The fission chain reaction in a nuclear reactor takes place when a neutron flies to a uranium-235 atom and collides with it, splitting the nucleus into two or three fragments. Several neutrons fly out of the whole process, slow down appropriately, collide with other uranium atoms, and so on. Each fission of uranium generates neutrons, which cause other uranium nuclei to fission. For this to be a controlled and therefore useful reaction, some of the neutrons must be captured by absorber rods. The reaction can then supply energy as long as there are enough uranium-235 nuclei available.

But when we have a reactor filled with uranium fuel at the very beginning and we want the fission reaction to start, the very first neutron has to come from somewhere and collide with the first uranium atom to start the fission chain reaction. Where does it come from?

Surprisingly, we can wait for one to “fly by”. There are a number of processes in nature that can produce a neutron. Common radioactive elements around and within us decay, and some of these processes can generate neutrons. Often the emission of a neutron follows beta-decay, when an unstable atomic nucleus is created and the expulsion of the neutron makes the configuration of protons and neutrons more stable.

Atoms with a high proportion of neutrons, such as beryllium-15, also undergo spontaneous neutron emission. Atoms with a high atomic number may even be subject to spontaneous fission, when their nucleus splits into parts and releases a few neutrons. Such an element is, for example, californium-252 or uranium-238. Last but not least, neutrons arrive on earth with cosmic rays.

Not all neutrons can lead to the fission of uranium-235. Most of them are too fast and just bounce off the uranium. So-called slow or thermal neutrons can penetrate the nucleus. A moderator, such as water, is used in a nuclear reactor to slow down neutrons that are too fast.

Neutrons that happen to pass by can start a fission chain reaction, but most reactors rely on a neutron source to deliver the neutrons. A small amount of neutron-producing material is inserted into a core full of fuel. Often, the above-mentioned californium is used. This source produces a known amount of neutrons at known energies and takes care of “igniting” the fission reaction. The neutron source is then removed from the reactor and the controlled fission reaction proceeds on its own.