How to Cool a Nuclear Reactor

In General on March 15, 2011 at 5:37 AM



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.

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.

It’s all meant to provide defense in depth. First you rely on the grid. If the grid is no longer available, you use diesel generators. If there is an issue with the diesels, you have a battery backup. And the batteries usually last long enough for you to get the diesels going.

The event we are looking to avoid is damaging the core. Once you start damaging the core, you are then releasing radioactive material into the coolant and thereby increasing the chances that something travels outside the 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.

Are there other coolants besides water? […] The reserve tanks at a reactor contain the same grade of water in terms of purity and chemical composition that are normally used in the core. It is possible […] to introduce […] regular water.

As a basic design feature in the U.S., plants are not literally self-powering. That’s by design, because you don’t want to end up in a situation where a problem at the plant cuts off its own power source. […] if you can’t use power from the grid, you shut down.

What kinds of events could knock out a diesel generator? […] It can be the case that diesel itself is running properly but the distribution system, the buses or the cabling could be misaligned to the point where the diesel detects that its power is not being accepted by the plant.


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