ML19259D282
| ML19259D282 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 09/11/1979 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19259D281 | List: |
| References | |
| NUDOCS 7910170559 | |
| Download: ML19259D282 (8) | |
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Safety Evaluation of the Overcressure Miticating System for Maine Yankee Atomic Power Plant 1.0 Introduction By letter dated February 24, 1978, Maine Yankee Atomic Power Comcany (MYAPCO) submitted the latest details on their proposed Overpressure Mitigating System (OMS). This infort: tion supplements other documen-tation submitted by MYAPCO over the past 2 years.
Staff review of all information submitted by MYAPCO in support of the proposed OMS is cocc?ete *nd we have found that the system provides adequate protection fram overpressure transients. A detailed safety evaluation follows.
2.0 Backaround Over the last few years, incidents identified as pressure transients have occurred in presst:rized water reactors. This term " pressure transients," as used in this report, refers to events during which the tenperature-pressure limits of the reactor vessel, as shown in the facility Technical Specifications, are exceeded. All of these incidents occurred at relatively low temperature (less than 200 degrees F) where the reactor vessel material toughness (resistance to brittle failure) is reduced.
The " Technical Report on Reactor Vessel Pressure Transients" in.'WREs-0138 (Reference 1) swanarizes the technical considerations relevant to this matter, discusses the safety cancerns 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 is presented here.
2.1 Vessel Characteristics Reactor vessels are constructed of high qua11ty steel made to rigid specifications, and fabricated and inspected in accordance with the time-proven rules of the ASME Soiler and Pressure vessel Cede. 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 *.ne pressure allowed during startup and st.atdown operations.
At operating temperatures, the pressure allowed by ADpendix G Ifmits is in excess of the set:oint of currently installed pressurizer code safety valves. However, nost coerating PWRs were not originally designed with systems to prevent pressure transients during cold conditions from exceeding the Appendix G limit.
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. 2.2 Reculatory Actions By letter duced August 11,1976 (Reference 2) the NkC requested that MYAPCO evaluate the potential for overpressurization of the reactor vessel as a result of a single equipnent failure or operator error and to implement those design and/or procedural modifications as required to preclude such occurrences. The MY plant was built with a key selected low pressure setpoint for the pressurizer relief valves. The low pressure setpoint was installed explicitly to provide protection from overpressure transients. However, the NRC single failure criteria for
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overcressure mit1 gating systems was not met by the existing system design which uses a single channel actuation circuit.
After initial examination of the installed system, we determined that proper adsinistrative controls would assure safe operation for the period of time prior to complete review of the CMS at MY.
In reference 3 MYAPCO proposed to remove the interlock which initiates automatic closure of the normal RHR system inlet motor operated isolation valves at a reactor coolant system pressure of 400 psig. This would allow pressure relief through code safety valves installed on the RHR system in the event of a pressure transient. The staff review of that documenu resulted in questions to MYAPCO transmitted in reference 4 The response to those questions was provided in reference 5.
References 4 and 5 provided the information needed for review of the methods used by MYAPCO for overpressure transient analysis. In addition, details of operating procedures were provided.
The NRC response to the initial MYAPCO proposal was transmitted to the licensee in reference 6.
The staff asked the licensee to submit a revised proposal that did not negate the safety aspects of the RHR isolation valves autoclosure feature. MY respon/ad in reference 7 with a proposal 6or a single keylock switch to disabit; the RHR autoclosure feature and to enable the low pressure setpoint of the pressurizer relief valves when at low temperature. As discussed in Reference 8 the staff found this proposal to be unacceptable because it did not satisfy the single failure criteria.
Subsequently (Reference 9) MYAPC0 submitted a proposal to raise the auto-closure set:cint to 600 psig and to use the existing relief capacity installed in the RHR system in conjunction with the pressurf:er relief valves as the CMS for MY.
The following detailed review is based on this latest proposal along with selected information provided in the earlier su bmi ttal s.
2.3 Cesien Criteria Througn a series of meetings and corres;endence witn pWR vendors and licensees, the staff developed a set of design criteria for an acceptable over-pressure mitigating system, which would orovide assurance that the mitigating lb2 b5b
. system would 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 performance are:
(1) Ocera tor Action: No credit can be taken for operator action for ten minutes after 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.
(3) Testability: The system must be testable on a periodic basis consistent with the system'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 coeren failure 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.
Another criterion required by the staff in the design of the pressure mitigating system was that the electrical, instrumentation, and contici systens provide alarms to alert the operator to (1) properly enable the system at the appropriate temperature during cooldowns and (2) indicate if a pressure transient is in progress.
In the initial letters to all PWR licensees, the staff also required the installation and use of permanent RCS pressure and tenperature recording devices.
