Modeling of irradiated graphite 14C transfer through engineered barriers of a generic geological repository in crystalline rocks

Abstract

There are two RBMK-1500 type graphite moderated reactors at the Ignalina nuclear power plant in Lithuania, and they are under decommissioning now. The graphite cannot be disposed of in a near surface repository, because of large amounts of 14C. Therefore, disposal of the graphite in a geological repository is a reasonable solution. This study presents evaluation of the 14C transfer by the groundwater pathway into the geosphere from the irradiated graphite in a generic geological repository in crystalline rocks and demonstration of the role of the different components of the engineered barrier system by performing local sensitivity analysis. The speciation of the released 14C into organic and inorganic compounds as well as the most recent information on 14C source term was taken into account. Two alternatives were considered in the analysis: disposal of graphite in containers with encapsulant and without it. It was evaluated that the maximal fractional flux of inorganic 14C into the geosphere can vary from 10−11y−1 (for non-encapsulated graphite) to 10−12y−1 (for encapsulated graphite) while of organic 14C it was about 10−3y−1 of its inventory. Such difference demonstrates that investigations on the 14C inventory and chemical form in which it is released are especially important. The parameter with the highest influence on the maximal flux into the geosphere for inorganic 14C transfer was the sorption coefficient in the backfill and for organic 14C transfer – the backfill hydraulic conductivity.