Assuming that suitable denser fuels are available, enrichment of the fissile uranium can be reduced to less than 50 % in the existing fuel assembly and with the same thermal power of the FRM II. An ancillary clause of the operating license of FRM II in 2003 states that this conversion must be completed by the end of 2010, if a suitable fuel is available. However, this conversion could not take place within this timeframe, as a suitable, qualified and denser fuel has not been found.
Worldwide, more than 70 reactors have been converted 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.
To date, no high-power research neutron source has been successfully converted. The following high-power research reactors and neutron sources with HEU are in operation worldwide:
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
TUM is conducting research in the European "HERACLES" network together with partners from France and Belgium. The USA is also conducting a very large amount of research, with research groups from both continents exchanging results intensively. Furthermore, TUM is in active exchange with partners in Korea. Worldwide, a number of research reactors (outside Russia) are currently still operated with HEU. For more than a dozen of these powerful research reactors, high-density fuel is not yet available to convert them to lower enrichment. About half of these research reactors that cannot yet be converted are located in the United States.
Immediately after the operating license was granted in 2003, a TUM working group started research into high-density uranium fuels. It is currently investigating two possible fuels made of uranium molybdenum: once U-Mo powder mixed with aluminum (dispersed) and once as a solid plate (monolithic). Work is also being done on further densification of the current fuel, U3Si2. In all cases, the actual fuel zone is still enclosed in a kind of picture frame made of aluminum. Test irradiations of the fabricated fuel plates are carried out in so-called material test reactors in Europe and the USA.
Together with the operators of the high-performance research reactors in Europe (SCK-CEN, ILL and CEA) and the fuel element manufacturer Framatome-CERCA, TUM is conducting research on all three solutions in the HERACLES cooperation. This is done in close coordination with partners from the USA and Korea.
Yes, in cooperation with international partners, including the HERACLES consortium, the research group at FRM II is investigating all three fuel candidates:
There are dispersed U-Mo and high-density uranium silicide (U3Si2) in addition to monolithic U-Mo fuel, which is the only fuel that enables an LEU fuel element at the FRM II.
The schedule is independent of which of the three fuel candidates is selected. A licensing process as well as extensive irradiation tests must be undergone by all three candidates. The time required to create an industrial manufacturing capacity at Framatome-CERCA is also independent of the material variant selected.
While each research reactor, and therefore each fuel element, is different for that particular reactor, the research work done at TUM, and the approval process that a new fuel goes through in Germany, also makes it easier to qualify the fuel in other countries and for other high-flux neutron sources.
To compensate for the lower enrichment, the fissile uranium must be physically packed more densely to continue to provide neutrons to users without disproportionate losses. Such fuel is not currently available worldwide and, once developed, must be qualified for use at FRM II. At FRM II alone, a twenty-member research group is investigating possible fuels and application scenarios for this purpose. This is being done, among other things, in a European research alliance funded by the EU with more than 10 million euros. In addition, there is close cooperation with the USA, because five high-flux reactors are to be converted in the USA alone.
When the agreement on the conversion came into force in 2003 for FRM II between the federal government and the state of Bavaria, the powdered uranium-molybdenum fuel appeared at the time to be a promising candidate for fuel development internationally. These hopes were first dashed at the end of 2004 when some test plates of this fuel swelled and burst open during test irradiations in test reactors in the USA and Europe. Subsequent metallurgical improvements to the fuel again proved insufficient for nuclear licensing in test irradiations around 2013.
Now, new plates made from altered material using further improved manufacturing techniques are being tested around the world. See also: Duration of a typical test irradiation.
Because of the high costs associated with irradiation testing, the tests are performed serially.
For the first time, reactor physics calculations show that operation of the Heinz Maier-Leibnitz Research Neutron Source (FRM II) with low enriched uranium (LEU, < 20 % uranium-235 enrichment) is theoretically possible. The existing FRM II fuel element contains uranium-235 with an enrichment of up to 93 %, which is referred to as highly enriched uranium (HEU).
The only fuel that now allows conversion to below 20 % enrichment of uranium-235 is monolithic U-Mo. Along with two other fuels, it is a possible candidate for FRM II conversion, but is favored by TUM because it has the highest uranium density and thus the lowest achievable enrichment.
The neutron flux of an LEU fuel element at FRM II will be a maximum of 10 % lower, averaged over all scientific instruments, than for the current HEU fuel element. This is also one of the prerequisites for the conversion to lower enriched uranium: FRM II will continue to be a high-flux neutron source and enable cutting-edge research with neutrons.
A fresh fuel element, regardless of whether it is made of LEU or HEU, does not pose a hazard to humans. It is only very weakly radioactive and emits almost exclusively easily shieldable α-radiation. Used fuel elements are highly radioactive due to the fission products produced and require special shielding measures even in the long term. The radioactive radiation from a spent highly enriched fuel element comes mainly from the fission products with a long-term dominant decay time of 30 years. A spent low-enriched fuel element contains substantially higher amounts of plutonium with a long-term dominant decay time of 24,000 years. Therefore, the radioactive burden of an original HEU fuel element is significantly lower.