Thermal and thermodynamic investigations of ceramic and molten salt systems relevant to the development of advanced nuclear reactors

Vitaliy G Goncharov

doi:10.7273/000005000

As the global energy economy begins to initiate decarbonization, nuclear power becomes an attractive option for sustainable and reliable energy production. Developments of novel experimental methodologies and accumulation of reliable data on materials that are related to the nuclear cycles, particularly those used in the Generation IV (Gen IV) reactor concepts are critical for the advancement of nuclear power systems. This graduate research work outlines four of our recent experimental efforts that investigate enthalpic, elastic and thermophysical parameters of several nuclear materials. These include (1) cerium-doped yttrium aluminum garnets (Y3- xCexAl5O12, Ce:YAGs), with applications as sensors and scintillators, (2) uranium mononitride (UN) and uranium monocarbide (UC) as advanced non-oxide fuel materials, and (3) f-element containing chloride molten salts, used for the development of pyrochemical processing of spent metallic nuclear fuels and as candidate salt fuels for molten salt reactors (MSR). First, in the study of Ce:YAGs, we examined the effects of cerium dopant on the elastic and thermophysical properties of YAGs. Resonant ultrasound spectroscopy (RUS) was used to determine the effects of Ce doping (0.025, 0.1, 1 at. %) on the C11, C12, and C44 elastic moduli of Y3-xCexAl5O12. Analysis of elastic moduli revealed that low Ce dopant concentrations (≤ 0.1 at. %) had negligible effects on elasticity, while higher Ce concentrations (≥ 1 at. %) caused significant softening. This was correlated to the change of the Ce:YAGs microstructures, observed for 1 at.% Ce:YAG by scanning electron microscopy (SEM). Furthermore, analysis of RUS obtained elastic parameters revealed that Zener anisotropy index (Az) increases, while heat capacity (Cp) and thermal conductivity (k) decreased at higher Ce loadings. RUS derived thermophysical parameters were used to simulate thermal stress distributions of Ce:YAGs, with implications of Ce:YAGs used as sensor components at elevated temperatures. The second and third work are centered around thermal oxidation and thermodynamic stability of UN and UC. Both studies used X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) to probe the long range and local structures (respectively) of UN and UC. The corresponding bulk thermal oxidation processes in air were examined using coupled thermogravimetric analysis-differential scanning calorimetry- evolved gas mass spectrometry (TGA-DSC-MS). The bulk oxidation of UN suggested of the existence of UO2 – UN1.5+x surface passivating layer, which hindered the onset of rapid bulk oxidation to 662 K. We also determined the sequence of UN oxidation as UN → UN-UN1.5+x-UO2 → UO2-UO3-Nk → UO3-Nk → UO3 → U3O8. Additionally, the standard enthalpy of formation (ΔH°f ) of UN was determined by performing high temperature transposed temperature drop and oxide melt drop solution calorimetry, through the formations of terminal U3O8 and UO3 phases, respectively. Both methods yielded consistent results (in agreement with previous calorimetric investigations) with ΔH°f = - 144.4 ± 5.9 kJ/mol·atom chosen as the new enthalpic value for the formation of UN. The bulk oxidation of UC was also studied by TGA-DSC-MS, suggesting a step-wise thermal oxidation process: 0.95UC·0.05UO2→ UO3·0.29(CxOy) + 0.66CO2 → UO3·0.20(CxOy) + 0.09CO2 → UO3·0.03(CxOy) + 0.17CO2 → U3O8 + 0.03CO2 + 0.166O2 (with the x:y ratio of ~1:2 and the bulk oxidation onset of 537 K). DSC was further used to examine the enthalpies of reactions (ΔHrxn) associated with the proposed oxidation process. ΔH°f of UC was determined as –50.7 ± 10.8 kJ/mol·atom by oxide melt drop solution calorimetry, again in good agreement with previous studies. Lastly, the enthalpic landscapes of U-N and U-C compounds were established based on our current results and previously determined ΔH°f of β-UN1.5-x, α-UN1.5+x, U2C3, and α-UC1.94. The resulting linear correlations revealed that UN may be stabilized at higher non-stochiometric N/U ratios, while UC is stabilized at lower C/U ratios. The last work presented in this dissertation is the evaluation of molar enthalpy of mixing (ΔHmix) of LaCl3 in molten eutectic 58mol% LiCl – 42mol% KCl. Lanthanides are common neutron poisons and also critical elements. Recycling them from spent nuclear fuels is crucial for the development of effective pyrochemical separation methods. La3+ can also be treated as a surrogate for U3+ . However, previous investigations of ΔHmix in molten LiCl-KCl lacked physically-based mixing models, making it difficult to bridge the gap between experimental thermodynamics and computational interrogations of salt chemistry and solvation structure. In this work, the molecular interaction volume model (MIVM) was used to evaluate experimentally measured ΔHmix of LaCl3 in eutectic LiCl-KCl, together with ab initio molecular dynamics (AIMD) to generate input parameters for MIVM and provide the solvation structure of the melt. Calorimetry-determined ΔHmix were used to benchmark the pair potential energies obtained from AIMD based on the MIVM model. This allowed for the resolution of appropriate distances corresponding to interatomic potentials of La3+ and Li-K + reflecting the averaged solvation structure. These parameters were also used to extrapolate excess Gibb’s energy (ΔGmix) and compositional dependence of La3+ activity coefficient in the system through MIVM, with a future goal of using the obtained values to optimize a phase diagram for the LaCl3 – LiCl – KCl system via the calculation of phase diagram (CALPHAD) method. The results suggest that MIVM can effectively correlate calorimetric experiments and AIMD simulations for molten salt systems.

Thermal and thermodynamic investigations of ceramic and molten salt systems relevant to the development of advanced nuclear reactors

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