Operating Experience Feedback Report, Solenoid-Operated Valve Problems at U.S. Reactors (Generic Letter 91-15)
UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON D.C. 20555
September 23, 1991
ADDRESSEES: ALL POWER REACTOR LICENSEES AND APPLICANTS
SUBJECT: OPERATING EXPERIENCE FEEDBACK REPORT, SOLENOID-OPERATED
VALVE PROBLEMS AT U.S. REACTORS
(GENERIC LETTER 91-15)
This generic letter informs addressees of a case study report of operating
experience problems with solenoid-operated valves (SOVs) prepared by the
Office for Analysis and Evaluation of Operational Data AEOD) and published
as NUREG-1275, Volume 6, "Operating Experience Feedback
Report--Solenoid-Operated Valve Problems," February 1991 (copy enclosed).
The case study integrates what has been learned over the past several years
and provides an extensive assessment of SOV operating experience. The study
describes deficiencies in design and application, manufacture, maintenance,
surveillance testing and feedback of failure data, and concluded that
problems with SOVs need additional attention by the industry. While the
recommendations in the case study are not intended to establish regulatory
requirements, many of the problems described in the report already are
addressed by current environmental qualification and quality assurance
rules.
In the study, several events are described in which SOV failures affected
redundant safety components, multiple trains of safety systems or multiple
safety systems. Three of the most significant events were isolated
occurrences involving the failure to close of both main steam isolation
valves (MSIVs) in the same line, the inability to start two redundant
emergency diesel generators, and simultaneous failure of several BWR control
rods to insert. The examples illustrate the vulnerability of safety-related
equipment to common mode failure or degradation of SOVs. The NRC is
concerned about the reliability of SOVs used in safety applications. As part
of NRC's ongoing regulatory activities, inspections such as Safety System
Functional Inspections (SSFIs) include the reliability of SOVs as well as
other components required by safety related applications. The NRC also is
providing technical advice to the Electric Power Research Institute's (EPRI)
Nuclear Maintenance Application Center (NMAC) to assist in preparing an SOV
maintenance guide. The first draft of the SOV maintenance guide is
anticipated to be available towards the end of 1991.
It has been estimated that many hundreds of SOVs are in wide-spread use in
each nuclear power facility. They are used in safety-related systems
indirectly as pilot operators working with control system fluid (such as
pneumatic or hydraulically operated isolation valves) and directly in fluid
systems (such as to vent the reactor vessel head or to supply air to the
starting system for emergency diesel generators). Many SOVs are also used
in nonsafety-related systems that can significantly affect safety systems
(such as plant instrument air drier systems). Over the years, many failures
of plant systems and components have been attributed to SOV problems. To
address specific SOV failures, the Nuclear Regulatory Commission (NRC) has
issued numerous information notices
.
Generic Letter 91-15 -2-
and bulletins that provide the immediately attributed root cause for the
failure. Because these communications frequently were focused on a specific
failure, licensees may have made assessments and taken corrective actions
that were focused on the specific failures and not on broader issues.
In the case study, the staff reviewed many SOV failures and degradations and
discussed those having a similar failure mechanism, thereby showing how only
slight differences frequently are all that separate operation from failure.
Correcting only one obvious and specific deficiency at a time without
awareness of other mechanisms for degradation may permit another problem in
a short time to lead to unnecessary recurrent SOV failures. In addition,
correcting problems only in SOVs used in the specific application in which
the problem was found can allow similar SOV degradation to develop in other
applications.
No specific action or written response is required by this generic letter.
However, it is expected that recipients will review the information
presented in the case study for applicability to their facilities and
consider actions, as appropriate, to avoid similar problems. Since this
generic letter and enclosure do not contain new or revised regulatory
requirements, the Backfit Rule, 10 CFR 50.109, does not apply. If you have
any questions about this matter, please contact one of the technical
contacts listed below or the appropriate NRR project manager.
Sincerely,
James G. Partlow
Associate Director for Projects
Office of Nuclear Reactor Regulation
Enclosure:
NUREG-1275, Volume 6
Technical Contacts: H. Ornstein, AEOD
(301) 492-4439
J. Carter, NRR
(301) 492-1153
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