Tritium Release Model Research Articles

Computer simulation of tritium release behavior of Li4SiO4 ceramic breeder with an improved packed bed model

In order to have a deep understanding of the tritium migration and release mechanisms in ceramic breeders, computer simulation of the tritium release behavior of Li4SiO4 ceramic breeder was performed by using an improved tritium release model. The influences of various factors, such as surface adsorbed water, water vapor in the purge gas, hydrogen in the purge gas, grain size and tritium production amount, on the tritium release process were systematically investigated. The simulation results have shown that: 1) The surface adsorbed water and water vapor in the purge gas can remarkably facilitate tritium release as tritiated water (HTO) in low temperature region (typically <450 °C); 2) Adding hydrogen to the purge gas is effective to promote tritium release as tritium gas (HT), but the effectiveness is sensitive to surface adsorbed water and water vapor in the purge gas; 3) Large grain size and tritium production amount can relatively weaken the effects of surface adsorbed water and water vapor in the purge gas. In general, the simulation results were consistent with the observations in tritium release experiments, which were helpful to further elucidate the complex behavior of tritium release from Li4SiO4.

Developing a tritium release model for Li2TiO3 with irradiation-induced defects

The annihilation kinetics of irradiation-induced defects in Li2TiO3 was evaluated by electron spin resonance. The radiation defects were stabilized with increasing defect density by interaction with neighboring defects. Subsequently, the tritium migration model in Li2TiO3 was established by integrating the kinetics of tritium diffusivity in Li2TiO3 crystalline grains, tritium trapping/detrapping at oxygen vacancies and hydroxyl groups, and annihilation of irradiation-induced defects. The contribution of hydrogen isotopes contained in tritium recovery gas was also considered in this model. The model can demonstrate an overall profile of out-of-pile tritium release for Li2TiO3 with various neutron fluences. Tritium release behavior under neutron irradiation was also estimated by the present established model. The tritium inventory increased under high neutron flux because of the continuous generation of tritium trapping sites, and the addition of hydrogen isotopes to the purge gas could reduce the tritium inventory for Li2TiO3.

Study on Tritium Release Behavior from Li2ZrO3

The present authors have developed the tritium release model to represent the tritium release behavior from solid breeder materials (Li2ZrO3, Li2TiO3, Li4SiO4, LiAlO2 and Li2O). It has been found that water is released from solid breeder materials into the purge gas due to desorption of physically and chemically adsorbed water and water generation reaction and that this water affects the tritium release behavior. In this study, the amount of adsorbed water and its desorption rate for Li2ZrO3 were quantified. It was found in this experiment that Li2ZrO3 has the largest adsorption amount among the solid breeder candidates. It was also observed that Li2ZrO3 has the largest water generation capacity among the solid breeder candidates. A unique reaction at around 550°C which made up approximately 80% of the capacity of water generation was also observed. It is considered that the phase change of ZrO2 at around 550°C supplies oxygen to promote water generation reaction. Tritium release behavior from Li2ZrO3 blanket was estimated using the properties obtained in this study under the operational condition of ITER or a commercial reactor.

Effect of surface water on tritium release behavior from Li 4SiO 4

The tritium release model to represent the release behavior of bred tritium from solid breeder materials has been developed by the blanket group of Kyushu University. It has been found that water is released to the purge gas from solid breeder materials and that this water affects the tritium release behavior. In this study, the amount of surface water released from Li 4SiO 4 is quantified by the experiment. In addition, the tritium release behavior from Li 4SiO 4 are estimated based on the tritium release model using parameters obtained in our studies under conditions of commercial reactor operation and ITER test blanket module operation. The effect of the surface water on tritium release behavior is discussed from the obtained results. Moreover, the tritium inventory of Li 4SiO 4 is discussed based on calculation under the unsteady state condition. Further, the effects of grain size and temperature on distribution of tritium inventory under the steady state condition are evaluated, and the optimal grain size is discussed from the view point of tritium release from Li 4SiO 4.

Characteristics of Tritium Release Behavior from Solid Breeder Materials

A tritium release model has been developed by the present authors. The tritium release curves estimated by this tritium model give good agreement with experimental curves for Li4SiO4, Li2TiO3, Li2ZrO3 or LiAlO2 under various purge gas conditions in our out-of-pile bred tritium release.The characteristics of tritium release behavior from various solid breeder materials carried out by us and in EXOTIC experiments at Petten are discussed in this study.

