What happened at the nuclear plant in Japan – in simple talk.

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Our friend Vic in Arizona sent along this link to a forum discussion that explains, in simple language, what happened at the nuclear power plant in Japan.

Why was Chernobyl so much more of a disaster?

Three reasons: 1) Chernobyl had no “last ditch” super-strong external containment. In the worst case in Japan, control rods would not have inserted (they did, AFAIK), the reaction would have run away, the core would have melted… and the whole lot would have been caught in the “core catcher”, the poured concrete base under the reactor, and been entombed. That’s the last ditch defense, which won’t be needed in Japan but which wasn’t even present in Chernobyl.

2) Chernobyl allowed heat-dissociated oxygen and hydrogen to build up within the reactor vessel, which then exploded. In the Japanese reactors, gases were vented into the space outside the containment, which meant the eventual hydrogen explosion did not affect the reactor.

3) Chernobyl was a graphite-moderated design. Graphite is carbon, and while it is very hard to get graphite to burn, it’s not impossible if you get it hot enough. Hot graphite will also react with steam to produce a flammable mixture of hydrogen and carbon monoxide. The Japanese reactors are pressurised-water designs, so there is no graphite, hence no risk of hot graphite reactions.

The situation in the Japanese reactors is that reactors shut down and the emergency cooling systems (that deal with the decay heat) kicked in. However, the tsunami took out the diesel generators running the emergency cooling, so then battery-powered emergency-emergency cooling kicked in.

However, for some reason they failed to restore the emergency cooling system before the batteries ran out and the reactor began to boil away the water immersing the fuel rods, which meant that steam had to be vented to control the pressure. The detection of traces of radioactive caesium and iodine in the steam meant that one or more fuel rod had become heat-damaged and ruptured its zirconium casing, at which point they gave up on trying to save the reactor and flooded the whole thing with seawater.

That is pretty much game over for the reactor. The sheer volume of water present has enough heat capacity to absorb all the decay heat as the reactor winds down, but they can probably kiss that particular reactor goodbye now.

In addition to the difference in design, the nature of the accidents is different. The reactors in Japan are being destroyed by a failure to manage decay heat. This is the same thing that happened at Three Mile Island – the reactor was shut down, but the residual radioactive elements in the core were still generating enough heat to melt the fuel rods once the cooling water drained out.

Chernobyl wasn’t destroyed by decay heat. The reactor at Chernobyl was through a combination of terrible design and an unwise test procedure put into a state where the core was briefly generating far more power than it was ever designed to.

It’s like if you took a car’s engine and removed the electronic governor and any other overspeed protection and then wired the throttle full open. By the time the operators realized what was happening it was too late for them to stop it, and their attempts to shut down the core may actually have made the situation worse.

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