Elsevier

Journal of Nuclear Materials

Volume 474, June 2016, Pages 163-173
Journal of Nuclear Materials

Thermodynamic assessment of the Pdsingle bondRhsingle bondRu system using calphad and first-principles methods

https://doi.org/10.1016/j.jnucmat.2016.03.025Get rights and content

Highlights

  • The mixing enthalpy of solid solutions in the Pdsingle bondRhsingle bondRu system was calculated using the DFT and SQS methods.

  • A thermodynamic assessment of the Pdsingle bondRhsingle bondRu ternary system was performed using the Calphad method.

  • The extrapolation based on only binary interaction parameters leads to a good agreement with the data on the ternary.

Abstract

Palladium, rhodium and ruthenium are abundant fission products that form in oxide fuels in nuclear reactors. Under operating conditions, these Platinum-Group Metal (PGM) fission products accumulate in high concentration at the rim of the oxide fuel and mainly precipitate into metallic solid solutions. Their thermochemistry is of significant interest to predict the high temperature chemical interactions between the fuel and the cladding or the possible precipitation of PGM phases in high level nuclear waste glasses.

To predict the thermodynamic properties of these PGM fission products, a thermodynamic modeling is being developed on the ternary Pdsingle bondRhsingle bondRu system using the Calphad method. Because experimental thermodynamic data are scarce, Special Quasirandom Structures coupled with Density Functional Theory methods were used to calculate mixing enthalpy data in the solid solutions. The resulting thermodynamic description based on only binary interaction parameters is in good agreement with the few data on the ternary system.

Introduction

Uranium oxide is the most common fuel of Light Water Reactors (LWR). During burnup, many fission products form and among them Pd, Rh and Ru are very abundant. According to Guillaumont [1], the fission yields of these fission products (in grams per ton of fissile uranium) are about 1245 g of Pd, 487 g of Rh and 2157 g of Ru for an UOX1 spent fuel 3.5% enriched in 235U with a burnup of 33GWj t−1. In this framework, numerous studies were undertaken to point out the chemical interactions between platinoids and fuel materials [2], [3], [4] or under waste disposal conditions [5], [6].

During reactor operation, the evolution of these platinoid based phases is of major importance. Pd, Rh and Ru fission products exhibit no solubility in the fluorite structure of the oxide fuel [2], [3]. They mainly form metallic precipitates revealed by post irradiation examination of the fuels. These so-called “white inclusions” are constituted of Pdsingle bondRhsingle bondRu generally alloyed with two other fission products: Mo and Tc [2], [7]. If chalcogen fission products (Se,Te) are present, these PGMs may also form chalcogenide rich solid solutions or intermetallic phases. The white inclusions can be single-phase or two-phase constituted; their compositions depend mainly on burnup, temperature gradient and oxygen potential of the oxide fuel [2].

In the glass matrix of high level nuclear wastes, these fission products partly precipitate either as metal or oxide phases as function of the oxygen potential [6], [8], [9], [10]. Palladium and rhodium preferentially react with chalcogen elements (Se,Te) to form complex intermetallic phases [11], [12], [13], [14], [15] whereas ruthenium forms both metallic Ru or RuO2 particles and/or RuO2 needles [9], [10], [11], [12], [13], [14], [15], [16]. But, in case of the occurrence of rhodium, a mixed solid solution of rhodium and ruthenium dioxide: (Rh,Ru)O2 can precipitate [6], [8], [17]. In order to predict the formation of these different phases, the description of the thermodynamic properties of all the competing phases is needed.

The thermodynamic and phase diagram data of the ternary system Pdsingle bondRhsingle bondRu were thus reviewed and a description of the stable phases was proposed in the present work. New Differential Thermal Analysis (DTA) experiments were performed to provide additional phase diagram data. Furthermore, the mixing enthalpies of the FCC and HCP solid solutions were calculated using the Special Quasirandom Structures (SQS) methodology coupled with Density Functional Theory (DFT) calculations to compensate the lack of experimental thermodynamic data for most of the systems under consideration. Using the Calphad method, all these results were used to describe thermodynamically the binary systems Pdsingle bondRh, Pdsingle bondRu and Rhsingle bondRu as well as the ternary system Pdsingle bondRhsingle bondRu.

Section snippets

Bibliography

As far as the authors know, no intermetallics form in any of the three binary systems. The solid phases are the solid solutions based on Ru (HCP) and Pd and Rh (FCC). To make the paper clearer these ternary extension of the solutions (Pd,Rh,Ru)-FCC and (Pd,Rh,Ru)-HCP solid solutions are merely noted FCC and HCP.

The three binary systems have been already reviewed by Tripathi et al. [18], [19], [20] and by Okamoto [21], [22], [23]. All these binary systems were thermodynamically assessed by

DFT and SQS methodologies

The SQS methodology [48] has been coupled with DFT calculations [49] in order to estimate the mixing enthalpy of binary and ternary solid solutions. The solid solutions have been treated by the SQS method: i.e. a random-like distribution of atoms into a given lattice is considered at a given composition and with a finite number of total atoms in a cell.

All SQS structures have been calculated in the frame of the DFT within a pseudo-potential approach using the VASP package within projector

DTA experiments

Differential Thermal Analysis (DTA) analyses were performed using a Setaram Setsys device calibrated using the melting point of pure gold, nickel and palladium. From this calibration, the temperature uncertainty is estimated to be ±3 K.

Due to temperature limitations, these DTA experiments were performed only in the Pd rich domain of the Pdsingle bondRh system. Several heating and cooling ramps were performed at: 20 K/min, 10 K/min, 5 K/min and 3 K/min. In all cases, the samples were produced in situ by

Conclusion

A thermodynamic assessment of the Pdsingle bondRhsingle bondRu system was performed to better predict and understand the formation of the metallic fission product precipitates observed in post-irradiation examinations in oxide fuels. A literature review was carried out on the Pdsingle bondRh, Pdsingle bondRu and Rhsingle bondRu binary systems and on the Pdsingle bondRhsingle bondRu ternary system.

As experimental thermodynamic data are scarce on these systems, DFT-SQS calculations were performed to determine binary mixing enthalpy data for the FCC and HCP phases and

Acknowledgments

DFT calculations were performed using HPC resources from GENCI-CINES (Grant 2015-096175).

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