• An element has unique chemical properties and is made of one kind of atom.

The nucleus of an atom contains positively charged particles called protons and particles with no charge called neutrons.

Electrons are negatively charged particles that orbit the nucleus.

Atoms are electrically neutral: they always have an equal number of protons and electrons.

The number of protons of an atom is its atomic number, and gives the element unique chemical properties.

The sum of the number of protons and neutrons make up its mass number.

• An element (with its unique atomic number) may have atoms with different numbers of neutrons, thus different mass numbers.

Atoms of an element with different mass numbers are called isotopes.

Uranium has an atomic number of 92; two isotopes are U-238 with 146 neutrons and U-235 with 143 neutrons.

• Some isotopes are radioactive: the nucleus is unstable and spontaneously decays into daughter atoms with release of subatomic particles as well as harmful gamma radiation and heat.

This decay occurs at a constant rate expressed as half-life - the time it takes for half the parent material to decay to daughter material.

The radioactive decay of an isotope such as Uranium-238 may undergo many radioactive daughter isotopes before reaching a stable product such as Lead-206.

• Fission reactions can yield 3 types of radiation.

• Alpha radiation (helium nuclei with 2 protons and 2 neutrons) are large and cannot penetrate skin.

• Beta radiation (electrons) can penetrate upper layers of skin but can be stopped by protective clothing.

• Gamma radiation is very energetic and potentially most damaging; only thick, dense material such as concrete and lead can stop this radiation.

Exposure to radiation can cause tissue damage, leading to radiation damage; it can also cause DNA damage, leading to cancer or birth defects.

• The decay of radioactive isotopes follow a half-life curve.

Starting with pure, 100% of parent material, after one half-life period, 50% of parent material remains.

Two half-lives leaves 25%, three half-lives leaves 12.5%, and ten half-lives leaves 0.1% of the parent material.

The parent material theoretically never totally decays to 0%.

• Uranium-235, with a half-life of 700 million years, is the isotope used in most nuclear reactors but is much rarer than Uranium-238.

Ores containing uranium (mostly U-238) must be mined from rock deposits.

Milling separates uranium from other minerals in an acidic or alkaline bath, yielding uranium oxide (yellowcake).

Yellowcake is enriched; reactor grade uranium is 3-5% U-235 formed into pea-sized pellets.

• Enriched Uranium-235 pellets are stacked into fuel rods.

The fuel rods are assembled into fuel rod assemblies, which are placed in a reactor core.

• The unstable nucleus of Uranium-235 can breakup in a process called fission.
Fission can be initiated by bombarding a U-235 atom with a neutron.
The U-235 is split into smaller (often radioactive) atoms, releasing free neutrons as well as heat.
The freed neutrons can bombard other U-235 atoms in a chain reaction.
The rate of the chain reaction is regulated by control rods that absorb excess neutrons.
The heat is harnessed by the nuclear reactor for energy.

• Nuclear reactors use two types of methods to harness heat from the fission reaction.

• In a Pressure Water Reactor, heated water (that is radioactive) from the reactor core is used to boil water from a separate source. The steam rives turbines connected to a generator to generate electricity.

• In a Boiling Water Reactor, the radioactive water from the reactor core is used to generate steam, making all parts of the reactor radioactive.

Warm water is cooled in cooling towers.

• Low-level radioactive waste from mining and crushing activities are covered with clay and rock for at least 200 years.

Other LLRW such as contaminated clothing and tools are buried in 3 underground facilities in the U.S.

• High-level radioactive waste contains mixtures of radioactive materials from fission reactors.

Many fission byproducts are much more radioactive, with shorter half-lives, than leftover U-235.

Storing spent fuel rods on-site is a temporary solution and can suffer from a catastrophic event such as the tsunami that disabled the Fukushima Daiichi plant in 2011.

A central storage such as one proposed for Yucca Mountain in Nevada faces political and logistical hurdles.