By Calder Phillips-Grafflin
Tokyo Electric Power Company, TEPCO, the operators of the Fukushima Daiichi nuclear power plant, have released the first analysis on the condition of the troubled power plant, and their conclusions are relatively grim. This analysis covers only Reactor 1, since it is the only reactor for which the necessary sensor data is available, but it is indicative of the problems faced at the other troubled reactors.
The analysis confirms what most already expected: the fuel in Reactor 1 melted down completely. According to the data from TEPCO, this melting occurred immediately, with almost the entire core having melted down by 16 hours after the earthquake. In a mere five hours or so, the temperature in the core rose from the stable operating temperature of 750 degrees Celsius to a high of nearly 3000 degrees. At this temperature, fuel rods melted completely and fell to the bottom of the reactor vessel.
This meltdown may have saved the reactor, as the bottom of the reactor vessel was the only area being cooled by water injection. Without this limited cooling, the heat might have caused a failure of the reactor vessel. Oddly enough, similar behavior has occurred in other near-meltdowns, in which fuel melting to the bottom of the vessel helped prevent the vessel from failing. This was the case with the partial meltdown of TMI-2 at Three Mile Island in 1979.
It is a sign of just how unstable the situation remains that the reactors have not yet reached cold shutdown. Cold shutdown is achieved when a reactor’s coolant can be depressurized, which requires that temperatures fall below 100 degrees Celsius. Despite two months of seawater and freshwater pumping, temperatures in parts of Fukushima Reactor 1 are still above boiling. So long as temperatures remain this high, water injection will have to continue, which is not ideal.
The problem with continued water injection is that major leaks in the reactor vessel have yet to be stopped. This means that radioactive water continues to leak at a high rate from the reactor, which poses the threat of groundwater contamination. If TEPCO is lucky, most of this water has been contained in the basement of the reactor building, but this still poses a threat to cleanup workers.
TEPCO’s analysis is a testament to the damage sustained by the plant in the first hours after the Tsunami, and somewhat of a relief: reactor vessels held despite temperature stresses above their design capacity, and all the radioactive fuel appears to have remained in the reactor. While TEPCO has not yet released data for the other two troubled reactors, it seems likely that they suffered similar fates to Reactor 1. If this is indeed the case, then Fukushima is a grim warning of just how bad things can go wrong.