Takaaki Tanifuji

Diffusion of tritium in single crystal Li2O

The release of tritium from neutron irradiated spherical samples of single crystal Li2O was measured by isothermal annealing experiments. The release is shown to be controlled by diffusion of tritium in the solid under appropriate experimental conditions. Deviations from solely diffusion controlled release were observed when traces of water were present in the He-purge gas used in the experiments. The diffusivity of tritium in single crystal Li2O is given by In(D/cm2s−1) = − (5.93 ± 0.48) − (81.73 ± 4.24)103J/RTfor 850 K < T < 1200 K. (R = 8.314 JK−1mol−1).

Tritium release from neutron-irradiated li2o; constant rate heating measurements

Abstract Based on the constant rate heating method, a study of tritium release from neutron irradiated Li 2 O has been-performed for sintered pellets and single crystals. About 8% of the tritiated species were released in gaseous forms and the other was released in condensible forms, both from sintered pellets (86% TD) and from single crystals. The rate limiting process of the release of condensible species from the sintered pellets was inferred to be due to the desorption mechanism. The apparent activation energies of the tritium release were determined from the maximum release temperatures of the release curves. The neutron fluence and flux effects on the tritium release were also examined.

Tritium release behavior from neutron-irradiated Li2TiO3 single crystal

Li 2 TiO 3 single crystals with various grain size (1-2 mm) were used as specimens. After the irradiation up to 4 × 10 18 n/cm 2 with thermal neutrons in JRR-2, tritium release from the Li 2 TiO 3 specimens in isothermal heating tests was continuously measured with a proportional counter. The tritium release in the range from 625 to 1373 K seems to be controlled by bulk diffusion. The tritium effective diffusion coefficient (D T ) in Li 2 TiO 3 was evaluated to be D T [cm 2 /s]=0.100 exp(-104[kJ/mol]/RT), 625 K < T < 1373 K. In this temperature region, the tritium effective diffusion coefficients in Li 2 TiO 3 are close to those of Li 2 O irradiated with thermal neutrons of 4 × 10 16 and 2 × 10 19 n/cm 2 . It indicates that the tritium release performance of Li 2 TiO 3 is essentially as good as that of Li 2 O.

Tritium release from neutron-irradiated Li2O: Diffusion in single crystal

The release of tritium from neutron-irradiated Li2O 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 (DT/cm2·s−1) = 0.116 exp(−(101±2) × 103J/RT).

Irradiation effects on lithium oxide

Lattice parameter change of Li2O irradiated with thermal neutron reactors and RTNS-II (14 MeV neutron) was measured. 0.15 % lattice expansion and a curious change of lattice parameter were observed in Li2O sintered pellets irradiated to 2 × 1023 thermal neutrons/m2 and Li2O single crystals irradiated up to 2.3 × 1021n/m2 by 14 MeV neutrons, respectively. In Li2O single crystals and sintered pellets irradiated by thermal neutrons and high energetic oxygen ions, F+-centers were found to be induced as predominant paramagnetic irradiation defects. The inducing rate of the F+-centers by oxygen ion irradiation was very higher than that by the thermal neutron reactors.

Release Behavior of Bred Tritium from Irradiated Li4SiO4

A model to explain tritium release behavior from irradiated Li4SiO4, in the model reported so far by the present authors, it is required to use so small reaction rates for the surface reactions as one several thousandth of the observed values reported in the previous papers to get the good fitting. In this study the mass transfer resistance between grain surface and surface water is newly introduced because it is preferable to use the same reaction rate as that reported previously. The estimated values using the new model give good agreement with the observed tritium release curves and also with the release curves estimated using the model so far.

Heat capacity and thermal decomposition of lithium peroxide

Abstract The heat capacity of Li 2 O 2 was measured by adiabatic scanning calorimetry from 301 to 566 K resulting in C p = 59.665 + 52.123 × 10 −3 T + 5.0848 × 10 5 T −2 (J/K · mol) . The thermal decomposition of Li 2 O 2 was studied by continuous calorimetric measurements. An endothermic decomposition was observed above 570 K. The enthalpy of the thermal decomposition was determined to be 25.8 kJ/mol. The effects of atmosphere and heating rate of the decomposition were also studied for the powder and the compact specimens.

Modelling of Tritium Release from Irradiated Li2ZrO3

ABSTRACT In this paper we propose a model to explain tritium release from irradiated Li2ZrO3 sample made by Mitsubishi Atomic Power Industries Inc. (MAPI). The release curves were obtained by temperature programmed desorption (TPD) techniques in a series of experiments in Kyoto University Reactor (KUR) and in the JRR-4 reactor of the Japan Atomic Energy Research Institute (JAERI). In the model a number of mass transfer steps were taken into account. There were diffusion of tritium in the grain, adsorption and desorption of water on the surface of grains, two types of isotope exchange reactions, water formation reaction in addition of hydrogen to the purge gas. Tritium release curves for different purge gas compositions (N2, N2 + H2O) were calculated to compare with data obtained in the experiments. Apparent diffusivities of tritium in crystal grain of Li2ZrO3 were determined.

Recoil range of 2.7 MeV tritons produced by the 6Li(n, α) 3H reaction in Li2O single crystals

Abstract The tritium release from Li2O single crystals due to a recoil process during neutron irradiation was investigated. The linear relationship between the number of tritium atoms ejected from Li2O and the 6Li(n, α) 3H reaction density was observed up to about 1 × 1024 reactions/m3 and the linearity was deviated above this reaction density. From this linear relationship, the recoil range of 2.7 MeV tritons in Li2O single crystals was determined and discussed in terms of the calculated value.

Fission gas release from UO2-dispersed graphite during irradiation

Abstract In-pile release of 85mKr, 87Kr, 88Kr, 133Xe, 135Xe and 138Xe from natural graphite was measured over a temperature range of 100 to 950°C and that of 133I and 135I was estimated. The effects of fission rate and fission density on the fission gas release were examined. The β-decay process of iodine within and outside the graphite was observed to control the gross release of xenon. Possible mechanisms of an anomalous sink of 88Kr in the plot of log R B versus log λ were discussed. A new model for knock-out release was proposed on the basis of its temperature dependence found. These results were compared with those of post-irradiation experiments and explained by a mechanism whereby fission gas is trapped in defects created in graphite by fission and β-decay energy, and released through annealing of the defects.