How to Cool a Nuclear Reactor

The 8.9 magnitude earthquake in Japan is causing problems for at least one of its fleet of nuclear reactors—and authorities have shut down 10 of the country's 55 units. Tokyo Electric Power confirmed that pressure had been rising inside reactor No. 1 at its Fukushima Daiichi nuclear power plant on the northeast coast, one of the largest nuclear power plants in the world. That means cooling water is not getting to the reactor core, causing a build up of steam inside the containment vessel. The problem, according to Japanese media reports, is a loss of grid electricity to run the pumps that bring in cooling water. The backup diesel generators that are supposed to provide emergency power in that case are out of order, according to the Japan Atomic Industrial Forum, but replacements were being taken to the plant. (Similar diesel generators were providing power to the nation's Rokkasho Reprocessing Plant, which recycles spent nuclear fuel.)

As a precautionary measure, the Japanese government has declared a nuclear emergency and asked people living within three kilometers of the facility to evacuate and people living within 10 kilometers to remain indoors. Tokyo Electric Power, for its part, planned to vent some of the radioactive steam from inside the containment building.

Scientific American spoke with Scott Burnell, public affairs officer at the U.S. Nuclear Regulatory Commission (NRC), the government agency charged with monitoring the safety of the 104 nuclear reactors in the U.S., about what it takes to cool down a reactor.

The reactor that was not cooling properly in Japan, the Fukushima Daiichi No. 1 reactor, was a boiling-water type. How are these different from pressurized-water reactors in terms of cooling?
Particularly useful to boiling-water reactors is a system that is steam driven. It does not require an outside power source. Steam generated by the heat of a cooling down reactor has enough force to run a turbine, which then runs a pump that provides coolant to the core. That sort of system is supposed to withstand an earthquake, and that can run for an extended period. It's a self-limiting condition. That system does use batteries for the controls, but it can also be operated manually. So even in the face of a complete station blackout—you don't have any power at all—there are methods for using the steam-driven pump to continue to keep cooling going.