Diffusion Coefficient Of Tritium Research Articles

Retention and Release of Tritium in B4C Irradiated as Control Rod of Fast Breeder Reactor

The release and retention of tritium generated in FBR control rods of B4C irradiated to high fluence were investigated for temperatures up to 800°C. The amounts of tritium generated in B4C specimens were released by reacting each specimen with a solid NaOH. Released tritium gas was introduced into water and then its radioactivity was measured with a liquid scintillation counter. The amounts of retention were expressed as a function of fluence, and the results agreed approximately with the theoretically calculated values. In addition, the amounts of tritium could be approximately evaluated by simply measuring the radioactivity of 54Mn using a germanium semiconductor detector, which is generated from Fe impurity in B4C. The temperature at which the tritium begins to be released from B4C was investigated by heating a specimen at a constant temperature increasing rate. It was found in this experiment that the tritium begins to be released from B4C at 400°C. From the results of this isochronal heating, diffusion coefficients of tritium in B4C were derived. Their temperature dependence was given using the following equation. The activation energy was expressed in J/mol and this result agreed approximately with results reported by other researchers.

Modeling of diffusivity of tritium interacting with F centers in Li2O

The diffusivity of tritium interacting with F centers in Li2O was modeled with a kinetic Monte Carlo simulation and classical diffusion theory. The migration energy and trapping/detrapping energy were set according to the previous studies using quantum mechanical calculations. The constructed mode clearly showed that the tritium diffusivity was lowered by F centers. It was also shown that the diffusion coefficient of tritium interacting with F centers depended not only on the concentration of F centers and on the trapping/detrapping energy, but also on the concentration of hydrogen isotopes (H+D+T). It was indicated that inclusion of hydrogen isotopes (H2, D2, etc.) in purge gas would suppress the influence of radiation defects on the tritium diffusivity, and thus reduce the uncertainty, which is caused by radiation defects, in tritium recovery rate under fusion reactor conditions.

Tritium transport studies with use of the ISEP NPA during tritium trace experimental campaign on JET

The neutral particle analyzer (NPA) known as ISEP (Ion SEParator) was applied to measure the tritium neutral flux during the tritium trace experiment (TTE) on JET. The energy dependence (in the 5–28 keV energy range) of the tritium neutral flux rise time after a short ∼100 ms tritium gas puff into deuterium plasmas has been observed for the first time. The dependence has been interpreted as being due to the penetration of the tritium ions from the plasma boundary into the core and has been used for the calculation of the tritium diffusion coefficient and convective velocity values.

Immobilization of tritium-containing oil wastes by their incorporation in a cement matrix

Tritium leaching from cement-based composites incorporating spent tritium-containing oil sorbed on zeolites, active alumina, or activated carbon was studied. Activated carbon is a preferable sorbent compared to the other examined materials, and with this sorbent the rate and degree of tritium leaching were the lowest. The leaching is due to diffusion mobility of water in the cement matrix. The tritium diffusion coefficient in the system is D = (4.7 ± 0.3) × 10−11 m2 s−1.

Optimisation of operating parameters for plasma facing components detritiation

Within the framework of management of waste produced by fusion facility, the presence of high tritium content in large pieces is very specific. This question is very sensitive in the case of maintenance of plasma facing components (PFC). In order to reach several goals (tritium recovery, limitation of tritium content in the waste), several treatment methods have been analysed and thermal detritiation is selected as a reference process. In the case of ITER facility a tritium recovery station based on this technique is foreseen. In order to better understand which parameters have important influence on the efficiency of the treatment (detritiation factor), a recall of the process principle and of the main data concerning tritium behaviour in different materials is done. For example, low tritium diffusion coefficient in Be which is the material of the first layer on some components can limit drastically the process performance. In this paper a recall of the different process parameters is presented. Some important parameters are selected in order to lead a sensitivity study on adequate variation domains. It allows studying several scenarios of treatment and analysing the consequences of them. The main parameters of this study are the level of temperature, the duration of treatment, the renewal rate of the furnace atmosphere, the presence of Be or not. The main response to this sensitivity analysis is the detritiation factor, however some other consequences such as tritiated water flow rate and also tritium concentration in water are evaluated. Then the consequences of change of operating parameters are compared in order to identify what could be the most appropriate operating domain.

Effect of Coexistent Hydrogen Isotopes on Tracer Diffusion of Tritium in Alpha Phase of Group-V Metal-Hydrogen Systems

Tracer diffusion coefficients of tritium in the alpha phase of group-V metal-hydrogen systems, � - MH(D)xTy (M ¼ V and Ta; xy), were measured in order to clarify the effects of coexistent hydrogen isotopes on the tritium diffusion behavior. The hydrogen concentration dependence of such behavior and the effects of the coexistent hydrogen isotopes (protium and deuterium) were determined. The results ob- tained in the present (for V and Ta) and previous (for Nb) studies revealed that tritium diffusion was def- initely dependent on hydrogen concentration but was not so sensitive to the kind of coexistent hydrogen isotopes. By summarizing those data, it was found that the hydrogen concentration dependence of the trac- er diffusion coefficient of tritium in the alpha phase of group-V metals could be roughly expressed by a single empirical curve.

