In a nuclear reactor, lowering control rods will result in

Slowing down the fission reaction rate.

Explanation:

In a nuclear reactor, nuclear fission reactions occur. Fission process is a type of nuclear reaction in which a big nucleus disintegrates into smaller fragments, with the release of lot of energy. This fission process is a chain reaction process happening in the nuclear reactor.

To prevent the nuclear chain reaction going beyond control, control rods that are made up of Boron are used. When these are pushed in (or lowered), the neutrons are absorbed and the chain reaction is slowed down or stopped entirely, as required.

When they are pulled out slightly again, the chain reaction can be accelerated. To summarize, the control rods control the rate of fission reactions.

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