Evaluation on migration of radioactive metal in irradiated graphite waste during an innovative thermal treatment based upon the Gibbs free energy minimization

A novel treatment scheme for irradiated graphite waste in nuclear power plants was proposed in order to remove the radioactive metal by adding chlorinating agent during the thermal treatment. The migration behavior of the radioactive metal was evaluated based upon Gibbs free energy minimization. The effects of chlorinating agent, temperature, and negative pressure on the decontamination of the radioactive metal were addressed. Most radioactive metals could be removed through volatilizing to gaseous species below 1273 K by adding AlCl 3 or NH 4 Cl. The selective removal of 55 Fe, 63 Ni, 60 Co, and 134, 137 Cs was available below 842 K after adding 10% NH 4 Cl. AlCl 3 exhibited strong chlorination. Especially, the complete volatilization temperatures for 55 Fe, 60 Co, and 90 Sr were even lower than 412 K with 10% AlCl 3 . Adding KCl was unsatisfactory for 60 Co, 63 Ni, 152, 154, 155 Eu, 239−242 Pu, and 235, 236, 238 U, which could require the complete volatilization temperature of radioactive metals up to 1473 K. Even at KCl of 30% and 0.07 MPa, the volatilization efficiency of 60 Co only lifted by 40% at 1273 K. This work could provide a new technical route and basic data for decontamination of the radioactive metal in the irradiated graphite.