Modelling of calcium leaching and its influence on radionuclide migration across the concrete engineered barrier in a NSDF

• The variation of porosity across the concrete barrier due to calcium leaching is non-linear.
• H-3, Cs-137, I-129 and Sr-90 have migrated higher than Tc-99, Co-60, Pu-239 and Am-241.
• Sorption played a dominant role compared to diffusion coefficient in radionuclide migration.
• Calcium leaching has shown considerable influence on radionuclide migration.
• The mean increase in migration distance due to Ca leaching is 0.69 ± 0.15 cm for 100 yrs.

A multi barrier concept of disposal is adopted for both low and intermediate level radioactive waste disposal to prevent the spread of radionuclides into the human environment. Concrete engineered barrier is a part of multi barrier system for radioactive waste disposal. As long as the concrete barrier is intact, the containment of radioactive waste within the disposal facility is assured. However, every concrete structure has a finite lifetime. Concrete barriers degrade with time with lifetime dependent on type of concrete, external and internal environmental factors. This degradation allows the free moment of the radionuclides out of the concrete barrier. Hence an attempt was made to model the Calcium (Ca) leaching from side wall of a concrete engineered barrier in a Near Surface Disposal Facility (NSDF) and its influence on radionuclide migration across the concrete barrier, limited to conditions specified in the paper. The conceptual model assumes that rain water seeps into the engineered barrier through the top cover and dissolves radionuclides from the nuclear waste matrix. The modelling was attempted with Finite Difference analysis and validated with the results available in literature. The influence of Ca leaching on radionuclide migration in concrete barrier has been discussed for eight different radionuclides (Cs-137, Sr-90, I-129, H-3, Pu-239, Am-241, Co-60 and Tc-99).