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Understanding Japan’s Nuclear Crisis


Understanding Japan’s Nuclear Crisis

Following the events at the Fukushima Daiichi nuclear reactors in Japan has been challenging. At best, even those present at the site have a limited view of what’s going on inside the reactors themselves, and the situation has changed rapidly over the last several days. Meanwhile, the terminology involved is somewhat confusing—some fuel rods have almost certainly melted, but we have not seen a meltdown; radioactive material has been released from the reactors, but the radioactive fuel currently remains contained.

Over time, the situation has become a bit less confused, as cooler heads have explained more about the reactor and the events that have occurred within it. What we’ll attempt to do here is aggregate the most reliable information we can find, using material provided by multiple credible sources. We’ve attempted to confirm some of this information with groups like the Nuclear Regulatory Commission and the Department of Energy but, so far, these organizations are not making their staff available to talk to the press.

Inside a Nuclear Reactor

Nuclear reactors are powered by the fission of a radioactive element, typically uranium. There are a number of products of this reaction, but the one that produces the power is heat, which the fission process gives off in abundance. There are different ways to extract electricity from that heat, but the most common way of doing so shares some features with the first steam engines: use it to boil water, and use the resulting pressure to drive a generator.

Radioactivity makes things both simpler and more complex. On the simpler side, fission will readily occur underwater, so it’s easy to transfer the heat to water simply by dunking the nuclear fuel directly into it.

In the reactor design used in Japan, the fuel is immersed in water, which boils off to generate power, is cooled, and then returns to the reactor. The pressure vessel and primary containment keep radioactivity inside. (Ars Technica)

Unfortunately, the radioactivity complicates things. Even though the fuel is sealed into rods, it’s inevitable that this water will pick up some radioactive isotopes. As a result, you can’t just do whatever you’d like with the liquid that’s been exposed to the fuel rods. Instead, the rods and water remain sealed in a high-pressure container and linked pipes, with the hot water or steam circulated out to drive machinery, but then reinjected back into the core after it has cooled, keeping a closed cycle.

The water recirculation doesn’t just let us get power out of the reactor; it’s essential to keeping the reactor core cool. Unless the heat of decay is carried away from the core, its temperature will rise rapidly, and the fuel and its structural support will melt.

The Fission Reaction

Uranium ore. (Marchin Wichary/Flickr)

On its own, the uranium isotope used in nuclear reactors will decay slowly, releasing a minimal amount of heat. However, one of the decay products is a neutron, which can strike another atom and induce that to split; other neutrons are produced as the products of that split decay themselves. At high enough densities, this chain reaction of neutron-induced fission can produce a nuclear explosion. In a nuclear reactor, the fuel density is low enough that this isn’t a threat, and the rate of the fission can be controlled by inserting or removing rods of a material that absorbs neutrons, typically boron.

Completely inserting control rods to limit uranium’s fission, however, doesn’t affect what’s happened to the products of previous reactions. Many of the elements that are produced following uranium’s split are themselves radioactive, and will decay without needing any encouragement from a neutron. Some of the neutrons from the reactor will also be absorbed by atoms in the equipment or cooling water, converting those to radioactive isotopes. Most of this additional radioactive material decays within the span of a few days, so it’s not a long-term issue. But it ensures that, even after a reactor is shut down by control rods, there’s enough radioactive decay around to keep things hot for a while.

All of which makes the continued operation of the plant’s cooling system essential. Unfortunately, cooling system failures have struck several of the reactors at Fukushima Daiichi.

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