Abnormal Hydrogen Generation in Fukushuma BWR Units Accidents

A simulation of loss of AC power in a 720 MWe BWR with Mark II containment has been made to compare with the sequence of events that occurred at Fukushima Daiichi BWRs in March, 2011. In this study, all AC power was considered lost after the tsunami swept through Fukushima Daiichi units following an earthquake with intensity of around 0.5g. The affected Fukushima units were BWR-1, with Mark I containment, Unit 1 rated at 439 MWe, and other three units rated at 784 MWe each. The simulation corresponds to BWR unit 2 and unit 3, which experienced hydrogen fire/explosion following excessive Zr-water reaction and radiolysis. In the simulation, actions and failures of all emergency coolant injection and recirculation systems, and containment vapour suppression systems of typical BWRs were simulated. The simulation also computed the extent of metal-water reaction in the reactor vessel. Even with severe starting assumptions, the hydrogen yield is grossly under predicted. The amounts of hydrogen that got generated during these accidents were greatly in excess of the design limits. The amounts of hydrogen and oxygen that got generated in units 1, 2 and 4 were enough to create massive fires in these units. In unit 3, these gases were enough to support a hydrogen explosion. A hypothesis is proposed which could explain the causes of excessive Hydrogen generations in reactor environments is presented. However, it is necessary to perform experiments to prove or disprove this hypothesis. Pending such verification, interim measures are necessary to minimise risks in the short term of repeat of Fukushima type events. Interim suggestions for controlling metal-water reactions in nuclear power plants, and for mitigating their effects under accident conditions are presented.