Neutron dose estimation by 24Na measurements in the triage of exposed individuals after criticality accidents: a case study of the Tokaimura criticality accident

We investigated the neutron dose estimation for the triage of personnel involved in criticality accidents by conducting 24Na measurements via the whole-body measurement method. For a case study, we examined the September 1999 Japan Nuclear Fuel Conversion Co. criticality accident (internationally known as "the Tokaimura accident"). We investigated the neutron-induced radionuclides produced in worker C's body due to the irradiation of the primary pulse. A total of 87 radionuclides were identified, with activities ranging from 1.3 × 10-19 Bq to 6.9 × 106 Bq for worker C by Monte Carlo simulations. The radionuclide with the highest activity was revealed by calculation results: 28Al (t1/2 = 2.2 min) 6.9 × 106 Bq. The specific activity of 38Cl reached a level that was three times that of 24Na. These radionuclides would severely affect the 24Na measurement in the whole-body measurements performed in the immediate post-accident period. Spectrometry would be preferable for determining 24Na specific activities by whole-body measurements at accident sites.