The redox variations and changes in local atomic environment of uranium (U) interacted with the magnetite nanoparticles were studied in a proof of principle experiment by the U L-3 and M-4 edges high energy resolution X-ray absorption near edge structure (HR-XANES) technique. We designed and applied a mixed flow reactor (MFR) set-up to maintain dynamic flow conditions during U-magnetite interactions. Formation of hydrolyzed, bi- and poly-nuclear U species were excluded by slow continuous injection of U(VI) (10(-6) M) and pH control integrated in the MFR set-up. The applied U HR-XANES technique is more sensitive to minor changes in the U redox states and bonding compared to the conventional XANES method. Major U(VI) contribution in uranyl type of bonding is found in the magnetite nanoparticles after three days operation time of the MFR. Indications for shortening of the U-O-axial bond length for the magnetite compared to the maghemite system are present too.

During reactor operation the fission gases Kr and Xe are formed within the UO2 matrix of nuclear fuel. Their quantification is important to evaluate their impact on critical parameters regarding the fuel behaviour during irradiation and (long-term) interim storage, such as internal pressure of the fuel rod and fuel swelling. Moreover the content of Kr and Xe in the plenum of a fuel rod and their content in the UO2 fuel itself are widely used as indicators for the release properties of I-129, Cs-137, and other safety relevant radionuclides with respect to final disposal of spent nuclear fuel. The present study deals with the fission gas release from spent nuclear fuel exposed to simulated groundwater in comparison with the fission gas previously released to the fuel rod plenum during irradiation in reactor. In a unique approach we determined both the Kr and Xe inventories in the plenum by means of a puncturing test and in leaching experiments with a cladded fuel pellet and fuel fragments in bicarbonate water under 3.2 bar H-2 overpressure. The fractional inventory of the fission gases released during irradiation into the plenum was (8.3 +/- 0.9) %. The fraction of inventory of fission gases released during the leaching experiments was (17 +/- 2) % after 333 days of leaching of the cladded pellet and (25 +/- 2) % after 447 days of leaching of the fuel fragments, respectively. The relatively high release of fission gases in the experiment with fuel fragments was caused by the increased accessibility of water to the Kr and Xe occluded in the fuel. (C) 2016 Elsevier B.V. All rights reserved.

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This paper presents the current status of the experiments performed at INE within the Work-packages 2 and 3 of the EURATOM FP7 Collaborative Project, “Fast / Instant Release of Safety Relevant Radionuclides from Spent Nuclear Fuel (CP FIRST-Nuclides)”. The segment N0204 from the pressure water reactor of Gosgen (KKG) nuclear power plant was selected to carry out this project. In a first approach and before the performance of leaching experiment, the fission gas release of this segment was determined to be 8.35% (in vol.). Leaching experiment of a cladded fuel pellet in bicarbonate water (19 mM NaHCO3 + 1 mM NaCl) under an Ar and H2 atmosphere with a total pressure of (40 ± 1) bar (pH2: 3 ± 1bar) were started. After a cumulative contact time of 57 days, 4.3% of the Xe and 17% of the Kr inventories of the cladded fuel pellet segment was released into the gas phase.

Low- and intermediate-level radioactive wastes are frequently solidified in a cement matrix. In a potential repository for nuclear wastes, the cementitious matrix is altered upon contact with solution and the resulting secondary phases may provide for significant retention of the radionuclides incorporated in the wastes. In order to assess the secondary phases formed upon corrosion in chloride-rich solutions, which are relevant for nuclear waste disposal in rock salt, leaching experiments were performed. Conventional laboratory batch experiments using powdered hardened cement paste in MgCl2-rich solutions were left to equilibrate for up to three years and full-scale cemented waste products were exposed to NaCl-rich and MgCl2-rich solutions for more than twenty years, respectively. Solid phase analyses revealed that corrosion of hardened cement in MgCl2-rich solutions advanced faster than in NaCl-rich solutions due to the extensive exchange of Mg from solution against Ca from the cementitious solid. Thermodynamic equilibrium simulations compared well to results at the final stages of the respective experiments indicating that close to equilibrium conditions were reached. At high cement product to brine ratios (>0.65 g mL−1), the solution composition in the laboratory-scale experiments was close to that of the full-scale experiments (cement to brine ratio of 2.5 g mL−1) in the MgCl2 systems. The present study demonstrates the applicability of thermodynamic methods used in this approach to adequately describe full-scale long-term experiments with cemented waste simulates.

In pulse radiolysis experiments at 22°C production and decay of Cl2− radicals are studied in 0.1 and 1.0mol dm−3 NaCl solutions by observing its optical absorption over a time span of 20ms. In addition to experiments with NaCl solutions equilibrated with N2O at ambient pressure, a series of experiments are conducted with NaCl solutions equilibrated at 10MPa hydrogen partial pressure and 0.1MPa N2O partial pressure. In the presence of hydrogen, the Cl2− yield is significantly reduced compared to that in hydrogen free experiments. The effect of hydrogen on the radiolytic yield is more pronounced in 0.1mol dm−3 NaCl solution than in the relatively concentrated NaCl solution. In parallel to the experiments the evolution of the Cl2− concentration is simulated using a kinetic model. Based on the comparison between measured and simulated optical absorption, which is mainly caused by the Cl2− radical, the rate constant of reaction Cl2−+Cl2−=Cl−+Cl3− is determined as 5.2(±0.8)×108Lmol−1s−1 at zero ionic strength. This value is within the range of published rate constants for the Cl2− disproportionation reaction.