Michio Iseki

Hydrogen solubility and diffusivity in neutron-irradiated graphite

Hydrogen solubility measurements and the analysis of absorption kinetics have been studied on graphite irradiated with neutrons at various fluences up to 5.4×10 24 n/m 2 . The absorption of hydrogen could be expressed as a diffusion-controlled process. The rate constant of hydrogen absorption was different from that for desorption. This difference may be ascribed to the effects of trapping sites in graphite. After neutron irradiation at 1.9×10 24 n/m 2 (~ 0.2 dpa), the hydrogen solubility was 20–50 times larger than that of unirradiated samples. The increase of hydrogen solubility was saturated above the damage level of ~ 0.3 dpa. The diffusivity of hydrogen was decreased by neutron irradiation up to 1.9×10 24 n/m 2 , and then increased above this fluence. This behavior can be ascribed to the production of the trapping sites for hydrogen, and the elongation of the distance between the basal planes by neutron irradiation.

Trapping and detrapping of hydrogen in carbon-based materials exposed to hydrogen gas

Abstract Measurements of hydrogen solubility have been performed for several unirradiated and neutron-irradiated graphite (and CFC) samples at temperatures between 700 and 1050°C under a ⋍ 10 kPa hydrogen atmosphere. The hydrogen dissolution process has been studied and it is discussed here. The values of hydrogen solubility vary substantially among the samples by up to a factor of about 16. A strong correlation has been observed between the values of hydrogen solubility and the degrees of graphitization determined by the X-ray diffraction technique. The relation can be extended even for the neutron-irradiated samples. Hydrogen dissolution into graphite can be explained by the trapping of hydrogen at defect sites (e.g. dangling carbon bonds) considering an equilibrium reaction between hydrogen molecules and the trapping sites. The migration of hydrogen in graphite is speculated to result from a sequence of detrapping and retrapping events with high-energy activation processes.

Microstructure evolution by neutron irradiation during cyclic temperature variation

Abstract Utilizing a technique to control the temperature which is not influenced by the operation mode of a reactor, an irradiation during which the temperature was alternatively changed several times between two temperatures ( T -cycle) has been performed. Some defect structures are understood as combinations of the defect processes at lower and higher temperatures, and some others are understood if the defect processes during the transient between the two temperatures are taken into consideration. However, the most remarkable characteristic of defect processes associated with the temperature variation is the reaction of point defect clusters induced by lower-temperature irradiation at the higher temperature. During lower-temperature irradiation, there is a greater accumulation of vacancy clusters as stacking fault tetrahedra in fcc metals than that of interstitial clusters as dislocation loops. Vacancies evaporated from the vacancy clusters at higher temperature can eliminate interstitial clusters completely, and the repetition of these processes leads to unexpectedly slow defect structure development by T -cycle irradiation.

Hydrogen behavior in carbon-based materials and its neutron irradiation effect

Abstract Hydrogen retention in graphites and CFCs (carbon fiber/carbon composites) has been studied with the crystallographic data obtained by the X-ray diffraction (XRD) technique. The amounts of retained hydrogen vary substantially among the samples by a factor of up to 16. After neutron irradiation at 1.9 × 1024 n/m2(∼ 0.2 dpa), the hydrogen retained becomes 20–50 times a larger than that of unirradiated samples. A strong correlation is observed between the values of hydrogen retention and the lattice constant c0. The size of crystallite also has a good correlation with the hydrogen retention. Hydrogen atoms will be trapped at dangling carbon bonds at edge surfaces of crystallite which are thermally stable even at high temperatures above 1000°C. Differences among the desorbed amount of hydrogen gas from graphite materials can be also explained well by this model.

