The fuel element of the FRM II is a hollow cylinder approx. 1.3 metres long, 24 cm in diameter and with a total weight of 53 kg. It contains around 8 kg of uranium, which is enriched with up to 93 % of the fissile uranium 235U. The fuel used is U3Si2, which is dispersed in aluminium.
On closer inspection, the fuel element contains 113 separate fuel plates, which are mounted between an inner and an outer tube with diameters of 118 mm and 237 mm respectively. Due to the compactness of the fuel element, the moderation of the neutrons takes place in the surrounding heavy water tank. This effect made it necessary to reduce the fuel density in the outer part of the plates from 3 g/cm³ to 1.5 g/cm³ in order to avoid an unacceptable power peak on the outer surface of the fuel element. The fuel plates are bent into an involute shape to ensure cooling slots of a constant width of 2.3 mm. Cooling is achieved by pumping light water through the cooling slots at a rate of 300 kg/s (11 bar), resulting in a temperature rise of the cooling water of about 15 K, i.e. from ~35 °C to ~50 °C.
Production of a fuel element
In the past, the uranium came from Russia. The fresh fuel elements are manufactured in a production facility in France (Framatome/CERCA). Conventional picture frame technology is used to manufacture the fuel plates:
1. Fusion/Grinding: Uranium metal is melted with silicon to obtain a uranium alloy. The alloy is then ground into an extremely fine powder in several steps.
2. Pressing/Framing: The powder is then pressed into a core, which is then inserted between an aluminium frame and cover plates to form a preplate or "sandwich".
3. Rolling/Sheet Metal Inspection: In the dispersion process, the fuel core, frame and cover plates are joined by hot rolling to form a solid metallic composite - a subsequent inspection guarantees that everything is perfect.
4. Assembly: The fuel plates are then assembled by welding and swivelled into a notched aluminium structure that forms the fuel element.
5. Assembly Check: Finally, the assemblies undergo a complete inspection.
Transport to Garching
The fresh fuel elements are transported by road from their production site in France to Garching in a highly secure special vehicle in accordance with the transport authorisations of the French authorities and the Federal Office for the Safety of Nuclear Waste Management (BASE).
Operation and utilisation
The FRM II is a heavy water moderated and light water cooled research reactor with a thermal output of 20 MW. Only a single cylindrical fuel element is used for a cycle of 60 days (1200 MWd). The goal is a high neutron flux of ~8×1014 neutrons/cm-2s-1 (~100 trillion neutrons per square centimetre and second).
Decay pool
At the end of the cycle, the spent fuel element is discharged under water (natural shielding against radiation) and transported to the decay pool of the FRM II, where it is stored for at least 6.5 years.
The spent fuel element then contains a total of approx. 6.9 kg of U, of which approx. 6 kg is still 235U. Thus its enrichment is approx. 88 % of the 235U and the gross mass approx. 44 kg (after cutting off the metal head). The maximum activity is approx. 8×1014 Bq, while the maximum residual heat (heat output) is approx. 55 watts. All statements refer to the end of the above-mentioned minimum decay time before transport is possible.
CASTOR® MTR3 transport and storage casks
The transport and storage containers must withstand thermal and mechanical loads, safely shield the radioactive inventory and dissipate heat.
The CASTOR® MTR3 cask, newly developed for this purpose in accordance with the safety standards of the International Atomic Energy Agency (IAEA), contains five FRM II fuel elements. It essentially consists of a ductile cast iron body, a support cage and a double lid system with metallic seals. The CASTOR® MTR3 is 160 cm high and weighs 16 tonnes. It was tested by the Federal Institute for Materials Research and Testing (BAM) and approved by the BfE in January 2019.
Transport to Ahaus
For the transport, a cask containing five fuel elements is loaded onto a special transport vehicle and driven to Ahaus, a town in western Germany near the Dutch border. This vehicle consists of a tractor unit and an articulated lorry, which are designed in accordance with the "Richtlinie zum Schutz vor Störmaßnahmen oder sonstigen Einwirkungen Dritter beim Transport von Kernbrennstoffen auf Straße und Schiene" (SEWD Guideline of the BMU).
Central Interim Storage Facility Ahaus
The Transport Cask Storage Facility (TBL) Ahaus is operated by the federally owned BGZ Company for Interim Storage (BGZ, eng. Gesellschaft für Zwischenlagerung mbH) and is the central facility in Germany for the interim storage of fuel elements from research reactors.
