One of the disadvantages of light water reactors is their low operating temperature. Thus the efficiency of the energy conversion process (Carnot factor) is limited.
High-temperature reactor
Temperatures are restricted due to the properties of the coolant, i.e. water, as well as due to the protection of the materials of the tubes, rods and pipes.
In pressurized water reactors the temperature remains below the boiling point of water, in boiling water reactors high temperatures lead to non-optimal heat dissipation due to the occurrence of vapour bubbles (“boiling crisis”). The operating safety may thus be endangered.
These circumstances drove research and development of high-temperature reactors. The nuclear fuel is embedded into temperature-resistance ceramics and covered by graphite. Using special alloying methods, the graphite cover can obtain fireproof properties.
Graphite serves as the moderator, while gaseous helium, which flows around the fuel elements, serves as coolant.
The fuel elements are arranged as bulk in spherical of prismatic form. However, fuel elements might also exist as rods.
The gas is heated by the fuel elements and transfers its thermal energy via a heat exchanger to water, which in turn evaporates. The steam then drives a steam power process, where electricity is generated via a turbine and a generator.
New fuel elements can be inserted into the reactor from above, while used fuel elements are removed from below. The spheres have a diameter of about 6 centimeter. In order to shut down the reactor, control rods can be inserted into the pile of spheres.
In Germany a research reactor of this kind has been in operation for 21 years in Jülich. A prototype with 300 MWel for commercial application has been constructed in Hamm. It was however deactivated in 1989 only 6 years after its commission. Technical difficulties were the main reasons for that step.
With this tutorial we end the series on nuclear energy and will be focussing on renewable energies from next week onwards.