The Effect of Water on Tritium Release Behavior from Solid Breeder Candidates

The authors have made a tritium release model to represent the release behavior of bred tritium from solid breeder materials using a series of studies.It has been observed that a large amount of adsorbed water and water produced by water formation reaction are released to the purge gas even though dry purge gas with hydrogen is introduced to solid breeder materials. According to our tritium release model, the presence of water in the purge gas and surface water on the material has a large effect on the tritium release behavior. In this study, the authors quantified the amount of adsorbed water and the capacity of the water formation reaction for various solid breeder materials (Li2TiO3, Li4SiO4, Li2ZrO3, LiAlO2). The effect of surface water on the chemical form of tritium released from the LiAlO2 blanket is also discussed in this study.

Tritium diffusivity in crystal grain of Li 2TiO 3 and tritium release behavior under several purge gas conditions

It has been reported by the present authors that behavior of tritium release from solid breeder grain is consisted of diffusion in grain, tritium transfer at surface layer and surface reactions on grain surface such as adsorption or isotope exchange reactions. Tritium release curves estimated using the tritium release model gave good agreement with observed tritium release curves from Li 4SiO 4, Li 2ZrO 3 or LiAlO 2. Tritium release behavior from Li 2TiO 3 under humid purge gas, dry purge gas and dry purge gas with hydrogen conditions is discussed in this study, tritium release curves using the release model that we proposed previously give a good agreements with experimental tritium release curves. Tritium effective diffusivity in the crystal grain of Li 2TiO 3 is also estimated in this study using a curve-fitting method applied to the release curves obtained under the humid purge gas condition. It is discussed that change of color of Li 2TiO 3 surface under hydrogen purge gas condition is observed and this phenomenon might affect tritium release behavior from Li 2TiO 3.

Chemical form of released tritium from solid breeder materials under the various purge gas conditions

Understanding of the release behavior of bred tritium from solid breeder materials is necessary to design tritium recovery system from blanket of a fusion reactor because permeation loss of bred tritium in the piping system or type of tritium recovery system depends on the tritium release behavior. It has been reported by the present authors that behavior of tritium release from solid breeder grain is consisted of diffusion in grain, tritium transfer at surface layer and surface reactions on grain surface such as adsorption or isotope exchange reactions. Chemical form of released tritium from Li 4SiO 4 (from FzK), LiAlO 2 (from JAERI), Li 2TiO 3 (from CEA) and Li 2ZrO 3 (from MAPI) under various purge gas condition is discussed in this study by using the data obtained from the out-of-pile tritium release experiment in JAEA and fitting results estimated by the tritium release model formed by the present authors. And then the tritium release behavior and chemical form of tritium in the test blanket module with solid breeder under the ITER condition is also discussed based on the estimation obtained using the tritium release model.

Estimation of tritium release behavior from solid breeder materials under the condition of ITER test blanket module

A tritium release model from solid breeder materials has been developed. Tritium release curves estimated using this model gave good agreement with tritium release curves from various solid breeder materials obtained by out-pile experiments under various purge gas conditions in previous study. In this study, tritium release behavior from Li 2TiO 3 and LiAlO 2 are estimated for the neutron irradiation condition and operation sequence of the ITER test blanket module test program (neutron wall loading of 0.78 MW/m 2 and repetition of burn time for 400 s with a dwell time of 1400 s). The results are evaluated to discuss the tritium release and inventory performance of different breeder materials.

Chemical behavior of Li 2BeF 4 molten salt as a liquid tritium breeder

Research results on chemical behavior of Flibe (LiF–BeF 2 molten salt) are reviewed, being focused on tritium release and corrosion against structural materials. Tritium release behavior depended on the REDOX potential in the system. The chemical form changed from TF to HT or T 2 with increasing H 2 partial pressure of the purge gas, and tritium release rate increased also with that, which can be explained well by a tritium release model with an isotopic exchange reaction in the salt. As for the corrosion behavior, a thermodynamic analysis and static experiment were carried out. It was suggested that some kinds of oxide layers can work as a protective scale for ferritic steel, while for V-based alloys, no stable oxide film can exist in the presence of Flibe. REDOX control and/or corrosion-resistant coating should be developed for the corrosion problem. It is concluded that the REDOX control is a key issue for Flibe blanket systems, and future R&D items are pointed out.

