An estimation of the chemical evolution of nitrate due to the metal corrosion and microbiological reaction

Uranium and Plutonium are planned to be recovered from spent fuel by the reprocessing in Japan. PUREX method is internationally dominant among the commercial reprocessing plants. PUREX method has been also employed in Japan. The low level liquid waste from PUREX process would contain NO3- as forms of soluble salts, if the special process for decomposing NO3- were not adopted. The nitrate will be possibly brought within the repository for TRU waste. The generation of H2 gas and the reduction of NO3- due to metal corrosion and the generation of N2 gas due to the microbiological reactions in repository for group3 waste were estimated. The uncertainty on the hydraulic conductivity of cementitious materials, corrosion rates, and microbiological reactions are reflected to the analyses through setting the multiple cases. The analysis results suggested followings. (1) The maximum concentrations of both NO2- and NH3 were given in the case where the hydraulic conductivity of cementitious material was assumed to be low (∼ 5.0 x 10-11 m/s) and the corrosion rate was constant at the initial value (0.1 μm/y). (2) The maximum concentration of NH3 was estimated to be 0.85 mol/kg. (3) The impact of microbiological reaction (denitrifying reaction) was negligible in terms of yield of NO2- and NH3 because the proportion of denitrified NO3- was low. (4) The existence of NO3- was strongly reduced the evolution of H2 gas because the NO2- reduction and NO2- reduction dominate the cathodic reactions. (5) The N2 gas evolution rate due to the denitrifying reaction by microorganisms dominated the total gas evolution rate, if time dependent reduction of corrosion rate was employed. However, the H2 gas evolution rate in the case of [NO2-]=0 is similar to that of N2 gas due to the denitrifying reaction mentioned above, if time dependent reduction of corrosion rate was neglected. (author)