Hiroshi Kudo

Kinetic studies of the trttium release process in neutron-irradiated Li2O and LiOH

Tritium produced in Li2O(s) and LiOH(s) by the 6Li(n,α)T reaction was released mostly in the form of HTO(g) through heat treatment under vacuum. From rate measurements of the HTO(g) release process in the temperature range 530–700 K, however, a difference was found in the kinetic data between the two materials. The HTO(g) release from LiOH(s) was revealed to be controlled by desorption of HTO molecules in analogy with the thermal decomposition reaction of LiOH(s). On the other hand, the rate-determining step of HTO(g) release from Li2O(s) was found to be diffusion of tritium in the solid. The diffusion coefficients determined for Li2O samples, different in neutron fluence (nvt), were D = 1.9 × 10−2exp{−(24.8 ± 2.5) × 103RT} cm2/s at nvt = 5.4× 1015/cm15, D = 1.2× 10 −2exp{−(24.9 ± 1.7) × 103RT} cm2/s at nvt = 8.1 × 1016/cm2, and D = 5.1 × 10t-3exp{−(23.9 ± 1.7) × 103RT} cm2/s at nvt = 8.9 × 1017/cm2.

Chemical states of tritium and interaction with radiation damages in Li2O crystals

Abstract The tritium produced in neutron-irradiated Li 2 O crystals was found to exist spreading over three different valence states of T + , T − and T 0 . The initial abundances of these species were 67–77%, 23–31% and 2 O crystals under thermal equilibrium with HT gas was found to exist only in the T + state. These experimental results suggest that the presence of T − species in neutron-irradiated Li 2 O crystals could be ascribed to radiation damages produced by neutron irradiation. When the neutron-irradiated Li 2 O crystals were heated above 570 K, almost all of the T − species was eventually converted to the stable T + species. The T − -to-T + conversion process was discussed by taking account of the interaction of tritium with F + centers (oxygen-ion vacancy occupied by one electron) induced by the 6 Li(n, α)T reaction in Li 2 O.

Chemical behaviors of tritium produced by the 6Li(n, α)T reaction in lithium oxide

Abstract The chemical forms of tritium released from neutron-irradiated crystalline powders of Li2O, LiOH and Li2CO3 have been investigated over the temperature range of 100–600°C with radio-gas chromatography and mass spectrometry. When the target materials are heated to 600°C under vacuum after the irradiation, more than 96% of tritium is released as hydrogen oxide, HTO, and collected in the cold trap cooled at −72°C. Tritium is also released in forms such as HT (and T2), CH3T and CnH2n+x − T (n = 2, x = 0, 1, 2), although their percentages are rather low. The release of HTO is interpreted by supposing that recoil tritium produced by the 6Li(n, α)T reaction is first stabilized as LiOT in the solids and then released by the reaction LiOT·LiOH(c)→Li2O(c)+HTO(g) at a temperature between 250 and 400°C.

Tritium diffusivity in lithium-based ceramic breeders irradiated with neutrons

The thermal release behavior of tritium produced in neutron-irradiated Li2O, γ-LiAlO2, Li2SiO3, Li4SiO4, Li2ZrO3 and Li8ZrO6 crystals was studied with emphasis laid on the tritium release kinetics. The tritium produced in these oxidic ceramic materials irradiated with neutrons was released mainly in the chemical form of tritiated water (HTO) when heated in a vacuum. The HTO(g) release process of oxidic ceramic materials was explained by a mixed regime effect where the bulk diffusion of T+ and solid-surface reaction of OT− compete for overall control of the release rate. The observed order of tritium diffusivity in these materials was D(Li8ZrO6) − D(Li4SiO4) >D(Li2O) >D(Li2ZrO3) >D (Li2SiO3) >D(γ-LiAlO2). The tritium diffusivity increased with an increase of the lithium-atom density in these materials.

Kinetics and mechanism of tritium release from neutron-irradiated Li2O

The chemical fate of tritium produced by the 6Li(n, α)T reaction in Li2O 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.

