The reactions inside a power plant are tightly controlled and moderated. Not like the nuclear explosions within a nuclear weapon that are out of control.
Here are the mass comparisons. Will use an estimated nuclear fuel mass for Fukishima at around 791 tons roughly. That is only the total in reactors 1-4 which includes the entire spent fuel pool contents. Worse case scenario where everything goes up in 1-4.
Now what is the mass of the nuclear fuel within a nuclear warhead ? The most powerful pure fission bomb used 60 kg of uranium to yield a 500 KTon blast. It was called the Ivy King. Shall we use that as the standard mass of fuel in a warhead exploded in the ocean ? So how many kg are in one ton ? About 907. So the total mass of nuclear fuel in Fukushima 1-4 is equal to roughly the mass of the nuclear fuel found within about 12,000 of our example nuclear warhead. And since a lot of that mass was converted to energy, we could roughly say 791 tons of nuclear fuel is approximately equal to the non-energy converted nuclear fuel in 18,000 warheads.
So lets say Fukushima so far has only given up 4 % of its #1 to #4 inventory. That would be roughly 25-30 tons of fuel. We had three reactors meltdown that contained 257 tons of fuel. And we have had reported fires in the spent fuel pools. Not to mention fuel blasted away in the explosions. Everything eventually ends up in the ocean one way or the other. So we are figuring about 10 % of the reactor fuel of # 1-3, minus what was burned or blasted out of the spent fuel pools of # 1-4. That would give us the nuclear non-energy converted fuel for 720 nuclear warheads, conservatively speaking. And that happened over a 2 1/2 year period. Much quicker then over a few decades.
This was done in haste so please double check if you can.