The current fuel element of the FRM II contains uranium with an enrichment of 93% and a maximum density of 3 g U/cm3. Such a fuel assembly is sufficient to operate the neutron source for 60 days at a power of 20 MW. If the enrichment is lowered and the external conditions remain unchanged, the lower fraction of the fissile isotope 235U has to be compensated by a higher overall Uranium density. The content of the second isotope in the uranium, 238U, is shifted from the current 7% to a higher value corresponding to the lower enrichment.
Higher density requires new fuel
If, for example, the enrichment of 235U of 93% is decreased to 50%, the proportion of 238U is increased from 7% to 50% accordingly. In order to maintain the amount of fissile material in the core, the total Uranium density would have to be increased from the current maximum of 3.0 g U / cm3 to about 3 g / cm³ × 93% / 50% = 5.6 g / cm³. However, to compensate for increased parasitic absorption due to the increased amount of 238U and other non-fissile materials in the core, the density has to be increased even further. This density exceeds the specification of current fuels in use, so a new fuel needs to be developed.
Technically feasible densities at 16 g U / cm³
With the introduction of a new fuel for the FRM II, the calculations of the neutron flux and the cooling capability of the fuel elements have to be updated. As set out in the conditions for the conversion, the scientific quality of the neutron source must be maintained. This includes maintaining the cycle length of 60 days. As mentioned above, the density of fissile material has thus to be overcompensated, so that not only 5.6 g U/cm3 but, in total, 8 g U/cm3 are needed. This overcompensation is larger the further the enrichment is lowered until a natural limit of about 19 g U/cm3 is reached. Densities up to about 16 g U/cm3 are technically feasible as additions to the pure Uranium are necessary to make it suitable for use in a reactor.
Enrichment lowered further
In addition to designing new and adapted fuel cores based on the development of fuels, the research group is involved in the development of new software and the maintenance and improvement of existing software systems for reactor calculation. This includes neutronics as well as burn-up and thermal-hydraulics codes. By using novel program systems and various optimizations as well as minor changes to the geometry, the research group has succeeded in reducing the enrichment required for a conversion down to 25% under certain conditions.