Determination of Radionuclides Induced by Fast Neutrons from the JCO Criticality Accident in Tokai-mura, Japan for Estimating Neutron Doses

概要

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A criticality accident occurred at a uranium conversion facility in Tokai-mura, Japan on September 30, 1999, and fission neutrons were continuously emitted for about 20 hours. Materials of stainless steel or iron, and chemical reagents were collected at places between 2 m and 270 m from the criticality accident site on October 25 and 26, 1999, November 27, 1999 and February 11, 2000. Neutron-induced radionuclides, such as ^<54>Mn and ^<58>Co, in the materials exposed to fast neutrons from the accident were measured to estimate the neutron fluences and energy distributions. Highly sensitive γ-ray spectrometry with a well-type Ge detector was performed after radiochemical separation of Mn and Co from the materials. An instrumental neutron activation analysis was mainly applied for determinations of the target elements and chemical yields. The concentrations of ^<54>Mn and ^<58>Co in a mesh screen of stainless steel collected at a location 2.0 m from the accident site were determined. The total number of fission events was evaluated to be 2.5 × 10^<18>by Monte-Carlo calculations of neutron transfer by considering the observed values of ^<54>Mn and ^<58>Co. The results presented here are fundamental to estimate the neutron doses at various distances.