In a fusion breeder blanket, tritium bred in lithium-ceramic pebbles must be recovered fast enough to sustain the fuel cycle. Kobayashi and Oya (2018) built a tritium transport code for a single Li2_2TiO3_3 pebble that couples grain-scale diffusion with trapping/detrapping at irradiation defects, Knudsen diffusion with adsorption/desorption in opened-pores, and vacant-core/shell kinetics in closed-pores, and showed that closed-pore trapping shifts tritium release to higher temperature. The code, however, assumes uniformly sized and distributed pores (which produces an unrealistically sharp release peak), treats one isolated pebble with no bed-scale purge-gas interactions, and depends on a trapping-reaction pre-exponential factor fitted to powder data, while its Knudsen-regime transport assumption rests on design arguments rather than direct measurement. The open problem is to turn this model into a validated predictive tool for actual blanket conditions: integrate realistic pore-size and closed-pore distributions so that the experimentally observed broadened thermal desorption spectra are reproduced, extend the treatment to multi-pebble/bed scale with coupled purge-gas flow, determine the key rate constants, and validate against experimental thermal desorption data for Li2_2TiO3_3 pebbles. A 2026 follow-up has partially advanced the closed-pore component; the remaining components are open.

Background

A deuterium–tritium fusion power plant must breed its own tritium: lithium-containing 'breeding blanket' modules lining the reactor vessel convert neutrons into tritium, and the tritium breeding ratio (TBR, tritium generated per tritium burned) is expected to barely exceed unity, so nearly all bred tritium must be recovered quickly. In solid breeder concepts, the breeding material is a ceramic — lithium metatitanate (Li2TiO3Li_2TiO_3) is a leading candidate — used as sintered spherical pebbles (typical relative densities 70–90% of the theoretical crystal density) packed into the blanket and swept by a helium purge gas, usually with a small hydrogen addition. Tritium generated by neutron reactions in the ceramic grains must diffuse out of the grain, cross the pebble's pore space, reach the pebble surface, and be carried away by the purge gas; how long it is retained determines both the plant's tritium inventory (a safety quantity) and the fuel-cycle recovery rate. The standard probe of these kinetics is thermal desorption spectroscopy (TDS), in which an irradiated sample is heated at a fixed rate and the released tritium (as HT, T2T_2, or tritiated water HTO/T2OT_2O) is measured versus temperature; such experiments are usually out-of-pile, i.e. performed outside the reactor on a previously irradiated sample, as opposed to in-pile measurements during irradiation.

Sintered pebbles contain two kinds of pores with qualitatively different transport physics: opened-pores, which connect to the pebble surface, and closed-pores, which are fully enclosed by grains. Gas-phase transport in opened-pores is classified by the Knudsen number (the ratio of the gas mean free path to the pore diameter); when it exceeds about 10, transport is Knudsen diffusion, in which molecules advance by successive collisions with the pore walls rather than with each other. Tritium trapped in a closed-pore can escape only by dissociative adsorption on the pore's inner surface followed by absorption into, and diffusion through, the surrounding grains — a path with a high activation energy.

Earlier work had established grain-scale diffusion as a key rate-controlling step, and Kobayashi, Oya, Munakata and Okuno (2015) had built a grain-scale release model for Li2TiO3Li_2TiO_3 combining diffusion with McNabb–Foster type trapping/detrapping (the classical framework in which mobile solute reversibly binds at traps) at irradiation defects — specifically oxygen hole centers, O-centers, the defects that strongly interact with tritium in Li2TiO3Li_2TiO_3 — plus defect annihilation kinetics. Kobayashi and Oya (2018) extended this to the pebble scale: their code couples the grain model with opened-pore Knudsen diffusion and wall adsorption/desorption equilibrium, and with a closed-pore 'vacant-core and shell' model in which a spherical gas-filled core is enveloped by a spherical shell of grains through which absorbed tritium atoms diffuse. Their simulations reproduced known features — a dominant release peak near 580 K controlled by grain diffusion, a hydroxyl-related peak near 680 K, and a pore-trapping peak near 750 K that grows as pebble relative density decreases — and supported the interpretation that closed-pore trapping shifts release to higher temperature. Compared with the out-of-pile pebble data of Kinjyo and co-workers and the modeling comparison of Munakata and co-workers, however, the observed release extends gradually up to about 900 K, whereas the code's uniform-pore assumption produces an unrealistically sharp release peak.

The 2018 paper itself states what is missing before the code is 'applicable to the actual fusion reactor': pore sizes and distributions in real pebbles are not uniform; the code treats a single isolated pebble and ignores interactions of released tritium with other pebbles in a packed bed; the pre-exponential factor of the trapping reaction was the only free parameter and was fitted to powder release spectra; and the Knudsen-regime assumption for opened-pores was justified by design arguments (small grains, high pebble density, high temperature, reduced purge-gas pressure in pores) rather than by direct measurement.

Problem Statement

Extend and validate the tritium transport code for Li2_2TiO3_3 pebbles of Kobayashi and Oya (2018) so that it becomes applicable to an actual fusion reactor blanket, by resolving the linked deficiencies that work identifies. (1) Pore-structure realism: replace the assumption of uniformly sized, uniformly distributed opened- and closed-pores with realistic pore-size and closed-pore size distributions, so that the code reproduces the experimentally observed broadened thermal desorption spectra of Li2_2TiO3_3 pebbles instead of the sharp single release peak the uniform-pore assumption produces. (2) Scale: extend the single-pebble treatment to multi-pebble or bed-scale simulation with coupled purge-gas flow, accounting for the interaction of tritium released from one pebble with other pebbles during transport through a packed bed. (3) Parameters: determine the key kinetic parameters — trapping/detrapping rate constants (including the pre-exponential factor currently fitted to powder release spectra), adsorption/desorption rates on opened-pore surfaces, and closed-pore dissociative adsorption rates. (4) Validation: validate the extended model against experimental thermal desorption data for Li2_2TiO3_3 pebbles, such as the out-of-pile measurements in which the release rate decreases gradually up to about 900 K, before using it for reactor-scale tritium-inventory and release predictions. All four components are jointly required for a resolution of the problem; advancing any single component, in particular the pore-structure realism of (1), does not by itself resolve it.

References

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