Bulletin 78-09: BWR Drywell Leakage Paths Associated with Inadequate Drywell Closures
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D. C. 20555
June 14, 1978
IE Bulletin No. 78-09
BWR DRYWELL LEAKAGE PATHS ASSOCIATED WITH INADEQUATE DRYWELL CLOSURES
Description of Circumstances:
Upon completion of the E. I. Hatch 1 plant refueling, the drywell head was
reinstalled on April 1, 1978 and a local leak rate test (LLRT) was
successfully passed, providing no indication of a potential problem at the
time. Subsequently, during the containment integrated leak rate test (CILRT)
at the E. I. Hatch 1 plant on April 6, 1978, high leakage around the drywell
head was experienced. After the high leakage rate was detected, the
containment was depressurized. Prior to detensioning the drywell head bolts,
a feeler gauge was used to determine that the clearance was greater where
the leakage was occurring.
Upon retorquing the drywell head for a second LLRT and CILRT, the tightening
sequence was started at the area of leakage and torque values of 1800
foot-pounds were used. Feeler gauge readings between the head and drywell
were recorded. The second LLRT was passed successfully and no leakage was
indicated around the drywell head during the second and successful CILRT
conducted at 59 psig.
Another instance was identified during the CILRT at Millstone Unit 1 on
November 22-23, 1976, when both the drywell head and the drywell head manway
cover "lifted" during a Type A test subsequent to successful Type B (LLRT)
tests.
Based on this experience, it appears that the LLRT following each
reinstallation of the drywell head or other closures, such as after a
routine refueling or other similar activity, cannot be the sole basis for
assuring a suitable degree of drywell closure leak tightness. It appears
that there must be controls to ensure that drywell closures are made in a
reproducible manner.
Action to be taken by all BWR power reactor facilities with an operating
license or construction permit:
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IE Bulletin No. 78-09 June 14, 1978
1. Determine if the procedural controls utilized at your facility are
adequate to assure that drywell head reinstallations achieve a degree
of leak tightness equivalent to that attained during the installation
immediately preceding the last successful CILRT. This determination
should include, but not be limited to, consideration of the following:
a. Comparison of the applied bolt torque (or for turn-of-the-nut
technique, the number of flats on each nut/bolt after
metal-to-metal contact has been achieved) after any head
reinstallation to that used during the reinstallation verified by
CILRT, including any additional local retightening needed to reach
LLRT acceptance criteria.
b. Comparison of head-flange clearances after any head
reinstallation, to that used during the reinstallation verified by
CILRT.
c. Determination of the suitability of the drywell head gaskets for
reuse.
d. Determination of the maximum allowable bolt torques to avoid bolt
overstressing conditions.
e. Determination of any special handling arrangements, tightening
sequence, gasket preparation, or other special considerations
caused by unique facility circumstances such as a warped drywell
head.
2. Should your determination called for in Item 1 above demonstrate a need
for procedural revisions, your response to this Bulletin should include
a description of the changes to be made and a timetable for their
implementation which must occur prior to the next drywell head closure.
3. Identify other bolted containment closures that would tend to unseat on
positive internal pressure and describe what actions are taken to
ensure that adequate leak tightness would exist at an internal pressure
of Pa.
4. Within 45 calendar days of the date of issue of this bulletin, report
in writing to the Director of the appropriate NRC Regional Office, the
results of your review and your plan of action with regard to Item 1
through 3. A copy of your report should be
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IE Bulletin No. 78-09 June 14, 1978
sent to the United States Nuclear Regulatory Commission, Office of
Inspection and Enforcement, Division of Reactor Operations Inspection,
Washington, D. C. 20555.
For all BWR power reactor facilities with a construction permit, this
Bulletin is for information only and no written response is required.
Approval by GAO, B180225 (R0072); clearance expires 7/31/80. Approval was
given under a blanket clearance specifically for identified generic
problems.
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