When does a nucleus become unstable?

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

Can unstable hydrogen isotope release alpha particles ?

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.

Why is nuclear fusion difficult to start?

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.