Why cool the reactros?


TEPCO says that the reactors were safely shutdown when the earthquake hit on March 11. Why do they need to keep cooling the reactors if the reactors are not running? (30s, Osaka)

The emergency shutdown system at the Fukushima Daiichi plant did successfully stop the nuclear fission reactions in all reactors immediately after the earthquake. The problem is that the radioactive substances produced in the fission process continue to emit heat by decaying into other substances. This is why the cooling system has to keep circulating coolant water in the reactors for as long as it takes for the nuclear fuel to cool—typically years. Otherwise, the fuel can meltdown due to its own heat, and in the worst case scenario, melt through the containment vessel, pouring out radioactive substances in the environment.

How much heat then is generated after shutting down a nuclear reactor? Suppose a reactor that can generate 800 megawatts of electricity. Since the reactor efficiency is about 33 percent, the fuel generates 2,400 megawatts of heat. Around 93 percents of this energy is generated by fission. The rest of the energy is produced by the decay of the fission products.

At the time of shutdown, the reactor generates about 170 megawatts (about 7 percent of the runtime heat) of heat. The amount comes down to about 70 megawatts (about 3 percent) after an hour, to about 10 megawatts (about 0.5 percent) after a day, then to about 6 megawatts (about 0.3 percents) after a week. (Depending on the length of reactor operation, the percentages can vary.) After a week, the percentage changes only very slightly, and the fuel continues to generate megawatts of energy for months or even years.

At the Fukushima Daiichi nuclear power plant, thousands of tons of water has been injected to cool the fuel in reactors and in the spent fuel pool. To put an end to the Fukushima disaster, in one way or another, it is critical that the cooling systems of the damaged nuclear reactors be recovered as promptly as possible.

Science Communicator: Misato Hayashida