Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6820
Accession No.:
8005050056
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
June 12, 1980
IE Bulletin No. 80-14
DEGRADATION OF BWR SCRAM DISCHARGE VOLUME CAPABILITY
During our review of BWR operating experience, two events have raised
concern on operations related to the control rod drive system scram
discharge volume (SDV).
Description of Circumstances:
At Hatch Unit 1, on June 13, 1979, while performing surveillance to
functionally test SDV high level switches, two switches (C11-NO13A, B) were
found to be inoperable. Redundant switches (C11-NO13 C, D) were operable.
The reactor was in the refuel mode and these switches had been modified
prior to this occurrence. Inspection of the inoperable level switches
revealed that the float rod was bent and binding against the side of the
float chamber on both switches. The licensee believes that the float rods
were bent during or prior to initial installation and that metal particles
from the modification caused binding of the float. (LER 79-038)
Brunswick Unit 1 reported that slow closure of the SDV drain valve during a
reactor scram on October 19, 1979 apparently caused a water hammer event
which damaged several pipe supports on the SDV drain line. Drain valve
closure time was approximately five minutes due to a faulty solenoid
controlling air supply to the valve. The damaged pipe supports were repaired
but repair parts for the faulty solenoid were not available. To prevent
possible damage from a scram, the unit started up with the SDV vent and
drain valves closed except for periodic draining. During this mode of
operation the reactor scrammed from high level in the SDV, without prior
actuation of either the high level alarm or rod block switch. Subsequent
inspection revealed that the float ball on the rod block switch was crushed
and the float ball stem on the high level alarm switch was bent such that
the switches would not operate. The water hammer event discussed above was
the reported cause of failure of these two switch assemblies. (LER 79-74)
As a result of these events and related anticipated transients without scram
(ATWS) studies, concern arises that the SDV function may be degraded by the
undetected presence of fluid in the SDV. The second event is significant in
that it indicates the potential for a common cause failure (faulty solenoid)
to result in operation of the SDV in a manner which could defeat both the
level switch function and the SDV draining function. The ATWS generic
studies (NUREG 0460) have led the staff to propose, among other
requirements, improvements in the SDV designs to reduce susceptibility to
common cause failures. By separate correspondence, the staff will provide
example Technical Specifications related to the action items discussed
below.
.
IE Bulletin No. 80-14 June 12, 1980
Page 2 of 2
A. GE BWR's With an Operating License
The following actions are to be taken by licensees of GE designed BWR
facilities with an operating license:
1. Review plant records for instances of degradation of any SDV level
switch which was or may have been caused by a damaged or bent float
assembly. Identify the cause and corrective action for each instance.
2. Review plant records for instances of degradation of SDV vent and drain
valve operability. Provide the closure times required and typically
observed for these valves and the basis for the required closing times.
Identify the cause and corrective action for each instance of
degradation.
3. By procedures, require that the SDV vent and drain valves be normally
operable, open and periodically tested. If these valves are not
operable or are closed for more than 1 hour in any 24 hour period
during operation, the reason shall be logged and the NRC notified
within 24 hours (Prompt Notification).
4. Review instances in which water hammer or damage which may have been
caused by water hammer has occurred in SDV related piping. Identify the
cause and corrective action for each instance.
5. Review surveillance procedures to ensure that degradation of any SDV
level switch due to a damaged float or other cause would be detected
and that inoperability from any cause would be reported to the NRC.
6. If no functional test or inspection which would detect degradation of
each SDV level switch has been performed during the past 3 months, make
provisions to perform an inspection and functional test of all SDV
level switch assemblies at the next reactor shutdown of greater than 48
hours duration.
B. Reporting Requirements
The action taken in response to the items in Part A shall be completed
and a written report on the results submitted to the NRC within 45 days
from the date of this Bulletin.
This report should be submitted to the Director of the appropriate NRC
Regional Office and a copy should be forwarded to the NRC Office of
Inspection and Enforcement, Division of Reactor Operations Inspection,
Washington, D.C. 20555.
Approved by GAO, B180225 (R0072); clearance expires 7-31-80. Approval was
given under a blanket clearance specifically for identified generic
problems.
