Structural features in fluorite compounds relevant for nuclear applications
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Structural features in fluorite compounds relevant for nuclear applications Gianguido Baldinozzi, Lionel Desgranges†, David Simeone, Dominique Gosset and Laurence Luneville SPMS, MFE, CNRS Ecole Centrale Paris, Châtenay-Malabry & DEN DMN SRMA, CEA, Gifsur-Yvette, France. † DEN, DEC, CEA, St Paul lez Durance, France. ABSTRACT Oxides with fluorite (or fluorite related) structures form a large class of compounds with a high radiation tolerance, somewhat related to their peculiar ability to accommodate a variety of defects and to form nonstoichiometric compounds with a large homogeneity range. Structural modifications are generally observed when the departure from the ideal composition is large. We discuss these structural features using an approach based on the crystal symmetry analysis based on the phase transition mechanisms in compounds relevant for nuclear applications. INTRODUCTION In the 1950s the designers of nuclear reactors had to find materials that not only had satisfactory neutron and thermo-mechanical behavior but also able to withstand irradiation. At that time, they chose materials with simple crystallographic structures and no phase transitions (at least within their operating conditions), thus avoiding volume changes that would compromise the integrity of the ceramics: the choice of uranium dioxide as fuel material was then quite straightforward. The desire to extend the life of ceramic components in the nuclear plants urges attention at reexamining the original choices in the more general framework of functional materials. Functional materials are passive or active materials that can perform certain functions when triggered by environmental changes such as stress, external fields, and temperature variations or by a control signal. The characteristic signature of a functional material is the appearance of an anomaly or a singularity in at least one physical quantity. In crystal systems, this anomaly is generally associated with a structural change and this large modification can be exploited to perform certain functions. In this respect, functional materials are quite at the opposite of the original parameters that led to the choice of uranium dioxide. The aim of this paper is to discuss the possible benefits of looking at nuclear ceramics as functional materials. Indeed, relaxing the requirement for single-phase pure compounds could provide supplementary degrees of freedom, and material complexity may be another key to engineer materials with a higher resilience to radiation defects. Here, we will focus at fluorite structures, and mostly at uranium oxides that are certainly not as simple as previously thought, and that represent the most important ceramic material for nuclear applications. One well-known example of passive functional material [1] with fluorite structure is partially stabilized ZrO2. The tetragonal-monoclinic phase transition [2] can produce a several % volume expansion, which can be used to prevent crack propagation [3] in ceramics. The complexity of the phase diagram of uranium oxide
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