Why Fukushima’s “Spent” Fuel Rods Will Continue To Catch Fire - FDL

[WillyT]

Why Fukushima’s “spent” fuel rods will continue to catch fire
By: Kirk James Murphy, M.D.
Tuesday March 15, 2011 4:26 pm

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Yesterday the spent fuel rod pool at Fukushima Daiichi reactor 4 caught fire. About that time instruments at the plant showed an exponential increase in radiation levels. After the fire was quenched, radiation levels fell. In the hour before I sat down to write this, there was an explosion at the same spent fuel rod pool. As I write, another fire is burning there. NHK reports the radiation level – 300 to 400 milliSieverts – is so high that firefighters cannot approach the area.

NHK reports that by Monday March 14 the temperature in the spent fuel rod pool was 84 degrees C: nearly double the usual temperature. NHK reports that there aren’t temperature readings for today: technical failure. We do know the pool temperature increased by roughly twenty degrees C per day after loss of power on Friday. And we know that water boils at 100 degrees C.

The spent fuel rod pool at reactor 4 is one of seven pools for spent fuel rods at Fukushima Daichii. These pools are designed to store the intensively radioactive fuel rods that were already used in nuclear reactors. These “used” fuel rods still contain uranium (or in the case of fuel rods from reactor 3, they contain both uranium and plutonium from the MOX fuel used in that reactor). In addition to the uranium and plutonium, the rods also contain other radioactive elements. These radioactive elements are created in the rods by the intense radiation around the rods when they are in the reactor core (before they are moved to the spent fuel pools).

Six of the spent fuel rod pools are (or were) located at the top of six reactor buildings. One “common pool” is at ground level in a separate building. Each “reactor top” pool holds up to 3450 fuel rod assemblies. The common pool holds up to 6291 fuel rod assemblies. Each assembly holds sixty-three fuel rods. This means the Fukushima Daiichi plant may contain over 600,000 spent fuel rods. The fuel rods once stored atop reactor 3 may no longer be there: one of the several explosions at the Fukushima reactors may have damaged that pool.

Now that we have partial meltdown in the reactor vessels – the part of the reactor where nuclear reactions are supposed to happen – in at least three of the Daiichi palnt’s six reactors, why bother with swimming pools for fuel rods? Simple. Even after they are no longer usable to drive nuclear fission in the reactor vessels, the “spent” fuel rods are still highly radioactive. Part of that radioactive energy is emitted as heat. That’s no surprise: heat from radioactivity is the how the reactor core vessels generate the heat that drives the nuclear plant’s turbines to generate electricity. The fuel rods don’t know whether they are in the core or in the pools: they keep emitting heat and radioactivity until the radioactive material decays into non-radioactive elements. That process can take years, which is why spent fuel rods are still dangerous years after they leave the reactor core.

How can we prevent the spent fuel rods from bursting into flame once they’re out of the reactor core? The Fukushima plant – like many other reactors – keeps the rods in water, which absorbs the heat energy. But the pools – like the water in a teakettle – will boil away unless new water is added. After the Fukushima plant lost power in Friday’s 9.0 earthquake and got hit by the tsunami, the plant was no longer able to keep the pools topped up.

How long does it take the water in spent fuel rod pools to boil down to dangerously low levels?

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More: http://my.firedoglake.com/kirkmurphy/2011/03/15/why-fukushimas-spent-fuel-rods-will-continue-to-catch-fire/

:kick:

[stockholmer]

1. K and R

[BeFree]

2. kicking

I now know more about nuke reactors than ever.
And now more than ever want all these old nuke plants closed down asap.

Problem is, when they close a plant down, they have to work for years and years to keep the sucker from blowing up. Which means outflow of cash and no income. They see it as billions down the hole. Guess who is going to have to pony up more bucks to keep GE solvent? Got a mirror?

[Vinca]

3. Rachel covered this very well last night.

In all honesty, she scared the bejesus out of me since I live about 10 miles from a nuclear plant.

[Lucinda]

4. kickety