Preparation of americium containing transmutation targets

    Research output


    A major challenge in the utilization of nuclear energy is the management of spent nuclear fuel, which holds a long-term radiological hazard. The long-term radiotoxicity and heat load of the spent fuel waste after U and Pu recycling is mainly caused by a group of elements known as the minor actinides (MAs). In particular, Am, constituting less than 0.1 wt% of spent fuel, dominates the heat emission and radiotoxicity for several hundreds of thousands of years. A possible solution to reduce the radiological hazard of spent fuel is to partition the MAs and to transmute them into lighter, short-lived elements in fast neutron spectrum reactors (FNRs). Transmutation targets may be manufactured in a way similar to that of commercial UO2 and MOX fuels, via powder metallurgical processes. However, a major drawback lies in the generation of dust containing radioactive MA isotopes, which will accumulate in filters and on the walls of shielded infrastructure, posing a radiological hazard. A possible remediation is the use of aqueous conversion routes to produce microspheres, which can be directly used as sphere-pac or vipac fuel, or alternatively be manufactured into pellets. One important disadvantage of these routes is the generation of liquid waste streams loaded with MA traces, which are often complicated and expensive to dispose. One particular innovation in response to all the aforementioned issues has been the application of infiltration (or impregnation) of host matrix materials with concentrated aqueous solutions containing MAs. The infiltration procedure may be repeated several times in order to reach the saturation limit of the matrix material. The MA loaded target materials are subsequently dried, calcined and sintered to obtain the desired chemical composition and physical properties.
    In this PhD, porous uranium oxide (UOX) host matrix microspheres, allowing efficient infiltration of Nd3+ (aq.) and Am3+ (aq.) dopant solutions, were fabricated by the internal gelation process. As a novel application, starch was used as a low-temperature burnable pore-former, and its effect was compared to that of graphite. Accessible porosity levels up to 20 vol% were obtained after calcination, without the use of a pore-former. By using starch as pore-former, a significant increase in the accessible porosity level up to 30 vol% was achieved. Moreover, the effects of a change in calcination temperature prior to infiltration were investigated by thermo-gravimetric analysis, density and surface area analyses.
    A multi-step infiltration method was first developed using Nd3+ as a surrogate for Am3+. After sintering, the dopant content was evaluated by powder X-ray diffraction and found to reach up to 25 mol% in solid solution with UO2. By further tailoring the accessible porosity of the UOX microspheres, a simplified single-step infiltration method could be proven effective and dopant levels reaching up to 30 mol% were obtained. The microstructural properties such as residual porosity, granularity, and the phase and dopant distribution inside the microspheres were comprehensively assessed by a variety of electron microscopy and X-ray spectroscopy techniques, i.e. scanning electron microscopy, transmission electron microscopy, energy-dispersive and wavelength-dispersive X-ray spectroscopy. A successful infiltration method and characterization methodology was first developed using Nd3+ as a surrogate for Am3+.
    The methodologies developed on the surrogate transmutation targets containing Nd3+ were subsequently implemented to fabricate and characterize Am-doped transmutation targets in shielded infrastructure. A stock solution of Am(NO3)3 (aq.) was first prepared by careful dissolution of AmO2 in nitric acid, and characterized by inductively-coupled plasma mass spectroscopy and ultraviolet-visible spectrophotometry. Infiltration by Am3+ solution was performed on porous UOX microspheres which were originally fabricated outside of the shielded infrastructure. As a result, no secondary, MA-loaded liquid waste was generated during the process. Furthermore, the use of microspheres avoided the handling of powders, which decreased the risk for local contaminations during processing. A comprehensive microstructural characterization was conducted. The results revealed that a single-phase (U,Am)O2 composition with average Am3+ levels up to 30 mol% was obtained, with a homogenous distribution of metal cations.
    For the first time at SCK CEN, (U,Am)O2 transmutation targets have been fabricated and characterized. The methods developed in this work were specifically adopted to the limitations imposed by working in shielded infrastructure for the handling of highly-radioactive materials. They were found to be safe and very effective in obtaining the desired end-product at a laboratory scale application.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    • KU Leuven
    • Vleugels, Jozef, Supervisor, External person
    • Leinders, Gregory, SCK CEN Mentor
    • Delville, Rémi, SCK CEN Mentor
    Date of Award27 Feb 2024
    StatePublished - 27 Feb 2024

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