Search

Detailed chemical analysis of the solidified molten fuel still residing in the stricken Chernobyl reactor unit 4 are inferred using multi-modal micro-focus X-ray analysis of a low-radioactivity proxy. A fascinating mixture of molten UO2, nuclear fuel cladding, concrete, stainless steel and other nuclear reactor components, these materials behaved like lava, solidifying to form a complex, highly radioactive glass-ceramic. Using element-specific chemical probes (micro-X-ray fluorescence and X-ray absorption spectroscopy), coupled with micro-diffraction analysis, the crystalline phase assemblage of simulants of these heterogeneous materials was established, which included “chernobylite” and a range of compositions in the (U1−xZrx)O2 solid solution. Novel insight to nuclear accident fuel chemistry was obtained by establishing the oxidation state and local coordination of uranium not only in these crystalline phases, but uniquely in the amorphous fraction of the material, which varied depending on the history of the nuclear lava as it flowed through the reactor. This study demonstrates that micro-focus X-ray analysis of very small fractions of material can yield rich chemical information, which can be applied to nuclear-melt down materials to aid decommissioning and nuclear fuel management at nuclear accident sites.

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.

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.

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.

Decommissioning of the damaged Chernobyl nuclear reactor Unit 4 is a top priority for the global community. Before such operations begin, it is crucial to understand the behaviour of the hazardous materials formed during the accident. Since those materials formed under extreme and mostly unquantified conditions, modelling alone is insufficient to accurately predict their physical, chemical and, predominantly, mechanical behaviour. Meanwhile, knowledge of the mechanical characteristics of those materials, such as their strength, is a priority before robotic systems are employed for retrieval and the force expected from them to be exerted is one of the key design questions. In this paper we target to measurement of the standard mechanical properties of the materials formed during the accident by testing small-scale, low radioactivity simulants. A combined methodology using Hertzian indentation, synchrotron X-ray tomography and digital volume correlation (DVC), was adopted to estimate the mechanical properties. Displacement fields around the Hertzian indentation, performed in-situ in a synchrotron, were measured by analysing tomograms with DVC. The load applied during the indentation, combined with full-field displacement measured by DVC was used to estimate the mechanical properties, such as Young's modulus and Poisson's ratio of these hazardous materials.

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.

International audience; 16 Single-phase monazite-type ceramics are discussed as waste forms for the safe disposal of 17 surplus plutonium or separated minor actinides. To gain a deeper insight into the mechanism 18 governing the dissolution of monazite-type compounds, the dissolution kinetics of synthetic 19 LaPO 4-monazite was systematically studied by dynamic dissolution experiments carried out 20 in the temperature range from 50 to 130°C in acidic media (0.01M HNO 3). The dissolution 21 rates at far from equilibrium conditions increased from 3.2×10-5 g m-2 d-1 at 50°C to 2.5×10-4 22 g m-2 d-1 at 130°C. Two different temperature regions were observed, in which the normalised 23 dissolution rates of LaPO 4 have a diverging temperature dependence, indicating two different 24 mechanisms of dissolution: namely surface-controlled dissolution (T = 50-90°C; E a = 44 25 kJ mol-1) and transport-controlled dissolution (T = 90-130°C; E a = 7.5 kJ mol-1). 26 Complementary thermodynamic modelling studies of the dissolution of LaPO 4 at the 27 experimental conditions show that La-rhabdophane (LaPO 4 ·0.667H 2 O) is the 28 thermodynamically favoured phosphate phase in aqueous environments below about 100°C. 29 Apparently, the hydration of monazite and the formation of a thin surface layer consisting of 30 rhabdophane is an intermediate step controlling the dissolution kinetics of the LaPO 4 31 monazite ceramics in low temperature aqueous environments. 32 33

