Information Notice No. 82-28: Hydrogen Explosion While Grinding in the Vicinity of Drained and Open Reactor Coolant System
SSINS No.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
July 23, 1982
Information Notice No. 82-28: HYDROGEN EXPLOSION WHILE GRINDING IN THE
VICINITY OF DRAINED AND OPEN REACTOR
All nuclear power reactor facilities holding an operating license (OL) or
construction permit (CP).
This information notice is provided as a notification of an event that may
have safety significance. It is expected that recipients will review the
information for applicability to their facilities. No specific action or
response is required at this time.
Description of Circumstances:
On April 10, 1982, a hydrogen explosion occurred at Unit 1 of Arkansas
Nuclear One while maintenance personnel were grinding a recently cut
high-pressure injection (HPI) pipe, approximately 18 inches from the nozzle
connecting the HPI pipe to the reactor coolant system (RCS) piping. At the
time of the explosion, the RCS was partially drained and the water level in
the reactor coolant piping was just below the HPI nozzle to permit
radiography of the nozzle and subsequent repair. (Information Notice No.
82-09 provides details concerning the cracking problem in HPI piping at
Babcock & Wilcox plants.) The reactor coolant temperature was being
maintained at approximately 100F by the decay heat removal system, and
nitrogen cover gas was being maintained in the reactor coolant piping. These
conditions existed since the RCS was depressurized and partially drained on
March 29, 1982.
At approximately 1240 hours on April 10, 1982, the craftsmen, who were
grinding on the HPI pipe in preparation for welding, observed a bright flash
at the outlet of the HPI line and heard a loud "bang". The craftsman
actually performing the grinding was physically blown away from the HPI pipe
a distance of about three feet. Personnel in other areas of the Unit 1
containment building heard the explosion and felt the resulting concussion
and mechanical vibration. Additionally, some personnel outside of the
containment building, including operators in the Unit 1 control room
reported that they heard the explosion and felt varying degrees of
vibration. Although there were no physical injuries as a result of this
event, it should be mentioned that the craftsman's life was endangered as he
was working on a scaffold that was over 30 feet high.
The most recent RCS measurement of dissolved gas in reactor coolant had been
taken on March 26, 1982, just before commencing the plant cooldown and
shutdown. It indicated 39 standard cc of total gas/liter of coolant. The
July 23, 1982
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concentration was 43% of this total. Because the total gas concentration
exceeded the maximum allowed by plant procedures (30 cc/liter) for RCS
depressurization, the RCS was degassed for approximately 14 hours as
cooldown progressed. The total gas concentration was not measured after
degassing had been terminated because the RCS had been depressurized and the
sampling method is effective only when the RCS pressure is greater than
several hundred psig. Atmospheric samples had not been taken to measure
hydrogen and oxygen concentrations in the vicinity of the open HPI pipe.
(This had been done at other Babcock and Wilcox plants which were undergoing
The reason for the presence of an explosive concentration of hydrogen is
unknown. It could have been caused by (a) inadequate degassing, (b) failure
to purge the HPI pipe with nitrogen, or (c) failure to temporarily plug the
open HPI pipe.
Subsequent inspection of the affected HPI line, the first upstream check
valve in the HPI line, and the corresponding nozzle and safe-end on the RCS
cold leg indicated no signs of damage as a result of the explosion.
No written response to this information is required. If you need more
information about this matter, please contact the Regional Administrator of
the appropriate NRC Regional Office or this office.
Edward L. Jordan, Director
Division of Engineering and
Technical Contact: W. Marinelli
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