The current fuel element of FRM II contains 8.1 kg uranium in the form of U3Si2 powder embedded in an Al matrix with an enrichment of 93 % and a maximum density of 3.0 gU/cm³. Such fuel element is sufficient to operate the neutron source for 60 days at a power of 20 MW. As part of the conversion and in coordination with fuel production, the working group is developing various reactor models and studies. It is of high importance to consider technical, and especially, physical aspects.
- The safety factors of the reactor must not diminish in any way. The cooling ability of the core must be ensured at all times, the fission gases resulting from nuclear fission must be safely confined in the fuel plates, and a new fuel must remain mechanically stable under irradiation.
- The scientific performance of the FRM II must remain a world leader. This includes an unchanged cycle length of 60 days and only marginal changes in usable neutron flux.
- The use of a lower enrichment fuel affects the reactor by increasing parasitic absorption in 238U and by other kinetics parameters.
- A new fuel assembly must be manufacturable on an industrial scale at an economical price. This results in limitations in manufacturing techniques and complexity.
To address all these issues in their studies, the working group uses state-of-art computer techniques that can cover the neutronics, burn-up, as well as thermo- hydraulics and -mechanics in a coupled fashion. Together with the international partners, the group is involved in the further development and validation of the computer codes used, and regularly presents the progress at international conferences and scientific publications.
Progressive success and scientific projects
Through various optimizations as well as marginal changes to the geometry of the fuel assembly, the working group has now succeeded in reducing the enrichment required for the conversion in theoretical model calculations to far below 50%. In order to reach the goal of a core with the lowest enrichment possible, new research projects have recently been launched:
- The validation of 3D Computational Fluid Dynamics (CFD) codes for use in high performance research reactors
- The validation of burnup codes using various experimental methods