ML20057D574
| ML20057D574 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 09/23/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20057D571 | List: |
| References | |
| NUDOCS 9310040393 | |
| Download: ML20057D574 (8) | |
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UNITED STATES
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NUCLEAR REGULATORY COMMISSION f
WASHINGTON, D C. 20555-0001 g,.....f SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NOS.175 AND 156 T0 i
FACILITY OPERATING LICENSE NOS. NPF-4 AND NPF-7 VIRGINIA ELECTRIC AND POWER COMPANY OLD DOMINION ELECTRIC COOPERATIVE NORTH ANNA POWER STATION. UNITS N0. 1 AND N0, 2 DOCKET NOS. 50-338 AND 50-339
1.0 INTRODUCTION
By letter dated April 27, 1990, as supplemented by letters dated December 21, 1990 and March 29, 1993, Virginia Electric and Power Company (licensee) proposed changes to the Technical Specifications (TS) for North Anna Units 1 and 2 to change portions of the surveillance requirements in TS 3/4 7.9. The purpose of the TS changes was to support removal of the Autoclosure Interlock (ACI) which provides automatic isolation of the residual heat removal (RHR) system from the reactor coolant system (RCS) by closing the RHR suction valves when the RCS pressure is above 582 pounds per square inch gauge (psig). The licensee also proposed to add a requirement to verify that the RHR suction valves are closed and de-energized prior to exceeding 500 psig in the RCS.
Removal of the ACI is expected to reduce the incidence of events involving loss of RHR cooling capabilities during nonpower operations due to inadvertent closure of the suction valves.
The December 21, 1990, and March 29, 1993, letters provided additional information requested by the staff. This additional information did not alter the proposed action or affect the staff's determination of no significant l
hazards consideration as noticed in the Federal Reaister on July 25, 1990 (55 FR 30316).
2.0 BACKGROUND
The RHR system is based on a design pressure of 600 psig and a design temperature of 400*F.
During normal operating conditions, a double barrier between the high pressure RCS and the low pressure RHR system is provided by two motor-operated suction valves. These valves are closed when the RCS is hot and pressurized (normal operating conditions) and open when the RHR system is in operation (cooldown or refueling).
Each of these suction valves is provided with manual controls on the main control board. There are two j
9310040393 930923 PDR ADOCK 05000338 P
. automatic interlocks associated with the control circuitry; the ACI and the Open Permissive Interlock (OPI).
The OPI prevents the suction valves from being opened when RCS system pressure equals or exceeds 418 psig. The OPI and the associated TS are not affected by the proposed amendment.
When the valves are in the open position, the ACI causes the valves to close automatically when RCS pressure increases to a value above the autoclose setpoint of 582 psig.
For RHR overpressure protection, there are two relief valves on the RHR l
suction piping to provide protection for the RHR system piping with a lift setpoint of 467 psig.
In addition, when the RHR system is in service, RHR overpressure protection is provided by the Low Temperature Overpressurization 1
Protection system (LTOP) from the pressurizer power operated relief valves (PORVs). However, at North Anna, the RHR suction isolation valves are closed in accordance with operating procedures prior to reaching the RHR pump suction relief valve setpoint of 467 psig to prevent inadvertent lifting of the relief valves.
The Comission and industry have previously recognized the safety benefits from removing the ACI circuitry from the RHR system. A disadvantage of the autoclosure feature is the possibility of an inadvertent valve closure during 1
RHR operation resulting in the loss of decay heat removal capability. The safety benefit of removing the ACI circuitry was stated in the Commission's case study on long-term decay heat removal, Case Study Report AE0D/C503,
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" Decay Heat Removal Problems at U.S. Pressurized Water Reactors," December 1985 and also in a study performed for the Commission by Brookhaven National Laboratory, NUREG/CR-5015, " Improved Reliability of Residual Heat Removal Capability in PWRs as Related to Resolution of Generic Issue 99," May 1988.
