ML19275A356
| ML19275A356 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 09/14/1979 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19275A343 | List: |
| References | |
| NUDOCS 7910040166 | |
| Download: ML19275A356 (25) | |
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UNITED STATES
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTING AMENDMENT NO. 59 FACILITY OPERATING LICENSE NO. DPR-3 YANKEE ATOMIC ELECTRIC COMPANY YANKEE NUCLEAR POWER STATION (YANKEE-ROWE)
DOCKET NO. 50-29 1.0 Introduction By letters dated December 1,1976, May 27,197, November 1,1977, and April 28,1978 (References 4, 10, 11, 15) Yankee Atomic Electric Company (YAEC) (the licensee) submitted to the Nuclear Regulatory Commission (NRC) plcat specific analyses in support of the proposed reactor vessel Low Temperature Overpressure Protection System (LTOPS) for the Yankee Rowe Nuclear Power Station (Yankee-Rowe). This information supplements other documentation submitted by YAEC (References 3, 8, 9).
To assure proper operation of the overpressure protection system, we requested nd YAEC submitted (Reference 16) proposed changes to the Technical Specifications that are in accordance with the requirements presented in Section 5.2 of this Safety Evaluation (SE). Our review of all the information submitted by YAEC in support of the proposed LTOPS including the proposed Technical Specifications has been completed.
2.0 Background and Discussion Over the last few years, incidents identified as pressure transients have occurred in Pressurized Water Reactors (PWR). The term " pressure transient,"
as used in this SE refers to events during which the temperature-pressure limits of the reactor vessel, as shown in the facility Technical Specifi-cations, are exceeded. All of these incidents occurred at relatively low temperature (less than 200 F) where the reactor vessel material toughness (resistance to brittle failure) is reduced.
The " Technical Report on Reactor Vessel Pressure Transients" in NUREG-0138 (Reference 17) summarizes the technical considerations relevant to this issue, discusses the safety concerns and existing safety margins of operating reactors, and describes the regulatory actions taken to resolve this. issue by reducing the likelihood of future pressure transient events at operating reactors. A brief discussion follows.
d 7910040 t i.1 100 050
. 2.1 Vessel Characteristics Reactor vessels are constructed of high quality steel made to rigid specifications, and fabricated and inspected in accordance with the time-proven rules of the ASME Boiler and Pressure Vessel Code.
Steels used are particularly tough at reactor operating conditions.
- However, since reactor vessel steels are less tough and could possibly fail in a brittle manner if subjected to high pressures at low temperatures, power reactors have always operated with restrictions on the pressure allowed during startup and shutdown operations.
At operating temperatures the pressure allowed by Aonendix G limits is in excess of the setpoint of currently installed pressurizer cede safety valves.
However, most operating PWRs were not originally designed to have pressure relief devices to prevent pressure transients dJring Cold conditions from exceeding the Appendix G limit.
2.2 Regulatory Action By letter dated August 11,1976, (Reference 1) the NRC requested that YAEC begin efforts to design and install plant systems to mitigate the consequences of pressure transients at low temperatures.
We also requested that operating procedures be examined and administrative changes be made to guard against initiating overpressure events.
It was our pos,ition that proper administrative controls are required to assure safe operation for the period of time prior to installation of the proposed overpressure mitigating hardware.
YAEC responded (References 2, 3) with information describing interim and long term administrative measures to prevent pressure transients at Yankee-Rowe.
The December 1,1976 YAEC submittal (Reference 4) provided a detailed descr.ption of the proposed LTOPS for Yankee-Rowe. The submittal contained a discussion of plant procedures, existing and proposed equipment used to mitigate postulated pressure transients, and the results of analyses of selected transients.
Our review of the document resulted in questions which we transmitted to the licensee in our February 16, 1977 letter (Reference 5).
By letter dated April 1,1977, (Reference 8) we requested YAEC to ensure that the likelihood of an overpressure transient as a result of improper Reactor Coolant Pump (RCP) operation is minimized.
The license:'s April 25, 1977 submittal (Reference 9) addressed our concerns regarding RCP operation.
5100051
YAEC provided the answers to our questions described in our letter dated February 16, 1977 (Reference 5) in their May 27, 1977 submittal (Reference 10). This submittal provided sensitivity studies and further LTOPS supporting calculations. A review of this document resulted in staff concerns which were discussed with the licensee in a July 29, 1977 telephone conversation.*
The licensee's November 1,1977 submittal (Reference 12) addressed our concerns raised in the July 29 telephone conversation.** After additional telephone conversations with the licensee, we issued cur January 31, 1978 (Reference 13) letter which stated that the LTOPS must be designed to prevent RCS pressure from exceeding the Isothermal Appendix G curve during a single LPSIP mass addition event. Our letter also contained the staff's justification for requiring the use of the Isothemal Appendix G curve.
YAEC contracted Energy Incorporated (EI) to perform a reliability assess-ment of the LTOPS with respect to the LPSIP mass addition event.
Based on the El study, YAEC submitted proposed equipment changes which would significantly lower the probability of the LPSIP mass addition event. The licensee's April 28, 1978 letter (Reference 15) contains the proposal and the El reliability assessment.
We visited the Yankee-Rowe plant during June 14-15, 1978 and observed the LTOPS. We also discussed with the licensee the proposed modificatfons and the impact on emergency core cooling system perfomance. YAEC proposed Technical Specifications in support of their LTOPS in their June 5,1978 (Reference 16) submitt.l.
2.3 Desian Criteria Through a series of meetings and correspondence with PWR vendors and licensecs, the staff developed a set of criteria for an acceotable over-pressure mitigating system. The basic criterion is that the mitigating system will prevent reactor vessel pressures in excess of those allowed by Appendix G for the design basis events discussed in Section 2.4.
Specific criteria for system perfomance are:
(1) Operator Action:
No credit can be taken for operator action for ten minutes aWer the operator is aware of a transient.
(2) Single Failure: The system must be designed to relieve the. pressure
- transient given a single failure of an active component in addition to the failure that initiated the pressure transient.
with YAEC's use of the Appendix G maximum pressure These concerns dealt curve for Reactor Coolant System (RCS) hydrostatic testing as a design basis for their LTOPS.
