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LWR Primary Coolant System Pressure Isolation Valves (Generic Letter 80-14)
GL80014 UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 February 23, 1980 TO ALL LWR LICENSEES SUBJECT: LWR PRIMARY COOLANT SYSTEM PRESSURE ISOLATION VALVES The Reactor Safety Study (RSS), WASH-1400, identified an intersystem loss-of coolant accident in a PWR which is a significant contributor to risk from core melt accidents (EVENT V). The design examined in the RSS contained two in-series check valves isolating the high pressure Primary Coolant System (PCS) from the Low Pressure Injection System (LPIS) piping. (Figure 1, attached, illustrates the applicable portion of the LPIS design). The scenario which leads to the EVENT V accident is initiated by the failure of these check valves to function as a pressure isolation barrier. This causes an overpressurization and rupture of the LPIS low pressure piping which results in a LOCA that bypasses containment. Prior to this letter, the Event V loss of pressure isolation was being addressed during the review of your ISI/IST Program as augmented by previous written and verbal communications. With respect to ECCS valve operability, testing will continue to be handled as part of your ISI/IST review. However, due to the safety significance of EVENT V and the need for timely resolution, the valve integrity concern will now be handled as a separate issue. Please note that the Event V concern is limited to component integrity and functionality as a pressure isolation barrier. At this time the following valve configurations located in the high pressure PCS portion of various system designs are considered explicit Event V arrangements: (1) two check valves in series, or (2) two check valves in series with a motor operated valve (MOV). Examples of such systems include the LPIS in PWRs as well as Core Spray and Low Pressure Safety Injection in BWRs. Your review should address, but not be limited to, these systems. In the RSS, the five year average failure probability of check valves was calculated to be a somewhat large 4x10 -6/reactor year with an estimated error factor of 10. Since the average was limited to five years of plant operation, it did not reflect the unacceptable increase in failure probabilities over the plant life. However, this probability is reduced significantly by periodic testing or by continuous surveillance of each valve. Acceptable methods to assure component integrity include: (1) continuous pressure monitoring on the low pressure side of each check valve, (2) periodic IST leakage testing on each valve every time the plant is shutdown and each time either check valve is moved from the fully closed position, (3) periodic ultrasonic examination on each valve every time the plant is shutdown and each time either check valve is moved from the fully closed position, or . - 2 - (4) periodic radiographic examination on each valve every time the plant is shutdown and each time either check valve is moved from the fully closed position. Various combinations of the above methods may be acceptable. Likewise, other equivalent provisions may also be found acceptable. At this time, we have no information to indicate that measures have been, or are being, taken by you to provide continuous surveillance or to perform periodic tests on a minimum of two isolation valves within the class 1 boundary in each of the high pressure lines connected to the PCS. Therefore, periodic valve tests or continuous surveillance should be accomplished as soon as possible. If tests or surveillance provisions necessitate a plant outage, every effort should be made to accomplish such tests/provisions prior to plant startup after your next scheduled outage. In addition, to determine whether your license should be modified to require either continuous surveillance or periodic testing, you shall provide in accordance with 10 CFR 50.54(f) the following information in writing within 20 days of receipt of this letter: 1. Describe the valve configuration at your plant and indicate if an Event V isolation valve configuration exists within the Class 1 boundary of the high pressure piping connecting PCS piping to low pressure system piping (1) two check valves in series, or (2) two check valves in series with a MOV; 2. If either of the above Event V configurations exist at your facility, indicate whether continuous surveillance or periodic tests are being accomplished on such valves to ensure integrity. Also indicate whether valves have been known, or found to lack integrity, and 3. If either of the above Event V configurations exist at your facility, indicate whether plant procedures should be revised or if plant modifications should be made to increase reliability. If we can be of further assistance, or if uncertainties remain, please contact Mr. Philip Polk of my staff at (301) 492-7872. Darrell G. Eisenhut, Acting Director Division of Operating Reactors Office of Nuclear Reactor Regulation Enclosure: Figure 1 "LWR-LPIS Check Valve Design"
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