In Generic Letter (GL) 88-17, " Loss of Decay Heat Removal," the Commission requested that TS that restrict or limit the safety benefit of actions identified in GL 88-17 should be identified and that appropriate changes should be submitted. One of the items listed by GL 88-17 that could limit such safety benefits was ACI.
In parallel with the Commission's activities, the Westinghouse Owners' Group (WOG) evaluated the removal of the ACI circuitry on Westinghouse designed plants and issued WCAP-Il736-A, " Residual Heat Removal System Autoclosure Interlock Deletion Report for the Westinghouse Owners Group," Volumes 1 and 2, Revision 0.0, October 1989. WCAP-11736-A documents the probabilistic analysis performed on the removal of the ACI circuitry in terms of:
(1) the likelihood of an interfacing loss-of-coolant accident (LOCA); (2) RHR system i
availability; and (3) low temperature overpressurization concerns. The results show that:
(1) the frequency of an interfacing system LOCA decreases with the removal of the ACI circuitry from the RHR system when accompanied by the addition of a control room alarm and procedural enhancements; (2) removal of the ACI increases RHR system availability; and (3) removal of the ACI from the RHR system has no effect on heat input transients, but will result in a 1
. small, but not significant, increase in the frequency of occurrence for some types of mtss input transients with a decrease in others. The net effect of ACI deletion from the RHR system is an improvement in safety.
The WCAP-11736-A report was based on four lead plants typical of the different RHR configurations and RHR design characteristics among Westinghouse plants.
North Anna Unit I was the reference plant for the Group 3 plants that had similar RHR configurations and design characteristics.
In the safety evaluation of WCAP-Il736-A, the staff found that the removal of the ACI for Westinghouse plants produces a net safety benefit provided that the following five key imprevements are in place.
(1) An alarm should be added to each RHR suction valve that will actuate if the valve is open and RHR system pressure is high (greater than the open permissive setpoint and less than the RHR system design pressure minus the RHR pump head pressure).
(2) Valve position indication to the alarm should be provided and power to the switches should not be affected by power lockout of the valve.
I (3) Proceoural improvements described in WCAP-11736-A should be implemented.
(4) Power should be removed from the RHR suction valves prior to their being leak-checked, if feasible.
(5) RHR suction valve operators should be sized such that the valves cannot be opened against full system pressure.
3.0 DISCUSSION In support of the. proposed TS change, the licensee referenced the WOG report WCAP-11736-A, which was approved by the Commission on August 8, 1989. The licensee's letters of April 27, 1990 and December 21, 1990 provided the results of analyses to demonstrate that the conclusions of WCAP-Il736-A, where North Anna Unit 1 is the reference plant for the similar plants in Group 3, are valid for both North Anna Units 1 and 2.
The letter also described how improvements identified by WCAP-11736-A will be implemented at North Anna.
These results take into account the impact of the removal of the ACI feature on the RHR inlet isolation valves. The licensee concluded that the implementation of their proposed design, TS, administrative control and procedure changes will reduce the frequency of an RHR overpressurization event and increase the RHR system availability at North Anna.
The staff has reviewed the licensee's proposal against the recommendations and guidance of WCAP-11736-A. The hardware change proposed for North Anna will be the removal of the ACI function from the RHR suction valves. The open permissive interlock will remain intact. The following discussion compares how the licensee addresses the five improvements, listed above in Section 2.0, that were recommended in WCAP-11736-A.
. 3.1 Additio.
Alarms The licensee has not added an alarm to each RHR suction valve that would indicate if one of the two suction valves was open when the RCS pressure was high. The North Anna plants do have high pressure annunciators that provide warning if both RHR valves are left in the open position when the RCS is being pressurized above the RHR design pressure. However, the existing alarms do not inform if one of the two valves is open.
In place of the alarm for each valve, the licensee proposes procedures and administrative controls to assure that both of the suction valves are closed. The licensee stated that the following actions are to be taken to ensure proper RHR suction valve position during unit startup:
1.
