Answer:
--In the first step, a uranium-235 atom absorbs a neutron, and splits into two new atoms (fission fragments), releasing three new neutrons and a large amount of binding energy. -- In the second step, one of those neutrons is absorbed by an atom of uranium-238, and does not continue the reaction.
OR
Nuclear fission begins when a neutron hits the nucleus of an atom. The nucleus splits in two and releases thermal energy and releases more neutrons. The neutrons hits the nuclei of more atoms, splitting them and releasing more energy and neutrons.
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