YAEC was asked to substantiate the use of this Also, YAEC was requested to analyze the RCS pressure response curve.
during the mass addition from the ECCS. This event was previously rejected by the licensee as a design basis for their LTOPS based on administrative controls.
- YAEC continudd to assert that the Hydrostatic Test curve was appropriate for LTOPS design, but gave insufficient justification. Also, the RCS prc sure from a single LPSIP mass addition event was shown to exceed the Appendix G Isothemal curve.
Loob 52
. (3) Testability: The system must be testable on a periodic basis consistent with the systErs's employment.
(4) _ Seismic and IEEE 279 Criteria:
Ideally, the system should meet seismic Category I and IEEE-279 criteria. The basic objective is that the system should not be vulnerable to a common failure mode that would both initiate a pressure transient and disable the overpressure mitigating system.
Events such as loss of instrument air and loss of offsite power must be considered.
We also requirea, for the design of the pressure mitigating syston, that the electrical, instm; mentation, and control systems provide alarms to alert the operator to (1) properly enable the system at the appropriate temperature during cooldowns and (2) 'ndicate if a pressure transient is in progress.
In the initial letters to all PWR licensees (Reference 1) we also required the installation and use of permanent RCS pressure and temperature recording devices.
2.4 Design Basis Events ine incidents that have occurred to date have been the result of operator errors or equipment failures.
Two varieties of pressure transients can be identified: a mass input from charging pumps, safety injection pumps or safety injection accumulators; and a heat addition type (which causes thermal expansion) from sources such as steam generators, reactor coolant pumps, pressurizer heaters or decay heat.
On Westinghouse designed plants, the most common cause of the overpressure transients to date has been isolation of the letdown path.
Letdown during low pressure operations is via a flowpath through the Residual Heat Removal (RHR) System.
Thus, isolation of RHR can initiate a pressure transient if a charging pump is left running.*
- Pressure excursion at H. B. Robinson 2 was caused by the securing of the RHR pumps,thile the RCS was in a cold, shutdown and water-solid condition.
The RCPs were left operating (heat input of about 4.2 MW) and the core was generating about 13.6 MW of decay heat. The loss of the low temperature heat removal capability, plus the possible partial loss of letdown, caused the pressurization.
. The Yankee-Rowe Emergency Core Cooling System (ECCS), contains three Low Pressure Safety Injection Pumps (LPSIP) and three High Pressure Safety Injection Pumps (HPSIP). The piping is arranged such that the LPSIPs can discharge to the HPSIP suction piping or directly into the RCS, and the HPSIPs discharge into the RCS.
Therefore, in the choice of design basis mass input events, the most limiting combination ot pumps is assumed.
The mas.: input from HPSIP and/or LPSIP is further discussed in Sections 3.4.1 and 4.0.
Based on the historical record of overpressure transients and the imposition of strict administrative controls, we conclude that the limiting events described above, and in Section 4.0 fom acceptable bases for the analysis of the proposed LTOPS.
3.0 System Description and Evaluation The Yankee-Rowe LT0oS is composed of the pressurizer Solenoid Operated Relief Valve (SORV) with a manually enabled low pressure set ASME code safety valves on the Shutdown Cooling System (SCS) point, two and the pressurizer bu;. bin.
The SCS safety valves (SVs) are connected to the RCS when the SCS is operating.
By procedure, the SCS is brought "on-line" at about 300 F during an RCS cooldown,* and is removed from service at the same temperature on a startup. Therefore, whenever the RCS is below 300 F, the two SCS SVs are available for RCS overpressure protection.
The 50RV setpoint is manually switched to its low pressure setpoint whenever RCS temperature is below 324 F.
A key lock switch located in the control room is used to lower the solenoid actuation point from its normal high setpoint (* 2500 psig) to the low pressure setpoint (500 psig).
The Yankee-Rowe pressurizer is required to have a bubble of at least 198 ft3 whenever RCS temperature is above 180 F.
Below 180 F the pressurizer is maintained in a water-solid condition.
An enabling alam is provided which senses RCS pressure.
Whenever RCS pressure is below 425 psig, the alarm annunciates to warn the operator to enable the LTOPS. The 50RV MOV is alarmed so that if the LTOPS is enabled and the 50RV MOV comes off its open seat (i.e., starts to close),
the operator is alerted.
Also, a high pressure alar-n at 400 psig alerts the operator to a pressure transient in orogress. The LTOPS equipment
'The ;rocedures allcw the SCS to be :: laced in operation when the RCS 0
temerature is ::etween 300 F and 33C0F.
i100054
. as a function of RCS temperature is shown below:
Temperature LTOPs Cproonents 300 F 3 T 3 324 F SORV + Pressurizer Bubble 0
0 180 F < T < 300 F SORY + 2 SCS SVs + Fressurizer Subble
- 0 0
0 T $ 180 F SORV + 2 SCS SVs Our evaluation of the Yankee Rowe LTOPS follows.
3.1 Electrical Controls YAEC's proposed overall approach to eliminating overpressure events incorporates administrative / procedural, and hardware controls, with reliance upon the plant operator for the principal line of defense.
Preventive administrative / procedural measures include:
(1) procedural precautions; (2)'de-energizing of essential components which are not required to be operable during the cold shutdown mode of operation; (3) maintenance of a non-water-solid reactor coolant system condition whenever possible; and (4) incorpor?. tion of a low pressure relief setpoint for the existing SORV contrpl logic and the use pf the SCS.
The basic design criteria that we applied in determinin:.. the acceptability of the electrical, instrumentation, and control aspects of the LTOPS relate to:
(1) Operator Action; (2) Single Failure; (3) System Testability; and (4) Seismic Category I and IEEE Std. 279-1971 Criteria, as defined in Section 2.3 of this SER.
In addition to complying with the above criteria, YAEC has agreed to provide an RCS temperature-recording capability compatible with the existing pressure-recording feature.
With this additicn, YAEC will meet all our requirements for an acceptable LTOPS.
