Structural changes of Nd- and Ce-doped ammonium diuranate during the conversion to U$_{1-y}Ln_y$O$_{2\pm x}$

The slides were presented at "Nuclear Fuel Cycle: A Chemistry Conference" (NFC3) on May 5, 2021 as keynote speech (K1) in the session "Actinide Materials: Nuclear Fuels and Radwaste Matrices" (MAT).

Abstract

In an advanced nuclear fuel cycle, partitioning and transmutation (P&T) is a key strategy to reduce spent nuclear fuel’s radiotoxicity and heat generation. Long-lived minor actinides (MA) are partitioned from spent nuclear fuel and subsequently converted into fuel or targets materials for use in fast reactor systems, where the actinides are fissioned to short-lived radionuclides. The sol-gel route via internal gelation is a process that is currently explored for the production of such MA containing transmutation fuel, by converting an actinide-containing solution into a homogeneous precipitate with spherical geometry. Those precursors are thermally treated and can be used as particle fuel or compressed into fuel pellets. The advantages of the process are to avoid handling of fine powder and to facilitate automation for the production of nuclear fuel precursors.

Within this study, the structural changes in Nd- and Ce- doped ammonium diuranate (ADU) gels, prepared by internal gelation during the conversion to U_1-yLn_yO_2±x were investigated. Nd and Ce molar metal fractions up to 30 mol% were introduced, acting as surrogates for the actinides Am and Pu. Nd was used in its trivalent oxidation state, while both trivalent and tetravalent cerium were tested. The dried Ln-doped ADU gels were characterised by optical microscopy and X-ray powder diffraction (XRD). Those precursors were thermally treated in two individual steps: (1) calcination under oxidising conditions at a maximum temperature of 900 °C, followed by (2) sintering in reducing atmosphere at a maximum temperature of 1600 °C (10 h). The behaviour of the dried gels during both thermal treatment steps was studied in-situ by high temperature scanning electron microscopy (HT-SEM) and thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC) and evolved gas analysis mass spectrometry (EGA-MS).

The material prepared with the trivalent dopant precursors behaved significantly different during the decomposition in air compared to mixtures prepared with Ce(IV), which indicates different crystallinities and/or initial compositions. The calcined products were identified as (Ln-doped) α−U₃O₈ for dopant contents up to 10 mol%. For higher Ln contents, a more complex mixed-phase behaviour was found. The introduction of a reducing atmosphere led to a sudden mass loss. Particle shrinkage occurred significantly later, pointing out that the shrinkage observed within the sintering is controlled by a general densification of the material and insignificantly by the volume change due to the transition of the orthorhombic (Ln-doped) α−U₃O₈ phase to the cubic (Ln-doped) UO_2±x phase. The sintered products were identified as single phase solid solutions according to the formula U_1-yLn_yO_2±x for Nd contents up to 30 mol% and Ce contents up to 20 mol%. In this contribution we will report and discuss the experimental results.