There is much fretting over whether the MOX fuel in Fukushima Reactor 3 – if it has melt down or does melt down – is a bigger problem than the other problems.
Let’s clear up some of the issues
MOX fuel is a combination of Uranium and Plutonium inside the fuel rods. MOX was developed by France and has been used as a way to destroy weapons grade plutonium originally created for nuclear weapons.
The MOX fuel rod has 5-7% plutonium as its fission source instead of U235. The reactor waste from nuclear reactors is not currently suitable for MOX fuel without a lot of further processing (there are multiple isotopes of Plutonium in the waste), so for now MOX basically means nuclear weapons grade plutonium. Savannah River in South Carolina is sitting in 34 tons of this stuff they would like to use up. Couter-intuitively, weapons grade plutonium is less problematic than spent fuel plutonium, which decays in a few years to an isotope that generates gamma rays.
Existing nuclear reactors require modification to use MOX, and then they can only use it in a small portion of the fuel rod assemblies. Only 32 of the 514 fuel rod assemblies in Reactor #3 are MOX, according to the New York Times article.
But it is important to remember that MOX fuel is inside the containment vessel, surrounded by radiation shielding. If the MOX fuel was in the spent fuel tank without cooling, it would be much more dangerous than where it is now.
We’ve never had a meltdown before with MOX fuel inside. Used fuel assemblies also have plutonium inside them, so it’s largely a matter of degree at this point. Whether a higher percentage of plutonium in a melted pool of nuclear fuel means it is easier to go critical on its own is something I guess we’ll get to find out if they lose the cooling battle.
In retrospect, the notion of putting MOX fuel into a 1970s era BWR reactor seems really short-sighted.
Duke Energy’s Catawba power plant is the only US plant that has used MOX fuel – the plant was finished in 1985, before the freeze on new power plants after TMI and Chernobyl. Catawba is located relatively close to the Savannah River facility, which probably was a factor in the DOE wanting to use it as the plant to learn how MOX behaves in existing commercial reactors that weren’t designed to use it. Catawba is a Westinghouse Pressurized Water Reactor (PWR), not a BWR like the ones in Fukushima.
In 2005, Duke installed 4 NOX fuel assemblies in Catawba #1 to test how they would work. During the 2008 fuel cycle, Duke removed the MOX fuel assemblies and did not put them back into the reactor and ended their involvement with MOX for now. Several fuel rods were sent to Oak Ridge to see why they didn’t perform as expected.
