Investigation of the influence of redox conditions on the conversion of actinide solutions into solid microspheres via sol-gel chemistry

Schreinemachers Christian

Promoter

Binnemans Koen, (KULeuven), koen.binnemans@chem.kuleuven.be

SCK•CEN Mentor

Verwerft Marc
marc.verwerft@sckcen.be
+32 14 33 30 48

SCK•CEN Co-mentor

Cardinaels Thomas
thomas.cardinaels@sckcen.be
+32 14 33 32 00

Expert group

Radiochemistry

PhD started

2016-10-01

Short project description

Spent nuclear fuel contains substantial amounts of fission products and transuranium elements, which are responsible ca. 99% of its radioactivity. Conventional industrial reprocessing (via the PUREX process) allows for selective extraction of only uranium and plutonium, which can then recycled into MOX fuel. In a more optimal scenario including partitioning and transmutation (P&T), also the minor actinides (MA) are recycled and burned into GenIV-type reactors.

To recycle plutonium and the MA, two types of recycling strategies are envisaged. The first approach is called “homogeneous recycling” and consists in mixing limited amounts of plutonium and MA to uranium-based fuel, which is then loaded as driving fuel of the reactor. For this specific case, innovative partitioning processes such as the “Group ActiNide EXtraction” (GANEX) process have been developed in the framework of the EU ACSEPT and SACSESS projects. The GANEX process extracts all transuranics from the spent fuel, after initial removal of uranium [1-4]. The second approach is “heterogeneous recycling”, involving immobilization of the plutonium and/or MA in an inert (i.e. U-free) matrix; examples of materials proposed inert matrices are ThO2, MgO, Mo-92 or yttria-stabilised zirconia. Such targets are intended to be loaded in limited number in the reactor core or periphery. Innovative extraction processes such as “Selective ActiNide EXtraction” (SANEX) and “EXtraction of Americium” (EXAm) have been developed in order to obtain selected MA (Am, Cm) from the spent fuel for this purpose [1, 5-7].

For homogeneous recycling, the R&D towards industrial scale production could benefit from the multi-decennial experience of SCK•CEN with MOX fuel production. However, as an alternative to conventional powder metallurgical processes of milling and mixing, other routes are proposed in which the desired composition is prepared in a liquid phase (sol), followed by sol-gel conversion, drying and calcination into free-flowing microspheres. These can be pressed and sintered into high density pellets; alternatively they are perfectly suited for the new sphere-pac fabrication concept [8]. A major advantage of these “wet” routes is the avoidance of dust formation which is inherent to the mixing/milling processes of the “dry” route. The wet routes also offer advantages in terms of automatization and remote operation, which becomes important for production of MA-bearing nuclear fuels with elevated gamma and neutron activity.

One of the major concerns in the “wet” routes however, is the homogeneity at the different stages: gel phase, dried microspheres, calcined microspheres and sintered bodies. In the case of high-Pu-containing MOX fuels, a tendency of phase segregation has recently been observed [9-13]. The influence of complex U-Pu redox mechanisms during gel formation are assumed to affect the local order in the final oxide phase [14-15]. Indeed uranium and plutonium can both exist in different oxidation states depending on the solution chemistry. A wide range of parameters influence their valence state, such as pH, nitrate and actinide concentration and also the presence of complexing or gelation agents and different amounts of MA will play a role [16-17].

Objective

In this project, sol-gel chemistry will be applied to produce actinide-bearing microspheres. Different binary (U-Ce, U-Pu, U-Nd) and ternary (U-Pu-Nd) systems will be studied where cerium and neodymium act as a simulants for plutonium and americium, respectively. The gel-microspheres will be produced via both internal and external gelation processes followed by drying and calcination of the gel-beads into solid microspheres [8, 18-19]. The objectives of the project are (1) to acquire expertise in sol-gel chemistry as part of the liquid-to-solid conversion technology and more specifically (2) to investigate the influence of the valence state of the starting materials on the sol-gel process and the homogeneity of the produced microspheres.

Methodology

The valence state can be tuned by using reducing and oxidizing agents. For instance in the case of plutonium, hydroxylamine hydrochloride can be used to convert Pu(IV,VI) to Pu(III) or KMnO4 can be used to convert Pu(III,IV) to Pu(VI) [15]. Also the concentration of the actinides (lanthanides) and acidity are parameters to control the valence states. An important issue to address is hydrolysis, which needs to be avoided (e.g. in non-complexing media, hydrolysis of Pu(III) occurs at pH 5, of Pu(VI) at pH 3-4 and of Pu(IV) at pH 1). Hydrolysis results in hydroxypolymers and oxopolymers which are more resistant to dissolution. The effect of the addition of complexing agents will be tested in order to reduce hydrolysis.

A wide instrumental pool will be used to characterize liquid and gel phases, and calcined microspheres: UV-VIS-NIR spectroscopy and HPLC-ICPMS will be performed on the liquid phase to analyze the valence states of the actinides (lanthanides), while XRD and SEM-EDS measurements will be performed on the calcined microspheres to investigate the crystal lattice structure and homogeneity. Particle sizes will be determined by optical microscopy and/or laser granulometry, while determination of specific surface areas will be performed by BET measurements. EXAFS and XANES measurements at synchrotron sources will also be part of this study (BM20, ESRF - Grenoble), in order to determine the first coordination sphere and valence state of the lanthanides and actinides.

This project is subdivided in three work packages (WP): each consists of synthesis and characterization (using techniques described above) of a specific binary or ternary actinide (or simulant) system.

WP1: synthesis and characterisation of U-Nd and U-Ce microspheres. As a first study, this will allow to determine and optimize synthetic sol-gel parameters for a simple U-Nd system (Nd having a fixed 3+ oxidation state) and more complex U-Ce system (Ce can exist in both 3+ and 4+ oxidation state).

WP2: synthesis and characterisation of U-Pu microspheres. The expertise and synthetic sol-gel parameters obtained from the U-Ce system will be tested on the U-Pu system. Several plutonium concentrations (1-30 wt.%) will be tested with a focus on the microhomogeneity of the microspheres containing a high plutonium concentration.

WP3: synthesis and characterisation of U-Pu-Nd microspheres. In the final stage of the project, the knowledge acquired during WP1 and WP2 will be used to produce ternary U-Pu-Nd microspheres with the challenge to control the uranium and plutonium valence states during the sol-gel process and to prevent phase segregation.

References

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