Uncertainty and sensitivity analysis of radionuclide migration through the engineered barriers of deep geological repository: Case of RBMK-1500 SNF

• Importance of uncertainty of main transport related parameters is nuclide specific.
• Parameter ranking varies for performance metrics (peak flux, time dependent flux).
• Uncertainty of defect enlargement is important for the mean flux of 129I.
• 129I and 226Ra peak flux is insensitive to defect enlargement time.

Like other nuclear countries, Lithuania is facing the problem of spent nuclear fuel (SNF) disposal from RBMK1-1500 type reactors at the Ignalina NPP (~22,000 of the SNF assemblies). The application of probabilistic and deterministic methods for uncertainty assessment and important parameter ranking is of high importance in order to build confidence in the assessment results on radionuclide migration and indicates directions for further uncertainty reduction.This study is devoted to the uncertainty and sensitivity analysis of radionuclide migration through the engineered barriers of a conceptual geological repository for RBMK-1500 SNF disposal. The analysis was performed using computer codes AMBER (United Kingdom) and MATLAB (Sweden). The analysis showed that the uncertainty of defect enlargement time had no significant impact on 129I and 226Ra peak flux from the engineered barriers yet it was an important parameter for time dependent mean flux of 129I. Sensitivity analysis revealed that in the case of 129I peak flux, further investigations should be oriented to more precise determination of diffusion coefficient in bentonite, instant release fraction and SNF dissolution rate. In the case of 226Ra, efforts to reduce the uncertainty of SNF dissolution rate, sorption coefficient in bentonite and equivalent flow rate should be made.