What does the crystallo-chemistry of UO$_2$ tells us to design innovative nuclear fuels?
Résumé
Nuclear fuel plays a fundamental role in the design of innovative nuclear systems. At the beginning of nuclear area, designers chose UO$_2$ as nuclear fuel because they thought it was a stable compound with no structural changes in operating conditions. Contrarily to these pioneer ideas, recent results evidenced that UO$_2$ has some internal degrees of disorder appearing when UO$_2$ is heated, or chemically doped. Here we are interested in understanding UO$_2$ disorder in order to derive driving force for the design of innovative nuclear fuels. First, when UO$_2$ is heated, the thermal motion of oxygen becomes anharmonic. Using neutron pair distribution function (PDF), we proved that this disorder could be modelled by a low symmetry local order different from average order. The existence of low symmetry domains could explain the increase of UO$_2$ heat capacity at high temperature, which is a factor increasing its resistance to irradiation damage.Second, when UO$_2$ is chemically doped, either by oxygen or by lanthanide fission products, the doping element is not homogenously distributed in the UO$_2$ crystalline lattice. In the case of oxidation, interstitial oxygen atoms are encapsulated in some clusters named cuboctahedrons [2]. In the case of neodymium doped UO$_2$, a miscibility gap having chemical phases with different Nd contents appears in the hypo-stoichiometric domain [3]. The manner by which UO$_2$ can retain radioactive fission products is closely related to the manner by which some chemical elements can be caged in UO$_2$.
Therefore, these recent results evidence that the existence of disorder in UO$_2$ can be beneficial for irradiation resistance and radionuclide retention. Understanding of this disorder is underway, but its existence is already a driving force for the design of innovative nuclear fuel. The design of such fuels should include some degrees of freedom to accommodate irradiation effects and changes in chemical composition.
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