3.1.1 Overpressure Protection System Design and Proposed Modifications YAEC's proposed design for the Yankee-Rowe LT0PS is based on the use of a pressurizer SORY in conjunction with two passive spring-loaded safety valves (SV's) which are part of the SCS. These valves, in conjunction with specific procedural controls, meet the following functional require-ments:
i100055/
- The pressurizer bubble is required by procedures, but not by Technical Specifications.
, (1) The existing pressurizer 50RV provides relief capability in excess of a single SCS SV. The pressurizer-SORV low pressure setpoir.t is 500 psig.
However, a new pressurizer 50RV of increased capacity is to be installed to replace the existing valve.
(2) The two SCS SV's taken together provide slightly less than double the relief capacity of the existing S0P.V. The SCS-SV pressure setpoint is 425 psig.
(3) A key-operated switch will be installed. The contacts of this switch will be connected to the pressurizer S0RV and the main coolant overpressurization alarm circuitry A new full range pressure recorder will alsc, *? installed on the main control board to continuously monitor the main coolant pressure.
The sensing line is the same as that used by tne main cool:nt pressure channel of the reactor protection system (RPS). The new channel will consist of a power supply, pressure transmitter, dual setpoint trip device, auxiliary relay, and a strip e nrt recorder.
One setpoint of the trip device will be connected to a panalann, will actuate at 450 psig decreasing main coolant system pressure, and will be used to signal the operator to activate the key-operated interlock switch.
The other setpoint of the tr y device will be adjusted to operate the pressurizer SORT', through the auxiliary relay, at 500 psig increasing.
An existing bistable located in the main control board will be set to actuate at 400 psig increasing.
It will be wimd via the permissive keylock switch to a main control board xnnunciator window to provide an alarm output to notify operatcrs of an impending low temperature overpressure condition.
(4) Below 300 F, the SCS SV's are available in conjunction with the pressurizer 50RV as pressure transient limiting devices. The SCS SV's have sufficient capacity to be considered as 100-percent backup for both the presert and planned pressurizer relief valves.
All the above proposed additicnal LTOPS featL.es have been installed, are protection grade and meet IEEE Std. 279-1971 Criteria. We, therefore, find these modifications to be acceptable.
3.1. 2 Single Failure Criterion and Operator Action The SCS SV's are available below 300*F, such that these valves and the pressurizer relief valve can be used as pressure transient limiting devices.
le.e SCS SV's have sufficient capacity to be considered as 100-percent backup for the new and larger pressurizer relief valve.
Therefore, we find that the single failure criterion is met at temperatures below 300 F without operator action.
Above 300 F, the SCS SV's are isolated from the main coolant system and, therefore, are unavailable to limit a pressure transient.
However, the main coolant systene is not solid at these temperatures and a pressurizer steam volume of > 198 cubic feet is available.
For all postulated events that could 1100 056
. lead to overpressurization at temperatures above 300 F, the pressurizer steam volume limits pressure buildup over the first ten minutes to a value less than the 10 CFR 50, Appendix G limits.
In this temperature range, operator action beyond ten minutes is an effective backup to the pressurizer relief valve low setpoint in limiting main coolant pressure.
Therefore, the single failure criterion can be met for all conditions requiring reactor vessel overpressure protection.
We, therefore, find that the use of the SCS SV's and the pressurizer relief valve in YAEC's proposed LTOPS is an acceptable method for preventing an overpressurization event initiation and for limiting the effects of such an event once it is initiated and that the LTOPS meet our criteria relative to single failure and operator action as defined in Section 2.3 of this SE.
3.1.3 S_eismic Design Criteria YAEC stated that Yankee-Rowe was not designed to specific seismic criteria.
Thus, the purchase specifications for the SCS SV's or the pressurizer SORV did not address seismic requirements.
Hot;ever, tha design of these valves is identical to valves that are seismically qualified.
The seismic requirements in the design changes will be in accordance with current YAEC specifications for additions or modifi-cations to plant equipment.
We conclude that the proposed Yankee-Rowe LTOPS conforms to the intent of our seismic dtsign criteria and is, therefore, acceptable.
3.1.4 IEEE Std-279-1971 Criteria The design of the Yankee-Rowe pressurizer relief valve system does meet the intent of the IEEE Std-279-1971 criteria.
The mechanical equipment was purchased for Safety Class I application.
The quality assurance requirements which would be imposed on the purchase of electrical equip-ment and instrumentation designed to the requirements of the IEEE Std-279-1971 criteria was imposed on the purchase of electrical equipment and instrumentation which is used in the pressurizer relief valve control system.
The pressurizer SORV depends only on electrical power to operate; it is not an air-operated valve.
The solenoid actuates a pilot which allows fluid pressure to open the valve. Thus, a loss of station or instrument air pressure will have no effect on the operability of this valve.
The SCS SV's, on the other hand, are spring-loaded, and the SCS-SV isolation valves are motor-operated.
In the event of an electrical failure, the SCS-SV isolation valves will " fail open," and the SCS SV's will still function.
'1100 057
. We conclude that the Yankee-Rowe LTOPS would not be susceptible to a Leon-mode failure involving loss of offsite electrical power and air supply and that it satisfies our position with respect to the IEEE Std-279-1971 criteria for low temperature overpressure protection. On this basis, the licensee's proposal is acceptable.
3.1.5 Alarm System YAEC has added electrical and control provisions for the LTOP to be used during low RCS temperature conditions. This system includes:
(1)A high pressure alarm to alert the operator of an impending overpressure transient and another high pressure indication associated with the actuation of the pressurizer S0RV, (2) Indications to alert the operator to open the pressurizer-SORV isolation valve and to confirm the opening of this valve at the main control board, (3) An alann associated with the plant cooldown process to ensure that the pressurizer-SORV will be operable in accordance with the Technical Specifications, and (4) An alarm that will activate to alert the operator that pressurizer-SORV is in the "open" position.
We have concluded that YAEC has provided all necessary alarms and indications for reliable LTOPS operation.
We have also concluded that the added alarm features are of the audio / visual type, ca 3able of providing the operator unambiguous information associated with t1e LTOPS operations. We, there-fore, find the alarm system to be acceptable.
