Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 87-38
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
August 17, 1987
Information Notice No. 87-38: INADEQUATE OR INADVERTENT BLOCKING OF
VALVE MOVEMENT
Addressees:
All nuclear power reactor facilities holding an operating license or a con-
struction permit.
Purpose:
This notice is provided to alert recipients to a potentially significant
safety problem pertaining to inadequate or inadvertent blocking of valves. It
is expected that recipients will review the information for applicability to
their facilities and consider actions, if appropriate, to preclude a similar
problem occurring at their facilities. However, suggestions contained in this
notice do not constitute NRC requirements; therefore, no specific action or
written response is required.
Background:
The NRC Office for Analysis and Evaluation of Operational Data (AEOD) issued
Engineering Evaluation Report AEOD/E706, "Inadequate Mechanical Blocking of
Valves," on March 31, 1987. This report, initiated by an event at a foreign
reactor, describes a number of events caused by inadequate or inadvertent
mechanical blocking of valve movement. Electrically disabling or locking the
valve handwheel with a lock or chain was not included in the study.
In addition to the event at the foreign reactor, there were nine events at
licensed U.S. power reactor facilities. Two of these events are discussed
below.
Description of Circumstances:
On December 21, 1985, an emergency core cooling system pump room at the Edwin
I. Hatch Nuclear Plant, Unit 1, was flooded to a level of 14 feet when a
maintenance isolation valve opened. At the time of the event, the plant was
in a refueling mode with a residual heat removal (RHR) loop suction valve
disassembled for repairs. The maintenance isolation valve, which was located
between the RHR loop suction valve and the torus, was shut, and its control
switch was "red tagged" (an administrative control that prohibits switch
operation) in the control room. The maintenance isolation valve is an air-
operated butterfly valve that opens on loss of power to its solenoid valve.
8708110476
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Power to the solenoid valve was lost when the station conducted a planned
loss-of-offsite-power test that also interrupted use of the plant's communi-
cations systems. Before control room personnel could be notified of the
flooding, the water level in the pump room had stopped rising because the
level had equalized with the torus water level.
On June 16, 1982, during a quarterly valve functional operational test at
Oconee Unit 2, an air-operated valve in the emergency feedwater system would
not fully open from the control panel. The valve was declared inoperable,
thereby making one of the emergency feedwater flowpaths to the two steam
generators inoperable. The emergency feedwater valve would not cycle to the
open position because its handwheel was engaged in the manual operation
position. The reason why the handwheel was in the wrong position could not be
determined. The valve's handwheel was disengaged from the manual operation
position to allow automatic movement of the valve, and the valve was tested
satisfactorily. To prevent recurrence of this event, the handwheels for the
valves in both loops of the emergency feedwater system at each of the three
units were locked in the disengaged position.
Discussion:
The AEOD report notes that the actual effects of the 10 events ranged from a
reduction in redundant protection without any actual consequences to signifi-
cant damage to safety-related equipment. Specifically, it reports that the
potential exists for (1) a reactor transient; (2) an unisolable steam and
water release; (3) a harsh environment with steam and flooding damage to major
safety-related equipment, components, and other control systems; and
(4) personnel injury.
The AEOD report concludes that the root cause of the inappropriate mechanical
blocking of valves was almost evenly divided between personnel error involving
both acts of omission and commission and inadequate procedures.
In conclusion, the AEOD report suggests that licensees
(1) verify the adequacy of the temporary procedures regarding modifications
to piping systems so that there is adequate assurance that a single
isolation valve will remain in the safe position when needed
(2) identify potential causes of unintended valve actuation (e.g., temporary
loss of power, surveillance tests) that could occur during maintenance,
which would result in an unblocked valve cycling to an unsafe position,
especially when this would result in an "open" system
(3) ensure that procedures for restoring blocked valves to service are clear
and provide for adequate verification.
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August 17, 1987
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No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the Regional
Administrator of the appropriate regional office or this office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Theodore C. Cintula, AEOD
(301) 492-4434
Richard J. Kiessel, NRR
(301) 492-9605
Attachment: List of Recently Issued NRC Information Notices
