Question

In a nuclear reactor, in the fission process, neutrons are emitted at high energies. Neutrons are to be thermalized, i.e., their kinetic energies have to be significantly reduced, so that you can increase the probability for the new fission processes. You have a choice of a number of materials such as, say, heavy, water, iron and lead. The neutrons experience elastic collisions with the atoms of these materials and lose their energies. Assuming the target atoms are at rest initially, out of the three materials given about which one would you prefer for the efficient thermalization of neutrons? Give reasons and explanation

Answer 1

The material used for the purpose of thermalizing neutrons is called a moderator. A good moderator reduces the speed of neutrons in a small number of collisions, but does not absorb them to any great extent. Slowing the neutrons in as few collisions as possible is desirable in order to reduce the amount of neutron leakage from the core and also to reduce the number of resonance absorptions in non-fuel materials.

Given the mathematics of elastic collisions, as neutrons are very light compared to most nuclei, the most efficient way of removing kinetic energy from the neutron is by choosing a moderating nucleus that has near identical mass.The ideal moderator is of low mass, high scattering cross section, and low absorption cross section (The probability of scattering of a neutron from a nucleus is given by the scattering cross section).

A collision of a neutron, which has mass of 1, with a 1H nucleus (a proton) could result in the neutron losing virtually all of its energy in a single head-on collision. Iron and lead have large Atomic weights and the only suitable material left is the heavy water.