Invisible and visible point defect clusters in neutron irradiated iron

Abstract Microstructure evolution in a low-dose neutron-irradiated iron was examined with a transmission electron microscope (TEM). The characteristics of the temperature dependence of defect structures in irradiated iron were as follows, (a) rather lower number density of defect clusters at 473 K than at 573 K, (b) development of dislocation loops and voids at an intermediate temperature of 623 K and (c) formation of irregularly shaped dislocation loops at 673 K. Comparing the defect structures produced by the irradiation with a conventional temperature control and that with an improved temperature control, nucleation of defect clusters was suppressed under neutron irradiation at 473 K. From the defect structures introduced by irradiations in which the irradiation temperature was cyclically changed between two temperatures (T-cycle irradiation), the accumulation of a large number of invisible vacancy type defect clusters by irradiation at 473 K was strongly suggested.

Hydrogen absorption process into graphite and carbon materials

Abstract In order to estimate bulk hydrogen retention and recycling in graphite and carbon materials, molecular hydrogen absorption has been studied. Hydrogen absorption rates significantly depend on samples which arise from grain size, trap concentration and so on. Absorption rate constants differ between the cases of low and high pressures. Trapping has a strong influence, especially in the low pressure range. Oxidation of graphite reduces hydrogen retention and enhances the absorption rate. This suggests that oxygen in graphite does not behave as trapping sites for hydrogen. Activation energies for apparent diffusion for H 2 and D 2 are determined to be 153 and 158 kJ/mol. They are smaller than those energies determined from desorption measurements.

Recoil energy spectrum effect in vanadium by electron and light- and heavy-ion irradiation

Abstract In order to understand defect reaction processes under cascade damage condition in vanadium, as one of the low activation materials for fusion reactor application, defect microstructures induced by light- and heavy-ion irradiations were investigated. The interest was focused to the difference in the primary recoil energy spectrum; collisions with small recoil energy are abundant in light-ion irradiation, whereas there are only large recoils in heavy ion irradiation. Electron irradiation with a high voltage electron microscope was utilized for the identification of the nature of ion irradiation induced point defect clusters, through the behavior of defect clusters under electron irradiation.

Defect structures introduced in iron under varying temperature neutron irradiation

Abstract Defect structures in iron under varying temperature neutron irradiation and those under constant-temperature irradiation were examined by a transmission electron microscope (TEM). For irradiation at a low temperature (473 K), microvoids were detected by positron annihilation lifetime (PAL) measurement. For irradiation at a high temperature (673 K), with increasing the irradiation dose a few interstitial (I)-type dislocation loops grew and the total number density of the loops decreased. In a varying temperature irradiation between 473 and 673 K, after a shift to the high temperature the absorption of interstitials to I-type dislocation loops was suppressed by microvoids introduced at the low temperature and the loops shrank or disappeared by vacancies released from microvoids. As the cycle of varying temperature irradiation became short, the effect of the suppression of the defect development became small.

Fast neutron damage in TiC1−x

Abstract Fast (14 MeV) neutron damage in titanium monocarbide (TiC 1−x ) was examined by measuring low-temperature electrical resistivity and magnetic susceptibility. Both physical properties changed remarkably with neutron damage. The damage process seemed to depend on the nonstoichiometry (the amount of the carbon vacancies), as had been revealed in an “in-situ” resistivity measurement at liquid He temperature. On the other hand, it is of interest that both properties (without damage) are considerably affected by the nonstoichiometry.

Surface erosion of substoichiometric titanium carbide under high-energy 4He+ ion bombardment

Abstract The surface erosion yield of substoichiometric titanium carbide coating and sintered titanium carbide bulk have been measured at normal incidence for 4He+ in the energy range 0.5 to 2.5 MeV by scanning electron microscopy. The erosion, such as multi-layer exfoliation, was observed on the irradiation surface area. The projected ion ranges were depended on the impinging helium-ion energy and agreed fairly well with the theoretical values. It was observed that the erosion yields of the sample strongly depended on the irradiation energy. We discuss these results on the substoichiometric carbon compositions and then estimate the erosion rate of 3.5 MeV 4He+-ion bombardment for titanium carbide from measurements with various high-energy, helium-ion irradiations.