Transforming a fertile nucleus into a fissile nucleus

Uranium-238 accounts for more than 95% of the uranium present in the core of reactors. It participates marginally to the chain reaction, but it nevertheless plays an important auxiliary role. By capturing a neutron, it becomes a nucleus of plutonium-239. Later on this plutonium-239 nucleus may undergo a fission if it again captures another neutron. Somehow, uranium-238 can be viewed as splitting with a delay or by proxy. It generates nuclear fuel by providing fissile nuclei: it is said fertile.

The mechanism of generation of plutonium involves several stages. By capturing a neutron, uranium-238 becomes uranium-239 that rapidly changes by beta decay into neptunium-239. This neptunium is transformed then again by beta radiation, after 3 days on average, into a new nucleus: plutonium-239. This radionuclide is fissile, like uranium-235.

Plutonium-239 generation
The capture of a neutron by a nucleus of uranium-238 leads to the formation within a few days of a nucleus of fissile plutonium-239. The nucleus of uranium-239 resulting from the capture of the neutron is radioactive beta-minus. By emitting an electron and an antineutrino, it becomes neptunium-239, which being itself unstable transforms through a second beta-decay into plutonium-239.
© IN2P3

Plutonium-239 is a good nuclear fuel: split by fast neutrons, it releases an average of 2.30 secondary neutrons per neutron captured. Of these secondary neutrons, one is required to maintain the chain reaction. Then 1.30 remains available.

If on average, one of these available neutrons is captured by a nucleus of uranium-238 transforming it into plutonium-239, the reactor produces as much fuel as it consumes. When more than one neutron produces plutonium (with an adequate configuration of the reactor core), the reactor is called breeder. Otherwise, the reactor is called a “sub-generator.” The handful of reactors that have operated as breeders are fast neutron reactors.

The conditions for regeneration are very far from being met for conventional reactors of nuclear power plants that use slow neutrons. However, significant quantities of plutonium are generated in their irradiated fuel.

The amount of plutonium produced from the origin of the worldwide fleet of reactors was estimated in the year 2000 at 1400 tons, of which 60% is plutonium-239. This amount was growing by 70 tons per year.


Learn more :
Slow and fast neutrons