Comparative study of fission product release of homogeneous and heterogeneous high-burn up MOX fuel by Knudsen Effusion Mass Spectrometry supported by EPMA, SEM/TEM and thermal diffusivity investigations

publishedVersio

Thermal properties of irradiated UO2 and MOX fuels

The thermal conductivity of UO2 and MOX LWR fuels decreases as a result of reactor burn-up. In this chapter, the phenomena causing this degradation are presented and the effect of the most influent parameters is discussed. Information useful for the interpretation and comparison of in-pile or out-of-pile measurements is given, especially as a function of burn-up and for samples having different irradiation temperatures, in-pile histories and microstructures. The impact of the introduction of plutonium or additives (Gd, Cr) in standard UO2 is also addressed. The evolution of the thermal conductivity as a function of burn-up is non-linear and the numerous approaches and approximations that are used to obtain thermal conductivity correlations are summarised and compared.JRC.E.3-Materials researc

The assessment of the fuels thermal conductivity by Molecular Dynamics.

This work is a review of the papers devoted to investigation of the thermal conductivity in single crystal and polycristalline UO2 and (U,Pu)O2 using molecular dynamics simulations, with inter-atomic interactions described by different potential models. Two main approaches are used to calculate the thermal conductivity by MD. In the first approach the thermal conductivy is given by the autocorrelation fuction of the energy current based on the Green-Kubo relations. In the second approach the thermal conductivity is defined from the energy current and temperature gradient using the Fourier law. The main results of the molecular dynamics simulation of the heat conductivity if the UO2 and (U,Pu)O2 are presented.JRC.E.3-Materials researc

Discussion About the Main Parameters Affecting the Thermal Conductivity of LWR UO2 and MOX Fuels

The thermal conductivity of irradiated nuclear fuels is usually extrapolated from the data acquired on fresh ceramics, incorporating in the formulation a burn-up dependency. However, the burn-up parameter covers several recoverable or not recoverable mechanisms. This methodology could be justified for low burn-up, but is no more applicable at high burn-up, mainly in case of recristallization. An example is the effect of the addition of plutonium in UO2. The common understanding is that the lower conductivity of the fresh MOX is a result of the differences in size and mass between the U and Pu atoms. However, this interpretation does not take into account the impact of the Pu valence, which leads to local fluctuations in stoichiometry in heterogeneous MOX, which in turn has a strong impact on the thermal conductivity. For unirradiated fuels, neglecting the effect of the stoichiometry fluctuations has no impact on the accuracy of the thermal conductivity correlations, because their parameters are adjusted to reproduce the experimental results which are available in sufficient number. However, when extrapolated to irradiated fuels, these correlations overestimate the effect of Pu because the oxygen distribution evolves differently between plutonium rich agglomerates, matrix and coating phase. The use of an appropriate model is strongly needed in interpreting experimental data, as far as the number of available characterizations is small, and their interpretation tricky. The investigation of the Pu and stoichiometry effects combines experimental results obtained on irradiated fuels at ITU and also in-pile temperature measurements conduced for EDFJRC.DG.E.3-Materials researc

The assessment of the fuels thermal conductivity by Molecular Dynamics

This work is a review of the papers devoted to investigation of the thermal conductivity in single crystal and polycrystalline UO2 and (U,Pu)O2 using molecular dynamics simulations, with inter-atomic interactions described by different potential models. Two main approaches are used to calculate the thermal conductivity by MD. In the first approach the thermal conductivity is given by the autocorrelation function of the energy current based on the Green-Kubo relations. In the second approach the thermal conductivity is defined from the energy current and temperature gradient using the Fourier law. The main results of the molecular dynamics simulation of the heat conductivity of the UO2 and (U,Pu)O2 are presented.JRC.E.3-Materials researc

Molecular Dynamics study of the effects of non-stoichiometry and oxygen Frenkel pairs on the thermal conductivity of uranium dioxide

In the present work, calculations of the thermal conductivity of UO2 were carried out applying classical Molecular Dynamics for the isothermal-isobaric (NPT) statistical ensemble, using the Green-Kubo approach. The thermal conductivity calculated for perfect stoichiometric UO2 is in good agreement with the literature data over the temperature range corresponding to heat transfer by phonons (up to 1700 K). The effect of non-stoichiometry on the thermal conductivity was calculated taking into account the presence of polarons. It was found that for the same value of the stoichiometry deviation, the effect of oxygen vacancies (hypo-stoichiometry) is more pronounced than the effect of oxygen interstitials (hyper-stoichiometry). Then, the saturation concentration of the oxygen Frenkel pairs (OFP) in UO2 was investigated and their influence on the thermal conductivity was calculated. The simultaneous impact of non-stoichiometry and OFP on the thermal conductivity was investigated and it was shown that the two effects can be combined using the interpretation obtained with the classical phonons scattering theory. Finally, simplified correlations were deduced for the calculation of the thermal conductivity of UO2 taking into account the effect of non-stoichiometry and of Frenkel pairs, these two effects being present during irradiation.JRC.E.3-Materials researc

Molecular Dynamics study of the Mixed Oxide Fuel thermal conductivity

There is still no clear understanding of the plutonium content influence on the thermal conductivity behaviour of the (U,Pu)O2 MOX fuels. In this work Classical Molecular Dynamics (MD) was used to investigate the (U,Pu)O2 thermal conductivity in the whole concentration range and in the temperature range from 400 K to 1600 K. The Green-Kubo approach was used for the thermal conductivity calculation and an algorithm was proposed to improve the accuracy of the calculation. The obtained results are in good agreement with the literature experimental data and results of modeling of other authors. On the basis of the obtained results we give recommendations for the MOX thermal conductivity evaluation in the concentration range from pure UO2 up to pure PuO2.JRC.E.3-Materials researc

Thermal conductivity of porous UO2: Molecular Dynamics study

Classical Molecular Dynamics was used to investigate the effect of nanometric size pores on the thermal conductivity of irradiated UO2. The Green-Kubo approach was used for the thermal conductivity calculation. The effects of pores size, volume fraction and separation were simulated. A comparison with existing theoretical models is presented and an analytical model adapted to irradiated fuel is obtained. The results demonstrate that, for realistic bubbles size and concentrations, the impact on the fuel thermal conductivity is higher than predicted by the correlations used to quantify the impact of porosity: at 500 K the impact of 0.3 vol.% of nanometric pores is of the same order of magnitude as that of 4.5 vol.% of micrometric pores.JRC.E.3-Materials researc

Diffusion of Helium in Non-stoichiometric Uranium Dioxide

Diffusion coefficients of helium in solid UO2+x at high temperature and various stoichiometric compositions are evaluated by using molecular dynamics simulations based on a partly-ionic model in conjunction with a polaron ¿Free Hopping Approximation¿ proposed earlier. The results are compared with existing experimental and simulation data. A strong dependence of the apparent diffusion activation energy on stoichiometry is found and the mechanisms of He migration in non-stoichiometric solid UO2+x are discussed.JRC.E.3-Materials researc

Computer Simulation of Defects Formation and Equilibrium in Non-stoichiometric Uranium Dioxide

Formation and stability of different types of clusters in hyperstoichiometric UO2+x, including Willis¿s 2:2:2 interstitial dimers as well as cuboctahedral tetra- and pentamers are investigated under static and dynamic conditions based on a partly-ionic model. A ¿Free Hopping Approximation¿ for small polarons is proposed and implemented in a molecular dynamic simulation computer code. Lattice parameter and other equilibrium properties of UO2+x are calculated and compared with existing experimental data in a wide range of temperature and stoichiometry.JRC.E.3-Materials researc