4.15.5.1 Tritium Release

A wide range of mechanisms play a role in the tritium transport and release processes of the lithium — containing ceramics’ of which an impression is given in Figure 48.1 2 Tritium generated from neutron cap­ture is first transported to the grain boundary by bulk diffusion. The bulk diffusion and trapping inside the grains are affected by the neutron radiation-induced defects. Via the intergranular diffusion, the tritium is then delivered to the grain surfaces’ which are exposed to open and closed porosity. The closed porosity frac­tion provides another means to build up inventory in

the material. At the surface isotope exchange with hydrogen (H2) and water (H2O) lead to desorption of tritium in molecular forms of HT and HTO, respec­tively. Further, the tritium in molecular form is trans­ported through the interconnected pores and enters the flow of the purge gas. In order to assess the tritium retention in the candidate ceramic breeder material, one needs to know which of the steps are rate deter­mining and which operation parameters are the most relevant for facilitation of the tritium release (Table 3).

The tritium release characteristics oflithium cera­mics are typically studied in two parameter ranges:

1. Out-of-pile. Tritium production through exposure to neutron irradiation, followed by out-of-pile tri­tium desorption through stepwise isothermal or ramp annealing tests in laboratory setups, also known as temperature programmed desorption (TPD). If irradiation doses are very low, such activity is typically called ‘tritium doping,’ and tritium transport parameters reflect beginning of life (BOL) conditions only because irradiation damage and lithium burnup remain negligible.

2. In-pile: In case of in-pile experiments, typically steady-state tritium production and release condi­tions. In general, such parameters are closer to breeding blanket conditions, as they allow the application of a wide range of temperatures and purge gas conditions, and the study of long-term performance issues such as irradiation damage and lithium burnup. At present, such data are limited in terms of fast neutron damage doses (thermal and mixed spectra materials test reactor (MTR) only).