Manufacturing technology

The fuel element of the FRM II is composed of 113 individual fuel plates. The plates consist of “meat” and cladding. The fuel itself is either of dispersed (left) or of monolithic structure (right).

In common with all high performance research reactors, FRM II has very high power densities and thus heat generation in the reactor core to produce the high neutron flux densities for its scientific users. However, to ensure good cooling capability of the reactor core, fuel plates as opposed to fuel rods are used at the FRM II. The design of these fuel plates corresponds to a sandwich structure with a fuel-containing core (=meat) and a cladding.

Two concepts are available for converting the FRM II to high-density, low-enriched uranium-molybdenum fuels: the dispersion fuel with fuel grains that are uniformly distributed in a carrier matrix, and the monolithic fuel where the core is a single solid fuel layer.

Currently, it is not possible to foresee which of the concepts offers the greatest advantages in application and will be adopted. Research in the field of manufacturing technology therefore aims to cover both concepts.

Monolithic fuel
At the FRM II, plasma processes are being developed which are able to treat monolithic U-Mo fuels. The left image shows a U-Mo fuel in full size (600 mm x 60 mm) during the plasma process. The right image shows the same fuel after plasma cleaning and coating.

Only since 2008 has an efficient, yet still laboratory scale production technique for monolithic UMo fuel become available. It was developed at the Idaho National Laboratory, USA. The industrialization of the process is associated with major difficulties, so it is not currently available for the production of UMo fuels in industrial quantities. Several research groups around the world are therefore collaborating to further develop the production technique.

Research at the FRM II examines alternative techniques and procedures that can be integrated into the existing production technique in order to improve it. The focus is primarily on techniques for surface treatment and modification of U-Mo fuel foils, since the surface of the foil represents the critical intersection between fuel and cladding.

Dispersion fuel
Techniques are being developed at the FRM II that allow for the provision of U-Mo fuel powder with a protective coating, thus making it more resistant to radiation and temperature. The picture shows the interior of a powder coating machine during operation.

Dispersed nuclear fuel with a carrier matrix of aluminium has been successfully used for decades and without any difficulties in many research reactors worldwide. The FRM II, for example, currently uses dispersed uranium silicide in aluminium. For the conversion to a lower enriched fuel, it was initially planned to use U-Mo dispersed in aluminium as well. However, various irradiation experiments over the last decade have shown that this nuclear fuel in its original configuration where some experience has already been gained presents difficulties at high temperature and challenging irradiation conditions in a high flux research reactor. Therefore, methods are being developed to stabilize the dispersed U-Mo fuel in the aluminium matrix.

Research at the FRM II examines techniques to increase the irradiation resistance of dispersed U-Mo. This includes on the one hand the optimization of the U-Mo particle geometry by advanced powder manufacturing processes and, on the other, the study of protective coatings for U-Mo grains.