Consideration of Valve Mispositioning in Pressurized-Water Reactors (Generic Letter 89-10, Supplement 7)
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
January 24, 1996
NRC GENERIC LETTER 89-10, SUPPLEMENT 7: CONSIDERATION OF VALVE
MISPOSITIONING IN PRESSURIZED-WATER
REACTORS
Addressees
All holders of operating licenses (except those licenses that have been
amended to a possession only status) or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this generic letter
supplement to notify addressees about a revised NRC position regarding
consideration of valve mispositioning within the scope of Generic Letter (GL)
89-10 for pressurized-water reactors (PWRs). Although this generic letter
supplement forwards a new staff position, no specific action or written
response is required.
Background
In GL 89-10 (June 28, 1989), Safety-Related Motor-Operated Valve Testing and
Surveillance, the staff recommended, among other things, that any motor-
operated valve (MOV) in a safety-related system that is not blocked from
inadvertent operation from the control room, the motor control center, or the
valve itself, be considered capable of being mispositioned (referred to as
position-changeable MOVs) and be included in licensee MOV programs. When
determining the maximum differential pressure or flow for position-changeable
MOVs, the licensees were asked to consider "the fact that the MOV must be able
to recover from mispositioning ..." Supplement 1 to GL 89-10 limited the
prevention of inadvertent MOV operation within the context of the generic
letter to the potential for MOV mispositioning from the control room.
The Boiling Water Reactor Owners Group (BWROG) submitted a backfit appeal on
the recommendations for position-changeable valves. The staff, with the
assistance of Brookhaven National Laboratory (BNL), reviewed and evaluated the
issues concerning the mispositioning of valves from the control room and
determined that the recommendations in GL 89-10 should be changed for BWRs.
The BNL study, which used probabilistic risk assessment (PRA) techniques, and
the NRC staff evaluation and conclusions were transmitted in a letter from the
9601190442 . GL 89-10, Supp. 7
January 24, 1996
Page 2 of 4
NRC to the BWROG dated February 12, 1992. The conclusions were communicated
to industry and the public at large via Supplement 4 to GL 89-10, also dated
February 12, 1992. Supplement 4 indicated that the NRC would perform a
similar review for PWRs and stated that GL 89-10 might be revised, if
warranted, to clarify the NRC position regarding consideration of MOV
mispositioning within the scope of GL 89-10 for PWRs.
Description of Circumstances
By letter dated July 21, 1992, the Westinghouse Owners Group (WOG) asked the
NRC staff to notify PWR licensees that the provisions of GL 89-10 for valve
mispositioning are not applicable to PWRs, based on arguments similar to those
made by the BWROG.
Discussion
Under contract to the NRC staff, BNL performed a study similar to the one
performed for BWRs of the safety significance of inadvertent operation of MOVs
in safety-related piping systems of three PWRs. Consistent with Supplement 1
to GL 89-10, the scope of the study was limited to MOVs in safety-related
systems that could be mispositioned from the control room. However, because
the available PRA models do not include active mispositioning of MOVs or the
physical phenomena that could inhibit repositioning, BNL's study of available
plant models was limited in its ability to address this issue. Given this
limited scope, BNL concluded that the risk insights from the mispositioning of
unlocked MOVs were similar for both PWRs and BWRs. Although PWRs tend to have
a higher core damage frequency (CDF) than BWRs, which would suggest that the
net increase in CDF from mispositioning of MOVs would be higher for PWRs than
for BWRs, PWRs typically have a lower conditional containment failure
probability, which would tend to balance the overall risk to the public.
The NRC is removing the recommendation that MOV mispositioning be considered
by PWR licensees in responding to GL 89-10, as was done for BWR licensees in
Supplement 4, in light of the following:
. Corrective actions have been taken by licensees subsequent to the
Davis-Besse event (i.e., detailed control room design reviews,
independent valve position verification programs, and operator
training improvements),
. Corrective actions are being applied to many of the most important
valves under the other provisions of GL 89-10,
. Other operational events are absent (other than Davis-Besse) in
which mispositioning MOVs from the control room actually set up
conditions that prevented repositioning, and
. The results of the BNL study for PWRs.
Implementation of this relaxation by licensees is voluntary.
. GL 89-10, Supp. 7
January 24, 1996
Page 3 of 4
Staff Position
The staff no longer considers the inadvertent operation of MOVs from the
control room to be within the scope of GL 89-10 for PWRs. However, the staff
believes that consideration of valve mispositioning benefits safety.
Licensees that have already taken action or made commitments related to valve
mispositioning may take advantage of this relaxed staff position provided the
licensees document this change in their GL 89-10 programs.
Modifying the provisions in GL 89-10 for valve mispositioning does not affect
the GL 89-10 recommendations for licensees to review safety analyses,
emergency procedures, and other plant documentation to determine the design-
basis fluid conditions under which all MOVs in safety-related piping systems
may be called upon to function. This position also does not supersede the NRC
generic recommendations or regulations on valve mispositioning that pertain to
such other issues as interfacing-systems loss-of-coolant accidents (ISLOCAs)
or fire protection (10 CFR Part 50, Appendix R).
Backfit Discussion
This letter represents a relaxation of recommendations set forth in GL 89-10
and prior supplements. Implementation of this relaxation is voluntary and
this generic letter supplement requests neither actions nor information from
licensees. Therefore, this generic letter supplement is not considered a
backfit and the staff has not performed a backfit analysis.
Federal Register Notification
The proposed generic letter supplement was published in the Federal Register
for a 30-day public comment period on July 26, 1995. Four comments were
received (from Nuclear Energy Institute, Florida Power Corporation, Centerior
Energy, and Virginia Power). All four comments supported issuance of the
generic letter supplement as written. Centerior Energy also suggested that
the supplement clarify how licensees should administratively handle any
commitments they might have made related to valve mispositioning. This
comment was accepted and is incorporated in the staff position section of this
supplement.
. GL 89-10, Supp. 7
January 24, 1996
Page 4 of 4
This generic letter requires no specific action or written response. If you
have any questions about this matter, please contact the technical contact
listed below or the appropriate Office of Nuclear Reactor Regulation (NRR)
project manager.
signed by
Dennis M. Crutchfield, Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contact: David C. Fischer, NRR
(301) 415-2728
Internet:dcf@nrc.gov
Lead project manager: Allen G. Hansen, NRR
(301) 415-1390
Internet:agh@nrc.gov
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021