Kiyoaki Taketani

Detection of Methyl Iodide upon Heating Irradiated Fuels

Laboratory scale experiments have been carried out to obtain information on the sources of CH3I, reported by various workers to have been detected under accidental conditions of fuels. Small pieces of slightly irradiated sintered UO2 and U metal turnings were heated externally up to 1,400° C in various carrier gases, and the carrier gases were analyzed for fission product iodine present in the form of CH3I. From the data obtained, it is concluded as follows: In the case of sintered UO2, some amount of CH3I appears to be released directly from the fuel below 600°C, whereas in case of U metal turnings, direct release does not apparently occur at any temperature. In oxidizing atmospheres, iodine released from either sintered UO2 or U metal is considered to change into CH3I only to a small extent, probably by gas phase formation.

Void Migration in Sodium Chloride Crystal under Temperature Gradient

A simulation experiment on columnar grain formation in UO2 was performed using NaCl single crystals, to examine the progress of void formation and migration. When two single crystals of NaCl were held together under a temperature gradient, plate-like or lenticular voids were observed to generate and migrate towards the hot side, forming columnar grains in the NaCl crystal. The crystallographic orientation of the colder crystal was found to affect the void shapes observed in the hotter crystal. This void behavior is explained by calculations concerning the evaporation and condensation of the NaCl molecule.

Release of Xenon from Sintered UO2 at Low Temperatures

Release of Xe from lightly sintered UO2 was measured by post-irradiation heating in vacuum at 400°~800°C. The release rate after initial burst followed the diffusion model. An Arrhenius plot of the diffusion coefficients revealed a discontinuity at 600°C. Activation energies were calculated to be 3.4 kcal/mol at temperatures lower than the discontinuity, and over 36kcal/mol in higher temperature range, which corresponded respectively to non-lattice and lattice diffusion. The discontinuity is explained by a changeover of the predominance of one process over another depending on temperature range. The initial burst coald not be attributed to surface oxidation. Based on analyses of such factors as dependence of burst on temperature and on specific surface area, as well as percentage of burst release, a kind of activated extrication from superficial layer, approximately 5A thick, was proposed as a concept for interpreting the burst phenomenon.

Xe Release from UO2 upon Oxidation

For estimating fission gas release from fuel under accident conditions, the present authors have previously measured Xe release from U upon oxidation by CO2, and have observed an Arrhenius type temperature dependence(1). Parker has measured Xe-Kr release from UO2 upon oxidation by air(2), and reported a small and rather complex temperature dependence for this case. The release percentage also differed between these two cases: In the former experiment 40 % of the Xe initially present was released at temperature as low as 700-C, whereas in Parkers case (400-~1,400-C) the largest release was only 18 % (at 1,100-C). The present note is on the results of an experiment on Xe release from UO2 upon oxidation, not in air, but in 02 at 600-~1,300-C. Broken pieces of sintered UO2 (95% T.D.), were vacuum sealed in quartz tubes and irradiated in the JRR-1 research reactor to an integrated neutron flux of 1 x 1016 nvt. After cooling for 1 to 2 weeks, the specimens were heated in an alumina combustion tube connected to charcoal traps with a stop cock inbetween. The apparatus was so arranged that the specimen could be heated to present temperatures within a few minutes. The

On the Microstructure of UO2 Pellets Irradiated at High Temperature

The microstructure of sintered UO2 pellets irradiated at high temperature in GETR were studied. Two groups of UO2 pellets prepared from the Mme lot were irradiated under similar thermal conditions for different periods. The effect of hurnup on the microstructure of the two groups were studied. An extensive development of columnar grains with lenticular voids were observed in the pellets from the lower burnup group, while those from the higher burnup group had large columnar grains characterized by many radial cracks. Detailed discussion is given. Autoradiographic techniques were also applied on the specimens to examine the microstopic structure. No appreciable migration of the fission products was detected.

Diffusion Coefficient of Fission Gas in Uranium Dioxide Powder Formed by Carbon Dioxide Oxidation of Uranium:Study on the Behavior of a Punctured Fuel Element for Calder-Hall Type Reactor, (4)

In connection with a program to study the behavior of punctured fuel elements for the Tokai Atomic Power Reactor, the diffusion coefficient of fission gas in uranium oxide powder formed by CO2 oxidation of U was determined by post-irradiation experiment, in which the fractional release of fission gas during isothermal heating of the powder was measured. The U was oxidized at 600° and 700°C, and in both cases the O/U ratio of the oxides, measured gravimetrically, was 2.0. The diffussion coefficients in the oxide powder formed by oxidation at 600°C were found to be 1.4× 10-20, 1.3×10-19, 1.1×1018 and 1.0×10-17, cm2sec-1, respectively at 450°, 550°, 650° and 750°C, and in the oxide powder formed at 700°C, 7.4×10-19 and 3.6×10-16cm2sec-1 at 600° and 700°C, respectively. Activation energies calculated for the two oxide powders were comparatively low.

Irradiation of Natural UO2 Pellets

Three kinds of capsules containing natural UO2 pellets were irradiated in the EFTL-2 irradiation facility of JRR-2. EFTL-2 is a hydraulic rabbit designed by the authors and completed in 1963. The purpose of the present irradiation is:(1) To assemble basic data pertaining to EFTL-2 when carrying fuel capsules under irradiation.(2) To ascertain the correctness of capsule design and to compare calculations with experimental data.(3) To gather information on the effects of irradiation on natural UO2 pellets of the same dimensions as fuel used in the JPDR.This is the first experiment undertaken in Japan on UO2 pellet irradiation, and consequently the irradiation capsule has been designed very conservatively to incorporate every precaution to ensure safety precautions.