What is used as a moderator in a nuclear reactor?

In a nuclear fusion process,the mass defect is $0.05\%$. How much energy will be released from $1 \, kg$ of mass?

In an atomic bomb,the energy is released due to

The binding energy of deuteron is $2.2 \, MeV$ and that of $_2^4He$ is $28 \, MeV$. If two deuterons are fused to form one $_2^4He$,then the energy released is ......... $MeV$.

In a nuclear reactor,heavy nuclei are not used as moderators because:

$1 \, g$ of hydrogen is converted into $0.993 \, g$ of helium in a thermonuclear reaction. The energy released is