An accident in a fuel reprocessing plant, caused by improper mixing of 235^U, produced a burst of fission energy liberating energy equivalent to the detonation of 7 kg of TNT (4.2 GJ/ton = 4.6 kJ/g). About 80% of the fission products were retained in the building, (a) How many fissions occurred? (b) Three months after the accident, what is the rate (W) at which energy is released by all the fission products left in the building?