Metallic fission products phases in nuclear fuels: Thermodynamic modelling of the Mo-Pd-Rh System
Abstract
Mo, Pd and Rh are fission products that accumulate with the burn-up of oxide nuclear fuels. They precipitate as metallic inclusions together with Ru and Tc forming so-called █white inclusions█. They mainly concentrate at grain boundaries but have been recently observed in the fuel-cladding interaction zone, deep in the structural materials. Their impact on fuel assemblies█ lifetime is not yet fully understood [1].
Predicting the thermodynamic behaviour of the █white phases█ is then required to better model the thermochemistry of the nuclear fuel. It is thus needed to assess the thermodynamic model of the Mo-Pd-Rh-Ru-Tc quinary system as well as its subsystems. The present work is part of a study that aims at assessing the full Calphad description of the whole quinary Mo-Pd-Rh-Ru-Tc system.
Since the thermodynamic model for the Pd-Rh-Ru system has already been published by Gossé at al.[2]. The presentation will focus on new assessments of both Mo-Pd and Mo-Rh binary systems showing significant improvement compared to the previous studies [3, 4].
The present Calphad modelling relies on a critical review of the literature, complemented by new experimental investigations such as differential thermal analyses, heat treatments, SEM-EDS analyses and crystal-chemistry investigations. Since experimental thermodynamic data are very scarce due to the refractory behaviour of such alloys, Special Quasirandom Structures (SQS) methodology coupled with Density Functional Theory (DFT) calculations were used to evaluate the mixing enthalpies of the bcc, fcc and hcp solid solutions.
[1] R.A. Clark et al., npj Materials Degradation, 4 (2020)
[2] S. Gossé et al., Journal of Materials 474 (2016) 163-173
[3] G. Ghosh and G.B. Olson, Journal of phase equilibria 21 (2000) 32-39
[4] R. Gürler and J.N. Pratt, Journal of Alloys and Compounds 89 (1992) 97-100