Experimental Characterization of the Chemical Behaviour of Cs, I and Te in UO₂

During a power transient, the nuclear fuel of Pressurized Water Reactors (PWR) undergoes major temperature gradients giving rise to strong Interactions, both mechanical and chemical, between UO₂ Pellets and the Zircaloy Cladding (PCI): the swelling of the fuel causes strong mechanical stresses on the cladding while corrosive chemical species, including iodine, are released. The combination of these two factors may lead to the cladding failure by Iodine induced Stress Corrosion Cracking (I-SCC). The corrosivity of iodine towards zirconium varies according to its chemical form, TeI_x or ZrI₄ compounds being more aggressive than the CsI species. In an irradiated UO₂ fuel including various chemically reactive Fission Products (FPs), the (Cs-Mo-Te-I)-UO₂ system controls the speciation, and thus the corrosivity, of Iodine towards Zr.<br/>The present study aims at clarifying the speciation of I, Cs and Te in UO₂ in conditions representative of a PWR, using a methodology combining experimental approach and thermodynamic simulation.<br/>Depleted UO₂ pellets are implanted with the elements I, Cs and Te and then thermally treated in conditions of (T, pO₂) representative of a fuel in nominal or transient operation. The samples are analyzed using complementary characterization techniques that reveal their microstructure (SEM, MET) as well as the concentration profiles (SIMS) and speciation (XAS, DX, EELS) of the implanted elements.<br/>Thermodynamic simulations are carried out using the TAF-ID and SGPS-SGTE Thermodynamic DataBases (TDBs), supplemented when necessary with relevant literature data. The calculations aim at both dimensioning the heat treatment conditions before the tests, and helping in the interpretation of the results afterwards. Lacks and gaps in the BDDs for key chemical systems of the irradiated fuel are identified shedding light on necessary future thermodynamic data acquisition.<br/>Various chemical systems of increasing complexity are studied i.e. I-UO₂, Cs-UO₂, I-Cs-UO₂, I-Cs-Te-UO₂, until addressing the (Cs-Mo-Te-I)-UO₂ system that is representative of the I-SCC. The cross-interpretation of the results confirms the existence of strong interactions between the FPs and the UO₂ matrix, depending on (T, pO₂) parameters, giving rise to solid precipitates and/or gaseous compounds in the form of bubbles. If the species Te(s) and CsI(g) are clearly identified, possibly more complex forms combining U-I-O, Cs-U-O or Cs-Te remain to be confirmed, in the absence of available reference spectra.