At 15.42 a nuclear emergency was declared. Twelve minutes later, staff lost water level readings.
At 16.36 emergency cooling system malfunction occurred. TEPCO’s figures show that, within hours, there was a huge loss of coolant within the reactors. The lack of electrical power reduced site personnel to carrying car batteries by hand, to make rudimentary temperature and pressure readings. This was dangerous, even in the daytime, as the site was now muddy and strewn with hoses, pipes, and other debris.
By 18.46 pm, just three hours after the tsunamis hit, core damage in Reactor 1 had already begun. Melt-through occurred the following day. With destruction of all cooling systems, any water coming in contact with the now critically hot fuel rods accelerated the oxidisation of the zirconium alloy cladding encasing the uranium oxide fuel. This created large quantities of hydrogen gas within the reactor building.
At midnight that first day, power was restored to the control room’s pressure gauge. The levels it showed caused TEPCO’s engineers increasing alarm. Rising heat in the fuel rods was generating hydrogen gas and steam within the containment vessel, preventing any injection of coolant onto the reactor core.
There was now no choice but to release radioactive gases into the atmosphere in order to reduce pressure within the reactor building. The alternative would have been a containment vessel explosion.
TEPCO now faced a situation it had never envisaged. Venting valves are driven by motors which need electricity. Though venting was achieved manually, and at great danger to staff – radiation levels within Reactor Building 1 were already dangerously high – their efforts were in vain. A hydrogen explosion ripped through the outer structure of the building.
Once the battery generators had lost power – in Reactor 1 even these had failed – the fuel rods at the cores rapidly reached temperatures between 2,300°C and 2,500°C. The now-incandescent fuel began to melt through the 16 cm thick steel pressure vessel, and drip to the bottom of the primary containment vessel. From there, in time, it melted through the steel and two metre thick concrete coating of the surrounding torus and dripped down onto and through the floor of the reactor building.
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