The MOX fuel use is recent, but he also points out that only some of the fuel rods in the reactor are MOX.

More here:
http://georneys.blogspot.com/2011/03/7th-interview-with-my-dad-nuclear.html

http://georneys.blogspot.com/2011/03/5th-interview-with-my-dad-nuclear.html#more

More bad news:
They didn’t proceed with the power lines as planned but had to stop

Still more bad news: Hydrogen over reactor 4

“Were able” was clarified to “have begun”, and “The operation was completed at 08:30 UTC.” was clarified to “The operation was continuing as of 20:30 UTC, Tokyo Electric Power Company officials told the Nuclear and Industrial Safety Agency.”

“Japan Earthquake Update (17 March 2011, 16:55 UTC) – CLARIFIED

Japanese authorities have informed the IAEA that engineers were able to lay an external grid power line cable to Unit 2. The operation was completed at 08:30 UTC. ”

So much for good news…

The U they use in MOX is probably depleted U238 which is cheap. That is the same depleted uranium used in anti-tank rounds because it is so dense (and pyrophoric when it goes through armor).

All reactors that use U238 in their fuel end up with Pu in their spent fuel. The danger from spent fuel is not from the Pu, it is from the fission products.

Chernobyl was a breeding reactor designed to make plutonium for Soviet weapons. The design was modified to generate power plus make plutonium. That was why Chernobyl used graphite as a moderator. When a neutron hits a nucleus, it recoils depositing some of its energy. Because mass and energy both are conserved, the â??bestâ? moderator would be something that has the same mass as a neutron, a proton. When there are lots of protons around (as in a water moderated reactor), the water is the moderator the neutrons slow down really fast (they start out at a couple MeV). The cross-section for fission is a pretty strong function of neutron energy, slower works better. In light water reactors, the design philosophy is to make the reactivity of the core â??stableâ?, which means if it gets hot from too much fission, you want the reactivity to go down. That is how these reactors were designed, when water in them boils, it generates a void, the void has fewer protons in it, the neutrons don’t slow down as much, fewer neutrons cause fission, the number of fissions goes down.

All breeder reactors use something other than hydrogen as a moderator. The Candu reactors use deuterium and natural uranium. Chernobyl and Sellafield used graphite. Fermi 1 which melted down and the Clinch River Breeder reactor used liquid sodium.

The â??problemâ? with using plutonium in reactors is that plutonium is really easy to make nuclear weapons from. Trivially easy because in fresh fuel, there are no nasty fission products to deal with. That â??problemâ? is only present before the fuel is put into a reactor. After a few minutes, the fuel becomes intensely radioactive (from the fission products). I think that as a rule, any MOX should be irradiated with neutrons before it leaves the fuel factory so it is intensely radioactive and can’t be picked up bare handed without lethal radiation exposure. That does increase the exposure risk for workers, but reduces the chance of diversion to weapons.

This is the entire problem with fuel reprocessing (done properly). All the processes were developed to recover plutonium for weapons, so they produce plutonium that is free from everything â??nastyâ?. All those â??nastyâ? things in there make it harder to make weapons from. The transuranics especially are difficult to remove, but essential to do so for weapons production because they have high levels of spontaneous fission, so there are always lots of neutrons floating around. That makes is a lot harder to get the supercritical Pu assembly that you need to get a good yield from a nuclear weapon. Leave those transuranics in the Pu you recycle for fuel and the fuel is somewhat safer and easier to use. You want a lot of neutrons floating around your core when you start it up.

There are 3 main isotopes of Pu that result from U238. They are Pu239, Pu240 and Pu241. Pu239 is the most desired for weapons, the others are much less desirable because they have high spontaneous fission rates. The longer Pu239 is in a reactor, the more of it becomes Pu240 and Pu241 and the crappier it is for making weapons. That is why Chernobyl was designed to be refueled while it was still running. They could put U238 in, irradiate it, take it out and reprocess it for Pu239 before it had built up much Pu240. As fuel in power reactors, they are essentially the same. Pu240 and Pu241 have much shorter half lives than does Pu239.