Fukushima

The water in the spent fuel pool does two things: through a heat exchange mechanism, it keeps the fuel assemblies at reasonable temperatures; and, it captures the radiation streaming out of the fuel rods. There's a lot of this spent fuel about, some 260,000 tons, most still in storage ponds and growing by 8-10,000 tons per year (the US has some 65,000 tons, Japan about 19,000 tons).

It's seriously nasty stuff. Spent fuel from light water reactors (like Fukushima's) is composed of 93.4% uranium (with only ~0.8% U-235, the fissile isotope), 5.2% fission products, 1.2% plutonium and 0.2 % other transuranic elements. It's the middle two that are the potential killers. Some of the fission products break down very quickly to more stable (i.e. less harmful) elements, but two hang around for a long time and are particularly dangerous: strontium-90 and caesium-137. Both have half lives of about 30 years and they mimic potassium and calcium respectively. They're therefore rapidly absorbed into the food chain and become concentrated in higher-order creatures, like us. Iodine-131 is similarly lethal; it mimics iodine and concentrates quickly in the thyroid. The good news is its half life is only eight days and so it's only a factor in nuclear explosions or reactor accidents.

All this radiation also generates heat. Once the spent fuel is removed from the reactor, the radiation (and heat generation) tails off dramatically; after one year, it's down by a factor of 10 and by 10 years it's reduced to about 1% of its starting level. For much of the first 100 years, the radioactivity comes principally from the fission products with strontium-90 and caesium-137 dominant after the first 10 years. After a few hundred years only the transuranics are still going strong: plutonium, americium, neptunium and curium.

Still, the radiotoxicity of spent fuels remains a potentially lethal hazard for hundreds of thousands of years. And, given the plutonium content, the fuel also of course has to be safeguarded for all that time.

What about the heat generation, though? This was a question I wanted to get to the bottom of. With radiation tailing off so quickly, maybe after a year or two even exposure of spent fuel rods to the air wouldn't be catastrophic, in which case Fukushima might be through the most dangerous phase.

Not so, unfortunately. Gundersen and co were not exaggerating. In "Technical Study of Spent Fuel Pool Accident Risk" (2001), the US Nuclear Regulatory Commission wrote:

In summary, 60 days after reactor shutdown for boildown type events, there is considerable time (> 100 hours) to take action to preclude the fission product release or zirconium fire before uncovering the top of the fuel. However, if the fuel is uncovered, heatup to the zirconium ignition temperature [900°C] during the first years after shutdown would take less than 10 hours even with unobstructed air flow. After five years, the heat up would take at least 24 hours even with obstructed air flow cases. [PWR is a Pressurised Water Reactor; BWR a Boiling Water Reactor. Both are light water reactors]

So, however one gets there (whether by leakage, a crack in the pool or, heaven forbid, another earthquake, or even a slow boiling off if the heat exchange mechanisms were to fail), even after five years those fuel assemblies would burn.

Before such a fire starts, the cladding around the fuel rods would almost certainly swell and burst, releasing "radioactive gases present in the gap between the fuel and clad." It's the next stage that's truly catastrophic.

If the fuel continues to heat up, the zirconium clad will reach the point of rapid oxidation in air. This reaction of the zirconium and air, or zirconium and steam is exothermic (i.e. produces heat). The energy released from the reaction, combined with the fuel's decay energy, can cause the reaction to become self-sustaining and ignite the zirconium. The increase in heat from the oxidation reaction can also raise the temperature in adjacent fuel assemblies and propagate the oxidation reaction. The zirconium fire would result in a significant release of the spent fuel fission products which would be dispersed from the reactor site in the thermal plume from the zirconium fire.

Unlikely as it is, this is the nightmare scenario at Fukushima.