), however, is a radioactive isotope, the other two being stable.

More than 1,000 radioactive isotopes of the various elements are known.

All minerals and raw materials contain radionuclides of natural origin.

The most important for the purposes of radiation protection are the radionuclides in the U-238 and Th-232 decay series.

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co.

In Tom Clancy's book "The Hunt for Red October," a Russian submarine has a nuclear reactor accident with radiation leakage that forces the crew to abandon ship.

Approximately 50 of these are found in nature; the rest are produced artificially as the direct products of .

Other radioactive isotopes are used as tracers for diagnostic purposes as well as in research on metabolic processes.

For most human activities involving minerals and raw materials, the levels of exposure to these radionuclides are not significantly greater than normal background levels and are not of concern for radiation protection.

However, certain work activities can give rise to significantly enhanced exposures that may need to be controlled by regulation.