Spent Fuel Leaching under Anoxic Conditions and the Effect of Canister Materials
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6SHQW)XHO/HDFKLQJXQGHU$QR[LF&RQGLWLRQVDQGWKH(IIHFWRI&DQLVWHU0DWHULDOV Daqing Cui 1, Jeanett Low1, Max Lundström1, Kastriot Spahiu2 Studsvik Nuclear AB, SE-61182, Nyköping, Sweden 2 SKB, SE-10240, Stockholm, Sweden
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$%675$&7 The results of a spent fuel leaching experiment in which a fuel pin (17.7 g) was contacted with 380 mL of a 10 mM NaCl, 2 mM NaHCO3 solution by taking special care to minimize atmospheric oxygen contamination are presented. During the first 287 days, the fractions of inventory in the aqueous phase per day (f/d) increased nearly constantly for all nuclides (except for 100Mo), but were higher for fission products f/d(137Cs)=1.2⋅10-6 , f/d(99Tc)=1.1•10-6 and f/d(90Sr)= 6.7•10-7 than for actinides: f/d (238U) =1.0 •10-7, f/d(237Np)= 2.6 •10-7and f/d(239Pu) = 5.1 -9 2 238 •10 . After adding iron, cast iron and copper foils (of ~30 mm size), the concentrations of U, 237 99 Np and Tc decreased by 80%, 97% and 88% to relatively stable levels (500ppb, 0.2 ppb and 0.6 ppb respectively). 239Pu concentrations increased from a level around 0.05 ppb to PuO2 solubility level, 0.5 ppb, and stabilized. The leaching process for 137Cs, 100Mo and 90Sr seems not to be influenced by the addition of metal foils. The observations in the present work contribute to an improved understanding of the behavior of spent fuel under near field repository conditions. ,1752'8&7,21 So far, the disposal of high level radioactive waste is still a challenge worldwide. The uncertainties in the assessment of radionuclide transport in the far-field (geosphere and biosphere) are high due to the difficulty of predicting environmental changes. It is relatively easier to gain confidence in performance assessment of a nuclear waste repository by improving our understanding of the long term behavior of high level nuclear waste and near field multibarriers. In Sweden, spent fuel (SF) will be placed in canisters with a corrosion resistant copper shell and a massive cast iron insert, embedded in compacted bentonite and disposed in a deep hard rock repository [1, 2]. In performance assessment it is conservatively assumed that some of the canisters are breached and SF will get into contact with groundwater in 103 years [2]. SF from light water reactors is a heterogeneous material, consisting of a matrix of nearly stoichiometric UO2(s). The actinides (as e.g. 237Np, 239 Pu) and the majority of fission and activation products (e. g. Sr and the lanthanide oxides) exist in the UO2 matrix as solid solutions. Only a small part of some fission products (e.g., 137Cs and 131I) migrate to the fuel-cladding gap or gain boundaries. Mo, Tc, Ru, Rh and Pd form separate alloy particles. Since 1982, the SKB spent fuel corrosion program has been performed in the Hot Cell Laboratory of Studsvik Nuclear AB, using a large number of SF segments, under oxic and anoxic conditions [3-5]. The redox conditions are by far the most important factor influencing the dissolution of the spent fuel. In the case of a canister failure, the dissolution of SF will be
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