Effect of Coexistent Hydrogen Isotopes on Tracer Diffusion of Tritium in Alpha Phase of Group-V Metal-Hydrogen Systems

Tracer diffusion coefficients of tritium in the alpha phase of group-V metal-hydrogen systems, α-MH(D)xTy (M = V and Ta; x >> y), were measured in order to clarify the effects of coexistent hydrogen isotopes on the tritium diffusion behavior. The hydrogen concentration dependence of such behavior and the effects of the coexistent hydrogen isotopes (protium and deuterium) were determined. The results obtained in the present (for V and Ta) and previous (for Nb) studies revealed that tritium diffusion was definitely dependent on hydrogen concentration but was not so sensitive to the kind of coexistent hydrogen isotopes. By summarizing those data, it was found that the hydrogen concentration dependence of the tracer diffusion coefficient of tritium in the alpha phase of group-V metals could be roughly expressed by a single empirical curve.

Tritium Diffusion in V-4Cr-4Ti Alloy

Characteristics of the tritium diffusion coefficient DT in V-4Cr-4Ti alloy, including a bending in the Arrhenius plot of DT, are examined. Based on a trap model, the possible trap sources and their binding energies for tritium in the alloy are evaluated using the experimental data of DT in pure V, which are measured with a tritium tracer method, and the literature data of protium diffusion in V-Ti and V-Cr alloys. The result of the evaluation suggests the presence of two trap sources in the alloy. The first would be attributed to a trap at each substitutional alloying atom which is likely to be Ti. The binding energy EB of 0.08 eV gives the best fit to the observed value of DT above 300 K. The bending in the Arrhenius plot below 300 K is caused by a second trap site with a higher EB, and a lower concentration than those of each alloying atom. The trap is probably formed by the alloying atoms presence to neighboring Ti atoms. The contribution of Cr atom to the trap effect seems to be rather small in this alloy.

Temperature-dependent diffusion coefficients fromab initiocomputations: Hydrogen, deuterium, and tritium in nickel

The temperature-dependent diffusion coefficients of interstitial hydrogen, deuterium, and tritium in nickel are computed using transition state theory. The coefficient of thermal expansion, the enthalpy and entropy of activation, and the pre-exponential factor of the diffusion coefficient are obtained from ab initio total energy and phonon calculations including the vibrations of all atoms. Numerical results reveal that diffusion between octahedral interstitial sites occurs along an indirect path via the metastable tetrahedral site and that both the migration enthalpy and entropy are strongly temperature dependent. However, the migration enthalpy and entropy are coupled so that the diffusion coefficient is well described by a constant activation energy, i.e., $D={D}_{0}\phantom{\rule{0.2em}{0ex}}\mathrm{exp}[\ensuremath{-}Q∕(RT)]$, with $Q=45.72$, 44.09, and $43.04\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕\mathrm{mol}$ and ${D}_{0}=3.84\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$, $2.40\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$, $1.77\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ for H, D, and T, respectively. The diffusion of deuterium and tritium is computed to be slower than that of hydrogen only at temperatures above $400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At lower temperatures, the order is reversed in excellent agreement with experiment. The present approach is applicable to atoms of any mass as it includes the full coupling between the vibrational modes of the diffusing atom with the host lattice.

Diffusional behavior of tritium in V–4Cr–4Ti alloy

Tritium diffusion behavior in a V–4Cr–4Ti (NIFS-Heat-2) alloy has been examined with a tritium tracer technique. Firstly, a small amount of tritium (T) was implanted into the specimen surface, and then the specimen was diffusion-annealed at temperatures ranging from 373 K to 573 K. The diffusion depth profile of T in the specimen was measured with a tritium imaging plate (IP) technique to determine the diffusion coefficient. The obtained diffusion coefficient of tritium in V–4Cr–4Ti is expressed as D t ( cm 2 / s ) = ( 7.5 ± 0.2 ) × 10 - 4 exp ( - 0.13 ( eV ) / kT ) , which is lower than that in pure vanadium, and is comparable with literature values of protium in a V–4Ti alloy taking the isotope mass effect into consideration.

