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The European Space Agency is funding the research and development of 241Am-bearing oxide-fuelled radioisotope power systems (RPSs) including radioisotope thermoelectric generators (RTGs) and European Large Heat Sources (ELHSs). The RPSs’ requirements include that the fuel’s maximum temperature, Tmax, must remain below its melting temperature. The current prospected fuel is (Am0.80U0.12Np0.06Pu0.02)O1.8. The fuel’s experimental heat capacity, Cp, is determined between 20 K and 1786 K based on direct low temperature heat capacity measurements and high temperature drop calorimetry measurements. The recommended high temperature equation is Cp(T/K) = 55.1189 + 3.46216 × 102 T − 4.58312 × 105 T−2 (valid up to 1786 K). The RTG/ELHS Tmax is estimated as a function of the fuel thermal conductivity, k, and the clad’s inner surface temperature, Ti cl, using a new analytical thermal model. Estimated bounds, based on conduction-only and radiation-only conditions between the fuel and clad, are established. Estimates for k (80–100% T.D.) are made using Cp, and estimates of thermal diffusivity and thermal expansion estimates of americium/uranium oxides. The lowest melting temperature of americium/uranium oxides is assumed. The lowest k estimates are assumed (80% T.D.). The highest estimated Tmax for a ‘standard operating’ RTG is 1120 K. A hypothetical scenario is investigated: an ELHS Ti cl = 1973K-the RPSs’ requirements’ maximum permitted temperature. Fuel melting will not occur.

Cesium and iodine, which are formed during a fission process in a nuclear reactor, are considered as major fission products responsible for the environmental burden in case of a nuclear accident. From the safety point of view, it is thus important to understand their release mechanism when overheating of the reactor core occurs. This work presents an experimental investigation of the behaviour of caesium iodide and caesium fluoride in fluoride based molten salt reactor fuel during high temperature events. It has been demonstrated that CsF will be retained in the fuel salt and thus its volatility will be significantly reduced, while CsI will not dissolve in the fluoride-based fuel matrix and will thus remain more volatile. The influence of the presence of CsI and CsF on the melting behaviour of the fuel has been investigated using calorimetry, revealing their negligible effects.

In order to progress in the development of Lead-cooled Fast Reactors, from the safety point of view it is essential to understand the chemical compatibility between liquid lead and uranium oxide. In the present work, entropy and heat capacity of Pb3U11O36, a possible ternary compound coming from fuel-coolant chemical interaction, were determined for the first time. Entropy at 298.15 K was obtained from low temperature heat capacity measurements using the Physical Property Measurement System (PPMS) in the temperature range 2–300 K, while the high temperature heat capacity has been measured by a drop calorimeter from 373 K to 1200 K. The experimental thermodynamic properties were compared with the values computed by means of DFT-GGA simulations, obtaining a very good agreement.

The low-temperature heat capacity of (U 1-y ,Am y )O 2− x solid solution with y = 0.0811 and 0.2005 and x = 0.01–0.03 was determined from a minimum of 12.52 K up to 297.1 K and from 9.77 K up to 302.3 K, respectively, using hybrid adiabatic relaxation calorimtry. The low temperature heat capacity results of the investigated system revealed the absence of the magnetic transition specific for UO 2 in the temperature region of 30 K. Since there are no experimental data available for AmO 2 in this temperature region, the results obtained for the intermediate compositions are validated based on the experimental data of UO 2 end-member and the low-temperature heat capacity computation of AmO 2 . In the measured temperature interval, excess heat capacity was observed for the two investigated intermediate compositions, which is concluded to be dominated by self-radiation effects at very low temperature.

PuF3 was synthetized by hydro-fluorination of PuO2 and subsequent reduction of the product by hydrogenation. The obtained PuF3 was analysed by X-Ray Diffraction (XRD) and found phase-pure. High purity was also confirmed by the melting point analysis using Differential Scanning Calorimetry (DSC). PuF3 was then used for thermodynamic assessment of the PuF3-LiF system. Phase equilibrium points and enthalpy of fusion of the eutectic composition were measured by DSC. XRD analyses of selected samples after DSC measurement confirm that after solidification from the liquid, the system returns to a mixture of LiF and PuF3.