3.1.6 Pressure Transient Recording To provide a required permanent record of pressure transients in the RCS, YAEC has installed a new full range pressure recorder on the control board.
This recorder continuously monitors reactor vessel pressure.
The associated pressure transmitter which provides the signal to this recorder also provides the input to the pressurizer relief valve low pressure setpoint.
To meet our position on pressure transient recording require-ments, YAEC has agreed to provide an RCS temperature recording capability to supplement the existing pressure recording.
YAEC has also agreed to verify tnat the response time of the pressure recorder is approximately 100 psig/sec.
Based on our implementation of YAEC's comitment, we find that the pressure transient recording provisions for the LTOPS are acceptable.
3.2 Testability Our position with regard to testability is that the system be tested prior to any reliance upon it for overpressure protection.
YAEC has stated that the SORV and its associated actuation electronics will be tested every refueling outage.
The two SCS SVs will have their setpoint verified in accordance with the requirements of the ASME Code,Section XI. Also, the pressurizer level instrumentation will be tested by performing a channel calibration at least every 18 months.
Testing requirements have
~
1100 D58
. been incorporated into the Technical Specifications as discussed in Section 5.2.
We find this to be acceptable.
3.3 Appendix G The licensee originally contended that the maximum pressure permitted by the Hydrostatic Test Curve of Appendix G was acceptable for LTOPS design.
We requested YAEC to provide justification for the use of the Hydro curve, and as a result of subsequent correspondence and telephone conversations, we maintained our position that the Isothermal Appendix G curve was more appropriate for PWR overpressure mitigating system desi :n. Therefore, the YAEC LTOPS was evaluated with respect to the Isothermal Appendix G curve,* as all other operating PWRs are. This curve has a 60 psig and a 10 F conservatism.
3.4 RCS Transient Pressure Analyses YAEC analyzed the RCS pressure response to a number of initiating events.
The results of these aralyses, presented in YAEC's December 1,1976 and May 27,1977 submittals (References 4,10), are given in such a manner that the proposed SORV and RHR SV setpoints are substantiated.**
The various mass input events are discussed below, with the exception of a Safety Injection System (SIS) train (300 F < T < 200 F) and a single LPSIP (T < 200 F).
For these events the LTOPS does not provide adequate relieving capacity to keep the RCS pressure below the Isothermal curve limit.
Section 4.0 provides further discussion of these events and their mitigation.
YAEC developed a computer code *** for the analyses of the RCP startup event (e.g., startup of a single RCP in an idle and water-solid RCS where the steam generator secondary is hotter than the RCS). The code itself and the results are discussed in Section 3.4.2.
Specifically, YAEC uspg the {sothermal Appendix G curve for a vessel irradiation of 9 x 10' n/cm.
See Section 6.0.
That is, if the peak RCS pressure is below the Isothermal curve limit, assuming a single failure and with the SORV SV setpoints proposed, then the setpoints are substantiated for that event.
- The code (PRESS) was submitted to the NRC for review.
The topical repcrt, YAEC 1124, was submitted February 1977.
~
1100 059
- 3. '4.1 Mass Input Events YAEC calculated the RCS pressure response to each postulated mass addition event.
In each case, the relieving paths provided by the two SCS SVs and the 50RV were assumed available for mitigation of the transient.
Since the SCS SVs are set to open at 425 psig and the 50RV is set to open at 500 psig, the SVs provide the first "line of defense" during a cressure transient (assuming the SCS and the RCS are connected).
Above 300 F, when the RCS and SCS are isolated, the pressurizer bubble 0
provides the first "line of defense."
Each transient is,, discussed in Reference 4 along with the peak RCS pressure. We compared these predictions to the Appendix G (Isothermal curve) limits to evaluate the LTOPS's ability to mitigate the various mass input events.
With the exception of the two SIS related mass input events, which are described further in Section 4.0, the worst case. mass input' event is the mass addition from a single SIS train (1 HPSIP + 1 LPSIP) when the RCS temperature is between 200cF and 300 F.
In this event, one SCS 0
SV is assumed to fail and the mass flow from the HPSIP is discharged out the remaining SCS SV and the 50RV.
Essentially, the RCS pressure.
goes to the shutoff head of the LPSIP.* The peak RCS pressure (about 700 psig) is below the Appendix G (Isothemal Curve) limit, 730 psig**
at 2000F.
The possible effe' cts of flashing were taken into consideration by assuming a constant backpressure of 75 psig in the calculation of the SV and 50RV flow capacities. The 75 psig was chosen since~ it corresponds to the rupture disc setting on the Low Pressure Surge Tank (LPST) which receives the discharge from the SCS SVs and the 50RV. More recent data ***
affims that the YAEC " constant backpressure" approach gives conservative flow values.
3.4.2 Heat Inout Cases The licensee evaluated several energy input type events.
In each case, the LTOPS equipment was that shown in the table in Section 3.0.
The effects of liquid flashing were included in YAEC's calculation of the relief path (SV or 50RV) flow capacity. The following scenarios were evaluated:
a.
Energy input from pressurizer heaters b.
Energy input from core decay heat Energy input from RCP (themal input) c.
In each case, the licensee evaluated the RCS expansion rate considering a number of initial conditions.
In some cases, the licensee combined energy inputs to evaluate a worst case event (i.e., core decay heat +
RCP thermal energy). YAEC compared the expansion rates with the relieving capacity available, and concluded that the RCS pressure would not exceed the allowable limits for these events.
The RCP startup transient was perfomed using the licensee's code PRESS, and is described below.
- If piping losst.s were considered, then the peak pressure would be lower (about 650 psig).
- The Appendix G limit at 210 F is about 760 psic, or 30 psig higher g gQ 0
0 0
than the limit at 200 F, so the evaluation at 200 F represents the worst 3 case.
. 3.4.2.1 Reactor Coolant Pump Startup Event The startup of a RCP in a water-solid and idle RCS can cause a rapid pressurization if the Steam Generator (SG) is initially hotter than the RCS.
The flow causes an increase in the transfer of energy from the SG to the RCS liquid and the liquid expands.
In a water-solid RCS, the expansion results in a rapid system pressurization.
