Yoshimasa Murata

Variation Of Uranium Isotopic Composition In Soil Within The Jco Grounds From The 30 September 1999 Criticality Accident At Jco, Tokai-mura, Japan

Following the 30 September 1999 criticality accident at JCO, 29 surface and 3 core soil samples were collected inside and outside the JCO grounds to evaluate possible contamination by 235U-enriched uranium (18.8%) being handled at the time of the accident. Uranium (234U, 235U, and 238U) and thorium (228Th, 230Th, and 232Th) isotopes were determined by alpha-spectrometry and ICP-MS after radiochemical separation. Concentrations of 238U and 234U ranged from 11.3 to 63.5 and 11.6 to 360 mBq g−1, respectively. Higher amounts of 238U and/or 234U were found in the vicinity of the uranium conversion building. The calculated 234U/238U activity ratios ranged from a 1.0 radioactive equilibrium value to an unusually high 5.7 value. Several of the soil samples showed considerably higher 235U/238U atomic ratios (1.06–4.37%) than 0.725% for natural uranium. Based on the assumption that measured U-series nuclides in soil samples taken from the JCO grounds were almost at radioactive equilibrium up to 230Th, excess uranium could be calculated for each sample. The results suggest that the excess uranium in the soils have lower 235U/238U atomic ratios (a few %) than the 18.8% enrichment of the precipitation tank uranium.

The JCO criticality accident at Tokai-mura, Japan: An overview of the sampling campaign and preliminary results

A criticality accident occurred on September 30, 1999 at the uranium conversion facility of the JCO Company Ltd. in Tokai-mura, Japan. A collaborating scientific investigation team was organized in two groups, the first to carry out research on the environmental impact (the environmental research group) and the second to assess the radiation effects on residents (the biological research group). This report concerns only the activities of the environmental research group. Four investigative teams were sent on different dates to the accident site and its vicinity to collect samples. About 400 samples were collected and subjected to analysis. An outline of the sampling campaign is presented here along with a brief chronology of the accident and the preliminary key results obtained by the independent research group are summarised in this Special Issue of the Journal of Environmental Radioactivity.

Neutron activation of chemical reagents exposed to the neutrons released by the JCO criticality accident

Induced radionuclides in the chemical reagents collected in the JCOs Wet Chemistry Testing Laboratory were measured using HPGe detectors in the Low Level Radioactivity Laboratory (LLRL) and in Ogoya underground laboratory. Radioactive nuclei produced by neutron capture reactions were detected in most of the reagents. Products of the (n, p) reaction were detected only in Raney-Nickel and metallic powder of stainless steel. Preliminary estimation of the neutron fluence was made on the basis of determinations of radionuclide concentrations.

Residual neutron-induced radionuclides in a soil sample collected in the vicinity of the criticality accident site in Tokai-mura, Japan: A Progress Report

Abstract Residual neutron-induced radionuclides were measured in a soil sample collected in the vicinity of the location where a criticality accident occurred (in Tokai-mura, from 30 September to 1 October, 1999). Concentrations of 24 Na , 140 La , 122 Sb , 59 Fe , 124 Sb , 46 Sc , 65 Zn , 134 Cs and 60 Co in the soil sample were determined by γ-ray spectrometry, and neutron activation analysis was carried out for selected target elements in the sample. Tentative estimates of the apparent thermal and epithermal neutron fluences which reached the sample were obtained through combined analyses of 59 Fe / 58 Fe , 124 Sb / 123 Sb , 46 Sc / 45 Sc , 65 Zn / 64 Zn , 134 Cs / 133 Cs and 60 Co / 59 Co .

Determination of low-level 54Mn in soils by ultra low-background gamma-ray spectrometry after radiochemical separation

An analytical method was developed for the measurement of low-level 54Mn in soil sample by ultra low-background g-ray spectrometry after radiochemical separation. The method consists of decomposition of sample by a mixture of HNO3 and HF, dissolution by HNO3, precipitation as hydroxide, solvent extraction with diisopropyl ether and anion exchange. Finally, for purification of Mn it was precipitated as MnO2 by adding KClO3. 54Mn in the precipitate was measured by ultra low-background well-type Ge detector at Ogoya Underground Laboratory. Measurements of 54Mn using 30-200 g of soil samples from the JCO grounds were successfully performed by the present method. The minimum detectable activity of 54Mn was about 0.01 dpm (0.2 mBq) for a 7-day counting period.

Estimation of fast neutron fluence released by the Tokai-mura criticality accident from 54Mn in soils collected from the JCO grounds

To estimate fast neutron fluence released by the JCO criticality accident in Tokai-mura, 54Mn, which is produced by the 54Fe(n,p)54Mn reaction, was determined in soil samples by ultra low background g-ray spectrometry after radiochemical separation. Activities of 54Mn, using 30-200 g soil samples were detected in soil samples from 13 points in all directions within a 20 m zone from the precipitation vessel we have used. The levels of 54Mn ranged from 0.015 to 2.12 mBq/g soil. By using these data and MCNP, the fast neutron fluences were estimated.