The authors regret to inform that the thermodynamic data [Formula presented] of the pure compound CsI(l) was not reported correctly in Table 3. The corresponding enthalpy of formation should read: [Formula presented]

A thermodynamic model for the Na-O system was developed for the first time using the CALPHAD method after review of the structural, thermodynamic, and phase diagram data available on this system. Differential Scanning Calorimetry measurements were moreover performed to assess the phase equilibria and liquidus temperature in the Na2O-Na2O2 composition range. A CALPHAD model for the Na-U-O system was furthermore developed on the basis of both reviewed experimental data, and thermodynamic functions of the sodium uranates derived by combining ab initio calculations and a quasi-harmonic statistical model. The phase equilibria in this ternary system are particularly relevant for the safety assessment of the nuclear fuel-sodium coolant interaction in Sodium-cooled Fast reactors (SFRs). The model predicts the stability of the ternary phase field UO2-Na3UO4-Na4UO5, which is consistent with the most recent literature data. Further optimization was moreover performed to fit the sodium partial pressures measured experimentally in the NaUO3-Na2U2O7-UO2 and NaUO3-Na2UO4-Na2U2O7 phase fields, yielding an overall consistent description. Finally, the oxygen content required to form pentavalent Na3UO4 and hexavalent Na4UO5 in liquid sodium at 900 K were calculated to be 0.7 and 1.5 wppm, respectively, which are levels typically encountered in SFRs.

In this study we present a comprehensive thermodynamic description of the binary CsF-ThF4 system. The phase equilibria of several intermediate compositions in this system have been determined using the DSC technique combined with a post-analysis using powder X-ray diffraction. Considering all the novel experimental data, a thermodynamic model for the CsF-ThF4 system has been developed for the first time using the Calphad approach. The present model reproduces very well the measurements performed. Knudsen effusion mass spectrometry (KEMS) has further been used to investigate the vapour pressure over the molten CsF-ThF4 salt and to determine the thermodynamic activities of CsF and ThF4 in the liquid solution. As part of this study, the vaporization of pure CsF was examined and the results were compared with the literature showing a good agreement. Next, the vapour pressure of CsF-ThF4 in the liquid solution was investigated by measuring three samples with compositions X ThF4 = (0.4, 0.6, 0.8) mol/mol. A strong negative deviation from Raoult’s law was observed for both species, more evident in case of CsF, and a good agreement with the predictions of our thermodynamic model was found. This article is a pre-requisite for the assessment of the ternary LiF-CsF-ThF4 system.

Mixed uranium and americium dioxides (U, Am)O 2±x are candidates as possible transmutation targets for generation IV reactors. In this work, the enthalpy increments of this solid solution were measured in the 470–1750 K temperature range by drop calorimetry for Am/(Am + U) ratios equal to 0.32, 0.39, 0.49, 0.58 and 0.68. Then, the heat capacity functions were obtained by derivation of the enthalpy data. The results of this work were compared to the heat capacity and enthalpy functions reported in the literature for the UO 2 [1] and AmO 2 [2] binary oxides and for the U 0.9 Am 0.1 O 2±x , U 0.8 Am 0.2 O 2±x mixed oxides [3]. From the obtained trend, it was found out that an excess contribution to the enthalpy increment appears for T > 1100 K in the compositions with Am/(Am + U)≥0.4 and a possible explanation attributing this effect to oxygen hypostoichiometry is provided. Finally, to verify the hypothesis, thermodynamic computations based on the CALPHAD method were performed for AmO 2-x under air and the results confirmed that the source of the excess contribution is the formation of oxygen vacancies.

