For unit 2, the first scenario suggests that water only recovered to about three metres below the top of fuel, and the second shows levels below the core entirely. In both cases the fuel temperature is calculated to have shot to around 2700ºC, well beyond the level at which the zircalloy cladding reacts with water to produce hydrogen and more heat. Under these conditions core damage is thought to have begun at 8pm on 14 March, three days and five hours after the tsunami hit.
The first scenario ends with about 54% of the top of the core melting to form a mass supported and surrounded by a number of damaged fuel assemblies. It would reach this steady state on around 18 March, according to the simulation.
The second scenario has only about 12% of the core remaining on the support plate, with the rest having fallen into the water in the the lower part of the reactor pressure vessel. This may have happened by 15 March.
The real situation is likely to be between these cases, but the prognosis compares unfavourably with the estimate of 35% core damage based on readings taken at the time.
Potential end states for units 2 and 3 as simulated by computer codes.
On the left, unit 2's scenarios; on the right, unit 3's. Undamaged fuel is
indicated by striped lines, damaged fuel is yellow and areas of melted
fuel pellets are orange.
For unit 3 the story is similar. Instrumentation at the plant showed water levels returning to about two metres below the top of fuel, whereas the two simulation cases put levels at about three metres below and just below the core. Temperatures reached damage-inducing levels on 13 March. The two possible core states for unit 3 put forth by the computer also follow the same pattern. The first has 42% of the core melted and merged with a mass of damaged fuel. This sits on a platform of 16% undamaged fuel at the bottom. In the second case, some 94% of the fuel is assumed to have dropped into the water below by the morning of 14 March.
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