2.4 Design Basis Events The overpressure 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 type from charging pumpr7 safety injection pumps, safety injection accumulator, and a heat addition ty:,e which causes thermal expansion from sources such as steam generators and decay heat.
Those transients which result in the most rapid pressure increases are identified as the design basis events. The most rapid mass input transient at Maine Yankee is inadvertent actuation of one of the centrifugal charging pumos which ilso serve as high pressure safety injection pumps.
The most limiting thermal expansion transient is the start of a reactor coolant pump with a 50 degree F tenperature difference between the water in the reactor vessel and the water in the steam generator.
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4 It should be noted that the limiting thermal expansion transient discussed in this section is based on RCS/SG tenperature difference of 50 cegrees F.
However, MYApCO performed the analysis assumir.g a temperature difference of 100 degrees F.
Since MYApCO has not proposed to initiate procedures to measure steam generator temperatures to limit the difference to less than 50 degrees F, we find the analysis based on 100 degrees F to be acceptable in lieu of procedures to maintain RCS/SG temperature differences of less than 50 degrees F.
3.0 System Descriction and Evaluatton The Overpressure Mitigating System adopted by Maine Yankee uses the pressurizer relief valves in conjuection with the RHR safety valves. Two pressurizer solenoid operated relief valves are installed in the pressurizer and are equipped with a low pressure setpoint feature. By means of a key lock switch, the operator enables the low pressure setpoint of 500 psig.
A single pressurizer pressure transmitter provides the pressure signal for both relief valves. During cooldown, the low pressure setpoint is selected after RCS pressure is less than 450 psig and prior to decreasing RCS temperature below 220 degrees F.
The RHR safety valves provide over-pressure protection in case a single failure disables the pressurizer relief valves. To assure continued availability of the RHR safety valves during a potential overpressure transient, the automatic closure setpoint of the RHR isolatiun valves was increased to 600 psig. Analysis of the postulated pressure transients shows a maximum pressure of 560 psig during these transients. The RHR isolation setpoint is sufficient to preclude automatic closure during a pressure transient.
An enabling alarm will be installed which audits RCS temperature, the position of the keylock switch, the position of the upstream isolation valves and the position of the RHR inlet isolation valves. This will alert the operator if the RCS temperature goes telow 225 degrees F with-out the overpressure mitigating system being completely enabled.
3.1 Electrical Controls The staff's review of the electrical, instrumentation and control design aspects of the OMS is attached to this evaluation.
3.2 Testability Testability of the pressurizer relief valves will be provided. The licensee has submitted a tentative Technical Specification in which a setpoint calibration test and a manual solanoid operation test would be performed on a schedule consistent with esca refueling outage.
3.1 Pressure-Tencerature Limit The pressure-tenperature ifmits sumnitted by Maine Yankee for purposes 1162 358
. of overpressure protection are based on the current Appendix Gcurve in the Technical Specification. The pressure limit of'589 psig is the maximum pressure allowed by the Technical Specifications at a RCS temperature of 70 degrees F.
At the present time, the reactor vessel can be pressurized to full design pressure at all temperatures above 220 degrees F without exceeding allowable limits. Thus, the overpressure mitigating system must be enabled prior to cooling below 220 degrees F and is required to limit the pressure during a postulated transient to below 589 psig. We find these values to be acceptable.
3.4 Setooint Analysis The licensee performed analyses of both design basis events, inadvertent HPSI initiation and inadvertent start of a reactor coolant pump with a hot steam generator. MYAPCO sutaitted detailed infonnation related to relief valve modeling and system modeling for the thermal expansion transient. We have reviewed these methods and find that they provide an acceptable technique for determining relief capacity and peak pressures,for the design basis transients at MYAPCO.
The pressurizer relief valves have a low pressure setpoint of 500 psig.
Based on the Isotopic Homogeneous Expansion model, the combined relief capacity of the two pressurizer relief valves is 800 gpm at 500 psig and 1200 sps at 600 psig. The licensee has stated that the maximum flow rate of a single high pressure safety injection pump is less than 800 gpa.
Since the flow capacity of the combined relief valves at 500 psig is sufficient to handle the maximum HpSI pump flow, we find the setpoint and relief capacity of the pressurizer relief valves to be adequate to maintain system pressure below 589 psig, as required by the Appendix G limits.
The RHR safety valves provide a backup in the event that a single failure would disable the pressurizer relief valves. The licensee has calculated the combined relief capacity of the two RHR safety valves to be 740 gpm at 400 psig and 1150 gpm at 500- psig. 5ecause the relief capacity of these valves at 500 psig is greater than the maximum flow rata of a sincie
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HPSI pump, we find the setpoint and the relief capacity of the RHR safety vaTves to be adecuate to maintain system pressure below 589 psig, as
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required by the Appendix G limits.