Low pressure tritium interaction with Inconel 625 and AISI 316 L stainless steel surfaces: an evaluation of the recombination and adsorption constants

The surface constants for the recombination (σk2) and adsorption (σk1) of tritium in Inconel 625 and austenitic stainless steel AISI 316 L were determined from the measurement of tritium permeation through engineering components (bellows) typical of those used on large fusion devices which will operate with tritium. Experimental permeation measurements were performed over the temperature range 450–620 K and an interpretation of the data was attempted based on a surface-limited tritium release model. At the tritium partial pressure of 0.1 Pa present in a machine such as JET, the flow of tritium is strongly influenced by surface reactions. Furthermore, it is often assumed that oxide layers, acting as permeation barriers, are present on such components. However, for effectiveness, such barriers must be intact and this may not necessarily be the case for engineering components in which mechanical stresses can lead to oxide cracking. The recombination (σk2) and adsorption (σk1) constants of tritium were estimated for both stationary and continually flexing bellows.

Tritium Inventory in Li2O Breeder Material for the SEAFP Helium Cooled Ceramic Blanket

The analysis of the tritium inventory in Li2O, carried out for the Safety and Environmental Assessment of Fusion Power (SEAFP) helium-cooled ceramic blanket, is based on a diffusion and desorption tritium release model. Within the specific range of breeder temperatures taken into account, desorption was the dominant mechanism so it can be defined as the rate controlling step. At steady state, the model for the tritium inventory in the solid Li2O breeder is supported by a computer code for several operating conditions. At reference conditions of breeder temperatures, by varying the mean grain radius from 1 to 5 µm, a tritium inventory from 0.5 to 2.8 g can be obtained. A helium purge gas velocity from 0.1 to 0.4 m/s gives rise to gas pressure losses from 0.22 to 0.9 MPa, which could probably be reduced by increasing the pebble diameter to 1 mm. This breeder configuration seems to ensure reactor safety.

Mass transfer coefficient of tritium from molten lithium-lead alloy (Li 17Pb 83) to environmental gas under neutron irradiation

The mass transfer coefficient of tritium from molten Li 17Pb 83 alloy to environmental gas was determined by in-situ tritium release experiments under neutron irradiation in the temperature range from 400 to 700°C. The tritium residence time in the sample in Al 2O 3 crucible was measured and a tritium release model was applied to obtain the mass transfer coefficient. The mass transfer coefficient decreased with increasing H 2 partial pressure in He sweep gas up to 10 4 Pa at 600°C, and its maximum value was 5.1 × 10 −3[m s −1]. At 10 3 Pa in H 2 pressure, it was expressed as follows: K D = 2.5 × 10 −3[m s −1] × exp(−30.7[kJ mol −1]/ RT). From the dependence of K D on H 2 pressure and the value of the activation energy in K D, it was concluded that the rate-controlling process is a liquid film diffusion above 10 3 Pa in H 2 pressure while, below 10 2 Pa, some surface reaction was considered to be a rate-controlling process.

Modeling Unusual Tritium Release Behavior from Li2O

Diffusion of tritium through the grain and desorption of tritium from the surface have been considered the important processes controlling tritium release from neutron-irradiated lithium ceramics. Modeling studies have focused on these two processes as the rate-controlling steps in tritium release from lithium ceramics. This paper presents a diffusion-desorption tritium release model in which the unusual tritium-release behavior observed in the CRITIC experiment is accounted for by an activation energy of desorption that is surface coverage dependent. Desorption and adsorption activation energies which are dependent on the amount of surface coverage have been reported. The current model is capable of reproducing both the unusual and the normal tritium release observed in CRITIC and predicts other regions where the surface-coverage-dependent release behavior may be observed. Results from the CRITIC experiment and the authors calculations imply that the details of the surface phenomena must be known to accurately predict the tritium inventory and changes in inventory that occur with changes in the breeder-material environment.