Tritium Diffusion Research Articles

Tritium diffusion and trapping associated with lithium in Al-Li systems

The chemical states and diffusivity of tritium in Al-1.1wt.%Li alloy, intermetallic LiAl and pure aluminium metal were investigated with a radio-chemical method. Most of the tritium incorporated into these materials existed in the T − state and was released in the form of HT when the materials were heated in a vacuum. The diffusivities for tritium determined from the HT release measurement were D = 7.9 × 10 −2 exp(−72.4 kJ mol −1/ RT) cm 2 s −1 in the alloy, D = 1.3 × 10 −3 exp(−63.9 kJ mol −1/ RT) cm 2 s −1 in LiAl and D = 2.0 × 10 −2 exp(−53.4 kJ mol −1/ RT) cm 2 s −1 in aluminium. The observed diffusivities for tritium in Al-1.1wt.%Li and LiAl were approximately 1 and 2 orders of magnitude lower than that in aluminium, depending on the concentration of lithium atoms in the materials. The impeded diffusion of tritium was explained in terms of a trapping effect associated with lithium in the material.

In-pile tritium extraction from samples of lithium aluminate

Several in-pile irradiation experiments on samples of lithium aluminate provide tritium retention and release data. Such as for all tritium breeders, the interpretation of results is not straightforward. All the results are not explained if only the kinetic contributions to the tritium inventory, diffusion or mass transfer resistance at the solid-gas interface, are considered. In particular, it is necessary to consider tritium adsorption at grain surfaces in order to explain that, at higher temperatures, small grain samples have a higher tritium inventory than samples with large grains. However, at lower temperatures, tritium retention is lower with smaller grains, and above 450°C and with the addition of hydrogen to the sweep gas, the tritium residence time is less than one day. Some nonreproducibility in the results shows that parameters playing a role in the tritium release are not all clearly identified. A more accurate characterization of the surface state of the samples is required.

Chemisorption and implantation of hydrogen isotopes in lithium-based fusion blanket ceramics and their thermal release: A study by nuclear reaction analysis

A brief summary is presented on tritium implantation techniques and depth profiling analysis by elastic recoil detection (ERD) and T(d, α)n nuclear reaction analysis (NRA), as applied to the studies of lithium ceramic materials to be used as tritium-breeding blankets in fusion reactors. New data about thermal behaviour of hydrogen and tritium near the surface of both poly- and monocrystalline Li 2O, and ceramic γ-LiAlO 2, are discussed. Ion beam techniques are especially suitable for studying near-surface diffusion and chemisorption of tritium, as well as for modelling the influence of irradiation damage on tritium transport phenomena.

The kinetics that govern the release of tritium from neutron-irradiated lithium oxide

We present a kinetic model that describes the release of tritium from a neutron-irradiated lithium oxide crystal. According to our model, the release is 1. (1) controlled by diffusion of tritium through the crystal when ak D > 10 2. (2) controlled by the desorption of tritiated water from the crystal when ak D < 1 3. (3) controlled by a combination of these processes when 1 < ak D < 10 , where a is a characteristic crystal size, k is the governing desorption rate constant, and D is the governing diffusion coefficient. Our model explains the lack of agreement among prior studies because it predicts both diffusion-controlled and desorption-controlled kinetics. As an initial test of this model, we performed preliminary release experiments in the desorption-controlled region. As expected, the observed release was consistent with a desorption-controlled theory. In addition, we found k to be (6.5 × 10 2e −(019kJ/mol)/RT cm/s.

Tritium release from neutron-irradiated Li 2O: Diffusion in single crystal

The release of tritium from neutron-irradiated Li 2O single crystal particles with sizes of 150 to 840 μm was measured by isothermal annealings. Time dependence of the release rate of tritium for various sizes of particles was well analyzed by a model of classical diffusion in a solid sphere. The diffusivity of tritium obtained was expressed in the temperature range from 573 to 950 K by ( D T / cm 2· s −1) = 0.116 exp(−(101±2) × 10 3 J/ RT).

Investigation of the electromigration of tritium in tantalum

The diffusion and electromigration of tritium in TaHT systems in α and ϵ phases has been investigated. The diffusion coefficient and the ratio of the effective valency to the thermodynamic factor ƒ were determined from radioactivity measurements of tritium. It has been found that during the phase transformation α → ge the effective valency changes in sign from positive to negative.

