FRM II's high thermal neutron flux density of approx. 8x10¹⁴cm^-2s^-1 is due to the compact core concept and the use of an uranium silicide-aluminium dispersion.

To achieve to duration cycle of approx. 52 days the fuel element must contain an adequate mass of the uranium isotope ²³⁵U. After this cycle at a nominal thermal power of 20MW the fuel element has to be replaced by a new one.

• On the one side highly enriched uranium must be used (enrichment factor approx. 93% ²³⁵U and approx. 7% ²³⁸U) for the fuel element for containing an adequate mass of this uranium isotope. ²³⁸U absorbs neutrons very good and changes into plutonium.
• On the other side the density of this highly enriched uranium must be relatively high, too.

That's the reason why FRM II's compact core concept includes new developed nuclear fuel based on uranium silicide-aluminium dispersion. In order to homogenise the power and fission density, the uranium content in the outer zone of each fuel plate is reduced from 3gcm^-3 to 1,5gcm^-3. The reactor core contains approx. 8,1kg uranium in total.

FRM II's new fuel elements include new technical know-how. Despite these innovations the nuclear safety must be and is ensured. Fuel elements of this geometry with involuted, curved fuel plates have been tested at the high-flux research reactors HFR (Grenoble, France) and HFIR (Oak Ridge, USA) for many years. Their power density or velocity of the cooling water is comparable to these characteristics of FRM II.