Explosion at the off-gas system of a nuclear power plant

During steady-state operations an explosion occurred in the off-gas system.

[Zalosh and Short, 1978]

DESCRIPTION OF THE OFF-GAS SYSTEM OF BWR
Historically, off-gas treatment consisted in:
(1) dilution of the stoichiometric hydrogen-oxygen mixture with steam or air.
(2) Recombination and removal of the water/moisture.
(3) Reduction of the radioactivity of the of the noble gases, by passing the gas through a charcoal bed or delay pipes.
(4) Abatement of the radioactive decay products in the stream by flowing the gas through a high-efficiency filter particulate to remove,
(5) venting the gas to the atmosphere through an elevated stack.
Older BWR power plants may be missing all but (5).

Hydrogen and oxygen are generated in a nuclear reactor by radiolysis of steam in the cooling system. If not properly extracted and handled, the two gases can build up a flammable mixture which can explode. Therefore, when the steam is condensed, the off-gases are extracted into an off-gas treatment system. Throughout the year 1960-80, explosions have been reported in various components of off-gas systems.

Zalosh and Short included in their report (“Comparative analysis of hydrogen fire and explosion incidents”, see references) 25 incidents of this type happened in the US in the 1966-77 period, caused by formation of flammable hydrogen-oxygen mixtures in cooling loops of boiling-water-reactor (BWR) nuclear power plants. A selection of them appears also in HIAD.
Despite the occurrences, equipment damage has been minimal in most of the off-gas explosions, probably because the Regulator required the system upstream of the recombiner to be designed to withstand hydrogen-oxygen explosions.
Most of the related injuries were due exposure to radioactive gases. Since the gases were usually short-lived radionuclides, contaminated workers recovered rapidly, i.e., within a day or two.

See incident HIAD_499 for the lesson learnt and related corrective measures following the occurrence of this type of incidents in the years 70’s , occurring in the off-gas system of BWR’s.