United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 80-14: Degradation of BWR Scram Discharge Volume Capability

                                                            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. 
Page Last Reviewed/Updated Tuesday, July 23, 2013