The fuel element is the heart of the reactor. Here, free neutrons are produced through the fissioning of uranium.
One fuel element for 60 days
At the core of FRM II is a single fuel element, which is in use at a nominal power of 20 MW for 60 days at a time. The so-called fuel irradiation supply is then consumed, i.e. the chain reaction comes to a halt and the reactor shuts down (in fact, the fuel irradiation supply lasts a little more than 60 days, but after 1200 MWd (60 d of 20 MW), the reactor is shut down in a controlled and well-defined way).
Control rod and shut-down rods stop chain reaction
The FRM II fuel element takes the form of a hollow cylinder of some 1.3 m in length and 24 cm diameter. It is located in a central position in the moderator tank of the facility. In the active zone, there are 113 involute curved fuel plates. A control rod is inserted into the inside of the hollow cylinder and its position regulates the thermal power during reactor operation. When the control rod is fully inserted, the chain reaction is stopped and the reactor shuts down. Five shut-down rods are positioned closely around the fuel element in the moderator tank as a further redundant possibility for shutting down the reactor.In both cases, Hafnium is used as a neutron absorber, which is highly effective in stopping the chain reaction through the absorption of the free neutrons.
The fuel element is located in a continuous vertical tube, the so-called “central channel”. A further absorber is installed in the upper part of the central channel (above the fuel element). It ensures a sufficiently safe subcriticality in the handling of the fuel element.
Fuel plates specially produced
The 113 involute curved fuel plates are produced using a picture frame technology, which is a conventional and well- established method for research reactors. The fuel core, which consists of a uranium-silicon compound and pure aluminium, is inserted into a frame made of aluminium and covered on both sides with aluminium plates. In order to avoid power peaks, each fuel plate comprises two radial zones with uranium densities of 3.0 g / cm³ (inner zone) and 1.5 g / cm³ (outer zone). The fuel core, frame and cover plates are rigidly connected in a metallic composite by heat-rolling.
Water flows through the fuel element from top to bottom, which dissipates the heat produced by the fuel plates during operation (see cooling circuits).