated due to installation of the SG nozzle dams or closure of the loop isolation valves.
If a large enough hot side vent is not available to limit pressurization of the hot side, core uncovery could occur starting within several minutes after boiling starts.
Hot leg (or upper plenum) injection from one high-head SI (or charging /SI) pump would be necessary to prevent or lim!t core uncovery.
For this case, hot leg (or upper plenum) injection should be initiated prior to or within minutes after boiling starts to avoid significant loss of RCS inventory.
Hot leg (or upper plenum) injection would also be required if SG no7zle dams are installed in all SGs and a cold side dam were to fail first due to the increased RCS pressure resulting from RCS boiling. The hot leg inaection flowrate is considered high enough if the core residual heat is less than the sensible heat required to raise the temperature of the makeup water to saturation.
This flow is typically within the capacity of one high-head SI pump for a 2-loop plant. Accounting for realistic RWST temperatures and decay heat levels several days af ter shutdown, this hot leg injection flow may even be within the capacity of one high-head SI pump for a 4-loop plant.
It should be noted that recovery using cold leg injection at comparable flowrates may not be effective since the amount of cold water reaching the core may not be adequate to suppress boiling if some or most of the cold leg injection water flows towards the cold leg opening. Thus, only hot leg injection is ARG-1
- 122 -
Rev. 0 a
t STEP OCSCRIPTION TAELE FOR ARG.:1 STEP S_. (Cont.)
recommended to increase RCS inventory.
Hot leg injection was selected as the only means to refill the RCS in this step since it guarantees that water will be injected ir'o the core for all cases of interest. This includes some configurations which were previously suspected of being " flooded limited" (refer to Section 2.5.2 for additional details).
For some of these cases, cold leg injection may be adequate but the operator would not necessarily have the information to confirm the effectiveness of cold leg injection (i.e., knowledge of all RCS openings, core inventory level trends when RCS level is offstale low).
A more complete account of the makeup requirements for various RCS configuration can be found in Section 3.0 and summarized in Section 3.11 of the mid-loop analysis report, WCAP 11916, LOSS OF RHRS COOLING WHILE THE RCS IS PARTIALLY FILLED. A summary of these cases is also given in the Roger A. Newton letter on "Early Notification of Mid-loop Operation Concerns", OG-88-21 dated 5/27/88.
Once hot leg injt. ion is initiated, it is continued until the RCS fills to a high level (i.e.,
idspan in the pressurizer) or until the injection flow stabilizes with the flow through the opening.
The RCS is allowed to refill above mid-loop to avoid potential cycling of the S' ' umps (for additional details, refer to the analysis for Case C presente. in Section 2.5.3).
To avoid the need for hot leg injection, a large hot side vent path can be provided to limit the RCS pressurization, if there are cold side openings, the vent path area must be large enough to limit the RCS pressure to less 2 than approximately 2 psig. More precisely, the vent path resistance (K/A )
must be less than a certain value, as defined in the Appendix to Section 4.1.
KNOWLEDG1:
N/A PLANT SPECIFIC INFORMATION:
o (6)
Enter the plant specific PR2R level corresponding to water level being midspan in the pressurizer.
o Valve alignments necessary to establish hot leg injection.
o For two-loop plants witmt hot leg injection capanility, upper plenum injection should be useo, o
for three-loop high pressure plants, the charging /$1 pump (s) are used for hot leg injection.
l i
ARG-1
- 123 -
Rev. O l
. _....=
l STEP DESCRIPTION TABLE FOR ARG STEP 9 LT12: -
Go To Step 11 1
EURPOSI:
To direct the operator to the proper step.
SASIS:
Hot leg injection was initiated in Step 8 to refill the P,CS.
The operator is then directed to Step 11 to begin the recovery steps for restoring RHR flow, KNOWLEDG1:
N/A PLANT SPECIFIC INFORMATION:
N/A i
~'
ARG-1
- 124 -
Rev. 0
I
}JILSISCR1piIONTAB],L,[ji!Jful STEP,,19 _
1Hf:
Refill RCS To (4) k EWLPQil:
To increase RCS level to at least the mid loop elevatico to allow eventual return to RHR cooling.
BA515:
The RCS level is on scalv but less than the mid loop elevation.
in order to sufficiently restere the RHR system, the RCS level is increased and the RHR system is vented in subsequent steps.
Since the operator can monitor level, 4
he can control charging flow as ne:essary to restore level 10 the mid-loop elevation.
Alternatively, if a charging pump is not available, makeup to the RCS can be accomplished using gravity reed from the RWST, VCT overpressure, or other plant specific sources.
Depending on the RCS pressurizat',on rate, gravity feed may 'e successful in restoring RCS level to allow restart of the RHR pumps. Other gravity feed lineups which offer paths of less resistance may be more succe=sful in recovering inventory (RHR and Safety injection).
The slower pressuri?.ation rate caused by the availability of SG condensation significantly improves the operators chances of successful gravity feed and
-subp14 dent RHR recovery: first, because retarding the pressurization allows a greater operator reaction time and, secondly, assuming a given operator action, SGs will prolong the time over which the RWST feed is effective.
