The radiochemical characterization of regular- and high-density concrete from a decommissioned reactor

Abstract

Samples of high- and regular-density concrete exposed to neutron and gamma irradiation as well as radioactive contamination were obtained from the Nuclear Power Demonstration (NPD) reactor. The radionuclide composition and distribution in these specimens was determined to provide important information on the management of decommissioning waste, concrete decontamination, and the behaviour of radionuclides in cement-based engineered barriers. With the exception of tritium, all radionuclides in regular-density concrete have specific activities less than 5 Bq/g. The tritium-specific activity varied along the length of the core section, but its maximum activity was 82 kBq/g. This was five times the maximum tritium activity found in high-density concrete (15 kBq/g). The effective diffusivity, D e , of tritiated water in high- and regular-density concrete is 5.2 × 10 −9cm 2 · 6.6 × 10 −8cm 2 · s −1 respectively. Radiation fields on contact with the ends of the high-density cores ranged from a low of 150 cpm above background at the boiler-room end of the core to a high of 125 mR/h at the vault-end closest to the neutron flux. The radiation field is due to Co-60, which has a specific activity of 1800 Bq/g when averaged over a distance of 20 cms from the reactor-vault end. Beyond 30 cm into the core the neutron flux attenuation was such that the Co-60 specific activity is <5 Bq/g. Other gamma-emitting radionuclides present in high-density concrete are Mn-54, Cs-134/137, Eu-152/154, and Ce-144, but the specific activity of these is <35 Bq/g. Of the pure beta emitters C-14, Ca-41, and Ni-63, C-14 was the most abundant, with a specific activity of 300 Bq/g. It originates from heavy water containing dissolved 14CO 2.