Synthesis, characterisation and corrosion behaviour of simulant Chernobyl nuclear meltdown materials

Understanding the physical and chemical properties of materials arising from nuclear meltdowns, such as the Chernobyl and Fukushima accidents, is critical to supporting decommissioning operations and reducing the hazard to personnel and the environment surrounding the stricken reactors. Relatively few samples of meltdown materials are available for study, and their analysis is made challenging due to the radiation hazard associated with handling them. In this study, small-scale batches of low radioactivity (i.e., containing depleted uranium only) simulants for Chernobyl lava-like fuel-containing materials (LFCMs) have been prepared, and were found to closely approximate the microstructure and mineralogy of real LFCM. The addition of excess of ZrO2 to the composition resulted in the first successful synthesis of high uranium–zircon (chernobylite) by crystallisation from a glass melt. Use of these simulant materials allowed further analysis of the thermal characteristics of LFCM and the corrosion kinetics, giving results that are in good agreement with the limited available literature on real samples. It should, therefore, be possible to use these new simulant materials to support decommissioning operations of nuclear reactors post-accident

Synthesis, characterisation and preliminary corrosion behaviour assessment of simulant Fukushima nuclear accident fuel debris

Nuclear fuel debris generated at the Fukushima Daiichi nuclear power plant during the loss of coolant accident in 2011, still resides within the reactor units, constantly cooled by water. Until it is retrieved, the fuel debris will corrode, releasing radioactive elements into the coolant water and the ground surrounding the reactors. To predict the corrosion behaviour of these materials, and to establish parameters for experiments with U-containing and real fuel debris, the corrosion of two surrogate fuel debris materials, with a composition of Ce(1-x)ZrxO2 (x = 0.2 and 0.4), was investigated. Materials were synthesised by a wet chemistry route and pellets were sintered at 1700°C in air atmosphere. Due to the slow corrosion kinetics, aggressive conditions were applied, and corrosion experiments were performed in 9 mol.L^-1 HNO3 under static conditions. The incorporation of Zr into the structure of Ce reduced the normalised dissolution rate; from (3.75 ± 0.15) × 10-6 g.m-2.d-1 to (4.96 ± 0.28) × 10^-6 g.m^-2.d^-1 for RL(Ce) of Ce0.8Zr0.2O2 and Ce0.6Zr0.4O2, respectively

Characterisation of a complex CaZr0.9Ce0.1Ti2O7 glass–ceramic produced by hot isostatic pressing

The behaviour of Ce-containing zirconolites in hot isostatically pressed (HIPed) materials is complex, characterised by redox interactions between the metallic HIP canister that result in reduction of Ce4+ to Ce3+. In this work, a glass–ceramic of composition 70 wt.% CaZr0.9Ce0.1Ti2O7 ceramic in 30 wt.% Na2Al2Si6O16 glass was produced by HIP (approx. 170 cm3 canister) to examine the extent of the material–canister interaction. A complex material with six distinct regions was produced, with the extent of Ce reduction varying depending on the distance from the canister. Notably, the innermost bulk regions (those approximately 7 mm from the canister) contained only Ce4+, demonstrating that a production-scale HIPed glass–ceramic would indeed have a bulk region unaffected by the reducing environment induced by a ferrous HIP canister despite the flow of glass at the HIP temperature. Each of the six regions was characterised by XRD (including Rietveld method refinements), SEM/EDX and linear combination fitting of Ce L3-edge XANES spectra. Regions in the lower part of the canister were found to contain a significantly higher fraction of Ce4+ compared to the upper regions. Though zirconolite-2M was the major crystalline phase observed in all regions, the relative abundances of minor phases (including sphene, baddeleyite, rutile and perovskite) were higher in the outermost regions, which comprised a significantly reduced Ce inventory

Influence of accessory phases and surrogate type on accelerated leaching of zirconolite wasteforms

