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Nuclear Safeguards Education Portal

Material Definitions, Part 1

There are a number of specific terms used in the nuclear fuel cycle. The following terms are some IAEA material definitions that are commonly used:

  1. Nuclear Material (NM) is any source material or special fissionable material.
  2. Source Material refers to natural uranium, depleted uranium, or thorium. This term does not apply to ore or ore residue, but does apply to ore concentrate.
  3. Special Fissionable Material (SFM) refers to any material that contains plutonium-239, uranium-233, or uranium enriched in U-235 or U-233.
  4. Radiological Material (RM) is any radioactive material other than Nuclear Material (NM).

Source material is a material that can serve as a source of SFM. A common source material is natural uranium. This material consists of approximately 0.7% U-235, over 99% U-238, and a very small amount of U-234. This can be converted into SFM either by enriching it in the isotope U-235 (which is commonly done for the production of nuclear fuel for power reactors) or irradiating it in a nuclear reactor to produce plutonium.

The following figure shows the production of plutonium from uranium via irradiation in a nuclear reactor:

the production of Pu from U via irradiation in a nuclear reactor

In this figure, each horizontal row represents an element (such as U or Np) and displays the various isotopes of that element (such as U-234, U-235, or U-236) in each individual box on a row. The bold outlines denoted those isotopes that exist naturally (in this case just U-234, U-235, and U-238). All other isotopes are man-made. The arrows denote a reaction or decay that would lead from one isotope to another (note: this does not display all possible reactions or decays, just the most prominent ones). So, for example, U-236 is produced via an (n,γ) reaction from U-235 (this type of reaction is commonly referred to as a radiative capture reaction and consists of the absorption of a neutron with the subsequent release of a gamma ray). Similarly, Np-239 is formed by a beta decay from U-239. Thus, we can see that Pu-239 is typically produced by absorption of a neutron in U-238 via a radiative capture reaction (creating U-239) and followed by two subsequent beta decays. Pu-239 can serve as a fuel for a nuclear reactor and as the core of a nuclear weapon. If we continue to irradiate the material in a reactor, the Pu-239 will absorb neutrons and produce the high mass Pu isotopes (e.g., Pu-240, Pu-241, and Pu-242).

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