To assure the availability of the RH!t safety valves to provide over-pressure ::rotection, the licensee has proposed to increase the set;oint of the RHR automatic isolation feature to 500 psig. We find this to be an acceptable msthod of preventing inadvertent isolation of the RHR system during a pressu: e transient because the maximum pressure revealed during an overpressure transient is limited less tnan 589 psig.
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. An analysis of the reactor coolant pump start transient was done assuming that the steam generator temperature was 100 degrees F above the reactor coolant systen temperature. The pressurizer relief. valves were used for purposes of a bounding analysis because the RHR safety valves provide greater relief and have a lower setpoint. The analysis showed a maximum pressure of 564 psig. Since the maximum pressure during this transient is below the Appendix G limit of 589 psig, we find the relief capacity and setpoint of the CMS to be adequate for mitigating the design basis thermal expansion pressure transient.
3.5 Imolementation Schedule The Maine Yankee plant was constructed with the keylock selected low pressure setpoint feature for the pressurizer relief valves. Current operating procedures use this overpressure mitigating feature during low temperature operations.
The licensee has stated that implementation of the changes required to meet the NRC single failure criteria will be made immediately following receipt of written approval of these changes from the NRC. We find this schedule to be acccotable.
a.0 Administrative Controls To supplement the hardware modifications and to limit the magnitude of postulated pressure transients to within the bounds of the analysis provided by the licensee, a dttense in depth approach is adopted using procedural and administrative controls. Those specific conditions re-quired to assure that the plant is operated within the bounds of the analysis are spellea cut in the Technical Specifications.
4.1 procedures A number of provisions to prevent the initiation of pressure transients are contained in the Maine Yankee operating procedures.
A precautionary statement is included recuiring the breakers for non-coerating RCps to be racked down wnenever RCS temperature is less than 220 degrees F.
An initial condition for plant heatup requires the computer alarm for RCS pressure to be operable and set to 375 psig.
A steam bubble is formed in the pressurizer and actual level ifmited to less than 80 percent before starting RCPs during a heat up.
One of the two operable charging pumo control swi.tches is placed in the PULL TO LOCX position wnen RCS temoer-ature is below 220 degrees F.
Current interim procedures recuire either the pressurizer relief valves or the RHR safety valves be connected to the system wnenever RCS tencerature is below 220 degrees F.
- However, proposed Technical Specifications will require both relief systems be coerable below 220 degrees F.
4.2 Technical Soecifications To assure adecuate coeration of :ne over;ressure mitigating system, we 16 360
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. are requiring the licensee to submit Technical Specifications consistent with the analysis provided. Maine Yankee submitted. tentative Technical Specifications as listed in Reference 9.
We find these to be acceptable with the following exception. The licensee must add a statement to assure that: A reactor coolant pump may be started (or jogged) only if there is a steam bubble in the pressurizer with a maximum level of 80 percent, or the SG/RCS temperature difference is less than 100 degrees F.
5.0 Conclusions The administrative controls and hardware changes proposed by Maine Yankee Atomic Power Company provide protecticn from pressure transients at low temperatures by reducing the probability of initiation of a transient and by limiting the pressure of such a transients to below the lir;ts set by Appendix G.
The staff finds that the overpressure mitigating system meets the criteria established by the NRC and is acceptable as a long range solution to the problem of overpressure transients. However, final acceptanace by the NRC is contingent upon t."e licensee incorporating Technical Specifications which comply with the statements in Section 4.2 of this SER. Also, any future revisions of the pressure-temperature Apcendix G limits for the Maine Yankee plant must be considered and the overpressure mitigating system setpoints adjusted accordingly with corresponding adjustments in the Technical Specifications.
Dated: September 11, 1979 9
i163 001
. Re ferences 1.
" Staff Discussion of Fifteen Technical Issues Listed in Attachement G November 3,1976 Memorandum from Director. NRR to NRR Staff." HUREG 0138, November,1976.
2.
USNRC Letter (Reid) to Maine Yankee Atomic Power Company (MYAPCO) dated August 11, 1976.
3.
MYAPCO letter (Naudenburgh) to USNRC dated December 2,1976.
4 USNRC Letter (Reid) to MYAPCO (Groce) dated January 28, 1977.
5.
MYAPCO Letter (John:en) to USNRC dated March 25, 1977 6.
USNRC Letter (Reid) to MYAPCO (Groce) dated August 22, 1977 7.
s'iYAPCO Letter (Johnson) to USNRC dated October 24, 1977.
8.
USNRC Letter (Reid) to MYAPCO (Groce) dated January 10, 1978.
9.
MYAPC0 letter (Johnson) to USNRC dated February 24, 1978.
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