A provision in the operating license of the FRM II from 2003 states that the conversion must occur by the end of 2010 provided that a suitable fuel is available. Only by using an appropriate, more dense fuel can the enrichment of fissile uranium be reduced to below 50% in the existing fuel element and with the same thermal power as the FRM II. This conversion could not occur within the planned timeframe because the necessary prerequisite – a suitable, qualified, and more dense fuel – was not met. Even today – as of 2024 – such a fuel is not qualified for use in high-performance research reactors such as the FRM II. TUM and its European and international partners continue to work hard to qualify the fuel selected for conversion.
No high-performance research reactor like the FRM II has been converted to date.
The following high-performance research reactors and neutron sources are in operation worldwide with highly enriched uranium (outside of Russia):
| Name | Type of reactor | Country | First criticality | Therm. Power | Fuel | Enrichment |
| FRM II | Beam tube | Germany | 2004 | 20 MW | U3Si2/Al | 93% |
| RHF | Beam tube | France | 1971 | 58,3 MW | UAlx/Al | 93% |
| BR2 | MTR* | Belgium | 1961 | 100 MW | UAlx/Al | 93% |
| MITR | Beam tube | USA | 1958 | 6 MW | UAlx/Al | 93% |
| NBSR | Beam tube | USA | 1967 | 20 MW | UO2/Al | 93% |
| MURR | Beam tube | USA | 1966 | 10 MW | UAlx/Al | 93% |
| ATR | MTR | USA | 1967 | 250 MW | UAlx/Al | 93% |
| HFIR | Beam tube | USA | 1965 | 100 MW | U3O8/Al | 93% |
*) Material test reactor
Over 70 reactors have been converted worldwide to date. In Germany, two reactors have already been successfully converted to LEU: BER 2 (1997 - 2000) and DIDO (2004). Both are already out of operation (Status: 2024).
Since the fuel decision in 2023, a cross-departmental project (HEU-LEU) has been working on the concrete technical implementation of the conversion of the FRM II. This project is scientifically supported by the TUM Center for Nuclear Safety and Innovation (TUM.CNSI), which, among other work, is carrying out the reactor physical design and computer simulation of the new fuel elements.
In parallel, TUM is working with partners from France and Belgium in the European HERACLES consortium to qualify the monolithic U-10Mo fuel. Partners in the USA are also conducting much research, with groups from both continents exchanging results intensively.
In order to develop and industrialise the production of the new fuel elements, TUM has been working closely with the French company FRAMATOME since 2019. Phase 2 of the project – optimising production techniques and preparation for industrialisation – was launched in April 2024.
Also, in October 2024, the EU-CONVERSION project of the HERACLES consortium, funded by the European Union (Euratom), will start. Among other work, this project is aimed at the reactor-specific qualification of the fuel selected for the FRM II and the validation of state-of-the-art computer simulation methods for reactor physics calculations.
A sum in the low double-digit millions. The Federal Republic of Germany and the Free State of Bavaria share the costs for the scientific work. To a lesser extent, these are covered by funding projects from the European Union. The Free State of Bavaria entirely bears the work on the actual implementation of the conversion.
Until the fuel decision in 2023, TUM worked on developing and qualifying the three leading fuel candidates in cooperation with international partners, including the HERACLES consortium. In addition to the monolithic U-10Mo fuel, which is the only one that enables an LEU fuel element for FRM II and has now been selected, these were also dispersed U-7Mo and high-density uranium silicide (U3Si2).
Since 2023, TUM has been working exclusively on monolithic U-10Mo fuel and is focusing its activities on the qualification and production of this fuel.
To compensate for the lower enrichment, the fissile uranium must be packed more densely to continue providing users with neutrons without disproportionate losses. The fuel suitable for this and selected for the conversion of the FRM II - monolithic U-10Mo - is currently being developed and qualified by TUM in cooperation with international partners, including the HERACLES consortium, and corresponding production techniques are being prepared. At the same time, a research group of around twenty people at FRM II alone is working on developing and optimising a fuel element design based on LEU.
There is a timetable for the conversion, which envisages concrete steps over the following years. For example, the license application for a new fuel element is to be submitted to the responsible authority at the end of 2025. Following the licensing procedure and the granting of the license, there will be a transition phase, followed by regular operation with LEU fuel elements.
When the agreement between the Federal Republic of Germany and the Free State of Bavaria on the conversion of the FRM II came into force in 2003, the dispersed uranium-molybdenum fuel (U-7Mo) appeared to be a promising candidate internationally. These hopes were dashed at the end of 2004 when some test plates of this fuel swelled excessively and burst open during test irradiations in test reactors in the USA and Europe. The subsequent metallurgical improvements to the fuel proved insufficient in test irradiations around 2013.
As a result, TUM intensified its work on the monolithic U-10Mo fuel, which is metallurgically more benign. It also made reducing the enrichment to below 20 % U-235 (low-enriched uranium, LEU) feasible. Extensive irradiation campaigns in material test reactors are also required for this fuel:
Duration of the serial irradiation tests
- Design and development: 1-2 years
- Manufacturing: 1 year
- Irradiation and decay: 1 year
- Post-irradiation examinations: 1-2 years
As the various irradiation tests build on each other, the tests are carried out in series.
There is now a timetable for the conversion (see previous question), according to which the license application for a new fuel element will be submitted at the end of 2025.
The French manufacturer of the current fuel elements for the FRM II, FRAMATOME, is available as a production partner for the upcoming conversion of the FRM II and considers the production of the planned new LEU fuel elements possible. Extensive cooperation with FRAMATOME has been in place since 2019 to provide the first low-enriched fuel element in the early 2030s.
The neutron flux of an LEU fuel element at the FRM II will be a maximum of 10 % lower averaged over all scientific instruments than with the current HEU fuel element. This is also one of the prerequisites for converting to lower-enriched uranium: the FRM II will continue to be a high-flux neutron source and enable cutting-edge research with neutrons.
Whether made of LEU or HEU, a fresh fuel element does not pose a radiological hazard. It is only very weakly radioactive and emits almost exclusively easily shielded α-radiation. Again, spontaneous fuel elements are highly radioactive due to the fission products produced and must be treated accordingly regardless of whether they are made of LEU or HEU.