Sunday, 13 March 2011

Another Chernobyl?

I've been closely following reports of threats posed to and by Japan's nuclear power plant since the earthquake. There seems to be much concern that the Japan nuclear power plant situation will turn into another Chernobyl: "US experts fear 'Chernobyl-like' crisis for Japan"  and here's a more "technical" description of the potential problem (source): "Without cooling water, there is a real chance of a meltdown of the reactor core that could result in a large release of radiation. Usually in case of a failure of the main cooling system, an 'emergency core cooling system' would extinguish the nuclear chain reaction by dousing the rods with water treated with boron. This element has a high affinity for the neutrons ejected from split uranium atoms, which are responsible for most of the steam-producing heat transfer."  Ed Markey has also taken this opportunity to argue that this demonstrates that nuclear power poses grave threats of a Chernobyl type the US. Now while the situation with these reactors may be grave, it's important to note key differences with Chernobyl.

Some of Japan's reactors are old but they're also of a different sort, light water reactors, than Chernobyl's  Reactor 4.  As Naoto Sekimura notes in an al jazeera article, "No Chernobyl is possible at a light water reactor. Loss of coolant means a temperature rise, but it also will stop the reaction."  That's because in a light water reactor, the coolant water also plays the role of "neutron moderator". A neutron moderator, counterintuitively given its name, facilitates the nuclear chain reaction because it slows fast neutrons thereby turning them into thermal neutrons that perpetuate the chain reactions of nuclear fission.  So, when the coolant gets too hot, it actually acts as its own release valve because the water density decreases, meaning it loses its effectiveness at "moderating" the neutron speed, i.e., losing its ability to turn fast neutrons into thermal neutrons. As that happens, the radioactivity is decreased because the moderator or the effectiveness of the moderator is undermined.  So, the very material that's used to do the cooling, also actually facilitates the chain reactions, losing one helps to prevent the other, a very elegant deadman's switch of sorts.  Compare this to Chernobyl in which the moderator actually ignited.  This isn't the only way in which Chernobyl and the Japanese plants differ, Chernobyl also had no containment mechanism, so the radioactivity simply dissipated into the air.

None of this is to argue that nuclear power plants are completely safe and foolproof, but I do think it's important to factor this important difference in when evaluating the likelihood that a Chernobyl will unfold in Japan or is likely or plausible in the US.

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