In addition to gravity feed, pressurized sources of borated water can also be u:ed to refill the RCS such as overpressurization of the VCT which would allcw transfer of water into the RCS.
However, to use the VCT, makeup capability tarough the blender must be available tn assu,J that adequate L
borated water can be added to the RCS, Other plant specific methods of refilling the RCS utilizing pressurized borated water sources or gravity feed systems should also be listed in this step.
ILNOWLEDGE:
Available RCS makeup sources.
PLANT SPECIFIC INFORMATIM :
o (4)
Enter the plant specific RCS level corresponding to mid+1oop elevation in the RCS hot legs including allowances for normal channel accuracies.
o Other plant specific methods of refilling the RCS.
o Low pressure plants typically cannot feed by gravity feed or VCT overpressure through positive displacement charging pumps.
ARG 1
- 125 -
Rev. O i-
- F
-STEP OtsCRipT10N TABLE FOR ltG 1 STEP 1L LT.II:
Identify And Isolate A.'.y Rc5 teskage PURPOSE:
To ensure any RCS leakage path is iso 16ted.
ILM11:
A possible cause for the lots of RHR cooling could be attributed to leakage from a drainage path that was inadvertently opened.
Since the control room operator is not able to completely verify the isolation of all potential.
drainage paths during mid loop operation it may be necessary to dispatch an operator to verify proper alignment of manual valves and to ensure all possible leakage sources are isolated.
KNOWLEDGE:
Location and status of potential RCS drain paths.
ELA.HLIPECIFIC INFORMATlqu:
N'A l
l
- 126 Rev. 0 ARG 1
+
l
.~-
HEP OESQLPliQ!i T ABLE FOB ARG 1 STLp 12_ NOT E-EQll:
To maintain RC$ pressure low enough for grasity feed, at least (7)
SGs must be refilled and utilized, f)f20EE:
To inform the operator that the ability to gravity feed the RCS is dependent on the availability of (7) SGs to remove heat from the RCS and thtes prevent pressurization.
[LAj)T:
The number of SGs available with water has a significant impact on the RCS If pressurization rate following loss of RHR cooling at mid loop operations.
the RCS pressurizes to about 25 psig (typical pressure for gravity fred limit from the RWST), then makeup to the RCS using gravity feed may not be effective within a reasonable time frame.
It thus is desiratie to establish a secondary heat sink using the proper number of SGs to prevent or limit the RCS pressurization.
MQWLEM(:
W The importance of maintaining the maxiraum number of SGs available for o
PLA4L 5fl0lf1(._ll((QEMATlfl{:
c (7)
Enter plant specific number of steam generators necessary to maintain RCS pressure low enough for gravity feed.
Refer to Background Document, The number of SGs necessary is discussed in the basis for Step 12.
o MG 1
- 127 -
Rev. O
STEP DESCRIPTION TABLE FOR ARG 1 STEP.lL SJH:
Determine If A Secondary Heat Gink Should Be Established PURPOSE:
To determine if a SG secondary heat sink is required to prevent or limit pressurization of the RCS.
RAEli:
Prior to establishing mid loop operating conditions, a decision is typically made on whether steam generators will be necessary for heat removal U the 4
RHR pumps are lost.
In order for SG condensation to become effect m, the RCS must reach saturation and the steam partial pressure at the Si tube entrance must exceed the saturation pressure corresponding to the temperature of the SG secondary water.
Unless a large vent is provided. RCS pressure will rapidly inc ease after the RCS reaches saturation until this condition is satisfied and the SGs start to become effectf..e heat sinks.
Referring to Figure 3.8.2 1 of WCAP 11916, the SGs with an initial secondary temperature of 140 F (Psat - 2.9 psia) start to become effective when RCS pressure 0
reaches approximately 25 30 psia or 10 - 15 psig.
For the Case A analysis presented in Section 2.5.1, the two active SGs start to become effective at a slightly lower RCS pressure (roughly 8 9 psig) since the SG initial secondary 0
temperature is only 100 F (Psat = 0.95 psia). When the secondary side reaches saturation, the RCS pressure is somewhere in the range of 25 psig (40 psia) for the various cases studied.
Based on these results, it is clear that a vent path limiting RCS pressure to roughly 2 psig (refer to figure 4.1 1 in Appendix 4.1) is too low a value to use to det'rmine :( SG condensation will or will not be effective.
A more reasonabl6 /alue of 20 psig should be considered.
By using the 20 psig figure, the operators could avoid needlessly refilling the SGs when SG reflux condensation would have limited value.
Figure 4.1-2 in the Appendix to Section 4.1 providos a typical curve of vent path area versus U me after shutdown for a RCS pressure of 20 psig.
If the vent path size is smaller and SGs are availabig (e.g., not isolated by SG nozzle dams or closed loop isolation valves), SG condensation will be effective and the operators should consider establishing a secondary-heat sink in at least half of the SGs.
To illustrate the effectiveness of SG reflux condensation, Case F.1 of WCAP 11916 is described.