Measurement of 10B (n,t) 2α Reaction Gross Section in the Energy Range of 2.5 to 10.6 MeV: Diffusion of Tritium in Boron Carbide

Measurement of ¹⁰B (n,t) 2α Reaction Gross Section in the Energy Range of 2.5 to 10.6 MeV: Diffusion of Tritium in Boron Carbide

The time dependence of in-situ tritium release from lithium oxide and lithium aluminate (VOM-22H experiment)

An in-situ tritium recovery experiment was conducted in the JRR-2 reactor on Li 2 O and LiAlO 2 spheres. A technique for obtaining tritium diffusion coefficients from the time dependency of tritium release after incremental temperature changes was developed. The tritium retention in LiA1O 2 was higher than in Li 2 O for the same temperature. The apparent diffusion coefficients for LiAlO 2 at temperatures up to 900°C was one order magnitude higher than the previous, lower temperature TRIO data.

Tritium Release from a Lithium Ceramic: Interpretation of the Trio Experiment

A new interpretation of the in situ TRIO experiment on tritium release from γ-LiAlO2 ceramic particles is presented. It is suggested that, due to a very small ceramic particle radius (R = 0.1 µm), the release-controlling phenomenon is desorption of tritium-containing molecules from the particle surface rather than tritium diffusivity in the particle. This hypothesis eliminates the large discrepancy with previous tritium diffusivity experiments with large particles, where the time-controlling phenomenon is clearly diffusivity.

Diffusion-controlled tritium release from neutron-irradiated γ-LiAlO 2

Thermal release behavior of tritium produced in neutron-irradiated γ-LiAlO 2 crystals has been studied with emphasis laid on the tritium release rate. All of the tritium produced in γ-LiAlO 2 was released on heating to 1170 K under vacuum. The predominant tritium species released was tritiated water (HTO). The HTO(g) release rate was controlled by diffusion of tritium (T +) and the diffusion coefficient determined in the temperature range from 630 to 930 K was D = 2.1 × 10 −5 exp( −90.3 RT )cm 2s −1 , where probable errors were ± 1.0 kJ mol −1 for the activation energy and ± 0.1 for the logarithm of the pre-exponential term. The diffusion coefficient for tritium in γ-LiAlO 2 was nearly two orders of magnitude smaller than that in Li 2O crystals.

Diffusion of hydrogen, deuterium, and tritium on the (100) plane of copper: Reaction-path formulation, variational transition state theory, and tunneling calculations

Diffusion of hydrogen, deuterium, and tritium on the (100) plane of copper: Reaction-path formulation, variational transition state theory, and tunneling calculations

Diffusion of hydrogen, deuterium, and tritium on the (100) plane of copper: Reaction-path formulation, variational transition state theory, and tunneling calculations

We have applied variational transition state theory in a reaction-path formulation, using semiclassical methods of calculating multidimensional tunneling probabilities, to the surface diffusion of H, D, and T on the (100) surface of fcc Cu cover the temperature range 100–1000 K using the empirical pairwise interaction potential of Gregory, Gelb, and Silbey. We show that the reaction-path formalism offers a tractable way to deal with multidimensional motion of an adsorbate across a many-atom substrate. Our calculations show that the variational transition states for these systems are located at the saddle point for all temperatures studied. We find very large quantal effects, due to zero point quantization and tunneling, with the largest effects, over four orders of magnitude, for H at 100 K. At this temperature, tunneling alone increases the surface diffusion coefficient by three orders of magnitude. Even at 140 K, the tunneling effect increases the surface diffusion coefficients of H, D, and T by factors 3.0, 1.6, 1.3, respectively. Unfortunately, there are no experimental data available for this system for comparison, but in light of the very large quantal effects, we are encouraged by the reasonably good agreement of our results for T ⩾ 110 K with those of Valone, Voter, and Doll, who included quantal effects by a completely different method.

Diffusion and Solubility of Tritium in Path a Prime Candidate Alloy

The tritium recoil injection technique was used to determine the tritium diffusion coefficient for Path A Prime Candidate Alloy (PCA) over the temperature range 323K to 523K. Dissolution tritium counting of samples saturated in a tritium-hydrogen mixture was used to determine the tritium solubility for PCA over the temperature range 673K to 973K. No evidence of high energy trapping due to the titanium carbide precipitates in PCA was seen.

Solid Tritium Breeder Materials-Li2O and LiAlO2: A Data Base Review

The fabrication, properties, and irradiation behavior of Li2O and γ-LiAlO2 are reviewed and assessed to determine the potential of these materials to satisfy the basic solid breeder blanket performance requirements. Based on the data analysis and theoretical modeling, a set of major technical uncertainties is identified. These uncertainties include1. fabricability of sphere-pac solid breeders2. highfluence and burnup effects on thermal conductivity and microstructural stability3. highfluence and burnup effects on tritium diffusion coefficients at low temperature4. relationship among purge flow chemistry, surface adsorption, and species of released tritium5. mechanical properties and the loads imposed on the structural materials by the breeder during blanket operation.Resolution of these issues is important in assuring that solid breeder blankets can be designed with confidence.

