Estimation of the effective surface area of UOX (MOX) spent fuel in Supercontainer disposal conditions (status 2010)

Since a few years, the Belgian agency for radioactive waste (ONDRAF/NIRAS) has selected the Supercontainer design with an Ordinary Portland Cement (OPC) buffer as the reference design for geological disposal of High-Level Waste (HLW) and Spent Fuel (SF) in the Boom Clay formation. To assess the long-term performance of the spent fuel disposal in these conditions, it is necessary to quantify the long term release rate of radionuclides, which is governed by the matrix dissolution rate. The effective surface area of the fuel is thus an important parameter to convert the surface normalized dissolution rates measured experimentally into fractional dissolution rates used in performance assessment calculations, and to allow comparison between tests with different fuel samples. The effective surface area measured on today’s fuels can be considered as the initial effective surface area, which is an intrinsic fuel property, independent from the disposal design. Based on a literature review on the initial specific surface area of various fuels from other countries, this topical report proposes a range of reference surface areas for the Belgian reference fuels (45 to 60 GWd.tHM-1), that can be used in performance assessment.