Permeability of HTR fuel element graphite for technetium, cesium, and neptunium

Spent fuel from the operating German High Temperature Gas Cooled Reactor THTR-300 (300 MWe) will not be reprocessed but stored in a final repository after suitable conditioning. An accidental ingress of water into the repository could lead to the formation of a brine solution which may corrode the multiple barrier system components and leach radionuclides from the fuel microspheres. Mobilized material would then migrate through the porous graphite hull which forms an integral part of the spherical fuel element. Although retardation capabilities of this graphite were not presumed, it was of interest to investigate its permeability for some typical long-lived radionuclides such as Tc-99, Cs-137, and Np-237 in order to supply data for the safety assessment of the final repository. It was shown that radionuclides permeate through graphite to a considerable extent, especially from fuel elements undergoing pressure cycles when stored under a brine solution.