Question

$D_{2}O$ is used as a moderator in nuclear reactors to slow down the neutrons.
If this is true enter 1, if false enter 0.

Answer 1

Heavy water is basically used as a moderator in nuclear reactors to slow down the neutrons so that they are captured and become effective to bring about the fission reaction.
The main reason why heavy water is used as a moderator is because it captures less neutrons than the normal water.