Takaaki Yoshimoto

Trace Elements with Large Activation Cross Section in Concrete Materials in Japan

Amounts of trace elements with large activation cross section in concrete materials were measured to offer the basic data for developing of low activation concrete. From the measurements, the quantities of the activated radioactivities in biological shielding concrete were measured and evaluated for the clearance level. The average concentrations of 60Co, 152Eu and 134Cs formed in concrete were 21.9, 1.08 and 3.21 ppm, respectively. The combination of the concrete materials for the most lowering concentrations of 60Co, 152Eu and 134Cs was the limestone as aggregate and the white Portland cement produced in specific places. The most lowering concentrations of this limestone concrete were 0.16, 0.049 and 0.060 ppm, respectively. The limestone concrete was excellent as biological shielding concrete, because the neutron shielding effect was excellent a little compared with ordinary concrete. If this concrete used for biological shielding concrete, concrete waste will be able to handle as follows. Usage of this limestone low-activated concrete makes almost all concretes satisfy the clearance level for 60Co after 20 yr cooling from decommissioning. In respect of 152Eu, radioactivation quantity in the biological shielding concrete is reduced up to a half of the average value or less. With regard to 134Cs, all concrete satisfies the clearance level.

Neutron-irradiation effect on the mechanical properties of alumina fiber

This paper describes the neutron irradiation effects on the deterioration of alumina fiber (made by Mitsui Mining Material, Almax), a typical electrical insulation material. The material was irradiated at the research reactor at Kyoto University Research Reactor Institute with a maximum fluence of 5.6×1023 n/m2 (energy: E>0.1 MeV). Tensile strength and tensile modulus of the specimen scarcely changed. Observation with a scanning electron microscope (SEM) and a transmission electron microscope (TEM) did not indicate any changes in crystal or pore structure. However, the Weibull coefficient of tensile strength decreased as the irradiation dose increased. This suggests an increase in the defect size distribution.

Neutron Capture Cross Section Measurements of Tc-99 and Rh at Energies below 40 keV by Linac Time-of-Flight Method

The neutron capture cross sections of 99Tc and Rh have been measured relative to the 10B(n, α) standard cross section at energies below about 40 keV by the neutron time-of-flight (TOF) method using a 46 MeV electron linear accelerator (linac) at the Research Reactor Institute, Kyoto University (KURRI). A pair of C6D6 scintillation detectors has been used for the relative measurement of the 99Tc(n, γ) cross section, and the result has been normalized to the reference cross section value (19.57b) at 0.0253 eV in ENDF/B-VI. A total energy absorption detector, which was composed of 12 pieces of Bi4Ge3O12 (BGO) scintillators, was employed for the absolute capture cross section measurement of Rh. Both of the measured cross sections have been compared with the existing experimental data and the evaluated data in ENDF/B-VI, JENDL-3.2 and JEF-2.2.

Uranium and Plutonium Isotope Ratio Measurement as a Tool for Environmental Monitoring - Experiences in Osaka, Gifu and Hiroshima, Japan

We measured U-234/U-238, U-235/U-238 and Pu-239/240 isotope ratios in soil samples from Osaka, Gifu, and Hiroshima, Japan by inductively coupled plasma – quadrupole mass spectrometry. Deviation in U-235/U-238 isotope ratio from natural ratio was not found in soil collected three days after A-bomb detonation in Hiroshima, where approximately 50 kg of U-235 should have been discharged. U-234/U-238 ratio variation in soil that was dependent on the soil particle size and soil digestion procedure was found. Variation in Pu-240/Pu-239 ratio due to incomplete dissolution of soil samples was found in soil from Osaka. The results indicated the necessity of further investigation on natural variation of U isotope ratios in Japanese environment, and selection of sample processing technique depending on sample characteristics in order to obtain accurate isotope ratio.