Diffusion and trapping of tritium in vanadium alloys

Tritium diffusion in a vanadium alloy (V–4Cr–4Ti) has been investigated at temperatures ranging from 230K to 573K. Tritium was loaded into the surface layers of the alloy specimen with an ac-glow discharge. Before and after diffusion annealing of the specimen, tritium diffusion profiles were measured by means of an imaging plate (IP) technique. Tritium diffusion coefficients (DT), which were evaluated by fitting a numerical solution of the diffusion geometry employed here to the obtained diffusion profiles, were a little smaller than those for pure V with the activation energy of 0.13±0.01eV. Below 320K, in addition, the Arrhenius plot of DT bent downwards showing a larger activation energy of 0.19±0.01eV, probably owing to the trapping effect of both of Cr and Ti. The effect of alloying elements on tritium diffusion and the influence of tritium release from the surface were discussed.

ＳＵＳ３１６からのトリチウムの脱離挙動

To understand the release mechanism of tritium from solid materials, release rate of tritium was measured when a tritium loaded 316 stainless steel specimen was put in dry argon gas flow of atmospheric pressure at room temperature. During blowing of argon gas released products from the specimen were collected in water bubblers which were set in the downstream of the blowing circuit, and the tritium content in the bubbler water was periodically measured as a function of time by scintillation counter. More than 99% of released species from the specimen was tritiated water, HTO. The measured result of tritium release rate showed that tritium is released chronically for a long time. The chronic release rate of tritium was evaluated using the diffusion model reported by Calder and Lewin, and it was found that when a reasonable value for the bulk diffusion coefficient of tritium is assumed, the tritium release rate can be described with the diffusion flux at the surface boundary of the specimen.

Study of tritium migration in liquid Li 2BeF 4 with ab initio molecular dynamics

We investigate the behavior of tritium in liquid Flibe (Li 2BeF 4) with molecular dynamics techniques. A major objective of the work is to estimate the diffusion coefficient of tritium. Preliminary results are reported in this paper. Ab initio molecular dynamics calculations yielded ionic distances of the molten salt in good agreement with values from the literature. A tritium ion arbitrarily introduced into the molten salt sample stayed bonded to a fluorine ion for the entire simulation run and BeF 4 2 − molecules were observed as a result of the calculation. The extracted diffusion coefficients from mean square displacements of Li, F, and T were lower than experimental values found in the literature.

Application of tritium radioluminography to the detection of hydrogen diffusion in Ti-Cr alloy

Tritium radioluminography has been applied to determine the hydrogen (H) diffusion coefficients in Ti50Cr50 and Ti40Cr60 alloys, both with two-phase structures of a body-centered cubic (bcc) phase and a Laves phase. Using radioluminography, H distributions in those phases have been observed, and H penetration profiles have been measured. The tritium diffusion coefficients in the Ti50Cr50 and Ti40Cr60 alloys have been successfully determined by analyzing the tritium penetration profiles to be 3.3±0.3 × 10−12 and 1.4±0.1×10−12 m2/s, respectively. These values suggest that the H diffusion in the Laves phase is slower than that in the bcc phase. It is also suggested that H diffuses by short-circuiting diffusion through the interface between the bcc phase and the Laves phase.

Density dependence of trace tritium transport in H-mode Joint European Torus plasma

Tritium transport in edge localized mode (ELM) high confinement (H-mode) plasmas is analyzed here as a function of density for discharges from the recent trace tritium experimental campaign performed on Joint European Torus. In this campaign small amounts of tritium have been puffed or injected (with neutral beam injectors) into deuterium plasmas [K.-D. Zastrow, J. M. Adams, Yu. Baranov et al., Plasma Phys. Controlled Fusion 46, B255 (2004)]. Information about the tritium has been obtained from the evolution of the profiles of neutron emission simulated via the TRANSP [R. J. Goldston, D. C. McCune, H. H. Towner, S. L. Davis, R. J. Hawryluk, and G. L. Schmidt, J. Comput. Phys. 43, 61 (1981)] and SANCO (L. Lauro-Taroni, B. Alper, R. Giannella, K. Lawson, F. Marcus, M. Mattioli, P. Smeulders, and M. Von Hellermann, Proceedings of the 21st European Conference on Controlled Fusion and Plasma Physics, Montpelier, France, 1994) codes. A strong inverse correlation of tritium transport with plasma density is found in this analysis. The low tritium transport at high density is close to neoclassical values while the transport becomes strongly anomalous in low density plasmas. The thermal transport does not exhibit such a strong density dependence, leading to a varying ratio of thermal to tritium transport in these discharges. An interpretation of the density effects on the trace tritium transport, partially based on the test particle simulations in plasmas with stochastic magnetic field, is proposed. A simple model for the tritium diffusion coefficient and convective velocity, which includes the modification of the neoclassical particle diffusion in presence of electromagnetic turbulence [A. I. Smolyakov and P. N. Yushmanov, Nucl. Fusion 35, 383 (1993)] completed with an empirical density dependence, is developed. This model has positive β dependence in agreement with the results of the similarity experiments performed for trace tritium transport.