Surface reaction and bulk diffusion of tritium in SUS-316 stainless steel

Diffusivities of tritium in SUS-316 stainless steel were measured by a gas-absorption method in the temperature range from 603 to 853 K. The measurements were carried out on clean and modified surfaces, which were carefully prepared by well characterized treatments. The diffusion behavior of the clean surface is successfully described by a simple diffusion equation and the bulk diffusion coefficient of tritium was determined as D(cm 2/s) = 4.2 × 10 2 exp( −64 ( RT kJ ) ) . The diffusion process for the modified surface was not explained by a simple diffusion equation but by a diffusion equation having an induction period. The microscopic explanation of the induction period is given.

Kinetics and mechanism of tritium release from neutron-irradiated Li 2O

The chemical fate of tritium produced by the 6Li(n, α)T reaction in Li 2O has been studied, with the emphasis on the thermal release behavior of the tritium. It was found that the tritium was released mainly in the chemical form of tritiated water (HTO) via the thermal decomposition of LiOT at the solid surface. At temperatures above 570 K the overall HTO(g) release rate was controlled by bulk diffusion of cationic tritium (T +), while the thermal decomposition of LiOT was the rate-determining step at lower temperatures ( < 570 K). The diffusion coefficients and the rate constants for the thermal decomposition are reported.

Tritium Percolation, Convection, and Permeation in Fusion Solid-Breeder Blankets

Models are developed to describe the steadystate percolation of tritium through the solid-breeder interconnected porosity to the purge stream, convection of tritium by the helium purge stream, and permeation of tritium through the structural material to the primary coolant system. Important parameters in the models are tritium generation rate, breeder microstructure, tritium species in the gas phase, temperatures, tritium diffusivities and permeabilities, and effectiveness of oxide barriers. The models are used to perform calculations for fusion-blanket conceptual designs. The results indicate that for isotopic swamping of the purge stream with protium and relatively high oxide impedance factors 100 for nonbreeder-side surfaces) the leakage rate from the blanket can be limited to less than 100 Ci/day for reasonable purge flow rates and pressure drops. However, for lower impedance factors and/or for decreased protium partial pressure in the purge, problems do arise with limiting the tritium leakage.

Diffusion and solubility of tritium in path a prime candidate alloy

The tritium recoil injection technique was used to determine the tritium diffusion coefficient for Path A Prime Candidate Alloy (PCA) over the temperature range 323K to 523K. Dissolution tritium counting of samples saturated in a tritium-hydrogen mixture was used to determine the tritium solubility for PCA over the temperature range 673K to 973K. No evidence of high energy trapping due to the titanium carbide precipitates in PCA was seen.

Temperature dependence of [formula omitted] for protium, deuterium and tritium in niobium

The thermal diffusion of protium, deuterium and tritium in niobium was investigated experimentally at average temperatures of about 70, 150 and 250 °C. The redistribution of hydrogen isotopes was examined as a function of the duration of the thermal gradient. The values of the heat Q ∗ of transport and the self-diffusion coefficient D were determined by analysing these data. It was shown that the values of Q ∗ decrease linearly with temperature and that the temperature coefficients for protium, deuterium and tritium are almost equal. These results are discussed on the basis of the biased diffusion model, and it is shown that the bias effect decreases with temperature. The diffusion coefficients determined for protium, deuterium and tritium are compared with those based on the Gorsky effect and good agreement is observed.

Studies on the permeation of Hydrogen and Tritium through heat resistant alloys

At temperatures of 750 to 950°C the permeation of hydrogen through bare, heat resistant alloys was studied from 1 to 40 bar, the diffusion and permeation of tritium in the partial pressure range of 10 −4 bar. Among the alloys studied were Incoloy 800, 800 H, 802, Inconel 617, 625, the Ni-based alloy Nimonic PE 13, and several non-commercial steels. At a given temperature, the permeation rate through samples with clean surfaces — substantially free of oxide films — was found to be proportional to the square root of the hydrogen pressure. The C and the Cr contents of the steels investigated proved to exert a strong influence on the activation energies of diffusion and permeation. In some alloys a reduction of the permeation rate was observed that can be attributed to the precipitation of intermetallic phases. In one case a decrease of the permeation rate occurred during the measurements due to a phase transformation.