PHYSICS

A nucleus becomes unstable when number of neutrons in the nucleus is  more than the number of protons

Unstable hydrogen isotope contains 1 proton 2 neutrons Now, alpha particles are the particles with 2 protons 2 neutrons What do you think? As there are insufficient particles in the hydrogen isotope, no alpha particles can be released.

Nuclear fusion is the process of combination of lighter nuclei (eg. hydrogen)  to form a heavier nucleus. When two lighter nuclei come close to each other, there is strong electrostatic force of repulsion between the nuclei. So this process is difficult. To carry out this process high temperature (=10000000 K ) and pressure is required. Hence it is also called thermo-nuclear reaction.