United States Nuclear Regulatory Commission - Protecting People and the Environment

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"
Page Last Reviewed/Updated Monday, June 17, 2013