This chapter will briefly describe the current state-of-the-art of molten salt reactor research and describe various concepts studied worldwide. A variety of fuel salts will be identified and their chemical aspects discussed with an emphasis on the importance of chemical behavior on smooth operation of the reactor in the context of performance and safety. As there is much more data on actinoid-containing fluoride systems, the current chapter mainly addresses fluoride-based fuels, but where appropriate comparisons to chloride systems are made as well as halides. It will address the relevance of various thermophysical properties on the performance of the molten salt reactor with insight on the fission product chemistry and its influence on the fuel behavior.

Basic thermodynamic and electrochemical data of pure actinide fluorides and their mixtures are required for the design and safety assessment of any presently studied molten salt reactor concept based on molten fluoride salt fuel. Since the actinide fluorides are usually not produced commercially, they have to be prepared from the available input materials, typically oxides. In this work, a specially designed facility for synthesis of pure actinide fluorides using pure HF gas is described, as well as a complete procedure of synthesis and characterisation of pure UF4 and ThF4. The fluorination installation consists of a glove box kept under a purified argon atmosphere, a high temperature horizontal fluorination reactor and a HF supply gas line connected to the glove box. The fluorides were synthesised from high specific surface oxides prepared from the respective oxalates by low temperature calcination. The fluorination was partly stationary and partly in a HF gas flow, based on a heterogeneous powder-gas reaction at high temperatures. The products were characterised by X-ray diffraction and differential scanning calorimetry, which confirmed high purity products obtained by this method. Moreover, the melting point of UF4 was revised using a pure sample and a new value is suggested.

Four fluoride fuel salt samples (78LiF-22ThF4) in graphite crucibles were irradiated in the HFR Petten for a duration of 508 Full Power Days under the name SALIENT-01 (SALt Irradiation ExperimeNT). Goal of the experiment was to gain experience with the design of liquid salt experiments and the handling of the salts before and after irradiation. Specific research goals for SALIENT-01 are (i) to confirm claims of good fission product retention in the salt, (ii) to obtain size distributions for noble metal particles using Transmission Electron Microscopy and (iii) to assess possible interactions between fuel salt and fine-grained nuclear graphite, as well as possible uptake of fission products by the graphite. Here the design and irradiation history of the experiment are discussed together with plans for post-irradiation examinations. Limitations in representativeness of this experiment and capsule irradiations in general are discussed as well as follow-up actions to improve the quality of future irradiations.

Investigations on the phase relation in the Pb-U-O ternary system have shown the formation of three lead uranium(VI) oxides: PbUO 4 , Pb 3 UO 6 , and Pb 3 U 11 O 36 . We present here the synthesis, thermal expansion, low- and high-temperature heat capacity, and thermal diffusivity of two representative phases (PbUO 4 and Pb 3 UO 6 uranates). Combining these results, a value for their thermal conductivity was derived and reported here for the first time.

The Raman features of neptunium dioxide (NpO2) and uranium–neptunium mixed oxides were investigated. Raman spectra of 16O and 17O-NpO2 enriched sample were measured with excitation energy range of Ei = 1.91–2.54 eV. Raman spectra of both NpO2 samples and the uranium–neptunium rich mixed oxides show an asymmetry at low energies of the Γ25′ (T2g) mode. The asymmetry was evidenced to originate from an electronic band at 431 cm–1 which originates from crystal field splitting. This assignment is in good agreement with theoretical models for NpO2 with the hypothesis that the electron–phonon bound state is decoupled at high temperatures. The asymmetry of the Γ25′ mode can be considered as a fingerprint of the NpO2 lattice.

Quantum chemical calculations of the molecular properties of the UO 2 (OH) 2 molecule are presented. From the results the thermodynamic properties of this gaseous species have been calculated, and these have been used to re-evaluate the existing literature study on the volatilization of U 3 O 8 in steam, allowing to derive the enthalpy of formation of the UO 2 (OH) 2 molecule.

