Enthalpy measurements of La 2 Te 3 O 9 and La 2 Te 4 O 11 M. Ali (Basu), S.R. Bharadwaj, D. Das * Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Received 12 June 2006; accepted 13 September 2006 Abstract Enthalpy increment measurements on La 2 Te 3 O 9 (s) and La 2 Te 4 O 11 (s) were carried out using a Calvet micro-calorimeter. The enthalpy values were analyzed using the non-linear curve fitting method. The dependence of enthalpy increments with temperature was given as: H°(T)  H°(298.15 K) (J mol 1 ) = 360.70T + 0.00409T 2 + 133.568 · 10 5 /T  149 923 (373 6 T (K) 6 936) for La 2 Te 3 O 9 and H°(T)  H°(298.15 K) (J mol 1 ) = 331.927T + 0.0549T 2 + 29.3623 · 10 5 /T  114 587 (373 6 T (K) 6 936) for La 2 Te 4 O 11 . Ó 2006 Elsevier B.V. All rights reserved. 1. Introduction The stoichiometry of the (U, Th)O 2 fuels does not change much during burnup, which is similar to the situation encountered in LWR UO 2 and (U, Pu)O 2 fuels, where the O/M shift is negligible during the burnup since oxygen transport across the fuel is very fast and the excess oxygen is gettered by the Zirca- loy. In (U, Th)O 2 fuel, however, the oxygen diffu- sion through the fuel is orders of magnitude less and so oxygen transport through the fuel is very slow. This results in a sharp built-up of the oxygen potential within the fuel without any appreciable change in the stoichiometry. Most of the fission products precipitate as multi- component oxide phases and multi-component metallic phases [1]. Rare earths (RE) and tellurium are among the fission products formed with burnup of the fuel. Usually the oxygen potential inside the oxide fuel does not go beyond the value of Mo/ MoO 2 buffer system and Te remains alloyed with the metallic fission products and the RE elements remain dissolved in the fluorite lattice of the fuel matrix and are components of the ceramic precipi- tates. The fission-generated oxygen is taken up by reactive fission products (RE, Y, Sr, Ba, Zr and Mo), ternary compounds of Cs/Rb, and the fuel itself. In thoria rich fuels, the oxygen uptake by the fuel is low. The concentration of the left-over oxygen is governed by its diffusion outwards to the Zircaloy clad [2]. In thoria rich fuels, oxygen diffusion takes place predominantly by self-diffusion, so the trans- port rate is orders of magnitude slower than in urania rich fuel. Under such circumstances Mo oxidation might not be as fast as oxygen accumulation inside the fuel leading to interaction of Te with RE oxide components in the presence of oxygen forming ternary compounds. The formation would depend on the oxygen potential and also on the tellurites stabilities. In the context of using thoria rich fuel, 0022-3115/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2006.09.006 * Corresponding author. Tel.: +91 22 25593700/05146; fax: +91 22 25505151/19613. E-mail address: dasd@apsara.barc.ernet.in (D. Das). Journal of Nuclear Materials 360 (2007) 99–103 www.elsevier.com/locate/jnucmat