Question

9. What is hydride gap? Why is heavy water used in nuclear reactors?

Answer 1

The metals of 7th, 8th and 9th groups in the periodic table do not form hydrides and this region of periodic table is referred to as the hydride gap.

Heavy water is used in certain types of nuclear reactors, where it acts as a neutron moderator to slow down neutrons so that they are more likely to react with the fissile uranium-235 than with uranium-238, which captures neutrons without fissioning. 

Answer 2

The metals of group 7th, 8th and 9th in the periodic table do not form hydrides so this region of periodic table is referred to as the hydride gap.