Source Trends for Performance Assessment of HLW Glass and Spent Fuel as Waste Forms
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source-term development, if both are independent from each other. During the dissolution of the waste form, the geochemical environment and related radionuclide solubility is altered due to temperature, radiolysis, and the release of other elements from the waste form. The consequence is to include source and sink-terms in an integral geochemical model, which also considers the effect of repository rock and near field materials. MECHANISTIC BASIS FOR SOURCE TERM DEVELOPMENT Though glass and spent fuel serve the same purpose, their properties are different. Spent LWR fuel is a heterogeneous substance with highest radionuclide inventories in the UO2 fuel matrix. The glass matrix is more homogeneous, it only contains few percent of small crystallites which probably do not significantly influence radionuclide release. In the language of chemical reaction kinetics, waste form alteration will occur if the affinity (A=Y•t0 is
Z :Z(ATreacti°n-t-A•transp°rt
k-AqPr°ducts"bAiOx generati°n)•>O
l
positive, i.e. if there exist a chemical driving force for dissolution. The overall affinity includes the reaction affinity, A.rea•ti°n= -Y'-•,pr°duct-•lireactant (implying saturation effects of silica with glass and of U(IV) for spent fuel, reaction over potentials, surface reaction rate constants, reactivity of radiolytic species), the affinity associated to chemical gradients At.... po, =Ejajf, c%•inear held(for mass transfer resistance of surface layers, grain boundaries, bentonite barriers etc.), the affinity imposed by sink-terms Aipr°aucts =E•+proauc• ] _•t+prodct z (e.g. pump effects imposed by zeolite formation during glass dissolution, 141 Mat. Res. Soc. Symp. Proc. Vol. 506 ©1998 Materials Research Society
coffinitization or U(IV) sorption on Fe for spent fuel) and the reaction affinity imposed by radiolytic oxidant generation Ai~o-esris. The alteration mechanism of glass and spent fuel under natural water conditions are quite different. Glass is a mixed covalent/ionic solid, whose degradation depends on ion-exchange, surface complexation and Si-saturation. In contrast, spent fuel is a redox sensitive semiconductor, whose dissolution behavior is mainly governed by the redox mass balance at the oxide/solution interface. Both for glass as well as for spent fuel the formation of alteration products and incorporation of radionuclides by precipitation, coprecipitation and sorption is a key mechanism for retention of radionuclides. Spent fuel With respect to radionuclide release from spent fuel one has to distinguish between release from cladding, structural fuel assembly parts, the oxide Rates of spent fuel matrix dissolution fuel matrix, its grain boundaries, fracture surfaces and the fuel sheath gap ("gap release"). The IE+2 Escaping a from cladding is considered to contain only activation products, and small quantities of fission products, 1E+1 resulting from recoil phenomena. The release T from fracture surfaces and the fuel sheath gap 1E+0 (gap release) comprises a significant fraction of a overall radioactivity releas
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