Resolution of Generic Safety Issues: Task II.D: Reactor Coolant System Relief and Safety Valves (Rev. 3) ( NUREG-0933, Main Report with Supplements 1–35 )
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The objective of this task was to demonstrate by testing and analysis that the relief and safety valves, block valves, and associated piping in the reactor coolant system (RCS) were qualified for the full range of operating and accident conditions; anticipated transients without scram (ATWS) could be considered in later phases of the testing. In addition, design changes or modifications that were necessary to provide positive indication of valve position were to be made.
ITEM II.D.1: TESTING REQUIREMENTS
DESCRIPTION
This TMI Action Plan48 item called for applicants and licensees to conduct testing to qualify reactor coolant relief valves, safety valves, block valves, and associated discharge piping for all operating conditions and design basis accidents.
CONCLUSION
This item was RESOLVED, requirements were issued, and MPA F-14 was established by DL/NRR for implementation purposes.
ITEM II.D.2: RESEARCH ON RELIEF AND SAFETY VALVE TEST REQUIREMENTS
DESCRIPTION
Historical Background
This TMI Action Plan48 item specified that RES contract with INEL to: (1) act as a systems integrator to technically monitor and analyze the planned industry valve test and analytical program at EPRI and collect, analyze, and compare information from foreign tests; (2) develop, improve, or verify available flow discharge and structural response models using the above information; (3) determine the need for a valve testing program by NRC, with the main focus to be on subcooled and two-phase discharge and on determining operability; and (4) conduct additional tests, as necessary, to ensure that the response to the full spectrum of fluid conditions that would be expected to result from anticipated operational occurrences and ATWS events had been adequately characterized. The above work, with the exception of the ATWS events, had been performed in conjunction with Item II.D.1 which was clarified in NUREG-0737.98
Safety Significance
The remaining concern under Item II.D.2 with respect to ATWS events was the capability to depressurize the reactor. Coupled with failure of the reactor protection system (RPS) following a transient, inadequate depressurization could result in rupture of the reactor coolant pressure boundary (RCPB) producing a loss-of-coolant accident (LOCA).
Possible Solution
To estimate the public risk associated with ATWS events, it was assumed64 that a possible solution would be to increase the sizing of the relief and safety valves. This modification was assumed to decrease the likelihood of an ATWS-induced rupture of the RCPB by enhancing the depressurization capability of the system.
PRIORITY DETERMINATION
Assumptions
Using Oconee-3 as representative of PWRs, PNL assumed64 that the dominant core-melt sequence representative of an ATWS event would involve a Power Conversion System (PCS) transient caused by events other than a loss-of-offsite power (LOOP) and failure of the RPS. The LOCA initiator was assumed to be a RCPB pipe rupture with an equivalent 4-inch diameter. Equipment failures included the containment spray recirculation system and emergency coolant injection and recirculation systems. The containment failure modes were assumed to be similar to other PWR Release Categories involving RCPB ruptures.
The Grand Gulf-1 reactor was assumed to be representative of BWRs. The dominant core-melt sequence used to model the ATWS event involved transients other than LOOP which require shutdown and a failure to achieve subcriticality. The LOCA initiator was assumed to be a RCPB rupture equivalent to an area of 1 ft2. The equipment failure assumed was loss of the residual heat removal (RHR) system after the LOCA. The containment failure modes were similar to other BWR Release Categories involving a LOCA and subsequent loss of RHR.
Frequency Estimate
Based on the above assumptions, the reductions in core-melt frequency as a result of modifying the safety relief valves (SRVs) were calculated to be 3.8 x 10-7/RY for PWRs and 7.1 x 10-9/RY for BWRs.
Consequence Estimate
The reduction in public risk was calculated64 to be 0.99 man-rem/RY for PWRs and 0.51 man-rem/RY for BWRs. Assuming at least one-half of the plants were affected (45 PWRs and 22 BWRs), with an average remaining life of 28.7 years for PWRs and 27.4 years for BWRs, the total public risk reduction was 1,300 man-rem.
Cost Estimate
Industry Cost: SRV modifications were assumed to require approximately 125 man-weeks/plant. At a rate of $2,270/man-week, the labor cost for this modification was estimated to be $284,000/plant. Equipment was estimated to be $100,000/plant. For backfit plants, the License Amendment Fee was $4,000. These costs resulted in a backfit cost of $388,000/plant and a forward-fit cost of $384,000/plant. For the forward-fit plants, it was assumed that only half of the plants scheduled to begin operation prior to 1986 would require modifications and, subsequent to that time, the modifications would be incorporated during initial installation. Based on these estimates, the total industry cost was $21M.
NRC Cost: Development and implementation costs were estimated to be $0.4M and $0.3M, respectively. The development cost was assumed to require 2 man-years of NRC effort and 2 man-years of contractor support. The implementation cost to monitor the hardware modifications at the affected plants was assumed to require 2 man-weeks/plant (36 backfit plants, 19 forward-fit plants). Based on these estimates, the total NRC cost was $0.7M.
Total Cost: The total industry and NRC cost associated with the possible solution was $(21 + 0.7)M or $21.7M.
Value/Impact Assessment
Based on an estimated public risk reduction of 1,300 man-rem and a cost of $21.7M for a possible solution, the value/impact score was given by:
CONCLUSION
With the exception of potential ATWS events, Item II.D.2 was integrated into Item II.D.1. Based on the above calculation, the part of Item II.D.2 that involved consideration of ATWS events was given a low priority ranking (see Appendix C) in November 1983. In NUREG/CR-5382,1563 it was concluded that consideration of a 20-year license renewal period could change the ranking of the issue to medium priority. Further prioritization, using the conversion factor of $2,000/man-rem approved1689 by the Commission in September 1995, resulted in an impact/value ratio (R) of $16,666/man-rem, which placed the issue in the DROP category. Consideration of new information1715 on the phenomenon of "microbonding," submitted by Region IV in April 1997, did not change this conclusion.1716
ITEM II.D.3: RELIEF AND SAFETY VALVE POSITION INDICATION
DESCRIPTION
This TMI Action Plan48 item called for all OLs and applicants for OLs to provide the RCS relief and safety valves with position indication in the control room.
CONCLUSION
This item was clarified in NUREG-073798 and requirements were issued.
REFERENCES
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