Sol–gel processes

The ‘vibro-sol’ and ‘sol-gel microsphere pelletization (SGMP)’ are dust free processes for manufacturing UO₂ and homogeneous MOX fuel pellets. These processes have so far been utilized only on a pilot plant scale in a limited number of countries [15, 16]. In these processes, first, free flowing hydrated gel-microspheres of the mixed oxide are prepared by ‘ammonia external/internal gelation process’ starting from the nitrate solutions of the heavy metals. The ‘ammonia gelation’ is achieved either ‘externally’ via NH₃ gas and NH₄OH or ‘internally’ via an added ammonia generator, namely hexa methylene tetra amine (HMTA). Figures 14 and 15 show the ammonia external gelation of uranium (EGU) and ammonia internal gelation of uranium (IGU) processes, respectively for preparation of hydrated gel microspheres of uranium oxide [17, 15]. The EGU and IGU processes could be extended for mixed uranium plutonium oxide. The major steps in EGU and IGU processes are:

— Preparation of sol or broth from nitrate solutions of uranium and plutonium;

• In the EGU process, the broth is prepared by mixing uranium plutonium nitrate solutions with urea and ammonium nitrate in the proportion of 1.0 mole per liter, 4.0 moles per liter and 2.5 moles per liter respectively followed by boiling for ~30 minutes. A small amount of polyvinyl alcohol (5 g/l) could be added to the mixed solution for mechanical stability of the gel.

• In the IGU process, uranium and plutonium nitrate solutions are mixed with HMTA and urea in mole proportions of 1.25 and 1.75 respectively and cooled to 0^oC. Urea prevents premature gelation of the solution.

— Droplet formation by vibrating nozzle.

FIG. 13. Schematic flow sheet of a simplified process under development in Japan for fabrication of MOX fuel pellets, involving microwave de-nitration.

FIG. 14. External gelation of uranium (EGU) process for preparation of hydrated gel-microspheres of uranium oxide or mixed uranium plutonium oxide.

FIG. 15. Ammonia internal gelation of uranium process for preparation of hydrated gel-microspheres of uranium oxide and mixed uranium plutonium oxide.

— Droplet gelation in NH₃ gas and NH₄OH bath (for EGU) or silicon oil bath at 90 ± 1^oC which decomposes HMTA to release ammonia for conversion of the droplets into hydrous gel-microspheres; in the IGU process, microwave heating could also be used in place of silicon oil.

— Washing of gel-microspheres; in the EGU process, the microspheres are washed in 1% ammonia solution for removal of NH₄NO₃; in the IGU process, the gel-particles are washed with CCl₄ to remove oil and in 3M NH₄OH solution to remove ammonium nitrate.

— Drying of gel-microspheres on a continuous belt dryer at 200–250^oC.

— Controlled sintering at ~1600^oC in hydrogen atmosphere to produce very high density (>99% TD)

‘non-porous’ UO₂ or (U, Pu)O₂ microspheres.

In order to produce ‘porous’ microspheres, carbon black pore former is added to the sol or solution prior to gelation and later removed by controlled calcination of the gel-microspheres at ~700^oC in air followed by hydrogen [16].

The gel-microspheres are subjected to controlled calcination and sintering, after which they are

‘vibro-compacted’ in fuel tubes. The porous microspheres are directly pelletized and sintered to obtain fuel pellets.

The ‘vibro-sol’ and SGMP processes are amenable to automation and remotization and well suited for manufacturing highly radiotoxic plutonium and minor actinide bearing mixed oxide fuels.

The advantages of sol-gel processes are:

— A high degree of microhomogenity is attained in MOX fuel because uranium and plutonium nitrate solutions are mixed prior to gelation;

— Generation and handling of fine powders of UO₂ and PuO₂ are avoided, thereby, minimizing the problem of radiotoxic dust hazard associated with the conventional ‘powder-pellet’ route;

— Dust free and free flowing microspheres facilitate remote and automated manufacturing of fuel rods by

‘vibro-sol’ or SGMP processes;

— The sol-gel plant could be easily integrated with the spent fuel reprocessing plant and could be utilized for preparation of MOX containing minor actinide oxides.

One of the major limitations of the sol-gel processes is the generation of large volume of high level of liquid wastes containing organic chemicals. However, this problem could be significantly minimized if the sol-gel plant is integrated with the spent fuel reprocessing plant.

The sol-gel derived oxide, carbide or nitride fuel microspheres have been used for manufacturing fuel rods of the following types on a pilot plant scale:

‘Vibro–Sol’fuel

In this process, high density oxide, carbide or nitride fuel microspheres of two or three size fractions (800–1 000, 80–100 and ~10 micron) are loaded in one end welded fuel cladding tubes and subjected to vibratory compaction. It is possible to prepare fuel rods of smear density in the range of 70–85% by this process [18, 19]. In the UK, several MOX fuel assemblies manufactured for PFR by the vibro-sol route were initially irradiated through the programme, which was given up later. The limitation of vibro-sol elements are lower operating limits on the linear power of fuel elements at beginning of life (BOL), concern about the fine fraction segregation in the fuel elements and fuel dispersion in the case of clad breach [20]. The Russian Federation opted for the DDP/DOVITA process which produces irregular particles and overcame some of the problems in vibro-sol fuel experienced with sol-gel derived microspheres.

Sol-Gel microsphere pelletization (SGMP)

In the SGMP process sol-gel derived porous or non-porous microspheres are directly compacted to pellets and sintered at ~1 700^oC in hydrogen atmosphere. The ‘non-porous’ microspheres retain their individual identity even after pelletization at high pressure (~840 MPa) and sintering at high temperature (1700^oC), resulting in ‘blackberry’

structures with microsphere boundaries and ‘open’ porosity. This is because of densification within the microspheres and not between them during the sintering process. The ‘porous’ microspheres have low crushing

strength, disintegrate easily during pelletization at ~350 MPa and yield sintered pellets without microsphere boundaries. Figure 16 shows SEM pictures of non-porous and porous microspheres and sintered pellets made from these microspheres [21].

3.2.3. DDP/DOVITA (Dimitrovgrad Dry Process/Dry reprocessing, Oxide fuel, Vibro-pack, Integral,