In this example, one of two SGs is filled with water in the secondary side and the initial temperatures of the RCS and SG are 140 F, A loss of RHR is assumed to occur 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after shutdown, the 0
core and upper plenum temperatures begin to increase and eventually reach saturation in about 10 minutes. A short while later, enough steam has been generated to increase the partial pressure at the entrance to the SG tubes to the saturation temperature of the SG tube walls and condensation begins.
The heat removed by the condensation process increases rapidly as the condensing steam generator draws more steam from the upper plenum than the non condensing steam generator.
ARG-1 128 -
Rev. 0
o.
SitP DESCRIP1104 TABtt FOR ARG 1 51[p JL (Cont.)
!Lalli (Cont. ):
As the SG tube temperature increases, steam condensation lessens and approaches a lower steady state value.
The secondary fluid also begins to slowly heat up to saturation.
The energy removed by condensation is less than the heat transferred to the core fluid, so the RCS continues to heat up and pressurize, but at a slower rate.
Eventually, th3 condensing SG reaches saturation and boiling begins.
At this point, the secondary side heat transfer coefficient inr.reases rapidly which causes the condensation heat removal rate to increase briefly until a new steady state condition is reached.
After this, the RCS and SG pressures both increase slowly. Assuming the operstor opens the SG PORV to reduce the SG pressurization rate, both pressures will equalize when the SG PORV is capable of relieving enough steam to match the core decay heat generation (7.3 MWt for this case).
The SG pressure would be approximately 35 psig when this occurs.
This mode of decay heat removal could persist for several hours (until most of the secondary water is boiled away) or longer if the SG makeup water can be provided.
The effect of increasing the number of condensing SGs for 2, 3 and 4 loop plants was analyzed assuming the initial RCS temperature and pressure were 140 F and 0 psig, respectively, and the initial SG levels covered the top 0
of the SG tubes. Without condensation, all 3 plants reached 25 psig in approximately 20 minutes.
(Note:
25 psig was thosen as the typical RWST gravity feed pressure head).
Increasing the vaber of SGs available for condensation resulted in an increase in the time to reach 25 psig.
- Ideally, the plant should consider maintaining 50% or more of the SGs available for condensation to increase the allowable operator recovery action time.
This would be 1 SG on a 2 loop plant, 2 SGs on a 3 loop plant, and 2 SGs on a 4 loop plant.
The pressu:ization rate is further reduced at lower decay heat rates, if the loss of RHR is assumed to occur 120 hours0.00139 days <br />0.0333 hours <br />1.984127e-4 weeks <br />4.566e-5 months <br /> after shutdown, the smaller decay heat results in an 11.7 minute delay before the RCS pressure reaches the ARG 1 129 -
Rev, 0 l
lw
4 STEP DESCRIPTION VABLI FOR ARG 1 STEP 11. (Cont.)
LAlli (Cont.):
typical gravity feed limit of 25 psig. For a 4 loop plant with water in one SG, it would take 15 minutes longer to reach 25 psig for the 120 hour0.00139 days <br />0.0333 hours <br />1.984127e-4 weeks <br />4.566e-5 months <br /> case j
when compared to the 48 hour5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> case.
YDSLEEf.;
Consideration could also be given to establishing blowdown to help in heat removal from the steam generators.
PLANT SPECIFIC lNFORMATIQH:
Plant specific means for determining if a secondary heat sink is required for responding to loss of RHR while at mid loop conditions.
ARG 1 130 -
Rev. 0
ilE.J1ESCRIPR04 TABLE FOR ARG j, STEP _13 NOTE L HQJI:
The time to boiling in the RCS should be taken into consideration when determining how much time should be spent venting the RHR system prior to taking additional actions for alternate cooling sources.
EBfQ31:
To inform the operator that there could be a time restriction for venting the RHR system.
5 E!11.5:
Depending on the plant configuration, the initial RCS temperature and water level. the time after shutdown, and the amount of air trapped in the RHR piping, there may not be sufficient time to completely vent the RHR system prior to boiling occurring in the RCS.
The operator needs to compare the estimated time to boiling to the estimated time required to vent the RHR system and determine the corresponding course of a. tion.
GQEllDi[:
N/A PL ANT _ SPECIFIC _lRf.QLP.:6110H:
N/A ARG 1
- 131 -
Rev. O L
STEP DESCRIPTION TABil FOR ARG 1 STEP 13 NOTE L ltQlit if adequate time to completely vent the RHR system is not available, air can be swept out of the RHR lines by filling the RCS to (8) and running an RHR pump at a flowrate greater than (9),
l PURPOSE:
To inform the operator of possible contingency actions if adequate time to completely vent the RHR system is not available.
SA$1$:
Depending on plant layout, a significant amount of air may be trapped in the RHR inlet piping.
If there is a long horizontal piping run, it could take over one hour for the air to bubble back into the RCS.
