As Japan's nuclear troubles continue, CISAC's Alan Hanson discusses the range of scenarios and how to prevent catastrophe.
CISAC: What is the range of events that could happen over the next several days and weeks?
Alan Hanson: The earthquake and resulting tsunami that hit northern Japan last Friday are unprecedented in modern times. These two nearly simultaneous natural disasters did significant damage to the Fukushima nuclear power stations operated by the Tokyo Electric Power Company (TEPCO). Despite this damage, including a total loss of off-site power and emergency backup power, TEPCO personnel have been making a heroic effort to bring the situation under control. To date off-site releases of radiation appear not to have had severe effects to the local population; because of prevailing westerly winds, the radiation releases have not been in the direction of major population centers.
It is impossible to predict the sequence of events that will unfold over the next few days and weeks. Under the best of circumstances, TEPCO will continue to take actions limiting further releases. To do so they must continue to cool the nuclear fuel in two separate regions of each reactor in the two stations; these regions are the reactor core itself and the used fuel storage pool. This means that enough water must be continually injected to keep the fuel covered. More dire circumstances could occur if the nuclear fuel is uncovered for a lengthy period of time. In the reactor core, this could lead to partial or total fuel melting, followed by failure of the primary steel containment due to excessive heat and pressure. Since some fuel has been uncovered for some time intervals, it is believed that partial melting may have already occurred and that the primary containment has been damaged in one or two of the reactors. If used fuel in the storage pools is uncovered, it could lead to fuel cladding failures from high temperatures releasing radioactive gases directly into the atmosphere. In a very unlikely scenario burning of the fuel cladding would release more gases and also particulate matter into the atmosphere. The reactor cores are of immediate concern because that is where the highest temperatures are located. The fuel pools become of greater concern over the intermediate term as water is boiled off or if some other event causes draining of a pool.
CISAC: What can be done to prevent the worst?
Hanson: Both worst-case scenarios described above can be prevented by keeping the nuclear fuel covered with water by any means available including the use of sea water, which has been initiated already. The weather will play an important role, too. As long as winds blow radioactive gases off shore and away from population centers, the public health effects should be minimal; if winds shift and blow toward Japanese cities, the situation would be significantly worse.
CISAC: How can we prevent this sort of thing from happening again?
Hanson: It is too early to speculate about the possibility of future accidents such as this one. Suffice it to say that the combination of a magnitude 9.0 earthquake and a 30-plus foot tsunami is a highly unlikely event. If early reports from the site turn out to be true, the reactors rode out the earthquake reasonably well and all of them shut down in the orderly fashion planned for an earthquake. Without the subsequent tsunami, it is very doubtful that the ongoing crisis at Fukushima would be anywhere near the magnitude we are witnessing. The nuclear industry has a good record of learning from accidents and making the appropriate changes to prevent reoccurrence or to at least mitigate the consequences should something similar happen in the future. Actions will certainly be taken by the industry and regulatory bodies in this vein once the immediate emergency is behind us.