A nuclear reactor is essentially a very intense source of free neutrons. These neutrons are produced during controlled fission reactions inside the reactor core. Because neutrons carry no electrical charge, they can penetrate atomic nuclei relatively easily.

Radionuclide production in a reactor relies on exposing stable target atoms to this high neutron flux.

There are two main mechanisms.

1) Neutron capture (activation)

In neutron capture, a stable nucleus absorbs a neutron. This increases its mass number by one. The new nucleus may be unstable and therefore radioactive.

For example:

• A stable molybdenum nucleus absorbs a neutron.

• It becomes a heavier isotope of molybdenum.

• That heavier isotope is unstable and undergoes radioactive decay.

This process does not split the nucleus. It simply adds one neutron to it.

Neutron capture generally produces radionuclides that remain chemically identical to the original target element. Because the radioactive atoms are mixed with large amounts of stable atoms, the specific activity (activity per mass) is often relatively low.

2) Nuclear fission

In fission production, something different happens.

A heavy nucleus such as uranium-235 absorbs a neutron. Instead of simply becoming heavier, it becomes unstable and splits into two smaller nuclei. These smaller nuclei are called fission products.

Some of these fission products are useful medical radionuclides. One of the most important is molybdenum-99, which decays to technetium-99m.

Unlike neutron capture, fission does not just modify the target atom, it creates entirely new elements. Because these new radioactive atoms are chemically different from uranium, they can be chemically separated. This allows production of radionuclides with very high specific activity, which is important for medical applications.