If the RHR pumps are needed before complete venting is accomplished, the RCS level should be increased to the top of the RCS hot legs and one RHR pump should be started at high flow rates to sweep the air through the pump. One of the conclusions in Section 2 of WCAP 11916, LOSS OF RHR$ COOLING WHILE THE RCS IS PAR 11 ALLY FILLED, July 1988, states that if air is entrained in the RHR system during partial loop operations, the quickest way to vent the system is to recover level and sweep entrained air from the system by operating the system at a relatively high flow rate where the Froude Number is greater than or equal to 1.5.
Caution must be exercised because if too much air is left in the piping, pump cavitation will reoccur.
KNOWLEDGE:
If RHR pumps are needed prior to completion of venting of the RHR system.
then an evaluation of the consequences of starting one RHR pump at high flow rates to sweep air through the pump should be made prior to taking that action.
PLANT SPECIFIC INFORMATIQN:
o (8)
Enter plant specific value for RCS level corresponding to the top of the hot legs including allowances for normal channel accuracies.
o (9)
Enter plant specific RHR pump flowrate including allowances for normal channel accuracies.
Refer to Background Document.
The flowrate necessary is one t$at would result in a Froude Number that is greater than or equal to 1.5 as discussed in Section 2.3 of WCAP-11916 L0f; 0F RHR$ COOLING WHILE THE RCS IS PARTIALLY FILLED, July 1988 132 -
Rev. 0 ARG.1
~
.w~__,.._,,m
i STEP DESCRIPTION TARLE,10R ARG 1 STEP _1L
}JIP:
Vent RHR System As Necessary
(
PURPOSE:
To remove entrained air in the RHR system.
B511:
If air has been ingested into the RHR system, then it is necessary to vent the air from the lines in order to reestabitsh normal system operation.
The air entrainment is the result of vortexing at the RHR drop line connection to the hot leg. Typically, if there is only one line from the RCS to the suction of the RHR pumps, then ingesting air into one RHR pump will also affect the other RHR pump.
If each RHR pump has its own suction line (two completely independent lines), then air ingestion into the running RHR pump should not affect the ' standby RHR pump unless both RHR pumps were in operation. Plant-specific procedures should spell out the high point vent locations for proper venting of the RHR pumps, if required by plant design, to minimize the time necessary to accomplish the venting operation.
An additional venting concern exists for plants that have vertical U tube RHR heat exchangers where the elevation of the top of the heat exchanger is higher that the RCS hot leg level for mid loop operation. With the top of the tubes higher than the RCS level at mid loop, the tubes will have a tendency to drain when all RHR pumps are stopped, creating a void in the top of the tubes that is comprised of a combination of water vapor and air drawn out of solution by the partial vacuum condition in the top of the tubes.
Data from one plant has shown that when the last RHR pump was stopped, the vessel level increased one inch almost immediately followed by a more gradual level increase of at least another inch. When the pump was restarted, vessel level was observed to drop at least two inches immediately, for plants with vertical U tube RHR heat exchangers, consideration should be given for eliminating or minimizing any void as a result of water vapor and an unknown volume of air coming out of solution by the partial vacuum condition in the higher elevations of the RHR heat exchanger, if this void is not collapsed prior to RHR pump restart, the potential exists for a severe water hammer event. Also, if the pump is started in the miniflow alignment, the present air volume would be recirculated and possibly cause pump failure. A possible strategy for shrinking this void would be to pressurize the inactive RHR piping by temporarily aligning the RHR pump suction to the refueling water storage tank. Once the pump suction is switched back to the RCS hot lag, the vacuum in the heat exchanger would return immediately.. However, if an RHR pump is started immediately, it is possible that only limited amounts of air could have time to fall out of solution and 'Serefore, the effect of water hammer would be minimized.
20CEDGI:
N/A PLANT SPECIFIC INFORMATION:
Plant specific location of RHR pt, vents and procedures for venting.
ARG 1 133 -
Rev. O L
o.
STEP OESCRIPTION TABLE FOR ARe 1 ST E P _UL c
11[p:
Establish Conditions To Start RHR Pump PURPOSE:
To establish adequate condit tons to support RHR pump operttion.
SAILi:
Once an RHR pump becomes available for restart, adequate level to support RHR pump operation should be verified prior to restarting the RHR pump.
If level is not adequate or an RHR pump is not available.
- hen the operator should start trending core exit thermocouple temperatures and implement any actions to establish alternate sources for cooling the RCS until RHR can be returned to service (refer to the key utility decision section of this document)-
If an RHR pump can be restarted, the RHR system lineup should be checked tu assure that 'there is a suction and discharge path available for the pump and the other valves are in their desired lineup. There typically is valve status indication for these valves in the control room.
The component cooling water system should also be checked to ensure that it is in service and ready to assume the decay heat load from the RHR system.
Prior to starting the RHR pumps, the core exit thermocouple temperatures are i
checked to determine if the RCS is below the boiling point.
If below the boiling point, then there are no voids to collapse when the RHR pump is started and the operator can proceed to start the pump.
If voids do exist (the loss of RHR transient has lasted long enough for the RCS to heat up above 200 F) then when the RHR pump is restarted the voids could collapse 0
quickly, resulting in the loss of adequate RCS level to support continued RHR pump operation. To prevent the loss of adequate RHR suction level during this evolution, RCS makeup flow is established at either maximum flow from one charging /S! pumn :ir one HHS1 pump until RHR cooling is later restored.
