Development of inversion code of radionuclide migration

One of key issues in assessing a safety of a deep geological disposal system for high-level nuclear waste is to evaluate the effects of model uncertainty and data uncertainty on the results of the radionuclide migration analysis. Therefore, it is crucial to develop a tool which evaluate appropriateness of a model and the reliability of data. In this work, inversion code of LTG, which solves radionuclide migration in fractured media owned by JNC, is developed. The code identify optimum parameters that explain observed data and calculates variance (correspond to uncertainty/reliability) of optimum parameters and model identification criteria. From the results of preliminary analysis, it is found that (a) identified parameter value is initial value dependent, and (b) solution is non-unique in case of weak-sensitive parameter or a certain combination of parameters such as velocity and sectional area of fracture. In order to obtain better solution, it is effective; (a) to decrease the range of identified parameter, (b) not to use the same initial values for multiple pathways, and (c) not to set initial values equal to upper and lower limit of the range. (author)