Possible Scenario Right Now at Fukushima Nuclear Power Plant

What is likely to be going on right now at Fukushima Nuclear Power Plant?

I contacted one of the authors for the forthcoming Wiley Encyclopedia of Nuclear Energy to seek a concise assessment of the situation.

Normally, in this type of reactor, water is constantly flowing to remove the heat from the core. It is this heat which produces steam and eventually turns turbines to make electricity. When the earthquake and tsunami hit, the normal flow of water stopped.

The nuclear fission process was halted by a procedure known as scramming.  The scram was triggered automatically by the quake without human intervention. The procedure involves inserting control rods into the core. As soon as this happened, the reactor went “subcritical.” The chain reaction, which normally keeps the reactor self-sustaining, slows down but it takes a while to come close to a full stop because of the delayed neutrons.

Even though the reactor scrammed, the core continues to emit some neutrons that cause new fissions, which are the source of the heating. So-called “delayed neutrons,” from the decay of fission products previously produced, keep a certain level of fission reactions going on for a while, thus generating more heat.

The Japanese plant workers are in a race against time. They have sustained failures to their primary and secondary cooling systems and have now implemented a third system to cool the reactor by using ocean water. They have to continue to cool the reactor long enough until the heat released by fissions and decay of fission products drop below a level which can cause structural damage.

News reports suggest the operators need another 24 to 36 hours of cooling to keep the reactor under control and bring the fission reactions to a nominal level.

This article from CNN is useful for additional information.