A thermodynamic assessment of the KF-ThF4 binary system using the CALPHAD method is presented, where the liquid solution is described by the modified quasichemical formalism in the quadruplet approximation. The optimization of the phase diagram is based on experimental data reported in the literature and newly measured X-ray diffraction and differential scanning calorimetry data, which have allowed to solve discrepancies between past assessments. The low temperature heat capacity of α-K2ThF6 has also been measured using thermal relaxation calorimetry; from these data the heat capacity and standard entropy values have been derived at 298.15 K: Cp,mo(K2ThF6,cr,298.15K)=(193.2±3.9) J·K-1·mol-1 and Smo(K2ThF6,cr,298.15K)=(256.9±4.8) J·K-1·mol-1. Taking existing assessments of the relevant binaries, the new optimization is extrapolated to the ternary systems LiF-KF-ThF4 and NaF-KF-ThF4 using an asymmetric Kohler/Toop formalism. The standard enthalpy of formation and standard entropy of KNaThF6 are re-calculated from published e.m.f data, and included in the assessment of the ternary system. A calculated projection of the NaF-KF-ThF4 system at 300 K and the optimized liquidus projections of both systems are compared to published phase equilibrium data at room temperature and along the LiF-LiThF5 and NaF-KThF5 pseudobinaries, with good agreement.

The mixture LiF-ThF4-UF4-PuF3 (77.5–6.6-12.3–3.6 mol%), a fuel option for the Molten Salt Fast Reactor (MSFR), has been measured by differential scanning calorimetry (DSC) for determination of phase transitions, and by Knudsen effusion mass spectrometry (KEMS) for measurement of partial vapour pressures. The boiling point of the mixture was determined by extrapolation of total vapour pressure to 1 bar. Thermodynamic calculations were performed and compared with the experimental results. Novel experimental data on pure plutonium trifluoride are presented: melting point, vaporization enthalpy, vapour pressure and ionization energies by electron impact.

The isobaric heat capacity of solid and liquid thorium tetrafluoride was determined by two calorimetry methods: drop calorimetry to obtain the enthalpy increments in the range from 430 to 1600 K an...

The low-temperature heat capacity of the (Th1−y,Puy)O2 solid solution with y=0.03, 0.08, 0.30, 0.54 and 0.85 was determined from 4 K up to about 300 K with a PPMS instrument using the hybrid adiabatic relaxation method. In addition, the heat capacity of 239PuO2 end member was also measured and compared with available literature data for 242PuO2, observing an indistinguishable match. The obtained heat capacity values of the intermediate compositions are greater than the data obtained from Neumann–Kopp's molar additivity rule based on the ThO2 and PuO2 end-members, and a substantial excess capacity is observed for all intermediate compositions, particularly at the lowest temperatures.

In this study we present the thermodynamic description of the binary CaF2–ThF4 system and the ternary LiF–CaF2–ThF4 system. In order to describe the general shape of the phase diagram, the phase equilibria of several intermediate compositions of the CaF2–ThF4 system have been measured for the first time in the present work using the DSC technique. These results have been combined with X-ray diffraction studies for the identification of the phases in equilibrium and for the determination of the structure of the CaThF6 compound. Considering all of the novel experimental data, the CaF2–ThF4 system has been assessed for the first time using the Calphad approach. Moreover, the ternary LiF–CaF2–ThF4 phase diagram has been calculated and optimized using the experimental data obtained in this study for the ternary mixtures. The model developed reproduces very well all the measurements performed and correctly predicts the lowest eutectic temperature and composition.

The chemistry of cesium and iodine is of main importance to quantify the radioactive release in case of a nuclear reactor accident, or sabotage involving irradiated nuclear materials. We studied the interaction of CsI with different metallic fission products such as Mo and Ru. These elements can be released from nuclear fuel when exposed to oxidising conditions, as in the case of contact of overheated nuclear fuel with air (e.g. in a spent fuel cask sabotage, uncovering of a spent fuel pond, or air ingress accidents). Experiments were performed by vaporizing mixtures of the compounds in air, and analysing the produced aerosols in view of a possible gas–gas and gas–aerosol reactions between the compounds. These results were compared with the gaseous species predicted by thermochemical equilibrium calculations and experimental equilibrium vaporization tests using Knudsen Effusion Mass Spectrometry.