An operator will close the RHR isolation valves by switches in the main control room which have light indications to show that the valves are in the full closed position.
2.
A second operator will independently verify that the valves have been closed.
3.
The unit supervisor will assign a third operator to open the power supply breakers for the RHR valves and lock the breakers in the open position.
4.
The unit supervisor will assign a fourth operator to independently verify that the power supply breakers are locked open.
In addition to the above administrative controls, procedures used during startup after the RHR isolation valves are closed and de-energized will be revised to now require a containment entry by two operators who will simultaneously verify that the RHR suction valves are closed by observing the mechanical indication on the valves. This revised procedure will give further assurance that the valves are in the correct position.
The licensee stated that other annunciators provide indirect indication that RHR pressurization is occurring and that one or both of the RHR high volume suction relief valves are relieving.
These annunciators include: high pressurizer relief tank (PRT) temperature, high PRT pressure and PRT level.
Also, the licensee stated that the RHR system for the North Anna plant is a unique design that varies significantly from the standard Westinghouse plants in that the RHR system is totally inside of containment.
They stated that the RHR system is not connected to, nor is it part of, the safety injection system and performs no emergency core cooling system function. The licensee stated that for their case (RHR system completely inside of containment), a LOCA inside containment is quite different from an interfacing LOCA outside containment by three major factors:
(1) radiological releases can be controlled by the containment; (2) water inventory remains within the containment for the long-term core cooling; and, therefore, (3) core damage can be averted through the use of engineered safety features.
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. The staff considers that, although the North Anna plant has no individual alarms on the suction valves, their design is acceptable because of the administrative controls and procedural steps that have been added to assure l
that each of the suction valves is closed and because their special design has the RHR system completely within containment to avoid a core melt due to an interfacing system LOCA.
3.2 Valve Position Indication to Alarm The licensee does not plan to use valve position indication to the alarm since they do not plan to add individual valve alarms as stated in Section 3.1.
However, as stated in Section 3.1, after opening the valve breakers and locking them in the open position after the valves are closed, two operators will simultaneously verify that the RHR suction valves are closed by observing the mechanical indication on the valves. The staff finds these measures to be acceptable.
3.3 Procedural Improvements The procedural inprovements recommended in WCAP-11736-A are mostly related to the alarms or, the suction valves. Since the licensee is not using alarms, these recommended procedures are not implemented. However, the licensee has identified procedural changes that will be implemented by the time the RHR ACI is deleted, that is, plant startup procedures and procedures for RHR system operation will be modified to require verification of the isolation of the RHR system by closing and de-energizing both remote operated RHR suction isolation valves and locking the associated breakers prior to exceeding 500 psig RCS pressure. The staff finds these procedures to be acceptable.
3.4 Power Removed From RHR Suction Valves Prior to Leak Check The licensee only has performance procedures for leak checks of the suction valves for Unit 2, for which the power is first removed. However, the licensee is using diverse methods, by using additional administrative controls and procedural measures, to assure that both the suction valves on each unit are closed as stated in Section 3.1.
Power is removed from the suction valves as stated in Section 3.1.
We find this to be acceptable.
3.5 Suction Valves Sized - Cannot be Opened Against Full System Pressure In addition to the hardware and procedural changes described above, the licensee has reviewed the sizing of the RHR suction valves. The licensee stated that the valve operators are sized so that they cannot be opened manually if the differential pressure is greater that 600 psi. At normal operating pressure, the minimum uf fferential pressure is greater than 1700 psi. This ensures that the valves cannot be opened against full RCS system pressure (2,250 psig). By procedure, both RHR suction valves are confirmed to be closed by utilizing the valve position indication lights in the control room. The breakers are then de-energized. Therefore, the valves cannot spuriously open. The staff agrees with the licensee that adequate
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consideration has been given for reasonable assurance that the valves will not be opened when RHR pressure is excessive.
