Melted nuclear fuel becomes granular

[jpak]

http://www.upi.com/Top_News/World-News/2011/04/15/Melted-nuclear-fuel-becomes-granular/UPI-16351302865363/

Nuclear fuel in the damaged reactors of Japan's Fukushima Daiichi nuclear plant has partially melted and settled in granular form, an analysis said Friday.

The grains have collected at the bottom of the pressure vessels of the No. 1, 2 and 3 reactors in the six-reactor plant damaged by a March 11 earthquake and tsunami, but do not pose any threat of a chain reaction, Kyodo News reported, quoting the Atomic Energy Society of Japan, an academic panel on nuclear energy safety.

An analysis by the panel said the pressure vessels have been kept at relatively low temperatures, thereby preventing any large buildup of the melted fuel grains, which could become hot and cause damage that could trigger radioactive emissions.

The panel's deputy chairman was quoted as saying it could take up to three months for the reactor fuel to be stabilized.

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[FBaggins]

1. "Granular" seems to be an odd term to use.

I presume that what they mean is that small drops of molten material resolidified when they hit the surface of the now-lower water. They then sank to the bottom.

Many have speculated as much over the weeks (even to the point of implying that it's the best way to go if you know some melting will occur), but "granular" seems much smaller than what I would expect.

[RC]

2. The fuel itself is in the form of ceramic pellets.

The fuel rod is a hollow medal tube, with the fuel inside. The tube can melt, but the fuel itself can never get hot enough to melt.
Probably what happen was the ceramic pellets disintegrated into granular form from the heat when cooling was lost.
The actual melting of the fuel is a misnomer.

[FBaggins]

3. I'm pretty sure that zirconium melts at a much higher temperature than uranium.

I don't agree that the fuel can never melt.

On edit - Scratch that. Of course it's the oxide form... which would have a higher melting point (and the cladding isn't pure zirconium).

But I still don't agree that it can't melt. :)