A fraction of the UK Pu inventory may be immobilised in a zirconolite ceramic matrix prior to disposal. Two zirconolite compositions, targeting CaZr0.80Ce0.20Ti2O7 and CaZr0.80U0.20Ti2O7, were fabricated by hot isostatic pressing, alongside a reformulated composition, nominally Ca0.80Zr0.90Ce0.30Ti1.60Al0.40O7, with an excess of Ti and Zr added to preclude the formation of an accessory perovskite phase. Materials were subjected to accelerated leaching in a variety of acidic and alkaline media at 90 °C, over a cumulative period of 14 d. The greatest Ce release was measured from CaZr0.80Ce0.20Ti2.00O7 exposed to 1 M H2SO4, for which 14.7 ± 0.2% of the original Ce inventory was released from the wasteform into solution. The extent of Ce leaching into the solution was correlated with the quantity of perovskite present in the wasteform, and associated with the incorporation and preferential dissolution of Ce3+. CaZr0.80U0.20Ti2.00O7 exhibited improved leach resistance relative to CaZr0.80Ce0.20Ti2.00O7, attributed to the decreased proportion of accessory perovskite, with 7.1 ± 0.1% U released to in 8 M HNO3 after 7 d. The Ca0.80Zr0.90Ce0.30Ti1.60Al0.40O7 composition, with no accessory perovskite phase, presented significantly improved leaching characteristics, with < 0.4%Ce released in both 8 M HNO3 and 1 M H2SO4. These data demonstrate the need for careful compositional design for zirconolite wasteforms with regard to accessory phase formation and surrogate choice

Short communication : the dissolution of UK simulant vitrified high-level-waste in groundwater solutions

Dissolution of a simulant UK nuclear waste glass containing Mg, Ca and Zn was investigated over 35 d at 50 °C in water and simulant groundwater solutions. The dissolution rates were influenced subtly by the groundwater composition, following the trend, from least to most durable: clay > water > granite ≈ saline. Solutions were rapidly silica saturated but boron dissolution rates continued to increase. This is hypothesised to be due to the formation of secondary Mg-silicate precipitates, preventing the formation of a passivating silica gel layer and allowing glass dissolution to proceed at close to the maximum rate

Synthesis, characterisation and preliminary corrosion behaviour assessment of simulant Fukushima nuclear accident fuel debris

Nuclear fuel debris generated at the Fukushima Daiichi nuclear power plant during the loss of coolant accident in 2011, still resides within the reactor units, constantly cooled by water. Until it is retrieved, the fuel debris will corrode, releasing radioactive elements into the coolant water and the ground surrounding the reactors. To predict the corrosion behaviour of these materials, and to establish parameters for experiments with U-containing and real fuel debris, the corrosion of two surrogate fuel debris materials, with a composition of Ce(1-x)ZrxO2 (x = 0.2 and 0.4), was investigated. Materials were synthesised by a wet chemistry route and pellets were sintered at 1700°C in air atmosphere. Due to the slow corrosion kinetics, aggressive conditions were applied, and corrosion experiments were performed in 9 mol.L-1 HNO3 under static conditions. The incorporation of Zr into the structure of Ce reduced the normalised dissolution rate; from (3.75 ± 0.15) × 10-6 g.m-2.d-1 to (4.96 ± 0.28) × 10-6 g.m-2.d-1 for RL(Ce) of Ce0.8Zr0.2O2 and Ce0.6Zr0.4O2, respectively.</p

Characterisation of a Complex CaZr0.9Ce0.1Ti2O7 Glass&ndash;Ceramic Produced by Hot Isostatic Pressing

The behaviour of Ce-containing zirconolites in hot isostatically pressed (HIPed) materials is complex, characterised by redox interactions between the metallic HIP canister that result in reduction of Ce4+ to Ce3+. In this work, a glass&ndash;ceramic of composition 70 wt.% CaZr0.9Ce0.1Ti2O7 ceramic in 30 wt.% Na2Al2Si6O16 glass was produced by HIP (approx. 170 cm3 canister) to examine the extent of the material&ndash;canister interaction. A complex material with six distinct regions was produced, with the extent of Ce reduction varying depending on the distance from the canister. Notably, the innermost bulk regions (those approximately 7 mm from the canister) contained only Ce4+, demonstrating that a production-scale HIPed glass&ndash;ceramic would indeed have a bulk region unaffected by the reducing environment induced by a ferrous HIP canister despite the flow of glass at the HIP temperature. Each of the six regions was characterised by XRD (including Rietveld method refinements), SEM/EDX and linear combination fitting of Ce L3-edge XANES spectra. Regions in the lower part of the canister were found to contain a significantly higher fraction of Ce4+ compared to the upper regions. Though zirconolite-2M was the major crystalline phase observed in all regions, the relative abundances of minor phases (including sphene, baddeleyite, rutile and perovskite) were higher in the outermost regions, which comprised a significantly reduced Ce inventory