The continuous release of nuclear energy caused when one fission reaction triggers more nuclear reactions is a
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I believe the correct words that will complete this sentence is:

The continuous release of nuclear energy caused when one fission reaction triggers more nuclear reactions is a “chain reaction”.

Chain reaction is a reaction characterized to be one in which a product of one reaction results in another chemical reaction and so on and so forth. In this case, the nuclear energy product of the first reaction becomes a catalyst in the occurrence of the second reaction hence a chain reaction.