Tomoo Kirihara

Irradiation induced lattice defects in UO2

Abstract Two kinds of sintered UO 2 having 2.5 and 5 μm grain sizes were irradiated below 150°C to fission doses between 1.14 × 10 14 and 2.92 × 10 18 fissions/cm 3 . The changes of lattice parameter and lattice strain with fission dose were studied. Three stages appeared in the dose dependence of lattice parameter and lattice strain which depended on grain size. In the beginning of the first stage the lattice strain decreased to some extent. The recovery of the lattice parameter within each stage was studied and defects produced in each stage were also discussed. The effective volumes per fission event were calculated at each stage based on kinetic consideration of defects.

Thermal recovery of defects in neutron irradiated UO2

Abstract Thermal recovery of both lattice and volume expansions of UO 2 irradiated to a dose of less than 9.97 × 10 17 fissions / cm 3 was studied in a temperature range from 200 to 1000°C. Two or three steps were observed in the recovery of lattice expansion. Volume expansion was also recovered in two or three steps. Recovery behavior of volume expansion of the specimen irradiated to 9.97 × 10 17 fissions / cm 3 was different from those irradiated to lower doses, and an abrupt volume increase was observed in the temperature range from 500 to 700 °C. This behavior might be related to the bubble swelling due to fission product gases. From the results on the recovery of lattice and volume expansions, the behavior of lattice point defects and defect clusters was discussed.

Irradiation induced volume change in UN2

Two sets of uranium dioxide samples having grain sizes of 2.5 and 5 μm were irradiated with different fission doses in the range of 1.1 × 1014 to 2.9 × 1018 fissions/cm3; the volume change was measured. A steep volume expansion was observed up to a dose of about 5 × 1016 fissions/cm3 which corresponds to the reported dose for maximum elongation of stack length of UO2 fuel elements in the starting period of BWR. At higher doses, an abrupt volume shrinkage was noted. The concentrations of interstitials and vacancies at various doses were evaluated from the changes of the volume and the lattice parameter. Kinetic equations for the densification of fuels based on the concentration of vacancies are proposed.

Neutron irradiation effects in uranium monosulfide

Uranium monosulfide (US) was irradiated to investigate the effects of fission damage. Post-irradiation examinations were done by measuring the electrical resistivity, and partly the magnetic properties, at low temperature. The lattice parameter and the electrical resistivity measured at room temperature just after the irradiations showed an increase starting at a fission dose of 1 × 1016 fissions/cm3 and attaining a maximum at 3 × 1016 fissions/cm3. After that, a saturation of both increases persiste until 3 × 1017 fissions/cm3. The low-temperature electrical resistivity in the magnetic ordered state (ferromagnetic transition, Tc, at about 180 K) increased remarkably, while decreasing drastically in the magnetization, with increasing fission dose, apparently corresponding to the lattice expansion. In addition, the Curie point (Tc) shifted to lower temperatures with accumulating fission damage.

Irradiation of Uranium Carbides in JMTR

Post-irradiation examinations were performed on three types of uranium carbide UC, UC2 and U2C3, that were irradiated with a relatively high thermal neutron flux (7×1013 n/cm2·s or 9×l012 fission/cm3·s for UC) and in a dose range between 2.7×l015 and 3.3×1018 fission/cm3 (i.e. 2.1×l016 and 2.4× 1019 nvt) in the JMTR (Japan Material Testing Reactor). On UC and UC2, trends similar to previous works were obtained in the irradiation effects. New preliminary results, however, were obtained on U2C3 for changes in the electrical resistivity and the lattice parameter which showed reduced values after attaining a maximum at 1017 fission/cm3. Successive annealing effects on the resistivity and the lattice parameter following the reactor irradiations were examined by pulse heating. Two major steps were observed, at around 400 and 600°C, in the recovery processes. A low temperature step, which was revealed previously at about 200°C in UC and UC2, was missing in this study, because of higher irradiation temperatures (...

Magnetic character of us with fission damage

Abstract Effects of neutron irradiation (fission damage) and successive annealing on the magnetic character of ferromagnetic uranium monosulphide (US) are evamined. Fission-induced defects reduce drastically the magnetivation of US in the ordered state, accompanied with a remarkable increase in the electrical resistivity due to magnetic disordering. The magnetic parameters also changed with fission damage. Successive annealing showed a recovery in all the magnetic parameters and the disordered magnetivation and resistivity, in accordance with an annealing of fission damage.

Magnetic properties of uranium mononitride with fission fragment damage

Abstract Magnetic disorder in neutron-irradiated (fission fragment damage) uranium mononitride (UN) has been investigated by magnetic susceptibility measurements. The Neel point (TN ) shifted to lower temperatures with increasing fission dose and attained the lowest value at a dose of around 1018 f/cm3. After further irradiations, however, it recovered to the non-irradiated value. The shift of TN was in accordance with the change of the concentration of fission-induced vacancies. Other magnetic parameters showed a similar trend. These fission-fragment damage results are discussed in terms of the electronic state of 5 f electrons.

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.

Electrical conductivity and thermoelectric power in irradiated UO2 + x

Two kinds of UO2 + x, the O/U ratios of which were 2.002 and 2.004, respectively, were irradiated to a dose range between 1.14 × 1014 and 1.90 × 1018 fissions/cm3, and electrical conductivity changes were measured. A steep decrease in conductivity was observed with increasing dose up to 1 × 1015 fissions/cm3, a gradual increase followed between 1 × 1015 and 1 × 1018 fissions/cm3 and above this dose the conductivity abruptly increased. Thermoelectric power measurements were also carried out for the specimens irradiated in the dose range up to 1.90 × 1018 fissions/cm3. It might be suggested that p-type conduction contributes to the electrical conductivity in irradiated specimens up to 1.90 × 1018 fissions/cm3.

Magnetic disorder in uranium compounds due to fission damage

Abstract Ferromagnetic US (Tc = 180 K) and antiferromagnetic UN (TN = 52 K), both with NaCl-type crystal structure, were examined with respect to the magnetic properties after neutron irradiation. Shifts of the magnetic transition point in both cases and a drastic reduction of the magnetization in US were accompanied with an increase of the lattice parameters due to fission damage.