Enrichment: The Basics

Quick 'Chemistry 101' Refresher

Naturally occurring Uranium (U)  is composed of 2 major isotopes, all have the same Atomic Number (92) but different Atomic Weights.  These isotopes have similar chemical properties but different nuclear properties.

  • U-238  The most plentiful/abundant; over 99% of nature uranium is U-238.
  • U-235  The only fissile uranium isotope; approximately 0.72% of natural uranium is U-235.
Uranium and the Periodic Table

Atomic Mass refers to 1 atom, describing a single Isotope of an element.

Atomic Mass Calc

Nuclear power reactors harness the energy and heat from nuclear fission to produce the steam that runs turbines which, in turn, generate electricity (see ).

Why Enrichment?

Uranium cannot be used as a fuel or weapon in its natural state.   To be used as reactor fuel or bomb manufacture, U-235 must be separated from natural uranium.

U-235 is the only fissile isotope in natural Uranium (U), therefore Uranium must be "enriched" to have a higher percent concentration of U-235.

% of U-235 comparison

U-235 Concentration needed for:

Power reactor:

  • 3-5% of U-235
  • This level of uranium is called Low Enriched Uranium (LEU)

Nuclear weapon:

  • 90% of U-235
  • Referred to as Weapons Grade or Highly Enriched Uranium (HEU)

All of the uranium isotopes are fissionable materials that can undergo nuclear fission reactions (see ).  Uranium-235 is a "fissile" material meaning that it is capable of undergoing a self-generated and self-sustained nuclear fission reaction.  Uranium-234 and U-238 are capable of a fission reaction only when their fission reaction is initiated by an outside energy source such as bombardment by neutrons.

Use of uranium as a reactor fuel or weapons component requires the U-235 isotope to be "enriched" until the U-235 abundance exceeds that of natural uranium (see ).  Typical U-235 enrichment abundance for use as a nuclear fuel is 2 – 5%.  One kilogram of enriched uranium-235 has the capacity to produce as much energy as 1,500,000 kilograms (1,500 tons) of coal.

Gaseous Diffusion Enrichment Process

Gaseous Diffusion is the method used at the Paducah Gaseous Diffusion Plant to enrich uranium.  See the process broken down to the molecular level.

Uranium Enrichment Stages

The PGDP Enrichment Process as presented to the Heath Middle School