High temperature measurements and condensed matter analysis of the thermo-physical properties of ThO2

Values are presented for thermal conductivity, specific heat, spectral and total hemispherical emissivity of ThO2 (a potential nuclear fuel material) in a temperature range representative of a nuclear accident - 2000 K to 3050 K. For the first time direct measurements of thermal conductivity have been carried out on ThO2 at such high temperatures, clearly showing the property does not decrease above 2000 K. This could be understood in terms of an electronic contribution (arising from defect induced donor/acceptor states) compensating the degradation of lattice thermal conductivity. The increase in total hemispherical emissivity and visible/near-infrared spectral emissivity is consistent with the formation of donor/acceptor states in the band gap of ThO2. The electronic population of these defect states increases with temperature and hence more incoming photons (in the visible and near-infrared wavelength range) can be absorbed. A solid state physics model is used to interpret the experimental results. Specific heat and thermal expansion coefficient increase at high temperatures due to the formation of defects, in particular oxygen Frenkel pairs. Prior to melting a gradual increase to a maximum value is predicted in both properties. These maxima mark the onset of saturation of oxygen interstitial sites

Greater tolerance for nuclear materials.

As interest in new generations of nuclear reactors is increasing worldwide, renewed research effort into new materials more tolerant to extreme conditions is crucial. © 2008, Nature Publishing Group

The management of separated plutonium: an introduction

This paper notes renewed enthusiasm in western countries for new nuclear power plant construction. Various energy and security challenges, however, remain contentious. One matter for which there is a wide variation of international practice concerns the management of separated plutonium arising from nuclear fuel reprocessing. This paper introduces a special issue of the journal Progress in Nuclear Energy dedicated to reporting on a conference held in Cambridge, England in July 2005. The workshop was tasked with considering long-term management options for separated civil plutonium. The authors express no opinion as to whether such plutonium is best regarded as a waste or an asset. Rather it is simply suggested that management practice may be improved via an open discussion of options originating from a wide range of contexts and perspectives. This paper introduces some of the relevant issues, which are considered in greater depth by the papers that follow in this special issue. (C) 2007 Elsevier Ltd. All rights reserved

Fabrication of transmutation fuels and targets the ECRIX and CAMIX-COCHIX experience

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A new method for the characterization of temperature dependent thermo-physical properties

The proposed method is based on the laser flash technique. Radially distributed thermograms are calculated via a finite element model and used in an inverse method by optimizing either specific heat or thermal conductivity of a material. These properties are evaluated as a function of radius and respective temperature. Two approximations are introduced inferring the dependence of each property as a function of radius – a polynomial (PNOM) approximation and an iterative gradient (IG) approximation. The method was tested using synthetic thermograms and both approximations were capable of yielding excellent results. The IG approximation was more universal and less sensitive to initial fitting parameters. The PNOM approximation was less computationally expensive but was prone to artefacts (such as un-physical minima or maxima) and more dependent on initial fitting parameters. Both approximations were successfully used on experimental data from UO2 and isostatically pressed graphite. Thermal conductivity was within 5% of the reference empirical correlation for UO2 and within 7% of the reference curve for graphite

Experimental evaluation of the high temperature thermo- physical properties of UO2

High temperature properties of UO2 are reported, in particular thermal conductivity, specific heat capacity, thermal diffusivity and melting point. All are measured with a single laser flash apparatus coupled with a numerical inverse method. The thermal conductivity, spectral emissivity, specific heat capacity, thermal diffusivity and melting point are in very good agreement with established literature values indicating the validity of the methodology and its potential for measuring these properties up to melting. The melting point was identified to be 3118 K ± 28 K. The thermal conductivity exhibits a minimum between 1800 K and 2100 K due to the competition between phonon scattering and an increase in the concentration of free charge carriers. The substantial increase in specific heat can be predominantly attributed to the formation of oxygen Frenkel pairs

Measurement and interpretation of the thermo-physical properties of UO2 at high temperatures: the viral effect of oxygen defects

Values are reported of specific heat, thermal conductivity and thermal diffusivity of UO2 from 1500 K to 2900 K based on laser flash measurements. Experiment is complemented by the development of solid state physics models that aid in the interpretation of the results. Specific heat is shown to exhibit a smooth maximum at 2715 K ± 100 K, consistent with a competition between two processes - oxygen defect interactions (net attraction) and saturation of oxygen interstitial sites. The specific heat model and measurements show, for the first time that a gradual pre-melting transition is consistent with high temperature literature values – enthalpy increment measurements and independently measured high temperature oxygen defect concentrations. Thermal conductivity exhibits a minimum consistent with: 1) an increase in electronic thermal conductivity via polaron production and mobilization and 2) degradation in lattice thermal conductivity due to phonon - phonon scattering and phonon - defect scattering. It is predicted that the high concentration of oxygen defects should contribute significantly to electrical conductivity and thermal expansion at high temperatures

A new numerical method and modified apparatus for the simultaneous evaluation of thermo-physical properties above 1500 K: A case study on isostatically pressed graphite

This paper presents a new numerical inverse method coupled with an improved apparatus based on the laser flash (LF) technique for the measurement of thermo-physical properties of materials at high temperatures. Using this thermal conductivity, specific heat capacity, thermal diffusivity and spectral emissivity have been measured at temperatures above 1500 K. The method improves the characterization of input parameters such as laser power profile, which was shown to impact thermal conductivity and specific heat capacity by 15–20%. Convective heat losses are characterized semi-empirically and are not fitted. The apparatus has been enhanced via the adoption of a spectropyrometer for the simultaneous measurement of spectral emissivity within an uncertainty of 5% (equivalent to 0.3–0.7% error in temperature in the range 1500–3000 K) The results obtained on isotropic, isostatically pressed, graphite are in good agreement with literature values (around 1500 K) and extend the available data up to 2800 K. Additionally, a model has been developed based on the theory of Debye and Klemens for predicting the temperature dependence of thermal conductivity, specific heat capacity and thermal diffusivity of isotropic graphite. The model is in good agreement with the new experimental data and previous lower temperature data and therefore provides confidence in the new experimental approach

Raman study of the surface oxidation in (U, Pu)O2 as a function of Pu content

International audienceThis work presents a systematic Raman study of the matrix oxidation in a variety of (U1-y, Puy)O2 compositions (0 ≤ y ≤ 0.46) at different temperatures, between 250 DC and 400 DC. Our results indicate that the increase in Pu content hinders the oxidation process of the dioxide matrix. Further oxidation of the uranium-plutonium mixed dioxides in air starts between 250 DC and 310 DC, on a time scale of several hours. M4O9 seems to be the most stable intermediate phase formed upon oxidation of all the investigated mixed oxides, before final oxidation to M3O8. In addition, X-ray diffraction measurements and SEM images confirm the trend observed by Raman spectroscopy, i.e. Pu stabilises the fcc structure of the dioxide