KNOWLEDGE:
The operator should know that if the RCS is voided RCS level could decrease rapidly when the RHR pump is restarted and that additional makeup flow is necessary to minimize the RCS level drop prior to starting the RHR pump.
If support conditions cannot be established such as CCW, then the operator may not want to start the RHR pump until support conditions are restored.
PLANT SPECIFIC INFORMATION:
o (4)
Enter the plant specific RCS level corresponding to mid loop elevation in the RCS hot legs including allowances for normal channel accuracies, List of RHR valves that can be checked from the control room.
o o
Any plant specific alternate cooling methods ARG 1
- 134 -
Rev. O
=
.*.o STEPDESCRIPTIONTABLEFORAdG1 STEP JLfjf'ljlQfL EaullES:
Starting an RHR pump may result in an RCS level decrease due to shrink or void collapse.
PURPOSE:
To alert the operator that additional RCS makeup may be required due to shrink or void collapse when an RHR pump is started.
DA_Sil:
With no forced flow through the RCS, voids may form in the hotter regions of the system. Also, all of the entrained air may not have been vented from the RHR system. These steam volds and air pockett will subsequently be collapsed and/or swept away whenever RHR flow is restored, resulting in a decrease in the indicated RCS level.
It may be necessary to add makeup to sustain RHR pump operation.
KNOWLEDGE:
N/A PLANT SPECIFIC INFORMATION:
N/A ARG 1 135 -
Rev. O L
STEP DESCRIPTION TABLE FOR ARG J STEP 15 NOTE EQ1[:
The RCS level necessary to operate RHR pumps is a function of RHR j
flow.
FIGURE 2 should be referred to in order to determine the required level necessary.
)
PURPOSE:
To inform the operator of the relationship between RHR flowrate and RCS level.
EA111:
Prior to the actual start of an RHR pump, the operator should be aware of the RCS level vs. flow limitations so that vortexing and air entrainment does not occur.
In addition if the operator increases RHR flow, he must assure he has adequate level to support the higher flow rates.
KNOWLEDGE:
N/A PLANT SPECIFIC INFORMATION:
Plant specific curve for RHR flowrato vs. RCS level, t
l l
l l
l ARG 1
- 136 -
Rev. 0
l e
STEP DESCRIPTION TA11LE FOR ARG 1 51[P,,.1L il[E:
Restore RHR flow i
PURPOSE:
To restart one RHR pump and restore cooling to the RCS, Bill:
Once adequate level has been restored to the RCS and the RHR pumps have been vented, the operator can restart one RHR pump to restore cooling to the RCS.
Prior to the actual start of the pump, the operator should be aware of the RCS level vs. RHR flow limitations to prevent the reoccurrence of vortexing and air entrainment. To restart one RHR pump, the operator would typically close the RHR heat exchanger flow control valves and start the pump on miniflow and then gradually increase the RHR finw on bypass until the desired flowrate is achieved. He would then adjust RHR system flow through the heat exchanger to establish the desired cooldown rate.
If the operator cannot restore RHR flow, then he should start trending core exit thermocouple temperatures and implement the remaining actions in this guideline in parallel with continued attempts to return RHR to service. Remaining actions include alternate sources for cooling the RCS which is discussed in the key utility decision section of this document.
KNOWLEDGE:
If the RCS is at saturation, RHR flow should be restored slowly to avoid a rapid loss of RCS level due to void collapse. Refer to the description for Analysis Case D in Section 2.5.4 of this document.
PLANT SPECIFIC INFORMATION:
Any plant specific alternate cooling methods.
ARG 1 137 Rev, 0
y---------.-
STEP DESCRIPTION TABLE FOR ARG 1 STEP 1 511E:
Check If RCS Hakeup Should Be Reduced PURPOS[
To provide criteria for stopping HHS! pumps and reducing charging flow once RHR cooling has be6n restored.
M i:
Once RHR cooling has been restored and the potential for rapid void collapse on the start of the RHR pumps has passed, it is no loteger necessary to maintain high makeup flowrates to the RCS for the 'surpose o{f and RCS levelcompens void collapse. RCS temperature is verified to be
>elnw 200 is checked to verify that it is stable or increasing. The operator can then reduce RCS makeup flow (stop HHS1 pun;s and/or throttle charging flow) to a value that is adequate to maintain level within the acceptable region of the RHR flow vs. RCS level curve.
KNOWLEDGE:
i N/A PLANT tPE GFIC INFORM M tj:
Plant specific curve for RHR flowrate vs. RCS level l
AR6 1
.-138 -
Rev, 0 1
1
STEP DESCRIPTION TABLE FOR ARG 1 STEP ll.
0 Elif:
Check RCS Temperature LESS THAN 140 f TogerifythattheRCStemperaturehasbeenreducedtolessthan PURPOSE:
140 F prior to exiting this guideline.
JLA11):
If the operator is successful in restoping RHR flow and reducing RCS temperature, then this guideline can be terminated and the operator can returntohisnormaloperatingproceduresfortheegnditionthattheplantis in (plant draindown, etc.). Cooling the RCS to 140 F demostrates that RHR cooling has been restored and that RCS conditions have been restored to pre loss of RHR conditions.
