What is Radiation?

Radiation is everywhere. It is a form of energy. Radiation is produced when the nucleus of an atom is broken apart. Huge amounts of energy are stored in the nucleus. When it splits (called fission) it produces heat and radiation. Nuclear power plants use the heat to turn water into steam and produce electricity. The radiation, and any radioactive materials generated are considered to be waste products. More than four-fifths of the radiation we receive comes from natural sources. This is called background radiation because it is present everywhere, all the time. Radioactive atoms are often called radionuclides.

Radioactive Decay & Half-Life
When atoms split apart naturally it is called decay.  Decay happens because certain elements have unstable nuclei - a situation occurring because they have a whole lot of neutrons and so the nuclear forces are not balanced.  When an atom decays it can form a different isotope of the same element (i.e. it loses neutrons) or it can become a new element (i.e. it loses protons). It is important to realize that, just because atoms have experienced one decay, it does not mean that they are no longer radioactive.  Some elements decay multiple times and go through a series of elements that are all radioactive.  Eventually though, enough time will pass and a stable elemental form will be reached.  An example of this is the decay series of which Radon Gas is an intermediate.  This series has multiple decay steps from the original Uranium atoms to final, stable lead isotopes.

One important property of radioactive elements is half-life. Half-life is defined as the amount of time it takes for one half of all of the atoms of that element to experience a decay.  Because you are halving the number of atoms each time, the amount of atoms of a particular element (and so the total number of decays) decrease exponentially! N is the number of decays/second (or atoms) at a particular time t. No is the initial amount.  Lambda is a decay constant. 

As an example, lets say you start with 1000 atoms of a radioactive element that has a half-life of 10 years.  In 10 years you will have 500 atoms; in 20 years you will have 250 atoms; in 30 years you will have 125 atoms and so on...until they have all decayed.

Half-life is an extremely important property of radiation.  It is used to determine how long a particular radioactive species will remain radioactive.  That is, it tells how long that species may present a danger to human health and the environment.  Half-life varies widely from element to element.   For example Uranium-235 has a half-life of 704 million years while Oxygen-9 has a half-life of only 26.9 seconds! Half-life is an important consideration when attempting to determine how to dispose of a particular radioactive waste.  This is because the waste must be contained for as much time as it takes to render it harmless.  In general, a radioactive species is considered to be harmless when it reaches background levels - usually after about 10 half-lives.

Sources of Radiation
Radiation is a natural part of our every day lives. There are all kinds of sources of radiation.  Some of these are:

• RADON GAS -this is a significant natural source.
• COSMIC RAYS -can act on nuclei in the atmosphere to produce radionuclides.
• COMBUSTION OF FOSSIL FUELS -fly ash, produced during the combustion of coal, contains a significant amount of radionuclides, even more than an equivalent nuclear power plant!
• NUCLEAR REACTORS / REPROCESSING SPENT FUEL -this is a significant source of Krypton-85 in the atmosphere.
• NUCLEAR WEAPONS -the above ground detonation of nuclear weapons releases tremendous amounts of radionuclides into the atmosphere.

In addition, people are exposed to radiation from manufactured sources such as color televisions and smoke detectors.

Types of Radiation
Radiation can occur either as high speed particles or as waves. Non-ionizing radiation is low energy radiation and cannot alter atoms. Examples of non-ionizing radiation are microwaves, infrared, and visible light. Ionizing radiation is high energy radiation that is capable of altering atoms.

ALPHA PARTICLES - the slowest, least energetic type. They do not travel far, and can be blocked by almost anything. In general alpha particles cause damage only if inhaled or swallowed.

BETA PARTICLES - are more energetic. Beta particles can pass through cloth or paper, but can be stopped by a sheet of foil, or glass. Like alpha particles, beta particles generally are harmful only if inhaled or swallowed. For example, Radon gas emits beta particles, and is harmful because it is inhaled.

GAMMA RAYS - are the most energetic, and dangerous. These are rays, not particles and they can pass through almost anything. It takes very, very thick concrete, lead or steel to block gamma rays. This type of radiation is dangerous and damaging.

Each radioactive element emits its own characteristic form of radiation.

Types of Radioactive Waste:
LOW LEVEL WASTES - do not present a significant hazard.  They emit low levels of radiation, usually in the form of alpha particles, and generally have short half-lives.  These wastes tend to be suitable for burial given a 500 year containment.  Some examples of low level waste are solution residuals from processing operations, contaminated equipment, contaminated clothing etc.  The historically preferred method for disposing of low level waste s was dilute and disperse.  This is no longer an acceptable practice.

HIGH LEVEL WASTES - These wastes are very radioactive and present a real threat to human health and the environment. They tend to be beta or gamma (or both) emitters. High level wastes are problematic to store, contain and dispose of.  Examples of high level wastes are the fission byproducts in spent fuel rods and fuel assemblies from nuclear reactors such as Krypton-85 (gamma and beta emitter;half-life=10.72 years), Strontium-90 (beta emitter;half-life=29.1 years) and Cesium-137 (gamma emitter;half-life=30.17 years).

TRANSURANIC WASTE - Transuranic wastes can be low level, high level or both, and consist of radioactive materials with atomic numbers greater than 92 (they have greater than 92 protons).

Another type of radioactive waste is MIXED WASTE which is a combination of radioactive and hazardous waste (for regulatory purposes these are taken as two different things

SOURCE - U, Th, or any other material that is determined by the AEC to be a significant source of radioactivity.   In particular, they regulate ores containing U, Th or other radioactive species in concentrations high enough to be useful for the manufacture of nuclear power or nuclear weapons

SPECIAL - any radioactive material that the AEC designates as significant enough to regulate, but that is not source material.

BYPRODUCT - any other naturally radioactive material , or material made radioactive via exposure to a radiation incident produced during such processes as the manufacturing of weapons, reactors, mining source material etc.