Elsevier

Journal of Nuclear Materials

Volume 456, January 2015, Pages 68-73
Journal of Nuclear Materials

Phase stability of an aged Pu–0.27 wt.% Ga alloy

https://doi.org/10.1016/j.jnucmat.2014.09.005Get rights and content

Highlights

  • Examination of the structure and phase stability of an aged Pu–0.27 wt.% Ga alloy.

  • The Ga content appears sufficient to retain the δ-Pu phase at ambient temperature.

  • Tenuous stability of the δ-Pu phase when cooled to sub-ambient temperatures.

  • The alloy transformed to more than 40% α-Pu when cooled to −50 °C for 1 h.

  • The alloy appears to have undergone δ  α′ transformation during hardness indentation.

Abstract

This paper describes the characterisation of a naturally aged Pu–0.27 wt.% Ga alloy 35 years of age. The alloy was subjected to bulk chemical analysis, density determination, differential scanning calorimetry (DSC), optical microscopy, electron probe micro-analysis (EPMA) and hardness measurements. Despite the Ga content being only 0.27 wt.%, it nevertheless appears to be sufficient to retain the alloy in the δ-Pu phase at ambient temperature. This was demonstrated by optical microscopy, density measurements and DSC. However, the ambient temperature stability of the δ-Pu phase is tenuous. This was demonstrated by the propensity of the alloy to undergo transformation to α-Pu when cooled to sub-ambient temperatures. Indeed, both density measurements and DSC indicate that when the alloy is cooled to −50 °C for 1 h the alloy has transformed to more than 40% α-Pu. Moreover, a comparison of the Vickers hardness (expressed as a mean pressure) with transformation pressures for Pu–Ga suggests that the alloy has undergone δ  α′ transformation during the indentation process.

Introduction

Owing to the radioactive nature of plutonium (Pu) the processes of Pu decay can lead to the evolution of transmutation products, lattice damage and helium bubbles, all of which have the potential to alter its properties with age. The alpha decay of each 239Pu atom leads to the generation of an α particle and a uranium (U) atom. Following the decay event the α particle can travel for up to 10 μm before coming to rest; the distance travelled by the recoiling U atom is significantly shorter (∼12 nm). However, during their travel through the lattice, both create damage in the form of vacancy–interstitial (Frenkel) pairs. Approximately 2500 Frenkel pairs are created by each decay event, although most recombine almost immediately owing to the mobility of the vacancies at room temperature: this process is known as “self-annealing”. Up to 70% of the Frenkel pairs recombine within 100 pico-seconds after the decay of a Pu atom [1]. The remaining lattice damage has been calculated to accumulate at a rate of 0.1 displacements per atom (dpa) per year [1].

During their travel through the Pu lattice the α particles (helium nuclei) acquire two electrons to form helium (He) atoms. In addition to He and U, americium (Am) and neptunium (Np) are also formed as a result of the decay of other Pu isotopes. Overall, it has been estimated that after fifty years of storage plutonium may contain 2000 ppm He, 3700 ppm Am, 1700 ppm U and 300 ppm Np [2].

Unlike U, Am and Np, He has little solubility in the Pu lattice; however, He atoms can readily enter lattice vacancies, enabling them to diffuse through the microstructure. Over time, this migration and coalescence of He atoms results in the formation of He bubbles. Such bubbles have been seen by transmission electron microscopy (TEM), which has shown that in Pu specimens up to 42 years of age that have been stored at ambient temperature the mean bubble density increased from 0.6 × 1017 cm−3 (16 year old Pu) to 2.0 × 1017 cm−3 (42-year-old Pu) [3]. In contrast, the mean bubble diameter was largely unchanged over the same timescale (1.4 ± 0.2 nm); bubbles smaller than approximately 0.7 nm could not be resolved [3].

The brittle nature of the monoclinic alpha-phase Pu (α-Pu) – the ambient temperature form of unalloyed Pu – has led to it being alloyed with elements such as gallium (Ga) in order to retain the more ductile face centred cubic (fcc) delta (δ-Pu) phase to ambient temperature. The minimum Ga content required to retain the δ-Pu phase to ambient temperature is approximately 0.3 wt.% Ga [4], although the precise composition is unclear. However, the δ-Pu phase in Pu–Ga alloys has been shown to be metastable, which, in addition to the products of radioactive decay, renders uncertain the long term stability of the δ-Pu phase with possible transformation to α-Pu. Both Chebotarev et al. [5] and Timofeeva [6] have suggested that δ Pu–Ga will decompose to a mixture of α + Pu3Ga, although it has been estimated that the timescale for this process is in excess of 11,000 years [7]. Nevertheless, obtaining further understanding of the behaviour of Pu alloys is of great importance. In the present study a naturally aged Pu–Ga alloy was subjected to comprehensive characterisation in order to understand its nature and properties over longer timescales and to study the phase stability of the alloy.

Section snippets

Experimental

The alloy examined as part of this study had the nominal composition of Pu–0.27 wt.% Ga; at the time it was produced it was cast into rods approximately 5 mm in diameter and 150 mm in length and subsequently homogenised (typically 450 °C for 100 h) to obtain an even distribution of Ga throughout the microstructure. Specimens were then cut from these rods and subjected to comprehensive characterisation comprising of bulk chemical analysis, density determination, differential scanning calorimetry

Chemical analysis

The results of the chemical analysis are listed in Table 1. It can be seen that the measured quantities are all significantly greater than the limits of detection for each element, which are also listed in the table. The mean Ga content was determined to be 0.274 wt.% (duplicate results were 0.274 wt.% and 0.272 wt.%), making the alloy close to the minimum figure necessary to achieve a wholly δ phase alloy at ambient temperature. It should be noted, however, that as well as the Ga content, another

Conclusions

This study has examined the structure and phase stability of a 35-year-old Pu–0.27 wt.% Ga alloy. The alloy was subjected to comprehensive characterisation, including bulk chemical analysis, optical microscopy, EPMA, density measurements, DSC and hardness measurements.

Despite the Ga content being only 0.27 wt.%, it nevertheless appears to be sufficient to retain the alloy in the δ-Pu phase at ambient temperature. Optical microscopy revealed a microstructure resembling that seen in other

Acknowledgements

The authors would like to thank the members of the Actinide Operations Team for their help during the experimental work described in this report and Mr P. Bayer for useful discussions. In addition, the assistance of Mr N. Thomas, Mrs M. Moore, Mr R. Danbury, Dr C. Puxley and Mrs F. Taylor in carrying out the chemical analysis is also acknowledged.

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