Plutonium have several isotopes, which have very different half times. If original amounts of different isotopes is known it is probably possible to identify source nuclide and time stamp for fission (I don't know how accurate this method is).
Identification should be easy in this case...
This Fukushima radiation contamination calculation seems to be VERY controversial issue. There is difference between ICRP and ECRR2010 risk valuations and that difference multiplication factor is about 1000!
ECRR2010 calculations can be found on fairewinds pages, ICRP calculations (probably)...
High volume of plutonium fuel is probably spread over Fukushima power plant. I have not exact details how much of plutonium is on that fuel. I think that depends how long this uranium fuel have been on reactor.
Anyway, that plutonium generates more or less heat and emits small particles...