Thermochemistry of metal-rich chromium telluride and its role in fuel-clad chemical interactions

Abstract

Vaporisation of Cr-Te alloys was studied by Knudsen-effusion mass spectrometry. The partial pressures of Te₂(g) and Te(g) over the two-phase field, Cr + CrTe_x, were determined in the temperature ranges 1015 to 1138 K and 1180 to 1285 K, respectively. The temperature dependencies of the partial pressures have indicated that nearly equimolar proportions of Te and Te₂ are present in the vapor phase and that there is a phase transformation in CrTe_x at 1160 ± 20 K. The Cr-rich phase boundaries of the nonstoichiometric CrTe_x were delineated at 1075 K (50.72 ± 0.7 at% Te) as well as at 1235 K (48.25 ± 0.9 at% Te) by a continuous monitoring of the intensities of Te⁺ and Te⁺₂ as a function of time, starting with samples having 55.63 and 57.22 at% Te. Enthalpies and Gibbs energy changes were derived for the equilibria. CrTe_x(s) Ai Cr(s) + (x/i)Te_i(g) [x = 1.029 and 0.932; i = 1 and 2] and Te2(g) ai 2Te(g). Enthalpies and Gibbs energies of formation of CrTe_1.0.29 and CrTe_0.932 were arrived at. The tellurium potentials which would be required for the formation of MTex (M = Fe, Cr, andNi) in Type 316 stainless steel and those likely to exist in the fuel-cladding gap of a mixed-oxide fuel pin were computed.