Multi-plant Issue B-56, Control Rods Fail to Fully Insert (Generic Letter No. 81-24)
UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D. C. 20555
June 15, 1981
LICENSEES OF ALL GE BWR FACILITIES (EXCEPT HUMBOLDT BAY)
SUBJECT: MULTI-PLANT ISSUE B-56, CONTROL RODS FAIL TO FULLY INSERT (Generic
Letter No. 81-24)
Gentlemen:
By letters dated August 29, 1978 and January 10, 1980, you were requested to
provide information on your experience with one or more control rods
stopping short of the fully inserted position on scram and then settling
back to notch position "02" or six inches short of full insertion.
We have evaluate your responses, along with those provided by other BWR
licensees, and have concluded that the problem of control rods inserting
only to notch position "02" on scram is attributable to leakage past worn
stop and drive piston seals. We have determined that this problem is a
maintenance problem, and is readily correctable by control rod drive
overhaul at a subsequent outage. We have further concluded that Technical
Specification requirements regarding control rod operability and shutdown
margin provide adequate assurance of the capability to place and maintain
the plant in a safe shutdown condition. Consequently, the failure to fully
insert events, as described herein, do not represent a significant safety
issue.
The enclosed safety evaluation provides the basis for this determination.
Sincerely,
Darrell G. Eisenhut, Director
Division of Licensing
Office of Nuclear Reactor Regulation
Enclosure:
Safety Evaluation
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UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D. C. 20555
SAFETY EVALUTATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION
DIVISION OF LICENSING
June 15, 1981
1.0 Introduction
Over the past several years, a number of instances of control rods
failing to fully insert on scram have occurred at GE BWRs. These
failures to fully insert on scram (FFIS) have involved one or more rods
stopping short of the fully inserted position (notch position "00") and
then settling back to six inches short of full insertion (notch
position "02").
2.0 Causes
Both General Electric (GE) and utilities experiencing such problems
have identified trhe apparent cause of the problem as leakage past worn
stop and drive piston seals which allows scram water to build up in the
buffer area of the drive. This results in a hydraulic lock of the
drives between notch positions "02" and "00". When a force balance is
achieved, the buffer area and the undreside of the drive piston
pressures equalize allowing the control rod drive to settle back into
notch position "02".
Orifices near the upper end of the control rod drive (CRD) piston tube
are progressively closed by the drive piston as the full rod insertion
postion is approached. This slows the CRDs and prevents seal damage as
a result of the drive piston slamming into the stop piston. However, in
the event of excessive leakage past the stop piston seals, the final
piston tube buffer orifice cannot pass all of the water during a scram.
Consequently, the hydraulic lock referred to earlier is created which
prevents the final increment of rod insertion.
General Electric Company has recommended a revised Control Rod Drive
Venting Procedure which required the CRDs to be vented until no air
could be detected with a Differential Pressure Cell in the CRD
Hydraulic System. By following this procedure, a water hammer condition
would not be created by the pressure of excessive air in the buffer
area. Such a condition could result in slamming the drive piston into
the stop piston resulting in seal degradation.
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An additional cause of seal deterioration has been attributed to high
crud levels in reactor coolant during operation. Some BWRs such as
Dresden 2 (which has had the highest number of FFIS events) require a
pressure breakdown of reactor coolant before purification, with a
resultant low volumetric cleanup rate. The high crud levels may inter-
fere with the tigh tolerances around the stop piston seals causing
binding, breakage, and subsequent FFIS events.
3.0 Evaluation
In all FFIS events reported in response to our letters of August 29,
1978 and January 10, 1980, control rode which failed to fully insert
inserted to notch position "02". In all cases, subsequent manual
insertion by the operator resulted in the rods being fully inserted.
Even with all rods inserted only to the "02" notch position instead of
the "00" fully inserted position, sufficient shutdown margin exists to
preclude FFIS from being a safety problem. The addition of reactivity
to the core as a result of all rods being withdrawn to six inches short
of full insertion is less than that of the most reactive rod being
stuck in the fully withdrawn position, according to calculations
performed by Brookhaven National Laboratories (BNL).
Although this problem appears to be a cumulative one (i.e., rods which
initially fial to fully insert continue to fail along with additional
rods on subsequent scram), it also appears to be a random type failure
(as opposed to a failure mechanism which results in "clustered" non-
insertion of rods such as that which occurred at Browns Ferry 3 on June
28, 1980). In addition, these failures are time dependent, which allows
for an increased likelihood of detection and correction of these
failures. This deficiency is typically corrected by removing and over-
hauling the affected CRDs at an outage subsequent to the recognition of
the FFIS event.
Technical Specifications pertaining to the reactivity margin required
to be abailable and to the operability of control rods provide adequate
assurance of the capability to place and maintain the plant in a safe
shutdown condition.
In summary, the failures of control rods to fully insert as described
herein does not present a safety problem since there is negligible
effect on reactivity even if all rods should insert to only the "02"
position, the capability to manually insert control rods is retained,
the overhaul of affected rod drives during subsequent outages rectifies
the problem, and existing Technical Specification requirements provide
adequate assurance of the capability to place and maintain the plant in
a safe shutdown condition.
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