Ceramic Breeder Materials

4.15.1 Introduction

4.15.1.1 Tritium Breeding

The fusion reaction of tritium and deuterium is con­sidered one of the most suitable options for near­term large-scale fusion power generation, through

D + T! 4He(3.56MeV) + n(14.03MeV)

Deuterium is a hydrogen isotope with an abundance of 1 out of 6500 atoms in seawater, implying virtually boundless resources. Tritium is the next hydrogen isotope, and it is radioactive with a half-life of 12.3 years under emission of a р-particle; it cannot be obtained from natural resources. Therefore, the D-T fuel cycle requires the breeding of tritium from lithium using one of the following reactions:

n + 6Li! T + 4He + 4.78MeV

n + 7Li! T + 4He — 2.47MeV + n

The neutron supplied by the D-T fusion reaction shown above is also the one that provides useful energy. The reaction with 6Li is exothermic, providing a small energy gain; on the other hand, the reaction with 7Li is endothermic but does not consume the neutron, though a more thermalized neutron is released. Natural lithium contains 7.42% 6Li and 92.58% 7Li.

In fact, lithium has been identified as the only viable element to breed tritium. 6Li has a very high cross-section to capture a neutron (see Figure 1), and through the use of isotope enrichment, the effective
6Li density can be raised from the natural 7.42% to about any desired value.