The Yankee-Rowe RCS has several design features which lower the likeli-hood of this event.
These features are electrical and administrative interlocks which deal with the reactor coolant loop isolation valves, and are described in Reference 4.
Even with these interlocks, Yankee-Rowe is susceptible to a pressure transient resulting from a RCS startup in an idle; non-isothermal, water-solid RCS.
For this reason, YAEC developed a computer code called PRESS to determine the RCS pressure transient during this event.
The major assumptions and conservatisms of the code are summarized below:
1.
The transient is initiated by reactor coolant flow acceleration in response to the opening of the loop isolation valves.
0 2.
A conservative RCS to SG aT of 100 F was assumed for the analyses.
3.
The relief rate assumed in the analyses is based on the pressurizer 50RV which has subsequently been replaced by a new SORV having a greater flow capacity (about 1-1/2 times the original SORV).
4.
No credit was taken for the two SCS SVs which would be available U
when the RCS temperature is below 300 F.
These valves are fully redundant to the original SORV, bet have lower setpoints.
Therefore, the peak RCS pressure would have been less assuming these valves were available.
5 The RCS is assumed water-solid, even though the administrative controls specifically prohibit RCP starts (or jogs) in this condition.
6.
The flow capacity of the 50RV are conservatively calculated (see Appendix C of Reference 4).
i
. 7.
No credit is taken for the thermal or pressure induced expansions of the RCS components (pressurizer, vessel, and coolant piping).
The licensee analyzed several cases assuming different flow accelera-tions, relief valve flows, initial temperatures and pressures, and relief valve availability.
It was determined that the nost limiting situation occurs 'for the RCP startup with a 1000F ST with the RCS temperature initially at 50 F.
The 50RV opens at about 500 psig, 0
and the peak RCS pressure is about 513 psig or about 47 psig below the Appendix G (Isothermal curve) limit.*
Based on these results, and the conservatisms associated with YAEC's code PRESS, we conclude that the SCS SVs and the 50RY provide adequate protection from the RCP startup transient.
4.0 SIS MASS INPUT EVENTS As discussed in Section 3.4, the YAEC LTOPS as proposed did not provide adequate mitigation during two postulated scenarios:
1.
The mass input from a single SIS train when the RCS temperature 0
is between 300 F and 3240F, and 2.
The mass input from a single LPSIP when the RCS temperature is below 2000F.
A brief description of the Yankee-Rowe SIS and its disablement during RCS cooldowns along with a discussion of each problem scenario and our evaluation follows:
4.1 Yankee-Rowe SIS and Disablement During Cooldown The portion of the Yankee-Rowe SIS, pertinent to RCS overpressure protec-tion, is' composed of three LPSIis, three HPSIPs, and the pipes and valves connecting the pumps to the RCS. This portion of the SIS is designed so that the three LPSIPs receive suction from the SI tank, and discharge to either the HPSIP suction header, or directly to the RCS via the LP injection header.
The HPSIPs can take a suction from either the SI tank or the LPS!P discharge (common header).
The HPSIPs deliver water into the RCS via the HP injection header.
The LP and HP injection headers are separate lines, each branching off into four lines.
Each RCS cold le; receives LP and HP injection water from one of these branch lines.
'This takes no credit for the two SCS SVs which would' limit RCS pressure to about 475 psig.
)00 Gb
The branch lines cre joined with an arrangement of check valves and MOVs, and each of the four HP header branch lines has a flow balancing throttle valve (newly installed).
The following SIS valves are lockec open and de-energized during normal plant operation, Four " Loop Injection MOVs" (in the line directing the combined a.
HP and LP flow to the RCS cold leg piping) b.
Four "LP Injection MOVs" (in the line directing LP injection flow to the individual injection lines) c.
6ne "HP Header MOV" and three "LP Header MOVs" (in the main HP and LP headers)
During an RCS cooldown, the two SIAS are manually blocked at 1800 psig.
This effectively disables all automatic initiation of the SIS components.'
Also, each of the LPSIP and HPSIP control switches is placed in the " trip pull-out" (TPO) position.** When RCS pressure is about 1000 psig, the power supply breakers to two HPSIPs and two LPSIPs are racked out or removed.
Also, the four loop injection MOVs are shut and re-energized.
0 When the RCS temperature and pressure have been lowered to about 330 F and 300 psig, the SCS is placed in service.
The third train of SIS pumps is de-energized (breakers racked out or removed) and the four LP injection MOVs are shut and re-energized. When the RCS temperature is below about 200 F, the three LP and one HP header MOVs are shut and U
re-energized.
The SIS is generally placed back in service in the reverse order during an RCS startup.
However, there are various surveillance tests perfo rmed on the SIS which may require adjustment of the sequence.
- Unlike more recent p!snts, contr.inment high pressure and steam /feedline break signals do not bypass the " block" feature. Only n.anual initiation of the Safety Injection Actuation Signal (SIAS) is still operative when the SIAS are blocked.
- This action prevents the pumps from starting even if there was an inadvertent SIAS.
m 1100 fl6 3 '~s
4.2 Mass Addition From One SIS Train (300 F < T < 324 F) 0 One scenario of concern is the mass input frem a train of SIS pumps when 0
the RCS temperature is between 300 F and 324 F.
Only one train of SIS pumps is energized in this temperature interval, and all control switches (in the control room) are in the TPO position.
Therefore, the mass addition from a full SIS train would require the folicwing separate actions:
1 Place the HPSIP control switch in Auto
- 2.
P' ace the LPSIP control switch in Auto
- 3.
Initiate the SIAS The RCS te'mperature is between 3000F and 3240F only during system heatup and cooldown.
There are no plant operations which recuire the RCS be maintained in this temperature interval.
The RCS is vulnerable during this interval because the SCS (with its two Sys) may not be available for overpressure prote: tion. The mass addition from a. SIS train would quick.ly fill the pressurizer (about 1-1/2 nin) and if the 50RV failed, the RCS pressure would increase up to the shutoff head of the SIS train (about 1550 psig). This is about 200 psig above the Isothermal limit at a temperature of 300 F.
