Multiple Equipment Failures in Safety-Related Systems (Generic Letter 79-24)

GL79024 

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                          WASHINGTON, D.C. 20555 

                               June 26, 1979 

ALL PRESSURIZED WATER REACTORS 

Gentlemen: 

Recently, because of operator error, an inadvertent reactor scram and safety
injection occurred during monthly surveillance tests of the safeguards 
system at a PWR facility. 

At the time of the event, train "A" of the safeguards system had been placed
in "test", and the operator, in addition to inserting a high steam flow 
signal required by the test, inadvertently incorrectly inserted a low steam 
pressure signal. This action resulted in a low steam pressure signal 
(signifying a main steam line break) in train "A" which initiated main steam
isolation valve (MSIV) isolation and a reactor scram. One MSIV, however, did
not close because of a valve solenoid failure. The resultant differential 
pressure between two steam lines initiated a safety injection signal from 
train "B". In the ensuing events several more equipment failures occurred. 
One feedwater regulation valve failed to close because of another solenoid 
failure. The steam driven auxiliary feedwater pump tripped on overspeed and 
one of the steam generator atmospheric relief valves failed to fully reseat 
upon closure. 

This occurrence, with its ensuing sequence of events, is of concern to the 
NRC staff because of the serious questions that are raised due to the 
multiple equipment failures and whether a very real problem could exist that
has not been analyzed. For example, the potential for common mode failures 
(in this case two apparently independent solenoid valve failures) should be 
investigated to ensure that a problem does not exist which could negate the 
criteria assumed in your previous accident analyses or which could lead to 
an overall reduction in system reliability. 

This occurrence is also of concern because of an apparent sense of 
complacency towards periodic surveillance requirements in general and on 
engineered safeguards systems in particular which has all too often resulted
in inadvertent reactor scrams and safety injection system actuations. From 
the standpoint of unnecessary challenges to the reactor trip and the 
safeguards systems and the imposition of unnecessary thermal stress cycles 
on reactor coolant system and its components, this is undesirable. 
.

                                  - 2 -          June 26, 1979 

You and your plant supervisors should review the events described in this 
letter, to determine whether similar errors have occurred or could occur at 
your facility and whether the potential exists for a problem associated with
occurrences that you have not previously considered. In addition, it is 
requested that management policies and procedures be reviewed and 
strengthened as necessary to assure that multiple equipment failures in 
safety-related systems will be vigorously pursued and analyzed to identify 
potential failure modes not previously considered that could lead to a 
significant reduction in the ability of safety systems to function as 
required. Finally, you are requested to review your engineered safety system
surveillance procedures to determine whether appropriate cautions are 
included and to ensure that plant operators and supervisors are aware of the
importance of avoiding challenges to the protective features of your 
facility. 

Within 30 days of receipt of this letter, please submit, in accordance with 
10 CFR 50.54(f) of the Commission's regulations, the results of these 
reviews. In addition to licensing reviews of these matters, we have 
requested that the NRC's Office of Inspection and Enforcement perform a 
followup inspection on these matters in the near future. 

                                        Sincerely, 



                                        Darrell G. Eisenhut, Acting Director
                                        Division of Operating Reactor 
                                        Office of Nuclear Reactor Regulation