The rates of thermal decomposition of LiOH(s), LiOD(s) and LiOT(s)

Abstract Kinetic studies on the thermal decomposition reactions, 2LiOH(s) → Li2O(s) + H2O(g), 2LiOD(s) → Li2O(s) + D2O(g), and LiOH(s) + LiOT(s) → Li2O(s) + HTO(g), have been carried out with mass spectrometric and radiometric methods over the temperature range 530–690 K. Those reaction rates were found to be of first order in the quantity of released water. The rate-constants, k, in s−1, were: kH2O = 1.8 × 108 exp(29 500/RT), kD2O = 1.7 × 108 exp(29 000/RT), and kHTO = 1.6 × 107 exp(30 700/RT), respectively. The apparent activation energy obtained for the thermal decomposition of LiOH(s), Ea = 29.5 ± 1.1 kcal/mol, coincided with a literature value of the enthalpy of reaction, ΔH°600° = 29.8 ± 1.0 kcal/mol.

Thermal release of tritium produced in sintered Li2O pellets

Abstract The thermal release behavior of tritium produced in a sintered Li 2 O pellet (76.5% TD) has been investigated with a radiometric method. When the neutron-irradiated pellet was heated under vacuum, more than 95% of the tritium was released in the chemical form of HTO. In the HTO release process, diffusion of tritium (T + ) in grains of the pellet was revealed to be a principal rate-determining step, although contribution of HTO diffusion through the grain boundary was also suggested. The Arrhenius relation of the overall diffusion coefficient determined in the temperature range from 570 to 680 K was D = 40.7 exp (− 154.1 × 10 3 / RT ) cm 2 s −1 , where probable errors are ± 8.7 kJ mol −1 for the activation energy and ± 0.77 for the logarithmic pre-exponential term.

Observation of gaseous Li4P: A hypervalent molecule

Abstract With Knudsen-effusion mass spectrometry, gaseous Li2P, Li3P, Li4P, LiP2 and Li2P2 species were observed in the equilibrium vapor over Li3P(s) at temperatures above 1100 K. The atomization energies of the Li2P, Li3P and Li4P molecules were determined to be 450±24, 648±26 and 873±35 kJ/mol, respectively. The enthalpy for the reaction Li4P(g)→Li3P(g)+Li(g) was 186±24 kJ/mol, indicating that the Li4P molecule was stable towards dissociation to form a Li3P molecule. The Li4P molecule, observed for the first time, was revealed to be a hypervalent molecule.

Diffusion-controlled tritium release from neutron-irradiated γ-LiAlO2

Thermal release behavior of tritium produced in neutron-irradiated γ-LiAlO2 crystals has been studied with emphasis laid on the tritium release rate. All of the tritium produced in γ-LiAlO2 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(−90.3RT)cm2s−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-LiAlO2 was nearly two orders of magnitude smaller than that in Li2O crystals.

The stability and structure of the hyperlithiated molecules Li3S and Li4S: An experimental and ab initio study

The thermodynamical properties of the hyperlithiated molecules Li3S and Li4S were investigated by means of Knudsen effusion mass spectrometry and their stability and structure were studied through ab initio molecular orbital calculations. The Li3S and Li4S molecules were found to be stable toward loss of one or two lithium atoms, respectively, to form the octet molecule Li2S. The experimental dissociation energies were D00(Li2S–Li)=33.1±1.6 and D00(Li2S–2Li)=83.9±2.7 kcal/mol. The atomization energies were determined as D00(Li3S)=161.3±3.8 and D00(Li4S)=211.9±4.2 kcal/mol. The ionization potential observed for Li3S was 4.4±0.2 eV. From the theoretical calculations, the occupancies of nine valence electrons in Li3S (C3v) and ten valence electrons in Li4S (C2v) were seen as (5a1)2(3e)4(6a1)2(7a1)1 and (6a1)2(3b1)2(7a1)2(3b2)2(8a1)2, respectively. The singly occupied 7a1 orbital of Li3S and the highest occupied 8a1 orbital of Li4S were found to be involved in the formation of a lithium ‘‘cage,’’ which should...