Several points are pertinent in our evalua-0 tion of this scenario.
1.
The RCS pressure is above the Isothermal limit only when the 0
RCS temperature is between 3000F and 318 F, and the plant would normally be in this temperature band about ten hours / year.
2.
If the 100F and 60 psig instrumentation conservatisms (asso-ciated with the Appendix G Isothermal curve) are removed, the RCS pressure during this postulated event does not exceed the allowable limits.
3.
The plant operating procedures governing RCS cooldown (OP 2107) and SCS operation (OP 2162) state that the SCS can be placed 0
0 in service between 300 F and 330 F.
If the SCS is in operation, no overpressurization would occur since the SCS SVs limit the peak RCS pressure to well below the Isothermal curve limits (with the instrumentation uncertainties included).
- Placing the HPSIP and LPSIP control switches in tne "0N" position and
. ass addition to the RCS.
7 1100 Dr64
. 4.
If the event were to occur at a RCS temperature between 300 F and 318 F, the Isothemal curve would be exceeded.
- However, the hydrostatic test curve would not be exceeded (the peak' RCS pressure would be at least 300 psig below this limit).
The staff is not allowing OPS design based on the Appendix G nydrostatic test curve, but we note that the RCS is allowed to be pressurized once per year up to these limits during a slow and controlled test. The SIS (single train) mass addition event is not a " slow and controlled" test.
Nevertheless, we note that the peak pressure of this relatively improbable event is significantly below the allowable pressure of a relatively frequent test.
The licensee's administrative controls significantly lower the likelihood of this event.
However, even if the event occurred, the peak pressure would exceed the staff's OPS design limits (Isothermal Appendix G curve) only if the RCS temperature was between 300 F and 318 F and the SCS SVs were not available. The instrumentation conservatisms associated with the Isothemal curve are such that the real or " Bare" Isothermal curve would not be exceeded.
Based on these arguments and the factors listed above, we conclude that this event can be omitted for Yankee-Rowe from consideration as an LTOPS design basis transient.
4.3 Mass Addition From One LPSIP (T < 200 F)
The other scenario of concern is the mass input from a single LPSIP when the RCS temperature is below 200 F.
At these temperatures, all SIS pump control switches are in the TP0 position, the pumps are de-energized with their breaners racked out (or removed), and all header and injection MOVs are shut and energized.
However, the plant conducts a surveillance test on each LPSIP and if the SIAS actuation logic were to fail during this test,* the SIS M0Vs would open and the LPSIP would inject water into the RCS.
The two SCS SVs and the pressurizer SORV are available for pressure relief when the RCS temperature is below 200 F.
However, the capacities of these vales are insufficient, and the RCS pressure, assuming the failure of the SORV, would rise to about 700 psig. The pressure limit (Isothermal curve) at 80 F is about 570 psig, therefore, the peak RCS pressure exceeds the Isothemal curve by about 130 psig.
Mhe SIAS actuation circuitry would not even have to " fail" for this to occur since testir.g and maintenance of the circuitry is conducted during unit shutdowns.
Since all the SIS valves are energized during shutdowns, an inadvertent SIAS would open them.
\\100065
. 4.3.1 SIAS Removal To reduce the likelihood of this event, YAEC has proposed the removal of the SIAS from the following valves:
a.
Four Loop Injection MOVs b.
Four LP Injection M0Vs c.
Two LP Header M0Vs These ten MOVs are locked open and de-energized during normal operation.
The SIAS during an actual Loss of Coolant Accident (LOCA) event performs no control function with these valves since they are, at that time, open and de-energized.* However, during a RCS cooldown, the valves are unlocked, re-energized, and shut, hence, an SIAS will open all of them.
YAEC's proposal to renove the SIAS would prevent their opening upon a spurious signal.
We reviewed the YAEC proposal with respect to low-temperature RCS over-pressure protection and with respect to LOCA mitigation.
We agree with the licensee that the removal of the SIAS from the ten SIS M0Vs would make the LPSIP event less likely to occur. Also, we agree that the proposal has no effect on LOCA mitigation during normal operation.
However, if a LOCA at low RCS temperature were to occur or even if the operator desired to expeditiously put water into the RCS without a LOCA, a significant number of operator actions would be necessary.**
This was. discussed with the licensee and a preferred approach was considered.
Rather than removing the signal from ten MOVs, the SIAS would be removed from only one LP header MOV (SIS-M0V-535).
If an inadvertent SIAS occurred while the LPSIP were running, the flowpath to the RCS would not be established since MOV-535 would not open.
If the operator wanted to put ECCS water into the RCS, only two switch operations would establish four flowpaths.***
Thus, the removal of the SIAS from only M0V-535 reduces the possibility of a LPSIP overpressure event and does not add a significant number of operator actions if ECCS injection into the RCS.is needed.
We find the removal of the SIAS from the SIS-M0V-535 to be acceptable.
- They receive an SIAS because the original design of the plant required them to be shut unless there was a LOCA.
With the SIAS, only one switch operation is necessary (manually initiating
" S I AS'.' ).
- The switch operations would be:
1.
Manually initiate SIAS; and 2.
Open SIS-M0V-535.
!!00 01i6
. 4.3.2 System Modifications to Fully Mitigate the LPSIP Ever,t The licensee's proposed LTOPS does not mitigate the LPSIP mass addition event.
Based on discussions with the licensee and other utilities, a nwnber of equipment changes could be made to various systems which would result in the LPSIP event being fully mitigated.
These changes are described below along with our assessment of their viability.
a.
Additional Pressurizer SORV Another pressurizer 50RV could be added to the existing 50RV in a parallel arrangement.
The setpoint of the new SORV could be the same as the existing SORV, or staggered to prevent unnecessary valve operation.
The new SORV discharge could be directed to the LPST eductor with the existing SORV discharge.
Analyses would have to be conducted to ensure the pressurizer itself could accommodate additional piping.
Also, analyses would have to be conducted to ensure the LPST internal hydraulics and reaction forces are not excessive and do not cause tank vibration and/or damage, b.
Increasing the Capacity of the Existina OPS This modification could be accomplished by the replacement of the newly installed SORY with a larger SORV and associated MOV and piping, c.
