The accident at the Fukushima Daiichi Plant demonstrates that inerting primary containment is not sufficient to protect plants against hydrogen explosions. If the containment fails during a severe accident, the hydrogen generated by the metal–water reaction in the damaged reactor core can be released into the reactor building, mix with air, and burn. For this reason, the most effective control strategy is to manage the pressure and thermal loads on containment to prevent its failure. This requires the capability to safely vent hydrogen in a timely fashion with a minimum release of fission products into the environment.
The maximum amount of hydrogen generated in a severe core accident is almost three times the volume of nitrogen present initially in the primary containment. This quantity of hydrogen overwhelms the inerting effect of nitrogen. When the hot hydrogen–nitrogen–steam mixture leaks into the reactor building, the steam will begin to condense, and a flammable mixture will be formed.
The explosions at the Fukushima Daiichi plant significantly degraded the ability of personnel at the plant to mount an effective accident response. Substantial structural damage occurred to the Unit 1, 3, and 4 reactor buildings, and particularly Units 3 and 4, creating concerns about the integrity of their spent fuel pools as well. The explosions also created pathways into the environment for radioactive material leaks from containment. An intact BWR building acts as a filter to trap fission products released from the damaged core during a severe accident. Filtering is effective only if the reactor building remains intact and fission products can be removed by passing the exhaust gas through the filters in the standby gas treatment system.
In the 1980s, researchers at Oak Ridge National Laboratory examined severe accidents in boiling water reactor plants and the mitigating role of reactor buildings (i.e., secondary containment) on fission product releases. Greene (1990) specifically examined the potential for secondary containment failure due to combustion of hydrogen. He noted that reactor buildings have complex structures and relatively low failure overpressures (the pressure resulting from even a low-speed combustion event will substantially exceed the estimated failure pressure of the building outer walls); consequently, combustion of large amounts of hydrogen in a reactor building “would probably challenge the integrity of the secondary containment” (Greene, 1990). Greene identified two key mitigation strategies that focused on maintaining primary containment integrity: primary containment sprays and primary containment venting.