eurogreen:

Plutonium comes from failure of primary containment subsequent to partial meltdown of fuel rods, probably those containing MOX.

I suspect a bayesian analysis will show the plutonium is more likely to come from regular fuel rods, in which

About 1% of the mass is ²³⁹Pu and ²⁴⁰Pu resulting from conversion of ²³⁸U, which may be considered either as a useful byproduct, or as dangerous and inconvenient waste.

That's after 5 years of operation, so after 1 year of operation a regular fuel rod will contain 0.2% plutonium, etc.

For instance, the spent fuel pool at reactor number 4 contained the regular amount of spent fuel as well as the full complement of fuel in the reactor. On average, 0.6% of that fuel is Plutonium.

The same is true of all the other fuel in all the other reactor buildings, except for reactor 3 which has 1/5 MOX fuel in the reactor core and none in the pool (is that right?) for an average of 1% plutonium, half of that coming from the fresh MOX in the core.

So you have 3 reactors at 0.6% and one reactor at 1%, 0.5% of which being from MOX. So the MOX plutonium is 0.5% / 2.8% = 35% of the plutonium in buildings 1-4.

So, in what may be my last act of "advising", I'll advise you to cut the jargon. -- My old PhD advisor, to me, 26/2/11