Basic Study on Radiation Degradation of Potassium Nickel Ferrocyanide

摘要

Selective removal of ~(137)Cs from radioactive liquid waste is essential for the waste management in nuclear facilities. According to many studies which have been carried out for separation of Cs from other fission products, potassium nickel ferrocyanide(KNiFC) is one of the most effective adsorbents for decontamination of ~(137)Cs in the radioactive waste solutions. Although the ferrocyanides are known to be chemically stable, HCN and H_2 gases are suspected to be generated by radiolysis. Generations of those gases should be suppressed or controlled in order to prevent hazards such as exposure to the poisonous gas, fire or explosion. In this study, gamma ray irradiation experiment on KNiFC absorbing Cs was carried out to evaluate stability of used ferrocyanides. Gamma ray irradiation was performed using ~(60)Co source, and the dose rate and integrated dose were 8.0-10 kGy/h and 0.13-6.0 MGy, respectively. After the irradiation, concentrations of H_2 gas, of CN~- and of Cs were analyzed by gas chromatography, colorimetry and atomic absorption spectrometry, respectively. The G-value of H_2 calculated from the concentration of H_2 gas showed that generation of H_2 gas prompted by adding KNiFC into water and that the Cs charge in the compound did not influenced on the value. Therefore, concentration of H_2 gas in the tank for the used KNiFC has to be attenuated by ventilation. Although analyses on CN~- and Cs revealed that KNiFC is stable up to 3 MGy irradiation, quite small amounts of CN~- and Cs were observed in the liquid phase after 6 MGy irradiation. However, poisonous HCN was not detected in the gas phase. Since 6 MGy irradiation simulates radiation-exposure during 10 years storage, immediate risk to health attributed to HCN gas generation can be neglected.