Actinide oxides used for nuclear reactor fuels have fluorite-type cubic structure. In this crystal structure, the self-diffusion coefficients of the constituent ions are closely related to its sublattice structure. Characteristics of the actinide oxides are reviewed with respect to self-diffusion and chemical diffusion.

抄録全体を表示

The nuclear inelastic scattering cross section of 232Th has been measured at 144 keV using a Si-filtered neutron beam from the University of Missouri Research Reactor (MURR). The energy spectrum of the scattered neutrons was measured with a spherical hydrogen-gas counter and a pulse-height-unfolding procedure. Data were taken at nine scattering angles from 30° to 150° in 15° increments. The results provide new experimental information below 250 keV. The angle-integrated cross section is 0.74 ± 0.05 barn, which agrees with the evalua-tion JENDL-2 and with a coupled-channel calculation where the inelastic scattering through the direct-excitation process of the collective rotational motion of a deformed nucleus is in-cluded as well as that through the compound-nucleus-formation process. Experimental re-sults have been obtained also for the angular dependence of the elastically scattered neutrons at 144 keV.

抄録全体を表示

The measurement of angular neutron spectrum in a quasi-spherical pile of Ti was carried out by the linac time-of-flight method for the assessment of neutron cross sections for Ti in the energy range from a few keV to a few MeV. The measured spectrum in the pile is generally in good agreement with the calculated one from ENDF/B-IV (MAT =1, 286 for Ti) except in the energy range from about 60 keV to a few 100 keV, where the calculation gives considerably lower neutron flux than the measurement.
In order to investigate the cause of this discrepancy between the measured and calculated spectra, the total cross sections for Ti were measured by the transmission method. The results give larger values of total cross sections for Ti by about 30% than ENDF/B-IV below 200 keV, and smaller values by about 10% above 200 keV. These results were ensured at 55 and 147 keV by the measurement using a Si-filtered neutron beam. The calculation based on the measured cross sections shows better agreement with the measured spectrum than that based on ENDF/B-IV. The discrepancy is still observed around 100 keV.
The sensitivity analysis shows the importance of cross sections above 1 MeV and elastic cross sections in the resonance energy region to solve the disagreement between the measured spectrum and the calculated one.

抄録全体を表示

When subcooled water in accumulator tanks is injected during a loss-of-coolant accident of a pressurized water reactor, violent condensation takes place in cold legs because of direct contact of steam with water. A flow model based on the multifluid model of the two-phase flow has been developed to formulate the condensation and mixing processes in the injection region. The measured liquid film temperature and pressure profiles in the injection region have been explained quantitatively with the model which includes (a) drop generation arising from atomization of injected water, (b) condensation of steam on the drops, (c) flow con-traction resulting from the formation of the dead water region, and (d) deposition of the drops. The calculated results show that the condensation rate depends mostly on the drop diameter generated near the water injection nozzle and the maximum drop mass fraction in the injection region. The present model can predict the effect of inlet thermal-hydraulic conditions on the condensation rate qualitatively, and then it has been confirmed that higher condensation rate with initial steam mass velocity is caused by the formation of finer drops and the higher drop mass fraction in the injection region.

抄録全体を表示

Knoop hardness tests were performed on irradiated UO₂ in the fission range of 10¹⁴-10¹⁸ fiss./cm³. Hardness values were obtained as a function of applied load (25-1, 000 g). Three different regions of load dependence on hardness [work hardening or plastic (25-70 g), plateau (70-100 g) and crack formation (over 100 g) regions] were observed in a similar way to those of non-irradiated ones. The elastic effect was not observed on the hardness values of specimens irradiated to a dose more than 7.8 x 10¹⁴ fiss./cm³ in the load around 25-30 g in contrast to those of non-irradiated ones.
Hardness change after irradiation was discussed on the basis of the variation of lattice strain, lattice volume and whole volume for the similar specimens at the same irradiation condition.
The hardness change in work hardening or plastic region and that in plateau region in-creased with dose up to 4.8 x 10¹⁷ fiss./cm³ and decreased above that dose, and correlated to the lattice volume change and lattice strain.

抄録全体を表示

Oxygen solubility in Na-K (22w/o-78w/o) was determined by a vacuum distillation method. A NaK sampling apparatus was installed in a forced convection NaK loop with a cold trap. Subtracting the oxygen blank from oxygen amount calculated from chemically determined quantities of Na and K in the distillation residue and using the least squares method, a following equation for oxygen solubility in NaK over a temperature range of 90210°C was derived
log₁₀S=7.09-2, 795/T,
where S is the oxygen solubility (wppm) and T the absolute temperature (K). This equa-tion was close to those derived for oxygen solubility in Na.

抄録全体を表示

In order to evaluate the release source of Co from heated tubes of the feedwater line of BWR, Co release rate measurements were carried out by measuring ⁶⁰Co released from ir-radiated stainless steel in contact with neutral water at an oxygen concentration of 20 ppb. The temperature- and time-dependences on Co release rate were studied for 670 h release experiments ; the rate was found to decrease in proportion to t^-1/2 (t : exposure time) and to have its maximum value around 240°C. An empirical equation was obtained for the Co release rates in the temperature range of 150240°C.
R_co =1.9 x 10⁶ exp (- 14, 300/ (RT))t^-1/2
where R_co Co release rate (mg/ (dm²•month)), R: Gas constant (cal/ (mol•deg.) ), T: Ex-posure temperature (K) and t: Exposure time (h). Decrease in the Co release rate with temperatures over 250°C, seemed to be ascribed to the formation of a protective oxide layer on the surface of stainless steel.

抄録全体を表示