Estimation of radionuclides migration mechanism in thermal decomposition of spent ion exchange resin in nuclear power plant

Massive spent ion exchange resins (SIERs) containing radionuclides are produced in nuclear power plants, which need to be treated reasonably. The migration mechanism of nuclides in SIER in both pyrolysis and gasification was evaluated from the chemical equilibrium analysis based on the Gibbs free energy minimization. The maximum volatility degrees of Co, Sr and Cs at 400–600 °C and 0.1 MPa were 0.01%, 0% and 0.1% in the pyrolysis. The maximum volatility degrees of Co, Sr and Cs were 0.015%, 0% and 100% at 600–1300 °C and 0.1 MPa in the gasification, and the steam concentration should be set mainly how to limit the volatilization of radionuclides. To reduce the volatility of nuclides as much as possible, the temperatures should be kept below 600 °C and 870 °C in the pyrolysis and gasification, respectively. The volatilization of Cs was greatly inhibited with lifting the pressure from 0.1 MPa to 1.5 MPa, which decreased from 0.103% to 0.007% in the pyrolysis (600 °C) and from 4.80% to 0.30% in the gasification (715 °C), respectively. Based on preventing the nuclides leakage, the negative pressure in the gasification should be controlled between 0.097 and 0.099 MPa. The volatilization of radionuclides was effectively weakened by adding a fixative, Zr(HPO4)2·H2O, since it could be helpful to reduce the gaseous cesium-based products in the pyrolysis and gasification. The volatility degree of Cs was close to 0 if Zr/Cs was great than or equal to 0.5. This study could provide basic data for the pyrolysis and gasification treatment of SIER.