This study presents an original approach of oxygen stoichiometry determination during high temperature (>2000 K) measurements of vapour pressure using the Knudsen effusion mass spectrometry technique. The method has been developed taking into account the vapour pressure measurements of series of (U1-x,Pux)O2-δU1-x,Pux)O2-δ samples with x(Pu) = 0.25, 0.5, 0.75 together with pure UO2-δUO2-δ and PuO2-δPuO2-δ end-members coupled with equilibrium calculations based on thermodynamic assessment of the U–Pu–O system. The presented method consists of two steps; in the first step the oxygen potential of the oxide phase is determined based on the measured partial vapour pressures of UO(g), UO2(g), PuO(g) and PuO2(g) gaseous species and during the second step the thus determined oxygen potential is linked with the matching oxygen stoichiometry of the sample. From the obtained results it has been demonstrated that it is possible to accurately estimate the oxygen stoichiometry of the mixed oxide fuel samples knowing the description of the oxygen potential of the corresponding end-members only.

A detailed physico-chemical investigation of molten system (LiF-NaF)eut-LaF3 up to 15 mol% of LaF3 has been done. The phase equilibria were measured by two independent methods (TA and DSC) and consequently Calphad modelling and calculation was performed in order to interpret obtained experimental data. Additional physico-chemical properties like density, electrical conductivity and surface tension were measured. It was found that all investigated properties reflect the ionic changes in the melt providing consistent data, i.e. the most significant changes in these properties are observed at ca. 10 mol% LaF3. The volume properties were calculated, as well, providing information about volume expansion. Also temperature anomalous behaviour was observed for the electrical conductivity and surface tension.

A selected composition of the initial fuel of the Molten Salt Fast Reactor (MSFR) is assessed by differential scanning calorimetry (DSC) for melting point determination and by Knudsen effusion mass spectrometry (KEMS) for vaporization behaviour. Partial vapour pressures and thermodynamic activities of the MSFR fuel mixture are discussed indicating departures from ideal behaviour, and further interpreted by phase equilibria calculations. The boiling point of the mixture is obtained extrapolating vapour pressure experimental results. New results on the vaporization behaviour of pure uranium tetrafluoride are presented, together with the ionization potentials of UF4 by electron impact.

The present study describes the full thermodynamic assessment of the Li,Cs,Th//F,I system. The existing database for the relevant fluoride salts considered as fuel for the Molten Salt Reactor (MSR) has been extended with two key fission products, cesium and iodine. A complete evaluation of all the common-ion binary and ternary sub-systems of the LiF-ThF4-CsF-LiI-ThI4-CsI system has been performed and the optimized parameters are presented in this work. New equilibrium data have been measured using Differential Scanning Calorimetry and were used to assess the reciprocal ternary systems and confirm the extrapolated phase diagrams. The developed database significantly contributes to the understanding of the behaviour of cesium and iodine in the MSR, which strongly depends on their concentration and chemical form. Cesium bonded with fluorine is well retained in the fuel mixture while in the form of CsI the solubility of these elements is very limited. Finally, the influence of CsI and CsF on the physico-chemical properties of the fuel mixture was calculated as function of composition.

The work presents the measured heat capacity of the (Li1−xCax)F1+x liquid solution. Four samples with different compositions have been prepared and measured using a Differential Scanning Calorimeter. Since this technique was newly adopted for measuring encapsulated fluoride samples, some modifications were introduced in the standard configuration of the instrument and they are described in this work as well. For comparison one of the analysed composition (xCaF2 = 0.5) was also measured using drop calorimetry, which has been previously used for similar studies. The reliability of the results obtained was confirmed by the good agreement between the two techniques. Moreover, the excess heat capacity of the (Li1−xCax)F1+x liquid solution was derived and a strong deviation from the ideal behaviour was observed.

