From HEU to LEU
Research reactors are the only facilities in the civilian sector that have used, and in some cases still use, highly enriched uranium (HEU). The goal of conversion of any research reactor is to minimize potential residual proliferation risks by reducing uranium-235 enrichment in the fuel used.
The reactor shutdown that is unavoidable in any conversion implies scientific cutbacks, interruption of isotope production essential for nuclear medicine, and high costs. Therefore, it is imperative to achieve this goal sustainably and thus in only one step.
Specific objectives for the conversion:
- Maintaining the current safety level
- Maintenance of the scientific performance
- Rapid conversion in a single step, if possible, without long downtimes
- Sustainable fulfillment of the conversion agreement, in the sense of non-proliferation
- Securing the future viability of FRM II
In an agreement between the Federal Republic of Germany and the Bavarian State, as well as in an ancillary clause of the operating license of FRM II from 2003, it is therefore stated that FRM II is also to be converted to a fuel with a uranium-235 enrichment < 50 %, provided that a suitable fuel is available. In addition, the obligation to convert FRM II is also written into the current MLZ Cooperation Agreement, between the Technical University of Munich and the Helmholtz Centers Jülich and Hereon, dated December 2020.
Beyond the target of enrichment below 50 % stipulated in the Federal-State Agreement and the operating license of FRM II, international efforts are being made, in particular by the International Atomic Energy Agency (IAEA) and the US Department of Energy (DOE), to convert research reactors exclusively to low enriched uranium (LEU, < 20 % U-235 enrichment). For this reason, as well as for the sustainable fulfillment of the above-mentioned conversion objectives, TUM has always strived for conversion to the lowest possible enrichment. The ultimate goal of the research and development work is therefore the conversion of FRM II to a fuel element with < 20 % uranium-235 enrichment (LEU).
Research achievements: LEU Design
Through various optimizations as well as marginal changes to the geometry of the fuel element, the working group has now succeeded in reducing the enrichment required for conversion to well below 50 % in theoretical model calculations, subject to certain reservations. In order to achieve the goal of the lowest possible enrichment, new research projects have been started in the recent past:
- The validation of 3D fluid dynamics codes for use in high-power research reactors
- The validation of burnup codes using various experimental methods
The search for a LEU (low-enriched uranium; < 20 % U-235 enrichment) design has produced models and methods that allow modern, efficient, and very accurate computation of FRM II. The only fuel that allows conversion to an LEU design is monolithic U-Mo.
From the reactor physics and technical point of view of TUM, as well as independently verified by the US Argonne National Laboratory, a conversion of FRM II to an LEU design appears possible.
Publication:
C. Reiter, A. Bergeron, D. Bonete-Wiese, M. Kirst, J. Mercz, R. Schönecker, K. Shehu, B. Ozar, F. Puig, J. Licht, W. Petry, P. Müller-Buschbaum,
A Low-Enriched Uranium (LEU) option for the conversion of FRM II,
Annals of Nuclear Energy, Volume 183, 2023, 109599, ISSN 0306-4549, https://doi.org/10.1016/j.anucene.2022.109599.
The task now is to put the theoretically established convertibility, which has been confirmed by independent teams of experts, into practice. To this end, a cross-departmental project team at FRM II will carry out further optimizations to the fuel element design in the next few years and prepare the initiation of the licensing procedure and the procurement of new fuel elements. Accompanying the technical and nuclear law implementation, the research work that continues to be necessary will be carried out at the TUM Center for Nuclear Safety and Innovation.