Life cycle of a fuel element
The fuel element of the FRM II is a hollow cylinder of some 1.3 m in length, 24 cm diameter and with a total weight of 53 kg. It contains approximately 8 kg of uranium enriched up to 93 % of the fissile uranium 235U. The fuel in use is U3Si2 dispersed in Al.
Looking closer at it, 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. Because of the fuel element’s compactness, the moderation of the neutrons takes place in the surrounding heavy water tank. An effect that made it necessary to decrease 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 at the outer surface of the fuel element. The fuel plates are bent to an involute shape in order to guarantee cooling slits of constant width namely 2.3 mm. The cooling is established by light water being pumped through the cooling slits at a rate of 300 kg/s (11 bar) and leading to a temperature increase of the cooling water of only about 15 K, i.e. from ~35 °C to ~50 °C.
Fabrication of FRM II fuel elements
In the past the uranium came from Russia. The fresh fuel elements are manufactured in a production plant in France (Framatome/ CERCA). The common picture frame technology is used for the plate fabrication:
1. Fusion/ Grinding: Uranium metal is melted with silicon to obtain a uranium alloy. The alloy is then milled into an extremely fine powder over several steps.
2. Pressing/ Framing: Next, the powder is compressed into a core, which is then inserted between an aluminium frame and cover plates to form a preplate or “sandwich.”
3. Rolling/ Plate inspection: In the dispersion process, the fuel core, frame and cover plates are joined together in a solid metallic composite by hot rolling an inspection afterwards guarantees that everything is perfect.
4. Assembly: Then, the fuel plates are assembled by welding and swedging into a notched aluminium structure that forms the fuel element.
5. Assembly inspection: Finally, the assemblies go through complete inspection.
Transport to Garching
The fresh fuel elements are transported by road with a highly secured special transport vehicle in accordance with the transport licenses of both the French authorities and the Federal Office for the Safety of Nuclear Waste Management (BASE) from their production plant in France to Garching.
Operation and use
The FRM II is a heavy water moderated and light water cooled research reactor exhibiting a thermal power of 20 MW. For a cycle of 60 days (1200 MWd), one single cylindrical fuel element is used. The goal is a high neutron flux of ~8·1014 n/cm-2s-1 (~100 trillion neutrons per square centimeter per second).
At the end of the cycle, the spent fuel element is unloaded and then transported under water (natural shielding against radiation) to the wet storage of the FRM II where it decays for a minimum of 6.5 years.
A total of approx. 6.9 kg U are then contained in the spent fuel element of which approx. 6 kg are still 235U. Thus, its enrichment is approx. 88 % of the 235U and the gross mass amounts to about 44 kg (after cutting off the metallic head). The maximum activity inventory is approx. 8·1014 Bq wheras the maximum residuel heat (thermal power) is approx. 55 Watt. All figures refer to the end of the above mentioned minimum decay time before a transport is possible.
Transport and storage cask CASTOR® MTR3
The transport and storage casks must withstand thermal and mechanical loads, shield the radioactive inventory safely and dissipate the heat emanating from the radioactive inventory.
The CASTOR® MTR3 cask, newly designed for this purpose in accordance with the safety standards of the International Atomic Energy Agency (IAEA), contains five FRM II fuel elements. It mainly consists of a ductile cast iron cask body, a fuel basket and a double lid system with metallic sealings. The CASTOR® MTR3 is 160 cm high and weighs 16 t. It was tested by the Federal Institute for Materials Research (BAM) and Testing and approved by the BfE in January 2019.
Transport to Ahaus
For transport, a cask with five fuel elements is loaded onto a special transport vehicle and driven to Ahaus, a town in western Germany near the Dutch border. It consists of a tractor and a semi-trailer combination that are designed in accordance with the “Directive for protection against disturbance measures or other effects by third parties during the transport of nuclear fuels by road and rail” (SEWD Richtlinie of BMU).
The Transport Cask Storage Facility (TBL) Ahaus is operated by the federally owned Gesellschaft für Zwischenlagerung mbH (BGZ) and the central facility in Germany intended for the interim storage of research reactor fuel elements.
The current storage capacity for spent fuel elements of the FRM II in Ahaus extends over seven storage locations for 21 casks. This results in a total capacity of 105 spent fuel elements because three casks can be stacked on top of each other.