The energetic tritium generated by the 6Li(n,alpha)T reaction in lithium oxide (Li2O) tritium breeder material is known to be trapped at irradiation defects in two chemical states (oxygen-associated T+ and defect-associated T-), with measured detrapping and chemical-state conversion kinetics, but the final two links of the migration chain were left unquantified: the microscopic diffusion of detrapped tritium through the defect-bearing lattice to the grain surface, and the surface reactions (recombination to HT/T2, isotope exchange with purge gases) together with the purge-gas composition and flow effects that control the release yield and released chemical form. The problem is to determine these diffusion kinetics and surface release kinetics and to integrate them with the known trapping/detrapping kinetics into a quantitative model that predicts tritium migration and release from Li2O. No post-2011 experimental resolution for Li2O is known in the literature: only partial computational treatments exist for each half (kinetic Monte Carlo diffusivity with F-center traps; first-principles surface energetics), and experimental effort has moved to other breeder ceramics (Li2TiO3 and Li4SiO4, lithium metatitanate and lithium orthosilicate), leaving the experimental core open.

Background

In a deuterium–tritium (D–T) fusion reactor the tritium fuel must be bred inside the blanket: lithium-bearing materials generate tritium through the 6Li(n,alpha)T reaction, in which an absorbed neutron splits a 6Li nucleus into a helium-4 atom and a triton — a tritium nucleus born with about 2.7 MeV of kinetic energy, hence the term "energetic tritium". Lithium oxide (Li2O) was long studied as a solid breeder ceramic because it has the highest lithium atom density of the candidate oxides. After the energetic triton slows down and thermalizes, it comes to rest in the lattice and interacts with the defects that neutron irradiation creates: F centers (oxygen vacancies occupied by trapped electrons, a classic type of color center), aggregates of F centers, and oxygen interstitials. Tritium is removed from the breeder by a purge gas, normally helium with controlled additions, that sweeps released tritium out of the blanket to the extraction system; the tritium leaves the solid grain as molecular species — HT, T2 and tritiated water (HTO) — formed when diffusing tritium atoms recombine at the surface or exchange isotopes with hydrogen-bearing species in the purge gas.

For reactor design one needs a quantitative model of this whole migration chain: production and thermalization of tritium, trapping at defects, thermal detrapping, diffusion to the grain surface, surface reaction, and release into the purge stream. Radiochemical studies of neutron-irradiated Li2O by Okuno's group (Shizuoka University) established the central microscopic facts. Tritium in the lattice occurs in two dominant chemical states: a positively charged state (T+) associated with oxygen, and a negatively charged, tritide-like state (T-) associated with lithium-site defects and specifically with F centers and their aggregates. During thermal annealing the two states interconvert, and the conversion kinetics were measured in isothermal annealing experiments at 449–549 K and expressed as first-order Arrhenius rate laws k(T)=Aexp⁡(−Ea/kBT)k(T)=A\exp(-E_{\mathrm{a}}/k_{\mathrm{B}}T), where EaE_{\mathrm{a}} is the activation energy and kBk_{\mathrm{B}} the Boltzmann constant. The same body of work tied these states to irradiation-defect dynamics: Frenkel pairs (vacancy–interstitial pairs) recombine at roughly 300–400 K, F centers migrate and aggregate into clusters stable between about 400 K and 500 K, and those aggregates decompose above about 500 K, releasing the trapped tritium.

Before and alongside this work, diffusion and release had been characterized only at the macroscopic level: tritium diffusion coefficients were measured in Li2O single crystals in the 1980s, before the role of irradiation defects was quantified; in-pile (in-reactor) and ex-reactor release experiments (such as the BEATRIX-II and CRITIC-I irradiation programs) mapped the release behavior of sintered pellets; modeling groups analyzed the order of the surface adsorption and desorption reactions from that legacy data and introduced surface-transfer steps into release models; and molecular-dynamics simulations of lithium and tritium diffusion in Li2O existed. What none of this provided was a closed, quantitatively predictive chain: the measured chemical-state kinetics stop at detrapping, while the two subsequent links — the microscopic diffusion of the detrapped tritium through the defect-bearing lattice to the surface, and the surface reactions with their purge-gas dependence that convert it into released HT/T2/HTO — were identified in the 2011 assessment by Okuno, Kobayashi, Kurata and Oya as the unmeasured work needed to complete the model.

Problem Statement

Two coupled kinetic gaps must be closed to complete a quantitatively predictive model of tritium migration and release in Li2O breeder material, as identified in the concluding assessment of the source study (Okuno, Kobayashi, Kurata and Oya, 2011):

(1) Diffusion of detrapped tritium. Determine the microscopic diffusion coefficients and migration mechanisms by which tritium, once detrapped (for example from F-center aggregates that decompose above about 500 K), moves through the Li2O lattice — via oxygen sites, lithium-site substitutions and lattice or vacancy pathways — to reach the grain surface, so that the coupling between detrapping events and the net release rate is quantified rather than assumed.

(2) Surface release chemistry. Determine the surface reactions that convert diffusing tritium atoms into released molecular species — recombination to HT and T2, and isotope exchange with hydrogen-bearing purge-gas components — together with the influence of purge-gas composition and flow rate on the release yield and the chemical form of the released tritium.

The objective is to integrate these two quantified links with the already-measured trapping/detrapping and defect-dynamics kinetics into a single model that quantitatively predicts tritium migration and release from Li2O — retained inventory, release rate, and released chemical species as functions of temperature and purge-gas conditions — and to validate that model against experimental release data. The two components are scientifically linked as successive steps of one release chain (detrapping, diffusion, surface reaction).

References

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