The most promising fuel candidate for upgrading the FRM II to lower enriched uranium, uranium-molybdenum (U-Mo), can be used in two different forms:
- As a dispersed fuel. Here, U-Mo is in the form of a fine powder embedded in an aluminum matrix. The fuel plate is obtained by wrapping this core in a frame cladding made of aluminum, also called cladding.
- As a monolithic fuel. This is a massive, metallic film made of U-Mo, which is covered by cladding.
The research of the working group "High-density nuclear fuels“ focuses on the production of monolithic fuels, since with these, naturally, the highest uranium density, and thus the lowest enrichment after conversion, can be achieved.
First of all, the U-Mo alloy is produced in the desired mixing ratio by means of alloying of uranium and molybdenum and then rolled into foils. To avoid unwanted diffusion between U-Mo and aluminum, these foils are coated with a diffusion barrier prior to application of the cladding.
The production of monolithic U-Mo fuels takes place in cooperation with international cooperation partners. While currently U-Mo foils are procured from US partners and the cladding application is performed with Framatome-CERCAs so-called C2TWP, TUM has specialized in developing a powerful and flexible process for coating U-Mo films with diffusion barrier layers. To this end, TUM operates a nuclear fuel laboratory specially set up and approved in accordance with §9 AtG on the grounds of the FRM II.
The physical vapor deposition (PVD) process used here involves a plasma process in which the material to be applied (e.g zirconium) is sprayed atomically in a low pressure plasma and vapor deposited onto the substrate (U-Mo).
In contrast to the alternative methods, e.g. methods used in US fuel development, the PVD method allows the application of nearly all layer materials in any thicknesses. Also the coating of complex geometries, e.g. U-Mo foils, with a thickness profile is easily possible with PVD process.
The complete manufacturing process for monolithic U-Mo fuels is at a transition from the prototype to the pilot development stage and is currently being validated under the EMPIrE irradiation test. The industrialization of this process involves major technical challenges, so it is not yet suitable for the production of U-Mo fuels on an industrial scale. In addition to the TUM working group “High-density nuclear fuels“, several research groups worldwide are working on further development and manufacturing processes.
First of all, the U-Mo alloy is produced in the desired mixing ratio by means of alloying of uranium and molybdenum. This alloy is processed by atomization to fine powder and coated with a diffusion barrier layer. After mixing with aluminum powder, a compacted core is pressed and cladded.
The production of disperse U-Mo fuels takes place at the French fuel element manufacturer Framatome-CERCA in close operation with the European partners, i.e. TUM.
For example, the prototypical atomization process was developed and optimized on site by employees of the “High-density nuclear fuels“ working group. In this process, a cylindrical U-Mo metal piece is rotated around its axis at an extremely high speed and, meanwhile, locally melted with an electric arc. Due to the high rotational speed, the melt atomizes into fine droplets which immediately solidify into a fine powder.
In contrast to alternative methods for the production of U-Mo powder, e.g. grinding, the atomization process provides exactly spherical (round) powder particles, which is thus suitable for coating with a diffusion barrier layer.
The complete manufacturing process for disperse U-Mo fuels, as well as for monolithic fuels, is now moving on from the prototype to the pilot development stage and is currently being validated under the EMPIrE test irradiation.