In the majority of cases, lithium ceramic samples are irradiated in research reactors with thermal or mixed neutron spectra. Extensive studies on tritium retention in neutron irradiated lithium ceramics using the TPD
method have been reported in the literature.152-178’201 The chemical form of released tritium from Li4SiO4 (from FZK), LiAlO2 (from JAERI), Li2TiO3 (from CEA), and Li2ZrO3 (from MAPI) was studied in the out-of-pile tritium release experiment under various purge gas conditions. The pebbles were irradiated for a few minutes in the fluxes 4 x 1017m-2s-1 at Japan Research Reactor 4 (JRR-4) or 1.65-2.75 x 1017m-2 s-1 at the Kyoto University Reactor. The tritium was

released in the purge gas of dry nitrogen, nitrogen with 0.1% of helium, and nitrogen with 0.1% water vapor. Even if hydrogen was added to the purge gas, a considerable fraction of tritium was released in the molecular form of water, HTO. Addition of water vapor to the purge gas greatly enhanced the release rate of tritium. It was concluded that the isotope exchange of tritium with water at the exposed surfaces of the grains is much faster than the isotope exchange of tritium with hydrogen. The water is adsorbed at the grain surfaces from the water vapor present in the purge gas. Even small traces of water of ^30 ppm in dry purge gas canbe enough to promote tritium release in the HTO molecular form. Another source of water at the surfaces of the grains is the reduction reaction that takes place in the H2 reducing atmosphere. Among the studied materials, Li2TiO3 showed the largest water formation capacity.161 Figure 49 shows out — of-pile annealing tests for Li4SiO4 with 0.1% H2/N2 sweep gas and 0.1% H2O/N2 sweep gas, and tritium doped for 2 min at a neutron flux of 2.75 x 10 cm s.

Later work on palladium deposited as catalyst on orthosilicate indicated that almost all tritium was released as tritiated water vapor from lithium ortho­silicate pebbles and tritium at higher temperatures remains slow.160 In contrast, it was also found that a considerably larger amount of tritium was released as the molecular form (HT) from the lithium

orthosilicate pebbles already deposited with palla­dium at lower temperatures (see Figure 50).

Alvani and coworkers172,173 correlated TPD after short irradiations in the Casaccia reactor (~2 x 1021m~2) with those from long-term irradia­tion in the HFR; Petten (thermal ^0.5 x 1025m~2) revealed two peaks, at 770 and 941 K (b = 5 Kmin-1) (see Figure 51). It was proposed that the second peak is related to tritium trapping at the oxygen vacan­cies located along the grain boundary interface. The concentration of these trapping sites is signifi­cantly increased by the reduction effect of the R-gas, which results in a shift in the release peak to higher temperatures for the pretreated pebbles. The observed effect is more pronounced for the pebbles with finer

grains. It was also found that a thermal pretreatment at 473 K for 2 h removes only the environmental H2O and CO2 contamination from the surface of the pebbles without affecting the H2O desorption at higher tem­peratures. The observed H2O release above 1173 K is ascribed to the reduction of Li2TiO3 to Li2TiO3_x with x reaching a steady-state value of xeq = 0.01.

Later work by Casadio eta/.174 concerned a batch of Li2TiO3 pebbles (ENEA code FN5) prepared following the ‘citrate’ route. Analysis of TPD spectra gave the correct order of magnitude of the time constants characterizing the main desorption sites, in rough agreement with the residence times obtained by the in-pile step-perturbation methods performed during EXOTIC-8/9 experiment (see Figure 52).175 Pure helium purge increases the tritium inventory; during the last cycle of this irradiation experiment, variations of the H2 concentration in the He purge showed an increase in tritium release rate from Li2TiO3 pebbles that was found to be proportional to PH34 at 473 °C, Figure 53.

The effect of open and closed porosity on tritium release behavior in Li2O single crystal and sintered pellets was studied by Tanifuji eta/.176-179 The pellets had densities in the range of 70-98% and grain sizes from 10 to 60 pm. Irradiations were performed by ther­mal neutrons in JRR-4, JAERI, up to 2 x 1023 m~2. The porosity dependence of tritium release behavior from the Li2O sintered pellets has been investigated through isothermal heating tests, and the results are shown in Figure 54.

For 88% TD specimens irradiated up to neutron fluences of 2 1 022 and 2 x 1023nm~2, no

irradiation effects on the tritium residence time have been observed.