KNOWLEDGE:
N/A PLANT SPECIFIC INFORMATION:
N/A ARG 1 139 -
Rev. O i
(i
STEP DESCRIP110N TABLE FOR ARG 1 ST E P,_lt.,
ET,[E:
Go To Appropriate Plant Procedure l
lo direct the operator to the proper plant procedure following BBEQjil:
the successful completion of this guideline.
B)1ll:
N/A KNOWLEDG1:
N/A PLANT SPECIFIC INFORMATION:
N/A Rev, 0 l
- 140 ARG.1
o
&ccendix To Section 4.1 Descriotion of Hot Side Vent Path Calculations 4
This appendix describes the vent path area and resistance calculations for a large hot side vent path. The step description tables for Steps 8 and 12 refer to this appendix.
The flow through a pressurizer or steam generator relief valve at high pressure is generally characterized as critical flow.
Critical flow is independent of the back pressure and largely independent of the resistance and flow losses in the piping. Critical flow also increases proportionally with the flow area.
Hence, for critical flow conditions, specification of the vent area and absolute pressure would be sufficient for constructing a figure relating a hot side vent path size with decay heat.
The flow rate would be determined by boil off due to the decay heat and the decay heat would be related to the time af ter shutdown.
For saturated steam, the critical pressure ratio is 0.545; at RCS pressures less than approximately 14.7/(.545) 27 psia (or 12 psig) the flow becomes subcritical. The RCS pressures used to determine the adequacy of the hot side vent (if there are cold side openings) could vary according to the specific application, however, typical values would be on the order of
? psig. At pressures this low, the vent flow would be suberitical.
If the vent path can maintain the pressure difference between the RCS and containment below this value, the PCS inventory loss through a postulated cold side opening can be minimized and a potential rapid core uncovery situation can be avo As explained in the following paragraphs, the vent pathresistance(K/Ajded.) is needed to accurately specify an adequate vent path size for this application.
A typical example is shown in figure 4.1 1.
ARG 1
- 141 -
Rev. O b
For subtritical flow, the RCS pressure (in psig) can be determined by the following formula:
)
2g v m g
p-288 ge A2 where m is the mass flow through the vent path (1bm/sec) 3 v is the specific volume of the steam (ft /lbm) g i
A is the flow or vent area (ft )
2 ge 32.17 lbm ft/lbf sec and K is the loss coefficient based on the flow area A (unitiess).
As noted in the above expression, it is possible to relate the vent path 2
resistance, K/A, to the pressure and flow (decay heat). However, unless a I
bounding loss coefficient can be assumed for the various possible vent paths in the RCS, it would not be appropriate to ignore the dependence on K and attempt to give an allowable vent size based only on the area.
I 2
To make the concept of vent path resistance easier to underscand, the.K/A calculations for the vent path provided by removal of three pressurizer safety valves will be outlined. The vent path through the safety valves is actually in " series" with the surge line. The vent path resistance is the j
sum of both contributions.
l With the 6" Schedule 160 safety valves removed and loop seal piping drained (at 2 psig, draining the pressurizer loop' seal piping can be very important),
a clear vent path through three 5,19" ( 43 ft) diameter pipes is provided.
2 The total flow area for this sub system is 0.44 ft. Approximate ' flow losses for each piping section consist of 0.4 contraction loss into the piping,1.0 expansion loss into containment, and 0.5 frictional loss (fL/0) in the piping (assuming each pipe is 10 ft long and the friction factor is 0.02). These values should be typical, however, the plant engineering staff should correct or refine them if more appropriate values apply. Combining these three contributions, the safety valve piping has a loss coefficient K - 1.9 and a vent path resistance K/A., 3,9f(_,44)2 - 9.8 ft*4 2
ARG 1
- 142 -
Rev. 0
For the surge line, a typical flow loss coef ficient of K 4 is used.
This loss is 2
based on the flow area of the surge line, A - 0.69 ft, which is appropriate for most 4 loop plants.
Again, the utility can evaluate the various entrance, exit.
bend and pipe friction losses to refine these flow loss estimates.
Based on the above values for the surge line, its resistance is K/A2 - 4/(.69)2 - 8.4 ft*4 This is cortparable to the resistance for the sub system formed by removal of the three safety valves. As might be expected, this estimate for the surge line resistance is roughly the same as that of the 16" pressurizer manway in the Figure 4.1 1 example.
This is because the manway loss itself is nuch smaller (A = 1.4 ft, K - 1.4, K/A2 - 0.7 ft*4).
2 Flow losses in the pressurizer vessel and the hot leg are neglected since the corresponding areas are large in comparison to those considered above.
Thus, the total vent path resistance for the surge line plus three safet..' valve system is K/A2 - 8.4 + 9.8 - 18.2 ft*4 This agrees approxi: 'tely with the value marked for the three safety valves in the Figure 4.1-1 example.
