ML18005A612
| ML18005A612 | |
| Person / Time | |
|---|---|
| Site: | Harris  |
| Issue date: | 09/29/1988 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML18005A611 | List: |
| References | |
| NUDOCS 8810030166 | |
| Download: ML18005A612 (8) | |
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+p*g4 UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 8810030166 880929 PDR ADOC)( 05000400 P
PNU SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO NATURAL CIRCULATION COOLDOWN SHEARON HARRIS NUCLEAR POWER PLANT CAROLINA POWER 8( LIGHT COMPANY DOCKET NO. 50-400
1.0 INTRODUCTION
Branch Technical Position (BTP)
RSB 5-1, "Design Requirements of the Residual Heat Removal (RHR) System," requires that test programs for pressurized water reactors (PWRs) include tests, with supporting analysis, to (1) confirm that adequate mixing of borated water added prior to or during cooldown can be achieved under natural circulation conditions and permit estimation of the times required to achieve such mixing, and (2) confirm that the cooldown under natural circulation conditions can be achieved within the limits specified in the emergency operating proce-dures.
In addition, the plant is to be designed so that the reactor can be taken from normal operating conditions to cold shutdown using only safety-grade systems.
A comparison of thermal hydraulic performance to that of another plant of similar design for which a natural circulation cooldown test has been performed may be used to justify the position that the NRC requirement is met by reference to that test.
This requirement applies to Class 2 plants such as Shearon Harris.
By Supplement 4 to the Shearon Harris Nuclear Power Plant (SHNPP) Safety Evaluation Report (SSER4),
the staff required the licensee to demonstrate that the Diablo Canyon natural circulation tests are applicable to SHNPP.
A natural circulation/boron mixing/cooldown test was performed at Diablo Canyon Unit 1 on March 28-29, 1985.
By memorandum dated February 4, 1987, the staff determined on the basis of the Diablo Canyon tests and sub-mittals and the Brookhaven National Laboratory (BNL) technical evaluation report (TER), that the Diablo Canyon Unit 1 systems meet the intent of BTP RSB 5-1 for a Class 2 plant.
By letter dated h1ay 17, 1988, the licensee submitted an analysis to show the applicability of the Diablo Canyon cooldown test results to SHNPP rather than conduct such a test at the plant.
The licensee provided the analysis entitled "Shearon Harris Nuclear Power Plant Natural Circulation Cooldown Evaluation Program Report,"
WCAP-11810, which evaluates the capability of SHNPP to successfully achieve cold shutdown conditions under the requiremIents of BTP RSB 5-1.
The Westing-house report includes the following:
l.
A comparison of the Diablo Canyon plant and SHNPP to demonstrate their similarity.
2.
A cold shutdown scenario for SHNPP using the constraints of BTP RSB 5-1.
3.
A quantitative thermal-hydraulic code (TREAT) analysis of the scenario to evaluate and justify the SHNPP cold shutdown capabilities under the requirements of BTP RSB 5-1.
The Westinghouse proprietary Transient Real-Time Engineering Analysis Tool (TREAT) computer code was used to perform the thermal-hydraulic analysis.
The staff finds the code use acceptable for this application on SHNPP.
By letter dated August 31, 1988, the licensee provided a response to staff questions on the natural circulation analysis and procedures.
The staff SER on the Diablo Canyon natural circulation test identified the plant parameter s that may affect application of the test results to other plants.
These parameters are the basis for the staff evaluation and are discussed in the following sections.
2.0 EVALUATION Natural Circulation Diablo Canyon Unit 1 is rated at 3338 Nwt and has four loops in its reactor coolant system (RCS).
SHNPP is rated at 2775 ltwt and has a
three-loop RCS.
The licensee has stated that the general configuration of the piping and components in each reactor coolant loop is the same in both SHNPP and Diablo Canyon Unit 1 even though SHNPP has one less heat transfer loop.
Significant parameters governing natural circulation are hydraulic flow resistance and thermal driving head.
To demonstrate similarity in design for natural circulation, these two parameters were compared.
Data from the Westinghouse report showed that the SHNPP hydraulic resis-tance coefficients at normal flow conditions were slightly lower than Diablo Canyon's.
Thermal driving head,
- however, because of a difference in steam generator tube lengths, was slightly higher for Diablo Canyon.
The report showed that the increased natural circulation driving head for Diablo Canyon and the lower overall piping flow resistance for Shearon Harris would decrease the natural circulation flow ratio to approximately 1.01.
Therefore, the licensee concluded that the natural circulation loop flow rate for either plant would be nearly the same.
Differences in reactor power and decay heat levels between the two plants are not expected to alter this conclusion.
, The staff questioned the applicability of flow resistance at normal flow conditions when significantly lower flows would exist during natural circulation.
The licensee stated that the hydraulic resistance coeffi-cient would slightly increase at lower flows but the expected flow ratio is expected to be valid for both normal and natural circulation.
The staff finds this explanation acceptable.
