Nuclear power plants use fission heat instead of decay heat. Its concentration is low, but it is fission. Fission is a chain reaction. You can't stop when you start, you can only conduct heat. The low concentration is to prevent the reaction from proceeding too fast.
Atomic bombs are different. Although the atomic bomb has high uranium concentration, it did not give it initial neutrons, and the reaction did not start. The decay heat of uranium at that time was nothing. But if you bombard him with neutrons and start fission ... this heat will ...
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Most nuclear power plants and atomic bombs use uranium -235. The decay of uranium -235 is alpha decay, releasing helium nuclei instead of neutrons. If you release neutrons, it's over. Isn't that the reaction before things are made?
The reactions of nuclear power plants and atomic bombs all started from neutron sources and were artificially added. It seems to be called "primary neutron source rod" That thing is a patented product and I don't know the principle. ...