The specific application should also be considered when determining the surge line resistance.
For the above example, it is assumed that the entire surge line area is free of water, i.e., passing only steam and initially some air. For a postulated cold side opening, this is a reasonable assumption since a slight pressurization on the hot side will depress the upp" plenum and hot leg mixture level and allow this to occur. Most of the water initially in the surge line should drain back to the RCS since the surge line typically slopes upward frr the hot leg over its entire length.
Any water that does not drain back could potentially be entrained by the high velocity steam " screaming" up the surge line and carried into the pressurizer vessel.
If the scenario is slightly differe.4, e.g., the surge line connection is initially blocked or partially blocked due to he.Wo swell and voiding in the upper plenum and hot leg, the surge line resistance would effectively be increased due to the presence of the mixture (this would reduce the flow area for the steam or increase the pressure drop if the steam must entrain the mixture and " hold" it in the pressurizer vessel). Alternatively, it may be possible for the surge line to support all vapor or steam flow if the mixture at the surge line connection locally depresses to allow this. Whatever the exact mechanism, the surge line resistance would effectively be increased by the presence of the mixture.
ARG 1
- 143 -
Rev. 0 l
l.
This sample calculation for three pressurizer safety valves removed illustrates that specifica*.ioi of the vent path resistance can be more I
involved than specification of a single vent area.
In many instances, however, a series calculation is not required and a typical but high value, e.g., K = 2, can be assumed.
If K = 2 is assumed for the three pressurizer safety valve case, an effective area using the above vent path resistance 2
(18.2 ft*4) is back calculated as A rt 0.33 ft ; this is only 25%
e smaller than the actual flow trea through the piping for the three safi:ty valves (0.44 ft ).
If the concept of effective area is easier to work with d
than vent path resistance, Figure 4.1-1 could simply be converted to a plot of effective area versus time after shutdown by the plant engineering staff.
The effective vent path area curve would then be used as a replacement or the vent path resistance curve. The acceptable egion would then be the For either method, plant engineering support may be region abcVe the curve.
2 calculations for the various vent needed to perform or verify the K/A paths available, if there are no cold side openings (or the size is limited and within the makeup capability of normal charging), the hot side vent path size can be I
At allowed to decrease and the RCS pressure can be allowed to increase.
approximately 20 psig (35 psia), the vent path flow is expected to be critical flow, so specification of a flow area is suf ficient.
Figure 4,1 2 shows a typical curve for this 20 psig limit, it should be noted that SG nozzle dams are normally tested tc pressures higher than this (routinely to about 28 psig, roughly twice the normal operating pressure when the refueling cavity is flooded). Therefore, a 20 psig curve may be useful for purposes of defining a large hot side vent, provided there are no cold side openings 3
larger than approximately 1 in.
As explained in the step description table for Step 12, a 20 psig curve can be used in the opposite manner to determine whether or not SG condensation will be effective (for cases where SGs are available, i.e., without SG nozzle dams or closed loop isolation valves on the hot leg side). The plant engineering staff may first want to make this determination prior to establishing a secondary heat sink using SGs previously drained for maintenance.
- 144 -
Rev. 0 ARG 1
"'h
%?10-100 F
-- T0=140 F 40.0 l
35.0
/
2 Safety Valves Removed
/ /
I 3
30.0 b'
[
Q NOT ACCEPTABLE REGION
/
p M
,/
25.0 d
E.
/
/
Y-- -
20.0 3 Safety valves Removed -f'j l
[
y ACCEPTABLE REGION e
g-7 (to Prevent loss Through Cold Side Opening) m tr l
g y -
2
/
EXAMP;_E 1
Pressurizer Manway Removed ONL j
p.--
1 5.0 1
0.0 0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0 800.0 900.0 o
f Time After Shutdown (hours)
Figure 4.1-1 Vent Path for RCS Pressure S 2 psig
Yn?
0.30 I
I g
2 Safety Valves Rrvisved i
\\
EXAMPLE
_cu 0.20
\\
ACCEPTABLE REGION ORY (to Keep RCS Pressure < 20 psig)
E
~
4:
E
\\
I Safety Valve Removed r
T "a
0.10 E
N i
f NOT ACCEPTABLE REGION 0.0 0.0 iM.0 200.0 300.0 400.0 500.0 600.0 700.0 800.0 900.0 Time Af ter Shutskmn thours)
Figure 4.1-2 Vent Path Area for M Pressure - 20 psig
.)
e
- e a
4.2 hjecuence Recuirernents This section consists of a table which presents the existing guideline
(
sequence and identifies the allowed interchangeability of guideline steps for the benefit of the utility proceudre writer, The Step Sequence Table for ARG 1 is proniced on the following page. The interchangeability of guideline steps is identified by the number in the column to the right of each guideline step.
Refor to the ERG Users Guide in the Executive Volume for information on use of tt>e step sequence tables.
147 -
Rev. O ARG-1 L
STEP SEQUENCE FOR ARG 1 SEQUE!4CE f
STEP g
1 1.
Check if RHR Pumps should Be Stopped 2
2.
Isolate Letdown And Known Drain Paths 3
3.
