Experimental evidence of oxygen thermo-migration in PWR UO2 fuels during power ramps using in-situ oxido-reduction indicators

Abstract

The present study describes the in-situ electrochemical modifications which affect irradiated PWR UO 2 fuels in the course of a power ramp, by means of in-situ oxido-reduction indicators such as chromium or neo-formed chemical phases. It is shown that sub-stoichiometric irradiated fuels under temperature gradient such as that occurring during high power transients are submitted to strong oxido-reduction perturbations, owing to radial migration of oxygen from the hot center to the cold periphery of the pellet. The oxygen redistribution, similar to that encountered in Sodium Fast Reactors fuels, induces a massive reduction/precipitation of the fission products Mo, Ru, Tc and Cr (if present) in the high temperature pellet section and the formation of highly oxidized neo-formed grey phases of U 4 O 9 type in its cold section, of lower temperature. To our knowledge, it is the first time that such a phenomenon, likely to greatly impact the corrosive fission gas release, is experimentally attested for PWR fuels. The parameters governing the oxidation states of UO 2 fuels under power ramps are finally debated from a cross-analysis of our results and other published information. The potential chemical benefits brought by oxido-reductive additives in UO 2 fuel such as chromium oxide, in connection with their oxygen buffering properties, are discussed.

Homogenization approach to model the thermal-mechanical behavior of melting fuel material

“…As a consequence the O/M ratio increases during the life of the fuel material under neutronic flux. Studies carried out by Riglet-Martial et al [32] showed that thermal gradient characterizing the fuel material during irradiation is the source of a strong reduction-oxidation reaction leading to migration of the oxygen from the center of fuel pellet to its periphery which induces a decrease of O/M ratio in the central zone. It has been showed by an expert group and then reported by the French Autority of Nuclear Safety [33] that in a hypothetical accidental case leading to fuel melting, this shall appear in the central region of the PWR fuel pellet and progressively evolve towards the periphery.…”

3D simulation of a power ramp including fuel thermochemistry and oxygen thermodiffusion

“…In this work, to by-pass these difficulties, we have identified the heat of transport and thermodynamic factor by inverse analysis using the weight fractions of chromium measured by EPMA along the pellet radius after several power ramps [8]. Figure 7 illustrate the measured evolution of chromium content at the center of a fuel pellet during the 12h long holding period of a power ramp (maximum power 470 W/cm).…”

“…In a recent paper on power ramped Cr-doped fuels [8], it was shown that the oxygen potential along the fuel radius decreases significantly during the high power period of a power ramp. These results were attributed to oxygen thermodiffusion (diffusion of oxygen from the hot pellet center to the cold pellet periphery), a phenomenon observed not only in Sodium Fast Reactor (SFR) fuels due to the high temperature gradients at hand in these fuels [9,10], but also in BWR power ramped fuel pellets as reported by Adamson et al [11].…”

Mikrostruktur Dan Komposisi Fasa Pelet Sinter UO2 Dengan Penambahan Dopan Logam Zirkonium

“…Kim [7] [20]. Deviasi nilai stoikiometri UO 2 sangat mempengaruhi kinerja pelet UO 2 di reaktor [20][21]. Pelet UO 2 yang hiperstoikiometri bersifat lebih plastis sehingga mempunyai laju mulur semakin meningkat dengan semakin besar nilai deviasi stoikiometri.…”

“…Pada temperatur tinggi, peningkatan laju mulur dan laju difusi dapat mengakibatkan terjadinya swelling pada pelet UO 2 yang mendegradasi kemampuan retensi produk fisi dan menurunkan integritas kelongsong karena terjadi PCI (pelletcladding interaction). Bila dikaitkan dengan pustaka diatas [20,21] …”