Installation of Relief Devices in the ECCS Piping Between the LPSIP and the RCS A system composed of safety,alves or 50RVs could be added to the ECCS piping such that thu RCS pressure would be limited by relieving LPSIP flow. This modification would have to be accom-panied by control circuitry or isolation valves which insured that the relief devictspl[Mt_. operate when the LPSIPs.are required during a LOCA.
~~
dT LPSIP Coktrol Circuit ECCS control system modifications could be made which would either limit LPSIP flow or trip the LPSIP in the event of an inadvertent injection.
'i100 4967
. e.
SC5 System Modifications Larger safety valves could be added to the SCS, involving removal of the existing valves and the addition of new larger piping and valves. The setpoint would remain the same as the existing SVs so the SCS perfomance is not compromised. The capacity of the new valves would have to be very large to accommodate the LPSIP flow. The new SV discharge could be directed to the LPST (like the existing SCS SVs).
However, a detailed analysis of the internal hydraulics and reactior, forces in the LPST would have to be perfomed.
The LPSIP flow is so large that the flow into the LPST could cause excessive stress and vibration.
It is, therefore, possible that the addition of new SCS SVs would have to be accompanied by a ww or modified LP5T.
We found the factors described below pertinent in our evaluation of the licensee's re<;uests that this event need act be considered when deter-minins LTOPS relief capacity:
1 If a LPSIP mass addition avent were to occur at a RCS temperature 0
below 200 F, the Appendix 3 (Isothemal curve) would be exceeded 0
by about 130 psig.
There are 60 nsig and 10 F instrument er'.or factors included for conservatisu. in the calculation of the Isothermal curve.
Removir.; these conservatisms, the Appendix G 0
curve at 80 F is about 650 psig, therefore, the RCS pressure during the LPSIP event exceeds the " Bare" Isothemal Apoendix G curve (i.e., all conservatisms removed) by only 50 psig.
2.
The " Bare" Isothemal curve increases n to about 700 psig 0
at an RCS temperature of about 140 F.
Therefore, t 6 RCS is susceptible to LPSIP mass input overpressure only wnen the RCS temperature is below this value. The LPSIP surveillance test (which could cause the RCS overpressure event) is normally 0
conducted at RCS temperature close to 200 F.
3.
If the 50RV and both SCS SVs were available for pressure relief the peak RCS pressure would be only 630 psig. Therefore, the Isothermal curve is exceeded at RCS temperatures below about l?5 F, and the " BARE" curve is exceeded below abcut 850F, The 0
RCS temperature is rarely below E5' F sith the vessel head in place and the LPSIP flow test is never conducted while tne RCS is in this condition.
1100 068
. 4.
If the 50RV failed during LPSIP mass addition event while the RCS temperature is below about 200 F, the Isothermal curve would be exceeded.
However, the hydrostatic test curve would not be exceeded and the peak RCS pressure would be at least 100 psig below the hydrostatic test curve. Our position is not to accept OPS design based on the Appendix G hydrostatic test curve, but we note that the RCS is allowed to be pressurized cnce per year up to these limits during a slow and controlled test. The LPSIP mass addition event is not a " slow and controlled" test. Never-the less, we note that the peak pressure of a relatively iaprobable event is significantly below the allowable pressure of an annually performed test.
5.
The licensee concludes, and we agree, that the most viable addition to the already existing LTOPS which would result in a new LTOPS that could totally mitigate a LPSIP mass addition event would be the installation of new large capacity SVs on the SCS. This modification would cost at least* $250,000, and no appreciable increase in plant safety would result.
6.
The YAEC proposal to remove the SIAS from SIS-M0V-535 results in a significant reduction in the likelihood of the LPSIP mass addition event.
Based on the factors summarized above, we conclude that the LPSIP mass addition event can be adequately prevented by the administrative controls described and the removal of the SIAS from SIS-MOV-535.
Since the removal of the SIAS from M0V-535 causes no degradation of the ECCS in its performance of LOCA mitigation, we conclude inat the SIAS removal from MOV-535 is acceptable. We also conclude that the LPSIP mass addition event can be excluded as a design consideration from the Yankee-Rowe-LTOPS.
5.0 Administrative Controls To supplement the hardware modifications and to limit the magnitude of postulated pressure transients to within the bounds of the analyses provided by the licensee, a defense in depth approach is adopted using procedural and administrative controls. Those specific conditions required to assure that the plant is operated within the bounds of the analyses have been included in the Technical Specifications.
- This does not take into account LPST analyses and/or modifications, and any indirect cost such as replacement power cost.
1100 069
. 5.1 PROCEDURES A number of provisions for the prevention of pressure transients are contained in the Yankee-Rowe operating procedures.
(1 ) The plant operating procedures fur shutdcwn, cooldown and heatup ' operations have been modified to recuce to a minimum the time the RCS is in a water-solid condition.
(2) The plant procedures for RCP startup have been modified to prohibit RCP starts (or jogs) in a water-solid condition even though the licensee's LTOPS has been designed to miticate the RCP startup transient with a 1000F aT.
(3) A number of administrative controls exist for the SIS to reduce the possibility of an inadvertent mass addition.
Some of these control's are discussed in Section 4.1, and the fol-lowing list supplements that discussion.
Testing of the SIS components and control systems is strictly controlled to minimize the possibility of inad-vertent mass addition.
The HPSIP and LPSIP Recirculation flow paths (from pump suctions back to Safety Injection Tank) are open whenever the RCS temperature is below 330oF.
This action effectively limits the RCS peak pressure from an inadvertent HPSIP or LPSIP operation.
The ECCS components are not re-energized (unless required for s,urveillance testing) until the RCS is being heated up.
We conclude that the procedural and administrative control described are acceptable. However, we have determined that certain procedural and administrative controls should be included in the Technical Specifications.
These are listed in the following section.
1100 070
. 5.2 Technical Specifiestions To assure operation of the LTOPS the licensee has submitted (Reference
- 16) proposed Technical Specifications to be incorporated into the license for Yankee-Rowe. These specifications are sunmarized below:
1.