The chemical durability of monazites LnPO4 (Ln = La → Gd) was investigated. The influence of several parameters on the normalized dissolution rates, including acidity and temperature, was investigated under dynamic conditions. The impact of the saturation processes of the solution was also examined through static experiments. Under the investigated conditions, the normalized dissolution rates were found to be very low for all the lanthanide elements considered (La → Gd). The partial order of the dissolution reaction related to the proton activity varied from 0.7 ± 0.2 to 1.5 ± 0.3 for LnPO4 (Ln = La, Ce, Nd and Gd). The apparent range of activation energies associated with the dissolution of monazites Eapp shifted from 54 to 103 kJ mol−1 below 313 K (characteristic values for dissolution process controlled by surface reactions) to 6–17 kJ mol−1 between 313 K and 363 K. These values are usually associated with mass transport as the rate-limiting step in the dissolution mechanism. This modification suggested an impact of the f(ΔRG) term included in the multiparametric expression for the normalized dissolution rate. Indeed, the Gibbs energy of the dissolution reaction, ΔRG, increased from 298 K to 363 K with a significant break at 313 K. This result indicated that the solutions approached thermodynamic equilibrium with respect to rhabdophane in experiments performed above 313 K. This could impact the normalized dissolution rates, and thus, the parameters associated with kinetics. The study on the long-term behavior of NdPO4 under static conditions (298 K, 0.1 M HNO3) confirmed this point. After 10 days of leaching, the normalized dissolution rate decreased by more than one decade, whereas the saturation index with respect to rhabdophane increased significantly. This strong decrease in the normalized dissolution rate suggests that the saturation index in the solution impacts the normalized dissolution rates, even for solutions that were macroscopically under-saturated with respect to the rhabdophane.

The standard solubility constant of TPHPH, Th2(PO4)2(HPO4)·H2O was evaluated at 298 K by performing dissolution experiments of a synthetic sample in different nitric acid solutions at a constant ionic strength of 1 mol L−1. The average value of the solubility constant and the associated expanded uncertainty U( log 10 K s , 0 ∘ (298 K), 0.95 level of confidence) were evaluated to log 10 K s , 0 ∘ (TPHPH, 298 K) = −(71.2 ± 1.3), while the Δ f G ∘ (TPHPH, 298 K) value reached −(5200 ± 23) kJ·mol−1. From the experiments performed at different temperatures, the standard molar enthalpy as well as the standard molar entropy of the dissolution reaction was determined. The solubility of TPHPH was found to be retrograde over the investigated range of temperatures (298–363) K. The values of the standard enthalpy of formation and the standard molar entropy of TPHPH are equal to: Δ f H ∘ ( TPHPH ) = - ( 5640.5 ± 16.0 ) kJ · mol - 1 and Sm = (298 ± 68) J K−1 mol−1, respectively.

Thorium incorporation in the rhabdophane structure as Pr 1-2x Ca x Th x PO 4 ·nH 2 O solid solutions was successfully achieved and resulted in the preparation of a low temperature precursor of the monazite-cheralite type Pr 1-2x Ca x Th x PO 4 . The rhabdophane compounds are considered as potential neoformed phases in case of release of actinides from the phosphate-based ceramic wasteforms envisaged to host radionuclides in the back-end of the nuclear fuel cycle. A multiparametric study was thus undertaken to specify the wet chemistry conditions (starting stoichiometry, temperature, heating time) leading to single phase Pr 1-2x Ca x Th x PO 4 ·nH 2 O powdered samples. The excess of calcium appeared to be a prevailing factor with a suggested initial Ca:Th ratio being equal to 10. Similarly, the recommended heating time should exceed 4 days while the optimal temperature of synthesis is 110 °C. Under these conditions, the stability domain of Pr 1-2x Ca x Th x PO 4 ·nH 2 O ranged from x = 0.00 to x = 0.15. After heating at 1100 °C under air during 6 h, rhabdophane-type samples were fully converted into the highly durable Pr 1-2x Ca x Th x PO 4 cheralite ceramic wasteform.