RCS Cooldown The plant's ability to cool the RCS at a specified cooldown rate,. assuming a sufficient supply of auxi liary feedwater and a subcooled RCS, is deter-mined by the capacity of the atmospheric steam dump (ASD) valves.
Steam flow through these valves removes the sensible heat and decay heat throughout the cooldown period.
The end of the cooldown period, when the steam generator pressure is low, provides the most limiting conditions for valve capacity.
The energy to be removed is determined by the water inventory, the amount of structural material in the
- RCS, and the level of decay heat.
The ASD valves for SHNPP are hydraulically operated and are safety grade.
Single failure analysis resulted in one steam generator and its steam release being unavailable for plant cooldown.
The analysis showed that approximately 14 hours1.62037e-4 days <br />0.00389 hours <br />2.314815e-5 weeks <br />5.327e-6 months <br /> are required to cool down the RCS to the RHR cut-in temperature.
The initial cooldown rate is limited to 25'F per hour to prevent the loss of pressurizer level because of RCS shrink.
RCS pressure generally decreased.
- However, towards the end of the cooldown period, the depressurization and cooldown rates decrease.
These decreases are due to the limiting capacity of the ASDs.
Throughout the cooldown period, the RCS subcooling is greater than 50'F.'igure 4.3-12 of WCAP-11810 shows the upper-head subcooling to be greater than about 40'F during this period.
The staff finds that there is reasonable assurance that the ASDs have sufficient capacity to perform an RCS cooldown to the RHR cut-in tempera-ture in a reasonable time, while maintaining sufficient subcooling, and the ASD capacity is therefore acceptable.
8 ass Flow and U
er Head Coolin A potential exists for void formation in the upper head of the reactor vessel during the cooldown/depressurization under natural circulation conditions if the upper head is relatively isolated from the rest of the RCS and its fluid temperature remains higher than the coolant temperature in the main flow paths of the RCS.
Upper-head cooling under natural circulation conditions is influenced by core bypass flow and mixing in the
.upper head.
Westinghouse plants may be divided into two groups according to the magnitude of the bypass flow:
T and T
plants.
For the T
- plants, such as SHNPP, sufficien( bypass PIIN exists to make the 3Mpera-ture of the upper head fluid essentially equal to the cold-leg tempera-ture.
On the other hand, for the Th t plants, which include Diablo hot
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- Canyon, the bypass flow is much smaller.
For 7 plants this circum-stance resuits in upper head temperature ranginl) between the cold-'leg and the hot-leg temperatures and raises a possibility of void formation in the upper head region.
The licensee stated that the reactor vessel spray nozzle between the downcomer and the upper head region for SHNPP has a flow area about nine times that of Diablo Canyon.
'This circumstance allows better flow communication and mixing in the upper head during natural circulation.
The upper head volume for SHNPP is slightly smaller than that of Diablo Canyon.
The NRC staff considers the upper head volume effect on cooling of the upper head to be small compared to the contribution of flow through the spray nozzles.
We would therefore expect a shorter cooling time for a T
ld plant compared with that of a Th t plant of the same size.
cold At the end of the cooldown period, corresponding to 14.1 hours1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> from the reactor coolant pump trip, the TREAT analysis indicated 170'F subcooling in the upper head.
This figure is based on the assumption of complete mixing of the bypass flow with the fluid in the upper head.
To further ensure upper head subcooling, the report indicated that with no upper head mixing, the upper head would have 15'F of subcooling at the end of the cooldoU(n period.
The licensee also demonstrated the capability of minimizing stratification in the upper head.
Before depressurization, the analysis considered the upper head vent to be opened for 10 minutes.
This circumstance resulted in a 200'F upper head subcooling.
The staff considers the results of the analysis to be based on a bulk or mixed temperature, and stratification can still occur.
The staff finds, however, that this venting capability is desirable because it should lower the bulk temperature arid thereby minimize voiding during depressurization.
This operation is not in the SHNPP natural circulation procedures;
- however, the licensee committed to reviewing and modifying plant procedures to incorporate features of the WCAP-11810 analysis.
The staff finds this resolution acceptable.
Boron Nixin The Diablo Canyon boron mixing test evaluation demonstrated adequate boron mixing under natural circulation conditions when highly borated water was injected into the RCS.
Contributing to the diffusion of the boron is the mixing effect created as the flow passes through the reactor coolant pumps and exits the steam generator tubes.
The plant's ability to achieve the proper shutdown margin,
- however, depends mainly on the injection rate of boron relative to the total inventory of water in the RCS.
The required concentration change for the test was achieved in less than 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />.
The boron injection for the SHNPP RCS was via reactor coolant pump seals at a flow rate and a boron concentration significantly less than that of Diablo Canyon.
These factors contributed to a time of approximately 6
hours from the initiation of injection to reach the desired concentration change.
