A Modeling study for the chemical evolution of nitrate accompanied with the corrosion of carbon steel

Honda, Akira; Masuda, Kaoru*; Kato, Osamu*; Nishimura, Tsutomu*; Tateishi, Tsuyoshi*; Imakita, Tsuyoshi*

Uranium and Plutonium are planed 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 is possibly brought within the repository for TRU waste. The specie of NO3- is an oxidizing agent which can be reduced to NO2- and NH3/NH4+ by the coexistence of reducing materials such as metals.In order to estimate the safety of the repository for TRU waste, the impacts of nitrate on the disposal system for TRU waste have to be estimated. Especially, NH3 can elevate the solubility and reduce Rd value through the formation of ammine complexes. The quantitative information of chemical evolution of nitrate is necessary for evaluating the impact of nitrate and the chemical species arising from nitrate on the safety of the repository of TRU waste. The evolution of chemical form of NO3- by the reducing reaction accompanied with metal corrosion was experimentally examined. The reduction of NO3- was considered to be a serial reaction, that is, NO3- - NO2- - NH3. The rate equations of cathodic reactions (water reduction and reduction of nitrate and nitrite) were experimentally determined through the electrochemical measurements. The rate equation of metal dissolution (anodic reaction) which must be balanced to the cathodic reactions in charge transfer is determined from the results of immersion tests without nitrate. The combination of the rate equations forms an assessment model of chemical evolution of NO3-. The model provided the interpretation of the results of immersion tests with nitrate.