Since geometry changes to the extremely compact core are almost impossible, a lower enrichment can only be achieved by increasing uranium density. However, in order to compensate for the parasitic absorption by 238U, the uranium density has to be increased disproportionally. Currently, there is no qualified fuel that can provide the required uranium density. Nevertheless, there are several promising fuel candidates which significantly differ in the achievable uranium density:
- A higher-density version of the currently used U3Si2 (high-density dispersed U3Si2)
- Alloyed Uranium-Molybdenum (U-Mo) powder in Al matrix (dispersed U-Mo)
- Alloyed U-Mo as solid foil (monolithic U-Mo)
While the currently used U3Si2 fuel at FRM II reaches an uranium density of up to 3.0 gU/cm³, international fuel research targets densities of 4.8 gU/cm³ and above. On principle, much higher values can be achieved with U-Mo. Thus, dispersed U-Mo reaches up to 8 gU/cm³ and monolithic U-Mo reaches even 15 gU/cm³ and above.
The FRM II actively researches on all three variants and also includes them in the theoretical core studies. Favored, however, is monolithic U-Mo, since the lowest enrichment is supposed to be achieved. The other two candidates are mainly investigated by the partners within the HERACLES consortium.
Evolution of an interdiffusion layer
An important part of the development of new nuclear fuels is test irradiation of fuel plates in specially designed research reactors, the material test reactors. Early test irradiation of high-density Uranium-Molybdenum based (U-Mo) metal fuels revealed the formation of an undesirable interdiffusion layer (IDL) at the interface between the fuel U-Mo and the surrounding aluminum. This amorphous layer poorly retains the gaseous fission products that are formed during nuclear fission – in contrast to U-Mo itself. This can lead to excessive swelling of the fuel plates under irradiation, making them unsuitable for use in reactors.
One of the key challenges of fuel development is therefore to prevent or delay the formation of this layer. For example, this is done by the targeted application of diffusion barrier layers between U-Mo and aluminum.
Radiation experiments with heavy ions
In order to be able to verify the effectiveness of these and other measures in a timely manner, test irradiations in material test reactors are necessary, as mentioned above. However, these not only mean a high financial cost, but the fuel plates themselves become highly radioactive through the irradiation itself, which requires the costly and lengthy examination of the plates in hot cells.
As an alternative to these test irradiations, TUM has developed the concept of heavy-ion irradiation. Here, fuels are not irradiated with neutrons but with iodine heavy ions, which are similar in energy and momentum to the fission products. This allows a simulation of nuclear fission on fuel, without the production of high radioactive fission products.
Using this technique, mini samples of the Ludwig-Maximilians University Munich (LMU) and TUM, are irradiated with different diffusion barriers at Maier-Leibnitz Accelerator Laboratory.
It has been shown that the formation of an interdiffusion layer can be greatly delayed or prevented with the help of sufficiently thick diffusion barriers of materials such as silicon (Si), zirconium (Zr), titanium (Ti) or molybdenum (Mo) as well as zirconium nitride (ZrN) or titanium nitride (TiN). Numerous US and European in-pile test irradiations confirm these results, but at the same time show that, to prevent excessive swelling, further measures, e.g. a homogenization of the molybdenum content in U-Mo, are required. The implementation and testing of these and other measures is one of the current key research tasks.
In addition to irradiation with heavy ions, the working group also participates in a large number of other material characterizations necessary for fuel qualification. These include the determination of thermo-physical properties such as the thermal conductivity of fresh and spent fuel, as well as the precise characterization of the phase behavior of U-Mo.