Fenomen Newsletter of the Department of Physics and Nuclear Engineering

The Fukushima accident sequence The earthquake and the subsequent tsunami that devastated part of Northeast Japan on March 11th, 2011, led to a series of accidents in the Fukushima Daichi nuclear plant, constructed and operated by Tokyo Electric Power Company (TEPCO). Only three of the six units of the plant (all of them boiling water reactors, BWR) were operating at that time. The sequence of events followed roughly the same pattern for the three operating reactors. During the earthquake, the reactors were automatically halted by the insertion of the control rods. The residual heat removal system, requiring power supply, provided cooling during nearly one hour, until the tsunami (far beyond the design basis of the plant) rendered the diesel generators inoperative. These generators had started up automatically when external supply from the electricity grid failed because of the earthquake. In these conditions, a passive system is able to evacuate the core thermal power into the containment; the drawback of this operating mode is that containment pressure builds up and after a time the water level in the reactor vessel shrinks to the point that partial dry-out of the fuel rods can occur. Steam venting into the upper floor of the reactor building, in order to reduce the pressure of the primary containment, resulted in a large hydrogen explosion. The presence of a large amount of hydrogen accompanying the steam is a symptom that core dry-out has caused an overheating of the fuel rods to the temperature of clad oxidation. Indeed, taking into account both the difficulties of maintaining a proper core cooling and the evidences of high radiation levels and fission product release, it must be assumed that a partial melt occurred in the core of the three reactors. This fact makes heat extraction from the fuel even more difficult, as the amount of accessible surface becomes smaller. Furthermore, molten fuel accumulates in the lower plenum of the reactor vessel which might break because of the heat. The cooling of the pools containing the spent fuel was also affected by the loss of electricity supply. Water evaporation because of residual heat eventually resulted in exposition of the spent fuel rods to air, with subsequent overheating, cladding degradation and release of fission products. Activity in the plant is now focused on restoring and maintaining the cooling capacity in the three units, to cool the core down and to maintain the integrity of the reactor vessel and of the primary containment in order to confine radioactivity, but also on keeping a sufficient water level in the spent fuel pools. Lluís Batet, Carme Pretel DEPARTAMENT DE FÍSICA I ENGINYERIA NUCLEAR SPECIAL ISSUE, MARCH 2011