Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: A nuclear reaction, such as those which occur in a nuclear power plant, can be classified as what type of reaction?
Nuclear fission or nuclear fusion, depending on the specific reaction. Nuclear power plants primarily use nuclear fission.