Feasibility and technology for volume reduction of concrete contaminated by <sup>14</sup>CO<sub>2</sub> using rubbing

A large amount of concrete contaminated by 14CO2 will be discharged during the decommissioning of aged nuclear power plants. Rubbing, which separates cements from concrete debris, is one method available to reduce the volume of the waste if most of the 14C is present in the cement inside the concrete. We confirmed that more 14CO2 adsorbs onto the cements than onto the aggregates by a factor of approximately 100. A rubbing test was performed to obtain the mass balance and decontamination factor (DF) using simulated concrete debris not contaminated with 14CO2. The cements and fine aggregates were removed from the debris as fines by rubbing using a Los Angeles testing machine. Steel balls with larger sizes and in greater quantities were used to increase the rubbing effect. In addition, the production rates of the fines for debris subjected to a heat treatment were compared with those of the fines for debris not subjected to a heat treatment. Residues in the mill of the Los Angeles testing machine were washed to remove deposits remaining on the surface. We concluded that an effective volume reduction could be achieved by rubbing the heat-treated debris using additional steel balls with a larger size. The DF was not improved by washing the surface residues. However, washing can resolve concerns regarding radiation protection because of scattering of fines contaminated with 14CO2 in the treatment of the residues after rubbing. If the volume reduction is performed using a system similar to that used in a concrete recycling plant, the production of class H aggregates is necessary to achieve a sufficient DF. In this case, air ventilation and removal of radioactive dust in the exhaust air using, e.g., a bug filter, should be required to reduce the radiation hazard to workers and to the surrounding environment.