The Fission Process in the Nuclear Reactor

A neutron colliding with a ²³⁵U nucleus causes it to split into two nuclei (fission fragments, typically one heavier and one lighter nucleus) and several free neutrons (fission neutrons). The fission fragments, like ¹⁴⁴Ba and ⁸⁹Kr, have a very high velocity and disperse their kinetic energy into the closest environment, heating it up. 80% of the energy released in a fission process is carried by the fission fragments that are radioactive and decay fast.

Fission neutrons striking the next nuclei of uranium may start next fissions, and the next, and the next .... building up a chain reaction.

Some neutrons are absorbed in the uranium isotope ²³⁸U which does not split. It captures a neutron becoming ²³⁹U, which after two beta decays becomes ²³⁹Pu. ²³⁹Pu is a long lived alpha-radioactive isotope.

²³⁵U

Fission, i.e. splitting of nuclei of heavy isotopes like ²³⁵U (or ²³⁹Pu) generates energy in a nuclear rector. A neutron collision with a heavy nucleus results in fission into two nuclei called fission fragments and into several neutrons.

Fission products and fission neutrons have a very high velocity but slow down quickly producing heat.

¹⁴⁴Ba

One of the most probable heavy fission fragments of ²³⁵U-fission is ¹⁴⁴Ba (Barium). ¹⁴⁴Ba is not stable, and similarly to its lighter companion ⁸⁹Kr (Krypton), it beta-decays very fast, through consecutive emissions of electrons and anti-neutrinos to very, very long-lived ¹⁴⁴Nd (Neodymium).

⁸⁹Kr

Fission fragments have masses around 90 and 140; most of them are radioactive. Decay of the fission fragments builds up a chain of other radioactive isotopes (fission products). As these processes continue also after the chain reaction in the reactor has stopped, some heat will continue to be produced after a reactor shutdown.

Plutonium Buildup

Plutonium isotopes, mainly ²³⁹Pu, are produced in reactor fuel when ²³⁸U absorbs a neutron and the resulting nucleus goes through two beta-decays. Part of the ²³⁹Pu is fissioned by neutrons directly in the reactor core, the rest remains in spent reactor fuel as toxic and unpleasant waste. It must therefore, be well isolated from the biosphere for a very long time.

²³⁹Pu can be used as a weapon material and therefore, must be under a strict control. Moreover, ²³⁹Pu can become a fuel for new types of reactors, like breeder reactors. It can even be reused in the existing reactors. This is a way to use the otherwise useless ²³⁸U for a long lasting energy production.