The enthalpy increment data for the (Th,U)O 2 and (U,Pu)O 2 solid solutions are reviewed and complemented with new experimental data (400–1773 K) and many-body potential model simulations. The results of the review show that from room temperature up to about 2000 K the enthalpy data are in agreement with the additivity rule (Neumann-Kopp) in the whole composition range. Above 2000 K the effect of Oxygen Frenkel Pair (OFP) formation leads to an excess enthalpy (heat capacity) that is modeled using the enthalpy and entropy of OFP formation from the end-members. A good agreement with existing experimental work is observed, and a reasonable agreement with the results of the many-body potential model, which indicate the presence of the diffuse Bredig (superionic) transition that is not found in the experimental enthalpy increment data.

Within Europe 241 Am is a feasible alternative to 238 Pu that can provide a heat source for radioisotope thermoelectric generators (RTGs) and radioisotope heating units (RHUs) to be used in Electrical power sources used in outer planet missions. In the EPSO-SPACE* project developed as an exploratory research at JRC, a new type of nuclear power source, to provide heat and electricity for deep space missions is considered. It will consist of a 241 Am based compound with aerogel thermal insulation and thermocouples in close contact with the heat source. The assembly is inside an iridium encapsulation to withstand all relevant accident scenarios. The design will be modular and combinable for required electrical output.Several candidate Americium compounds will be investigated for chemical stability at high temperature and for self-irradiation damage. New thermo-electric converter materials containing actinides will be assessed, and a robust encapsulation designed. Safety analyses will be performed including launch explosion and re-entry accidents. The research will conclude in a conceptual design of a prototype power source. In the first part of this study, americium dioxide will be considered from the point of view of its chemical durability and of its behavior against radiation damage and helium formation, two aspects to be carefully investigated due to the high alpha-activity of the americium. Transmission electron Microscopy (TEM) and helium thermal desorption spectrometry (TDS) experiments will be described and results on aged (more than 30 years) AmO2 reported. Some comparison with 238 PuO2 based RTG’s will be discussed.

Basic thermodynamic and electrochemical data of pure actinide fluorides and their mixtures are required for the design and safety assessment of any presently studied molten salt reactor concept based on molten fluoride salt fuel. Since the actinide fluorides are usually not produced commercially, they have to be prepared from the available input materials, typically oxides. In this work, a specially designed facility for synthesis of pure actinide fluorides using pure HF gas is described, as well as a complete procedure of synthesis and characterisation of pure UF 4 and ThF 4 . The fluorination installation consists of a glove box kept under a purified argon atmosphere, a high temperature horizontal fluorination reactor and a HF supply gas line connected to the glove box. The fluorides were synthesised from high specific surface oxides prepared from the respective oxalates by low temperature calcination. The fluorination was partly stationary and partly in a HF gas flow, based on a heterogeneous powder-gas reaction at high temperatures. The products were characterised by X-ray diffraction and differential scanning calorimetry, which confirmed high purity products obtained by this method. Moreover, the melting point of UF 4 was revised using a pure sample and a new value is suggested.

Knudsen effusion mass spectrometry measurements on neptunium dioxide are reported in this work, which have allowed to improve the understanding of its vapourization behaviour and solved discrepancies noticed in the literature: the enthalpy of formation of NpO2(g) has been re-assessed and the composition of neptunia at congruent vapourization has been determined at 2262 K. In addition, a thermodynamic model for the neptunium-oxygen system has been developed using the CALPHAD method. The non stoichiometric NpO2−x phase is described herein using the compound energy formalism with ionic constituents ( Np 3 + , Np 4 + ) 1 ( O 2 - ,Va ) 2 , while the liquid phase is represented with the ionic two-sublattice model ( Np 4 + ) P ( O 2 - , Va Q - ,O ) Q . The reliability and consistency of all optimized Gibbs energies have been verified by calculating the phase equilibria, thermodynamic data, oxygen chemical potential and equilibrium partial pressures. Finally, a number of ill-defined data in the Np–O system have been identified after critical review of the literature and comparison with the present experimental results and CALPHAD model.

In the aftermath of a nuclear reactor accident, understanding the release of fission products from the fuel is key.