Explosions in a nuclear power plant

Two hydrogen explosions occurred at a boiling water reactor (BWR).
The first explosion occurred at 9:30 a.m. and was mostly confined to the off-gas system. Damage was relatively minor.
The second explosion occurred at 13:00 outside the off-gas system in the two-level room at the base of the plant stack. This explosion damaged considerable the stack however, the overall structural capability of the stack was not impaired. One man was injured.
The review of the event concluded that the action taken to restore off-gas system drain line loop seals in the stack base space had not been successful. Without these seals, gases from the off-gas 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.

The INITIATING CAUSE was defective seals in the off-gas system, which allowed release/accumulation of hydrogen outside the system.
CONTRIBUTING CAUSES were the inadequate ventilation and the absence of gas monitoring equipment.
The ROOT CAUSE could be a combination shortcoming in management, design and possibly also installation/maintenance.

loop seals, off-gas system,

One man was injured by the blast and contamination. He was hospitalized because of the concussion and was released four days after the event.
The damage of the first explosion was relatively minor, including the breaking of glass faces on off-gas system flow differential pressure gages, the blowout of a rupture disc and the evacuation of loop seals between the off-gas system drain lines and stack base sump.
The second 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 off gas 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.

In 1978, following the investigation of this and previous similar cases, the NRC recognised that the BWR off-gas system design was considering the possibility of accumulation of explosive hydrogen mixtures. Thanks to this, the design has prevented major releases of airborne radioactivity in the approximately 25 known hydrogen gas explosions that had occurred so far within the off-gas systems of operating BWRs.
However, cases of hydrogen gas accumulation outside the off-gas system occurred. In five cases, including this one, this resulted in five explosions, extensive mechanical damage to equipment and structures, and uncontrolled release of radioactive material.

Action requested by the authority:
For all BWR power reactor facilities with an operating license, NRC required the following:
(1) Review the operations and maintenance procedures related to the off-gas system to assure proper operation in accordance with all design parameters. Include measures to prevent inadvertent ignition causes.
(2) Review the adequacy of the ventilation of spaces and areas around the off-gas system piping, considering ventilation losses and off-normal off-gas operation (dilution steam, lost loop seals, blown rupture discs, bypassing recombiners) and the possibility of leakage.
(3) Consequently take action to assure that explosive mixtures cannot accumulate, that monitoring equipment is in place to warn for accidental accumulation and that disposal of such mixtures can take place safely.
(4) Loop seals are potential off-gas leakage paths following a pressure transient in the off-gas 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 off-gas system explosions.