What are fast neutrons

Fast breeder

In a special class of nuclear reactors, the fast breeders, fast neutrons convert the isotope uranium-238, which is not suitable as a nuclear fuel, into the new nuclear fuel plutonium-239. This technology is rarely used today, but it could gain in importance worldwide if nuclear energy is expanded.

The uranium isotope 235 is used in current power reactors for power generation, which can only be split with neutrons that have been slowed down to thermal speeds after nuclear fission. Since uranium-235 only occurs with 0.7 percent in nature, the lion's share of uranium remains unused for energy generation. This is because the uranium-238 cannot be used as a fissile material in reactors with thermal neutrons.

Uranium-238 has another property that can be used to generate energy: it can capture a fast neutron that is released immediately after a fission process. It can then transform itself into plutonium-239 through several decay processes. This isotope, in turn, is also a good fission material for fast neutrons. One disadvantage: Plutonium can also be used to build nuclear weapons.

Creys-Malville nuclear power plant

Two important processes are taking place at the same time: On the one hand, energy is generated through the fission of plutonium-239 and, on the other hand, the non-fissile uranium-238 is incubated with the help of fast neutrons, the new fission material plutonium-239. As fast neutrons are used in these types of reactors and new fissile material is generated in the process, they are called fast breeder reactors or more simply “fast breeder”.

The core of such reactors can be constructed in such a way that more plutonium-239 is produced from uranium-238 than is consumed by the fission of the plutonium. In this way, an excess of fissile material is achieved that can be used for energy generation in other reactors. This breeding process makes it possible to use the natural uranium around sixty times better than with the light water reactors that only use uranium-235. The energy potential of uranium can be increased by a factor of 60 using this technology.

Since breeder reactors use the fast neutrons to generate energy, these particles must not be slowed down after the plutonium has been split. Moderators, as in thermal reactors, are therefore not used as a way of transporting the released heat away. Liquid sodium is therefore used as the coolant, for example in the former French breeder reactor Superhénix (running from January 15, 1986 to December 31, 1998) or the Russian Beloyarsk 4 breeding reactor (beginning December 10, 2015). It has a melting temperature of 98 degrees Celsius. Its boiling temperature is 883 degrees Celsius. Since the temperatures at which the sodium is used in fast breeder reactors are between 400 and 550 degrees Celsius, the coolant is far from its boiling point, so that these types of reactors under the relatively low pressure of only about 10 bar in the primary circuit can be operated, in contrast to pressurized water reactors with about 160 bar.

In Germany there were two types of fast reactors: in the former Research Center Karlsruhe (today KIT) from April 9, 1978 to August 23, 1991 the compact sodium-cooled nuclear reactor facility Karlsruhe, KNK II and in Kalkar between 1973 and 1985 the fast breeder reactor SNR-300, which never went into operation and was finally given up in 1991.

According to the World Nuclear Association from July 2017, a total of eleven breeder reactors have been built in nine countries since 1962. Of these, eight plants were shut down by the end of 2016, three are still in operation: in Russia the reactors Beloyarsk 3 (560 MWe, since April 1980), Beloyarsk 4 (789 MWe, since December 2015) and in the People's Republic of China the breeder reactor Cefr (20 MWe, since July 2011). In India, the twelfth Pfbr breeder reactor with a capacity of 470 MWe was under construction at the end of 2016.