Determine The Time To Boiling Based On
[xisting Conditions 4
4.
Initiate Actions To Protect Personnel Working in Containment 4
5, initiate Actions To Establish Containment Closure 4
6.
Start Avail 6 bis Containment Fan Coolers 5
7.
Check RCS Level 6
8.
Refill The RCS As follows 7
9.
Go To Step 11 8
- 10. Refill RCS To (4) 9 11.
Identify And Isolate Any RCS Leakage 9
- 12. Determine If A Secondary Heat Sink
)
Should Be Established 9
- 11. Vent RHR System As Necessary 10 14.
Establish Conditions To Start RHR Pump 11 15.
Restore RHR Flow 12
- 16. Check if RCS Makeup Should Be Reduced 13
- 17. Check RCS Temperature - LESS THAN 0
140 7 14
- 18. Go To Appropriate Plant Procedure Rev. 0
- 148 ARG.1
s 0
6 FREQUE!4T QUEST 10f45 The following are questions which have been frequently asked about ARG 1, LOSS 0F RHR WHILE OPERATitiG AT HlD LOOP CONDITIONS:
4 Q.
If RCS level is offstale low, why is hot leg injection initiated instead of first trying cold leg injection?
A.
If RCS level is offscale low, there is no direct way of verifying that water is being injected into the core for all cases. Cold leg injection would be adequate for many cases. However, if there was a large cold leg cpening without an adequate hot side vent path, cold leg injection may ficw towards the break rather than into the core if the RCS is pressurized.
When this guideline was developed, the WOG Operations Subcommittee did not expe-t the operator to know whether an adequate site hot side vent path existVd.or whether a large cold leg opening existed.
Since hot leg injection provided adequate RCS makeup for all cases, it was selected as the preferred actiori rather than requiring the operator to try to determine what type of openings exist in th9 RCS.
Q.
Why doesn't Step 1 of this guideline try to start the standby RHR pump first before initiating other guideline actions?
A.
When this guideline was developed, it was assumed that procedures written based on this guideline would be integrated with other plant alarm response procedures and abnormal operating procedures.
The Operations Subcommittee felt that actioris to start the standby RHR pump wou'Id typically be handled by utilities as part of the initiol alarm response.
Therefore it was deemed not necest,ary to include starting the standby f.Hk pump as part of the generic guideline.
e ARG-1 149 -
Rev. O A
l.
[
e 6.
REFERENCES
- 1) NUREG/CR 2792, e.n. Assesment of Residual Heat Removal And Containment ipfj y Pumn Performance Under Air And Debris inoestino Conditioni, September, 1982.
- 2) U S. Nuclear Regulatory, Loss of R,nidual Heat Removgl, (RHR1 While the Reactor Coolant S." stem (RCS) is Partially filled, Generic Letter 8712, July 9, 1987.
- 3) Letter from Roger A. Newton to Westinghouse Owners Group Primary Representatives, [arly Notification of_Jid Looo Operation Concerns, OG 88 21. Hay 27, 1988.
- 4) Westinghouse Electric Corperation, loss of RHRS _while the RCS 1,s Partialh_
f.iUS.d. WCAP-11916, July, 1988.
5)
U. S. Nuclear Regulatory, Lg1Lp10tcay Heat Removal, Generic Letter 8817, October 17, 1908.
- 6) Westinghouse Electric Corporation RCS Level Gratiitpli, WCAP 12111, January 30, 1989.
- 7) Letter from L. A. Walsh to Westinghouse Owners Group Primary Representatives Operations and Analysis Subcommittee Members, and Hid loop Operations Workshop Attendees, tilddogt Qps. rations Interim Guidpng, t to WOG48156, November 7,1988.
- 8) Letter from L. A. Walsh to Westinghouse Owners Group Primary Representatives and Operations Subcommittee Meinbers, loss of RHR While at Mid-looo Conditions Guideline (Phase 2 guidance), WOG 89-ll8, May 31, 1989,
- 9) Yeh, H. C. and L. E. Hochreiter, 'Hass Effluence During FLECHT forced Reflood Experiments,' Nuclear _ Enoineerine and Desian, Volume 60, 1980, pp.
413 429.
la's.
^
- 10) Hochreiter, L. E., and others, PWR f TECH 1 SEASET 5.nisps Ef fects Nalgr_(L Circulation and Reflux (ondensation NUREG/CR 3654. EPRI NP 3497, and Westinghouse Electric Corporation Report WCAP 10415, September 1984,
- 11) American Nations) Standard
- Decay Heat Power in Light Water Reactors,"
AtiSI/Aks 5.1 1979.
- 12) Meyer, P. E., NOTRUMP - A Nod;l Transient Small Oreak And General NetworL.
(.01g. Westinghouse Electric Corporation Report WCAP 10079 P A (Westinghouse Proprietary Class 2), August 1985.
- 13) Wallis, G. B., Qne-dimtrdj.pnal TW21.chue fin, Section 12-10, McGraw Hill, New York, NY, 1969.
.ARG 1
- 151 -
Rev. 0 l
i
....,