NRC PERSPECTIVE ON RELIEF AND SAFETY VALVES Charles G. Hammer Component Performance & Testing Branch Division of Component Integrity Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Pressure Relief Device Interest Group January 2006

2 NRC regulatory activities relating to pressure relief devices have involved design, qualification, maintenance, and testing issues. Beginning in the 1970s, issues were raised regarding capabilities and performance of relief and safety valves for all operational and accident conditions, such as: –Capability of pressurizer safety valves (PSVs) for extremely high pressure liquid conditions (ATWS) –Stuck open SRV events at some BWRs

3 The TMI-2 accident in 1979, which involved a stuck-open power-operated relief valve (PORV), and the followup generic activities (NUREG- 0737, Item II.D.1) resulted in qualifying primary system relief and safety valves for all design- basis conditions. Major findings were: –Adjusting rings on some safety valves were not set properly. –Some safety valves performed poorly with liquid. –Discharge piping loads at some plants were greater than design loads.

4 Other generic safety issue (GSI) activities have focused on improving reliability of pressure relief devices, including: –GSI B-55 was created to address problems with 3- stage and 2-stage Target Rock SRVs in BWRs. The 3-stage SRVs were prone to inadvertently opening, and the 2-stage SRVs were prone to corrosion bonding and high setpoint drift. B-55 was closed in 1999 based on significant improvements to performance. A regulatory issue summary was issued (RIS ).

5 –GSI-70 and GSI-94 were created to increase the reliability of pressurizer PORVs and block valves and resulted in issuance of Generic Letter While PORVs are not fully safety-grade devices at all facilities, it was determined that they are important in accomplishing specific safety functions including low temperature overpressure protection and system cooldown. –GSI-126 was created to address potential problems with PWR main steam safety valves. Issues included possible sticking open and misadjusted ring settings. An industry test program resulted in proper adjustments to rings to assure adequate performance. GSI-126 was closed in 1987 based on probabilistic risk assessment involving secondary system rupture.

6 –GSI-165 was created to address potential problems with relief valves in systems other than primary and secondary systems. Main concern was possible diversion of system fluid through relief valves. GSI- 165 was closed in 1999 based on probabilistic risk assessment and improvements to valve testing required by the ASME code (OM Code, Appendix I). A regulatory issue summary was issued (RIS ).

7 Over the past several years, there have been several events relating to relief devices. Some of the more significant events have involved: –Thermal effects involving differential thermal expansion of the internal valve parts causing changes to the setpoints (Information Notices and 96-03). This has emphasized the importance of setpoint testing at actual operational conditions. –Corrosion bonding and mechanical sticking has affected some valve designs. This has resulted in material changes to some internal parts. –Excessive wear of some internal parts. This emphasized the importance of periodic inspection and maintenance (Information Notice ).

8 –Excessive leakage of BWR SRVs has resulted in heating of suppression pool water and excessive operation of RHR system in pool cooling mode. This can result in a significant waterhammer for a loss of offsite power event (Information Notice and Supplement 1). –Equipment calibration and accuracy has been a problem in performing inservice testing. Testing of main steam safety valves with on-line testing devices requires input of the disk area, which has been found to be in error (Information Notice 94-56). Also, use of non-temperature compensated test equipment can result in significant setpoint errors (Information Notice 96-22).

9 –The adequacy of position indication devices (PIDs) has been an issue. At one facility, the internal parts of a pressurizer PORV were installed upside down such that the main disk would not open. However, the installed acoustic PID indicated that the valve was opening properly since the opening of the pilot valve alone provided sufficient input noise to actuate the PID (Information Notice 96-02). In another event, when water was discharged through SRVs, none of the installed pressure-actuated switch PIDs indicated that the SRVs had opened (Information Notice ). –Events involving thermally-induced pressurization of isolated piping segments and the issuance of Generic Letter resulted in several relief valves being installed at many plants.

10 There has been decreased overpressure protection margin at several plants due to power uprates, with some need for additional capacity and increased emphasis on setpoint tolerances. In recent years, there has been a reduction in numbers of licensee event reports involving relief and safety valves. It is believed that this trend is due, in part, to improved maintenance and testing. The NRC staff continues to participate with the nuclear industry in development of codes and standards. The ASME OM Code, Appendix I, requirements for relief and safety valves have been significantly improved by incorporation of industry operational experience. NRC recently issued Supplement 1 of NUREG-1482, which includes some guidance on testing of relief and safety valves.

11 NRC issued 10 CFR rule for risk-informing treatment of structures, systems, and components (SSCs) based on safety significance, and specifies treatment to provide appropriate confidence in design capability according to risk categorization. –Most special treatment requirements for low-risk safety-related (RISC-3) SSCs can be eliminated. –Licensees required to ensure with reasonable confidence that RISC-3 SSCs are capable of performing their safety functions. –Periodic inspection and testing activities required to determine that RISC-3 SSCs remain capable of performing their safety-related functions under design-basis conditions.

12 Ninth NRC/ASME Symposium on Valves, Pumps, and Inservice Testing –Symposium provides for exchange of information on technical, programmatic, and regulatory issues for valves, pumps, and inservice testing –July 17-19, 2006, at L’Enfant Plaza Hotel in Washington, DC –OM Code Meeting will be conducted at same hotel on July –For more details, see