4.0 Technical Specification Chances 1
The following TS changes were reviewed:
l 3/4.7.9 Residual Heat Removal System - (RHR) 4.7.9.1 Surveillance-Requirements for Modes 1, 2 and 3.
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- f A requirement was added to surveillance.for operating (Modes 1, 2
' i and 3) which required verification that there was isolation of the RHR system prior to the RCS pressure exceeding 500'psig by closing.
l and de-energizing-both remote operated RHR suction isolation valves and locking the associated breakers.
At least once per 18 months the RHR pump is to be verified.to be operable according to. Specification 4.0.5.
These modifications were found to be acceptable as discussed in Section-3.0.
4.7.9.2 Surveillance Requirements for Modes 4 and 5.
A requirement was added to surveillance for shutdown.(Modes 4 and 5) which required verification that there was isolation of the RHR system prior to the RCS pressure exceeding 500 psig by closing and de-energizing both remote operated RHR suction isolation valves and locking the associated breakers.
At least once per 18 months each remote or automatically operated valve in the subsystem flowpath is to be cycled through one complete cycle of full travel.
At least once per 18 months'the RHR pump is to be verified to be operable according to Specification 4.0.5.
These modifications were found to be acceptable as discussed in Section 3.0.
Editorial Chanaes a
Throughout the specifications 3/4.7.9.1 and.3/4.7.9.2, the licensee has replaced the term " residual-heat removal" by "RHR."
We find this acceptable as an editorial change. ' Also, other minor editorial changes were made to provide continuity to the North Anna Units 1 and 2 TS.'
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'l 5.0 EVALUATION Based on the above, the removal of the RHR ACI has.a positive impact on safety. The licensee has proposed to remove the ACI capability and to modify the TS surveillance specification associated with TS 3/4.7.9 by removing-the -
requirement to verify automatic isolation of the RHR system.
In lieu of this requirement, there would be a requirement for verifying the isolation of the RHR system prior to the RCS pressure exceeding 500 psig, by closing and de-energizing both remote operated RHR suction isolation valves and locking the associated breakers. The staff finds these changes to be acceptable for the-
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reasons given above.
In addition, the licensee has proposed to delete the requirement of periodically checking that the RHR pumps develop a differential pressure of.
greater or equal to 123 psi. This would be replaced by a statement that each RHR pump is to be operable per Specification 4.0.5, which is a specification in accordance with the ASME Section XI code of 1980. The staff finds this-to be acceptable as the test for operability is fulfilled with the requirements of the ASME code.
l Therefore, based on all of the above, the staff finds the proposed changes to the TS to be acceptable.
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6.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Virginia State official-was notified of the proposed issuance of the amendment. The State official had no comment.
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7.0 ENVIRONMENTAL CONSIDERATION
-l These amendments change a requirement with respect to installation or use of a facility component located within the restricted area-as defined in 10 CFR l
Part 20. The NRC staff has determined that the amendments involve no l
significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no-1
- ,ignificant increase in individual or cumulative occupational radiation i
exposure. The Commission has previously issued a proposed finding that these amendments involve no significant hazards consideration and there has'.been no l
public comment on such finding (55 FR 30316). Accordingly, these amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR-1 51.22(c)(9).
Pursuant to 10 CFR 51.22(b) no environmental impact statement or i
environmental assessment need be prepared in connection with the issuance of-the. amendments.
8.0 CONCLUSI0t!
The Commission has concluded, based on the considerations discussed above, a
that:
(1) there is reasonable assurance that the health and safety of the--
public will not be endangered by operation in the proposed manner,-(2) such 1
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l 1 activities will be conducted in compliance with the Commission's s egulations, and (3) the issuance of the amendments will not be inimical to the common i
defense and security or to the health and safety of the public.
Principal Contributor:
H. Balukjian l
1 Date: september 23, 1993 I
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