THERMOCHEMISTRY OF METAL-RICH MANGANESE TELLURIDE AND ITS ROLE IN FUEL CLAD INTERACTIONS

Vaporisation of Mn-Te alloys was studied by Knudsen-effusion mass spectrometry. The partial pressures of Te(g) over the two-phase field, Mn-MnTe, were determined in the temperature range 1120-1250 K. Two samples of initial composition of 29.1 and 40.1 at% Te were used in the experiments. The vapour phase consists of Mn(g) and Te(g). Partial pressure-temperature relations for Te(g) were found to follow the equation log((p)/Pa) = -(16099+/-285)/ T(K)+(10.525 +/- 0.240). Mn-rich phase boundary of MnTe was determined from continuous vaporisation experiments starting with a sample from two-phase field Mn + MnTe. The boundary composition was found to be 44.3 +/- 0.5 at% Te in the temperature range 1205-1280 K. Enthalpy of the following reactions was obtained: MnTe0.8(s) reversible Mn(s) + 0.8Te(g), MnTe0.8(s) reversible Mn(g) + 0.8Te(g) and Mn(s) reversible Mn(g). The standard molar enthalpy and Gibbs energy of formation of MnTe0.8 were arrived at. The tellurium potential which would be required for the formation of MnTe0.8 in AISI 316 stainless steel was calculated and the possibility of such a telluride formation in the fuel-cladding gap of a mixed-oxide fuel pin is discussed.