Bulletin 78-03: Potential Explosive Gas Mixture Accumulations Associated With BWR Offgas System Operations
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
February 8, 1978
IE Bulletin No. 78-03
POTENTIAL EXPLOSIVE GAS MIXTURE ACCUMULATIONS ASSOCIATED WITH BWR OFFGAS
Description of Circumstances:
On December 13, 1977, two hydrogen explosions occurred at the Millstone
Nuclear Power Station Unit No. 1, a boiling water reactor (BWR).
The first explosion occurred at 9:30 a.m. and was mostly confined to the
offgas system. Damage was relatively minor, including the breaking of glass
faces on offgas system flow differential pressure gages, the blowout of a
rupture disk and the evacuation of loop seals between the offgas system
drain lines and stack base sump.
The second explosion occurred at 1:00 p. m. outside the offgas system in the
two-level room at the base of the plant stack. This explosion blew the stack
base space door into a warehouse about 200 feet away, breached the
reinforced concrete ceiling between the stack chimney and the stack base
space, extensively damaged the ceiling beams, dislodged the 2-ton concrete
plugs in the floor above the offgas system particulate filters, damaged the
stack radiation monitor isokinetic probe supports, and produced cracks in
the stack. These cracks were vertical, with a maximum separation at the
surface of 1/16 inch; however, the overall structural capability of the
stack was not impaired.
One man was injured by the blast, receiving a concussion, skin abrasions,
and contamination. He was hospitalized because of the concussion and was
released four days after the event.
The licensee's review of the event concluded that the action taken to
restore offgas system drain line loop seals in the stack base space had not
been successful. Without these seals, gases from the offgas system
accumulated in the space, resulting in an explosive mixture which was
probably ignited by a spark from the level switch in the stack base sump.
Inadequate ventilation of the stack base space and the lack of equipment
installed to monitor explosive gas concentrations within the enclosed area
were concluded to be contributing factors.
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IE Bulletin No. 78-03 February 8, 1978
By way of background, it is recognized that accumulation of explosive
hydrogen mixtures was considered in the design of the BWR offgas system. As
a result, the design has prevented major releases of airborne radioactivity
in the approximately 25 known hydrogen gas explosions that have occurred
within the offgas systems of operating BWRs. Extensive mechanical damage to
equipment and structures, however, in addition to uncontrolled release of
radioactive material, have resulted from five explosions, including the
Millstone event, where hydrogen gas accumulated outside of the offgas
Action to be Taken by Licensees:
For all BWR power reactor facilities with an operating license:
1. Review the operations and maintenance procedures related to the offgas
system to assure proper operation in accordance with all design
parameters. Include in this review measures you have taken or will take
to prevent inadvertent actions (such as arc strikes) which might cause
ignition of the mixture of gases contained in the offgas piping.
2. Review the adequacy of the ventilation of spaces and areas through
which offgas system piping containing explosive mixtures of gases pass.
The review should consider ventilation losses and off-normal offgas
system operation, such as lack of dilution steam, lost loop seals,
blown rupture disks, bypassing recombiners, and leakage of offgas into
isolated portions of systems.
3. For those spaces and areas identified, describe what action you have
taken or plan to take to assure that explosive mixtures cannot
accumulate, that monitoring equipment will warn of such an
accumulation, and that disposal of such mixtures will be controlled
without resulting in a damaging explosion.
4. Loop seals are potential offgas leakage paths following a pressure
transient in the offgas system piping. Describe your design features to
minimize and detect the loss of liquid from loop seals and describe
operating procedures which assure prompt detection and re-seal of the
blown loop seals.
5. Review operating and emergency procedures to assure that your operating
staff has adequate guidance to respond properly to offgas system
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IE Bulletin No. 78-03 February 8, 1978
6. 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 Items 1
through 5. A copy of your report should be 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.
Approved by GAO, B180225 (R0072); clearance expires 7/31/80. Approval was
given under a blanket clearance specifically for identified generic
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