Studies on induced activities and target nuclei in low-activation concrete structure for thermal neutron irradiation

Eight types of low-activation concrete have been developed and some types of low-activation reinforcement have been selected. These materials have been irradiated by thermal neutron with some reference materials. The induced activities have been measured with a Ge detector and a liquid scintillation detector. Based on these measured data, we analyzed the content of the target nuclei and estimated the residual radionuclides for cooling time of 1 day, 1 month and 5 years under the assumption that the thermal neutron reactor was operated for 40 years. It was found that the dominant activities during maintenance such as ²⁴Na can be reduced to 1/100 by using the limestone aggregates, the silica stone aggregates and the white portland cement, and that the residual radionuclides weighted by the Clearance Levels during decommissioning can be reduced to 1/20-1/100 by using the limestone aggregates, the silica stone aggregates, the synthetic aggregates, the white portland cement and the alumina cement. It is concluded that we can reduce the greater part of the radionuclides induced in the concrete structure by thermal neutron irradiation to the Clearance Level by using the low-activation concrete structure thus-obtained. (author)