Environmental behaviors of spent nuclear fuel and canister materials

Abstract

The world's first spent nuclear fuel repository concept (Swedish KBS-3) is illustrated and the results of experiments on environmental behaviors of spent fuel and canister materials under a potential canister breaching at early stage of disposal are reported. In a deoxygenated synthetic groundwater (2 mM NaHCO₃) under radiation (γ 0.9 Gy h⁻¹), inventory fraction leaching rates of fission-products (¹³⁷Cs, ⁹⁰Sr and ⁹⁹Tc) and actinides (²³⁸U, ²³⁷Np) from a spent fuel segment were found to be around 10⁻⁶ and 10⁻⁷ per day, respectively. A cast-iron canister surface was found to be able to immobilize ²³⁸U, ⁹⁰Sr, ⁹⁹Tc and ²³⁷Np dissolved from spent fuel, but a copper surface could not. In the presence of the oxidative species generated from water radiolysis, the corrosion rates of waste canister materials, copper and cast-iron were found to be 1 and 30 μm per year, respectively. The observation of insignificant dissolution of spent fuel in the leaching solution equilibrated with 0.1 atm H₂ is explained by the reducing effects of H₂ in the presence of fission-product alloy particles (Mo–Tc–Ru–Rh–Pd) as catalysts and dissolved Fe(II) in groundwater. The coating effect of ferric precipitates on spent nuclear fuel dissolution is also discussed.