High temperature phase transition of mixed (PuO 2 + ThO 2 ) investigated by laser melting

Abstract

a b s t r a c tA laser heating approach combined with fast pyrometry in a thermal arrest method was used to provide new data for the melting/solidification phase transition in mixed (PuO 2 + ThO 2 ) at high temperature. At low concentration of ThO 2 in PuO 2 a minimum in the solidification temperature in the pseudo binary (PuO 2 + ThO 2 ) was observed. The minimum was found around a composition with 5 mol% ThO 2 . Phase transition temperatures of other compositions are closer to an ideal solution behaviour. To detect changes in the material a complete investigation with electron microscopy, Raman spectroscopy and powder X-ray diffraction was done. Raman vibration modes were found, that are characteristic for materials containing PuO 2 , and high temperature segregation effects during solidification were described. The results obtained in the present work are compared to other mixed actinide dioxides and compared to the ideal solution case for this system. The presented results show the importance of the hightemperature oxygen chemistry in this actinide oxide phase.

Melting behavior of (Th,U)O2 and (Th,Pu)O2 mixed oxides

“…The MD calculations suggest that the melting temperature of PuO2-5 atom% ThO2 is lower than pure PuO2 and PuO2-15 atom% ThO2. This result is in agreement with a previous set of experimental melting temperatures by Böhler et al [33], which are consistent with a minima in the phase diagram at ~ 5 mol% of ThO2.…”

“…In this definition it is assumed that the heat capacity is same for solid and liquid phases in the vicinity of melting and only configurational entropy is contributing to the Gibbs free energy. [33].…”

An original approach for Raman spectroscopy analysis of radioactive materials and its application to americium‐containing samples

“…The same technique can be used for a broad variety of radioactive or otherwise hazardous compounds. In particular, it has recently been tested also for a local microscopic analysis of other mixed oxides well representative of traditional and advanced nuclear fuels, whereby important segregation effects depending on the sample's thermal history could be successfully observed by Raman spectroscopy …”

“…Raman spectroscopy is an especially well adapted method for the investigation of nuclear materials, thanks to its flexibility permitting remote and in‐situ analyses and its relative ease of use which requires only limited effort for the sample preparation. Recently, it has been used particularly for the identification of different uranium oxides and oxygen defects and for the study of spent nuclear fuels and mixed actinide systems . The main challenge in the field of nuclear materials is of course the sample radioactivity, potentially exposing the user to significant radiation doses but also to the risk of ingestion or incorporation of radiotoxic nuclides.…”

Raman spectrum of plutonium dioxide: Vibrational and crystal field modes

“…At the end, it is of importance to note that these modes were observed also for PuO 2 -ThO 2 and PuO 2 -UO 2 solid solutions, 6,42 and their intensity increases with the increase of the plutonium content. This feature is very helpful to identify PuO 2 when mixed or dissolved in other compounds, since the vibrational T 2g mode, usually overlap with other active vibrations in the phonon spectral region.…”