Nuclear Fission and Chain Reaction
Research reactors like the BER II at the Helmholtz Centre Berlin, use fission of atomic nuclei of the uranium isotope with the mass 235 (the number of protons is Z = 92, that of neutrons is N = 143, written 235U). Uranium is fissioning, as shown in the figure on top, into two fragments approximately half the size of uranium ("fission fragments") plus free neutrons (about 2.4 per fission), which have kinetic energies of more than 1 MeV. Natural uranium contains circa 0,7% of the isotope 235U, the rest of 99,3% is the isotope 238U. For the use in a research reactor one has to enrich the 235U-component in a complicated procedure to a concentration of approximately 20%; the rest remains the isotope 238U, which does not fission. Nuclear fuel with less than 20% of 235U-component is called LEU (low-enriched uranium), fuel with more than 20% is called HEU (high-enriched uranium). The irradiated uranium-isotope 238U does not fission but leads by capture reactions to some long-living radioactive reaction products, e.g. new elements like neptunium und plutonium. These are the main components of the so-called nuclear waste.
Every nuclear fission releases energy, which is transported mainly by the motion of the outgoing fragments, these heat their surroundings by collisions with the water molecules in the reactor vessel. Therefore, one has to cool nuclear reactors. Reactors can thus be classified by their thermal output (measured in units of Watts, e.g. 10 MWatt for the BER II-reactor at Helmholtz Centre Berlin).
The neutron flux in a reactor is related to its thermal output, it depends on the concentration of the rare isotope 235U and on the geometry of the reactor core. To enhance the efficiency of a reactor one surrounds the reactor core by a 30 cm thick layer of beryllium, which reflects the neutrons back into the core. Beam tubes guiding the neutrons from the outside of the reflector to the experiments. The reactor at the Helmholtz Centre Berlin achieves at 10 MWatt thermal output a neutron flux density of approximately 1 to 2·1014 neutrons/s·cm2, as measured close to the reactor core. Thereby about 2.75 kg uranium (235U) are "burned" per year. The strongest European research reactor is at the Institut Laue-Langevin in Grenoble/France. Its flux density is about 1015 neutrons/s·cm2 at a thermal output of 60 MWatt. Hier wird bei 60 MW thermischer Leistung eine Flussdichte von etwa 1015 Neutronen/s·cm2 erreicht.
Texts: Th. Wilpert , Ch. Schulz