The injection was postulated to begin 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> after the reactor trip; thus, boration is accomplished within 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> following reactor trip. Since the natural circulation cooldown is calculated to take 14
- hours, the staff finds that there is reasonable assurance that sufficient time exists for boron injection and mixing to achieve the required shutdown margin.
De ressurization The Diablo Canyon test, demonstrated that the RCS could be depressurized from cooldown conditions to the RHR initiation pressure under natural circulation conditions using the auxiliary spray and/or pressurizer power operated relief valves (PORVs).
At SHNPP, depressurization may be accomplished through the use of pressurizer PORVs pressurizer auxiliary spray pressurizer vent reactor upper-head vent The pressurizer PORV controls are not qualified and thus the PORVs are not available for depressurization per BTP RSB 5-1.
The SHNPP= pressurizer auxiliary spray valve wi 11 fail closed upon loss of air.
The licensee has taken credit for local operator action to hook up a
portable compressed gas supply to the auxiliary spray valve in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, assuming the loss of normal air to the valve.
This approach is acceptable since SHNPP is a Class 2 plant and reasonable manual actions are allowed by RSB BTP 5-1.
In this mode, however, overfill of the pressurizer is a
staff concern since normal letdown is unavailable.
The report,
- however, states that a qualified letdown path can be established through the reactor vessel head vent line to either containment or the pressurizer relief tank.
The WCAP-11810 TREAT analysis for depressurization assumed the use of the pressurizer auxiliary spray system at SHNPP.
A spray flow of 50 gallons per minute was used in the analysis to depressurize the RCS at approximately 0.4 psi/second.
During this period, the pressurizer level would increase and any faster depressurization may require use of the upper-head vent to avoid overfilling the pressurizer.
At the end of the depressurization, 15.3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> from the reactor trip, the RCS is at 375 psia.
The RHR system may now be placed in service, and the cooldown to cold shutdown condition continued.
On the basis of further review, the licensee determined that the pressur-izer vent and/or the reactor upper head vent are qualified for use as a
preferable alternate means of depressurization.
Subsequent to the analysis presented in the Westinghouse
- report, an additional analysis was performed using the TREAT code to model the RCS depressurization via the pressurizer vent line instead of the auxiliary spray.
The licensee's letter of May 17, 1988, states that RCS pressure decreased at approxi-mately 9 psi/minute with only a slight increase in pressurizer level.
Approximately 16 hours1.851852e-4 days <br />0.00444 hours <br />2.645503e-5 weeks <br />6.088e-6 months <br /> are needed to attain RHR initiating conditions with this method of depressurization.
In the event of high pressurizer levels
the upper head vent could be used as a means of pressurizer level control.
A further review indicated that the reactor vessel upper head vent could also be used for the depressurization.
The pressurizer/.
reactor vent systems are safety grade and can be operated from the control room.
The staff finds the use of the vent systems acceptable to meet Branch Technical Position RSB 5-1.
The licensee committed to modify the present natural circulation procedures to incorporate the depressurization through the use of the vents.
The staff finds this commitment acceptable.
Coolin Water The primary auxiliary feedwater supply to the steam generators is provided by the condensate storage tank (CST) at both Diablo Canyon and SHNPP.
The emergency service water system is a backup seismic Category I source for the auxi liary feedwater system at SHNPP.
Utilizing the pressurizer auxiliary spray for depressurization, 230,000 gallons of cooling water are required according to the TREAT analysis.
On the basis of the requirement for an additional hour of cooling time if the pressurizer vent is used for depressurization, the staff estimates a cooling water requirement of 235,000 gallons.
The BNL TER estimated a 360,000-gallon cooling water requirement for Diablo Canyon on the basis of a 43-hour cooling time for the upper head.
This calculation was based on assumptions of no heat loss from the upper head to the containment and a limited amount of bypass fluid mixing with fluid in the bottom of the upper head.
For the thermal power output of SHNPP and the same cooling time, the staff estimates that the cooling water required would be 320,000 gallons because of a reduced decay heat.
We would further conclude that had Diablo Canyon been a T
- plant, and with heat transfer from the upper head to containment conB3)red, the cooling requirement would have been significantly less than 320,000 gal 1ons.
The SHNPP CST, which is safety grade, has a capacity of 415,000 gallons with a technical specification (TS) minimum of 270,000 gallons.
In addition, the backup supply provides an essentially unlimited cooling water supply.
Therefore, we conclude that there is reasonable assurance that sufficient cooling water inventory exists to meet the proposed plant cooldown method.
- 3. 0 CONCLUSION The staff assessed the capability of SHNPP to meet the requirements of RSB BTP 5-1.
We have identified and evaluated the plant parameters that may affect application of the Diablo Canyon natural circulation test results to SMNPP.
On the basis of the licensee's submittals, commitments to modify existing procedures, and our evaluation as previously discussed, we conclude that SHNPP has demonstrated that the Diablo Canyon natural circulation tests are applicable to SHHPP and that they comply with the requirements of BTP RSB 5-1.
Principal Contributor:
D. Katze Dated:
September 29, 1988