I made some rough sketching of core burnup distributions assuming 12 month equilibrium cycles, 80 % availability, 6 irradiation cycles for each assembly prior to final discharge, and 170 kg uranium weight per assembly. For different batches I assumed the relative power fractions of 1.1, 1.2, 1.1, 1, 0.9 and 0.8. Roughly, for a 400-assembly unit with 1380 MWth around 140 EFPD I got the batch average burnups 3, 10, 17, 23, 28 and 33 and for a 548-assembly 2381 MWth unit about 4, 12, 21, 28, 35, and 42 MWd/kgU at the time of shutdown.
Then I took the decay heat data from burnup/shut down cooling calculations made (by someone else - credits due) for a "generic" BWR fuel assembly with different void histories, and calculated core-average power-weighted decay heats at different cooldown times. The results are in the attached file. This approach should give somewhat overestimated values, since it does not take into account the cooldown periods at refueling outages, but rather burns the fuel with constant power density starting from fresh fuel, and ending at 5, 10, 15 MWd/kgU etc., and then continuing with the cooldown calculation for 1800 days.
This is just an exercise I made in order to get some kind of an idea of the decay heats one could assume at this time. Therefore I have not had the time to do any double-checking of the results. Qualitatively, however, they seem to appear sensible. A decent decay heat calculation would follow the decay heat from each nuclide separately and take into accound the different saturation and cooldown periods during plant operation, but that's beyond my resources at the moment. It would, however, be nice to hear what kind of estimates others have for the decay heat of the earthquake-stricken reactors.