Tritium transport behavior in the molten lithium–tin alloy

Using the in-reactor tritium release experiments, the tritium transport behavior in the Li 20Sn 80 alloy was studied, including the tritium diffusion in the alloy and the mass-transfer at the interface between the alloy and the gas phase. The tritium diffusion coefficient is 1.27×10 −9 m 2/s at 873 K. The mass-transfer coefficient obeys the following temperature dependence: K D = 2.82 × 10 − 4 exp ( − 27.2 ( kJ / mol ) / R T ) ( m / s ) ( 673 – 873 K )

Tritium transport experiments on the JET tokamak

An overview is given of the experimental method, the analysis technique and the results for trace tritium experiments conducted on the JET tokamak in 2003. Observations associated with events such as sawtooth collapses, neo-classical tearing modes and edge localized modes are described. Tritium transport is seen to approach neo-classical levels in the plasma core at high density and low q95, and in the transport barrier region of internal transport barrier (ITB) discharges. Tritium transport remains well above neo-classical levels in all other cases. The correlation of the measured tritium diffusion coefficient and convection velocity for normalized minor radii r/a = [0.65, 0.80] with the controllable parameters q95 and plasma density are found to be consistent for all operational regimes (ELMy H-mode discharges with or without ion cyclotron frequency resonance heating, hybrid scenario and ITB discharges). Scaling with local physics parameters is best described by gyro-Bohm scaling with an additional inverse beta dependence.

The effect of helium generation and irradiation temperature on tritium release from neutron irradiated beryllium

The effect of neutron irradiation condition on tritium release from beryllium is described in this paper. Beryllium samples were irradiated in the SM reactor with neutron fluence ( E>0.1 MeV) of (0.37–2.0) × 10 22 cm −2 at 70–100 and 650–700 °C. Mass-spectrometry technique was used in out of pile tritium release experiments during stepped-temperature anneal within a temperature range from 250 to 1300 °C. The total amount of helium accumulated in irradiated beryllium samples varied from 521 to 3061 appm. The first signs of tritium release were detected at temperature of 406–553 °C. It was shown that irradiation temperature and helium generation level significantly affect the tritium release. A fraction of 44–74% of tritium content in samples irradiated at low temperature (70–100 °C) is released from beryllium at an annealing temperature below 800 °C, whereas for samples after high temperature irradiation (650–700 °C) tritium release did not exceed 14%. Majority of tritium (∼68%) is released within a temperature range from 800 to 920 °C. The increase of helium generation from 521 to 3061 appm results in lowering the temperature of maximal tritium release rate and the upper temperature of tritium release from beryllium by 100–130 and 200–240 °C, correspondingly. On the basis of data obtained, the diffusion coefficients of tritium in beryllium were calculated.

In-reactor experiment and the tritium diffusion coefficient in molten lithium–tin alloy

The measurement of the tritium diffusion coefficient in molten Li 20Sn 80 alloy was carried out using an in-reactor tritium release experiment. The analysis of the measurements is based on a diffusion-controlled steady state model. When the hydrogen partial pressure was higher than 1100 Pa in the purge gas, and the temperature was 873 K or higher, it satisfied the requirements of the model. At 873 K, the tritium diffusion coefficient in the alloy is of order 10 −9 m 2/s, and the Sieverts' constant in Li 20Sn 80 is below 10 −5 Pa −0.5, based on the rough estimation which is supported by the relation of the tritium residence time to the diffusion coefficient and the Sieverts' constant in the model.

Recent progress in the modelling of helium and tritium behaviour in irradiated beryllium pebbles

One of the key issues of the European Helium Cooled Pebble Bed blanket is the behaviour under irradiation of beryllium pebbles, which have the function of neutron multiplier. An intense production of helium occurs in-pile, as well as a non negligible generation of tritium. Helium bubbles induce swelling and a high tritium inventory is a safety issue. Extensive studies for a better understanding, characterisation and modelling of the behaviour of helium and tritium in irradiated beryllium pebbles are being carried out, with the final aim to enable a reliable prediction of gas release and swelling in the full range of operating and accidental conditions of a Fusion Power Reactor. The general strategy consists in integrating studies on macroscopic phenomena (gas release) with the characterisation of corresponding microscopic diffusion phenomena (bubble kinetics) and the assessment of some fundamental diffusion parameter for the models (gas atomic diffusion coefficients). The present work gives a summary of the latest achievements in this context. By an inverse analysis of experimental out-of-pile gas release from weakly irradiated pebbles, coupled to the study of the characteristics of bubble population, it has been possible to assess the thermal diffusion coefficients of helium and tritium in and to improve and validate the classical model of gas precipitation into bubbles inside the grain. The improvement of the description of gas atomic diffusion and precipitation is the first step to enable a more reliable prediction of gas release.