The pressurizer SORV shall be operable in the low pressure setpoint mode whenever the RCS temperature is below 324 F.
The SCS SVs shall be available for RCS overpressure protection whenever the RCS temperature is below 300 F.
The pressurizer surge volume must be greater than 198 ft3 whenever the RCS temperature is between 300 F and 324 F.
2.
The LTOPS components are to be tested at the following frequency; The 50RV setpoint is verified every 18 months, a.
The SCS SV setpoints are verified in accordance with the b.
frequency specified by the ASME Lection XI requirements, The pressurizer level instruments are calibrated at least c.
every 18 months.
If the RCS temperature is below 300 F and the 50RV or one SCS 0
3.
SVs becomes inoperable, restore to an operable condition within seven days or depressurize and vent
- the RCS to atmosphere, the LPST or the Primary Drain Collecting Tank (PDCT) within eight hours.
4.
A RCP may be started only if there is a bubble in the pres-surizer or if the steam generator /RCS AT is below 100 F.
- The RCS is vented by the installation of a pipe section (bypassine two cao'llary tubes) which conr.ects the cressurizer to the LPST (or PDCT).
I100 A/
, 5.
Whenever the RCS temperature is between 2000F and 3240F, two of the three LPSIPs power supply breakers are OPEN and either racked out or removed.
When the RCS temperature is below 200 F, all HPSIP and LPSIP breakers are OPEN and either 0
racked out or removed.*
0 6.
Whenever the RCS temperature is below 324 F, all four loop injection MOVs and all LP injection MOVs, are shut, and the LPSIP recirc valve, MOV-532, is open.
The following Technical Specifications were not croposed by the licensee in Reference 16, but have been discussed with YAEC and are mutually a g ree a bl e..
0 7
If the RCS temperature is below 300 F and the 50RV and one SCS SV becomes inoperable, or if two SCS SVs becomes inoperable, depressurize and vent the RCS to atmosphere, LPST or to the PDCT within eight hours.
0 8.
If the RCS temperature is between 300 F and 324 F, and if thL SORV becomes unavailable, then within eight hours either 0
reduce the RCS temperature to below 300 F and place the SCS SVsin service, or raise the RCS temperature to above 324 F.
If neither of these actions can be completed within the allowed time, cooldown and depressurize the RCS to either the atmos-
,phere, LPST or PDCT within an additional eight hours.
We have reviewed the licensee's proposed Technical Specifications described above and concluded that they are acceptable based on the analyses and methods described in Section 3.4 and Section 4.0.
6.0 Suma ry The administrative controls and hardware changes made by YAEC provide protection for Yankee-Rowe from pressure transients at low temperatures by reducing the probability of initiation of a transient and by limiting the pressure of such a transient to below Appendix G limits.
Based on the factors described in Section 4.0, we agree with YAEC that the following scenarios can be excluded in the determination of LTOPS relief capacity:
(1) the mass addition from a single LPSIP (TRCS ~< 200 F) and (2) the mass addition from a single ECCS train (300 F < T < 324 F).
However, any future revisions of the Appendix G limits for Yankee-Rowe must be considered and the LTOPS setpoints adjusted accordingly, with corresponding adjustments in the license.
Based on the above, we have concluded that the LTOPS would provide adequate protection from overpressure transients.
Therefore, we find the electrical, instrumentation, and control aspects of the overall LTOPS design and the related Technical Specification changes proposed by YAEC, as modified by us, acceptable on the basis that they meet the criteria relative to (1) Operator Action, (2) Single Failure, (3) System Testability, and (4) Seismic Category I and IEEE Std-279-1971, as defined in Section 2.3 of this SE.
1100 072
- Special surveillance tests are allowed with special restrictions specified in the proposed Tech Specs.
7.0 Environmental Considerations We have determined that the amendment does not authorize a change in effluent types or total amounts nor an increase in power level and will not result in any significant environmental impact.
Having made this determination, we have further concluded that the amendment involves an action which is insignificant from the standpoint of environmental impact and, pursuant to 10 CFR Section 51.5(d)(4), that an environmental impact statement or negative declaration and environmental impact appraisal need not be prepared in connection with the issuance of this amendment.
8.0 C_onclusion We have concluded, based on the considerations discussed above, that:
(1) because the amendment does not involve a significant increase in the probability or consequences of accidents previously considered and does not involve a significant decrease in a safety margin, the amendment does not involve a significant hazards consideration, (2) there is reasonable assurance tiat the healt and safety of the public will not be endangered by operation in the proposed manner, and (3) such activities will be conducted in compliance with the Commission's regulations and the issuance of this amendment will not be inir.iical to the cormion defense and security or to the health and safety of the public.
Da ted: September 14, 1979 kl00073
- 25.-
YANKEE R0WE REFERENCES 1.
NRC (Schwencer) to YAEC (Groce), August 11, 1976.
2.
YAEC (French) to NRC (NRR), September 3, 1976.
3.
YAEC (French) to NRC (NRR), October 26, 1976.
4 YAEC (French) to NRC (NRR), December 1,1976.
5.
NRC (Schwencer) to YAEC (Groce), February if,1977 6.
YAEC (Vandenburg) to NRC (Reid), February 4, 1977.
7.
YAEC (Groce) to NRC (NRR) March 31, 1977.
~
8.
NRC (Schwencer) to YAEC (Groce), April 1,1977.
9.
YAEC (French) to NRC (Schwencer), April 25, 1977.
10.
YAEC (Vandenburg) to NRC (NRR), May 27, 1977.
11 NRC and YAEC telephone conversation July 29, 1977, 12.
YAEC (Vandenburg) to NRC (NRR), November 1,1977.
13.
NRC (Schwencer) to YAEC (Groce), January 31, 1978.
14 YAEC (Groce) to NRC (NRR), March S,1978..
15.
YAEC (Johnson) to NRC (NRR), April 28, 1978.
16.
YAEC (Johnson) to NRC (NRR), June 5,1978.
17.
" Staff discussion of 15 technical issues listed in Attachment G November 3,1976 Memo from Director, NRR, to Staff" NUREG-0138, November 1976.
.