J.-P. Hiernaut

Premelting transition in uranium dioxide

Thermal analysis of the cooling curves of small samples of UO2±x—initially laser heated (in a high-pressure autoclave to inhibit evaporation) on a subsecond time scale to temperatures just below their melting points [Tm(x)]—reveals, in the case of nominally stoichiometric UO2.00, a significant, λ-like, heat capacity [Cp(T)] peak near 2670 K; the cooling curves of samples exposed to a reducing environment, on the other hand, exhibit undercooling, characteristic of a first-order phase transition, while under oxidizing conditions it is found that the premelting transition readily disappears. These findings confirm Bredigs original prediction of a premelting transition in this material, in common with that found in other (nonactinide) fluorites near 0.85Tm. A simple model is presented in terms of which the observed behavior can be rationalized. The model is based on the hypothesis that the premelting transition is due to Frenkel disordering of the oxygen sublattice—a process which is rendered cooperative by attractive interactions between complementary Frenkel defects (oxygen interstitials and vacancies); these interactions are treated in a “mean-field” approximation. The quantitative degree of maximum disorder (realized just above the transition) is, on the other hand, controlled by repulsive interactions between like defects—the inclusion of which, solely through their effect on the configurational entropy, satisfactorily reproduces the values inferred from recent high-temperature neutron diffraction experiments. Assuming that the phase transition in stoichiometric UO2.00 is of second order, the model predicts a divergent heat capacity, Cv, which approximates well to the experimental (λ-like) Cp peak. Crucial to reproducing the observed behavior away from stoichiometry is the introduction of a (linear) dependence of the nonconfigurational partial entropy of formation on the prevailing concentration of intrinsic Frenkel defects in UO2±x; interestingly, it is found that the line of calculated (but unrealized) second-order transitions in UO2+x intersects the U4O9 phase boundary near to where a high-temperature diffuse order-disorder transition has been observed in the oxygen superlattice, suggesting that the second-order, λ-transition in UO2.00 is the stoichiometric counterpart of this transition in U4O9.

Equation of state of uranium oxide

Abstract The total and partial pressures over liquid UO2 have been measured and calculated up to 5000K. A review of previous work is given. The equation of state of UO2 as the main constituent of the fast breeder oxide fuel is required up to at least 5000K in order to estimate the energy release in a loss of flow (LOF) driven hypothetical core disruptive accident (HCDA) of the liquid metal fast breeder reactor (LMFBR). Two models, a macroscopic “mixture” model and a microscopic “defect” model have been developed to determine the oxygen potential of UO200 up to 5000 K. A combination of mass spectrometric, Langmuir probe and high tension diode studies, applied for the first time to the laser vaporization process, revealed large quantities of ions emitted directly from the surface, and resolved previous discrepancies between measured and calculated vapour pressures by an enhanced rate of evaporation due to ion emission. As shown theoretically intrinsic ion emission can contribute to the net evaporation rate only if the resulting positive space charge can be neutralised. It is proposed that this can be accomplished by the presence of “hot” electrons in the plasma. The recommended equilibrium total pressure over liquid UO2.00, valid between the melting point and 5000K, is log p (MPa) = − 2.717 − 20131/T + 1.925 log T .

Helium and tritium kinetics in irradiated beryllium pebbles

High-precision measurements of the release rate of helium and tritium from weakly irradiated beryllium pebbles were carried out by means of a Knudsen-cell technique. The samples were heated in a high vacuum to temperatures above the melting point and the evolution of gas release was measured by a mass spectrometer. Different gas diffusion and release stages were recorded and the ruling mechanisms were analysed up to complete exhaustion of the gas inventory. A minor part of the helium content was released by atomic diffusion to free surfaces, starting at about 700 K with a temperature ramp of 10 K/min and about 850 K with 30 K/min. Most of the helium precipitated into bubbles and was released only during abrupt pore venting events, which started at approximately 1500 K. Tritium release was mostly diffusion-dominated. A small fraction of tritium was trapped by helium bubbles during their nucleation phase and was consequently released corresponding to the appearance of the helium peak.

Volatile Molecule PuO3 Observed from Subliming Plutonium Dioxide.

Abstract Mass spectrometric measurements of effusing vapours over PuO2 and (U, Pu)O2 indicate the presence of volatile PuO3 (g) molecules. The formation of plutonium trioxide vapour is due to a chemical process involving oxygen adsorbed during oxidation of the sample. Although in the examined samples, the fraction of trioxide effusing in vacuo was of the order of 0.02 ppm of the plutonium content, under steady-state oxidation conditions it has been shown that the process can have a relevant effect on the sublimation rate of the dioxide.

Studies on Spent Fuel Alterations During Storage and Radiolysis Effects on Corrosion Behaviour Using Alpha-Doped UO2

UO2 containing different fractions of short-lived alpha-emitters, the so-called alpha-doped UO2 simulates the level of activity of spent fuel after different storage times, and can be used to study the effects of radiolysis on the corrosion behaviour of aged spent fuel exposed to groundwater in a geologic repository. Furthermore, the integral over time of the alpha-decay in alpha-doped UO2 can simulate the decay damage accumulated in spent fuel during storage. This allows investigating property modifications occurring to the fuel during storage periods of interest (e.g. in view of spent fuel retrieval or in view of final disposal) within a laboratory-acceptable timescale. Periodical measurements of lattice parameter are performed on high activity alpha-doped UO2 to investigate the build-up of radiation damage and evaluate possible dose rate effects. Additionally, annealing tests combined with He-release measurements using a Knudsen cell and with microstructure examination using TEM are performed to establish a correlation among the annealing of damage in the microstructure (mainly characterized by dislocation loops) and the release behaviour of He. The effects on the microstructure due to the accumulation of He and α-decay damage are of interest as they may considerably affect the mechanical integrity of the fuel rods, by causing e.g. swelling or cracking in the fuel and/or overpressurization of the cladding. Alpha-doped UO2 with specific activities spanning over three orders of magnitude and undoped UO2 were used in static leaching experiments at room temperature in deionized water under nominally anoxic conditions. Under these experimental conditions (single effect studies) a clear dissolution enhancing effect of alpha-radiolysis was observed coupled with the establishment of higher redox potential due to the radiolytic process. An alpha-activity dependence of the dissolution behaviour was observed.Copyright