Rare earth moderated materials

Neutrons in thermal neutron reactors need to be moderated. According to the principle of reactors, in order to achieve good moderation effect, light atoms with mass numbers close to neutrons are beneficial for neutron moderation. Therefore, moderating materials refer to those nuclide materials that contain low mass numbers and are not easy to capture neutrons. This type of material has a larger neutron scattering cross-section and a smaller neutron absorption cross-section. The nuclides that meet these conditions include hydrogen, tritium, beryllium, and graphite, while the actual ones used include heavy water (D2O), beryllium (Be), graphite (C), zirconium hydride, and some rare earth compounds.

The thermal neutron capture cross sections of rare earth elements yttrium, cerium, and lanthanum are all small, and they form corresponding hydrides after hydrogen absorption. As hydrogen carriers, they can be used as solid moderators in reactor cores to slow down neutron rates and increase the probability of nuclear reactions. Yttrium hydride contains a large number of hydrogen atoms, equivalent to the amount of water, and its stability is excellent. Until 1200 ℃, yttrium hydride only loses very little hydrogen, making it a promising high-temperature reactor deceleration material.