A free neutron decays spontaneously into a proton, an electron, and an antineutrino, but a free proton does not spontaneously decay into a neutron. This is primarily because:

An unknown nucleus has a nuclear density of 2.29 × 1017 kg/m3 and mass of 19.926 × 10-27 kg. Its mass number A is approximately: (Take R0 = 1.2 × 10-15 m, 4π = 12.56)

In which stage of India's three-stage nuclear power programme do thorium-based reactors play the primary role?