Radiation is not a modern creation. Most radiation comes from natural sources - the air, rocks, earth, sun, building materials and even the food we eat.

The term radiation is very broad and includes such things as light and radio waves. But it is most often used to mean ionizing radiation, which is radiation that can produce charged particles (ions) in the materials it strikes.

Natural sources of ionizing radiation include cosmic rays from the sun and outer space, and radioactive elements that occur naturally in the soil, rocks and building materials, such as concrete.

Nuclear radiation is particles or rays given off by unstable atoms. Radiation is produced inside a nuclear reactor when the uranium atoms split or "fission". This also produces heat which is used to generate electricity.

The three basic types of ionizing radiation produced by radioactive atoms are alpha particles, beta particles, and gamma rays. Alpha particles and beta particles have low penetrating power. Alpha particles can be blocked by a piece of paper. Beta particles can be blocked by a thick piece of cardboard or by a piece of aluminum foil. Gamma rays are electromagnetic energy, similar to visible light and X-rays, but they have a higher penetrating power. Heavy shielding of lead or concrete, such as a nuclear power plant's containment structure, will stop gamma rays.

The unit most commonly used to measure human exposure to radiation is the rem. Because most normal exposures involve only a fraction of a rem, the most commonly used unit is the millirem, one thousandth of a rem.

The amount of natural radiation to which people are exposed depends upon where they live and the concentration of radioactive materials in the ground. The table on this page gives you an idea of sources of radiation that people are exposed to. The effects of human exposure to radiation depend upon how much is received, the length of exposure and the person's general health and age. The risk from radiation exposure can be reduced by shortening the time of exposure, getting farther away from the source, and shielding or blocking the source. It is known that whole-body radiation doses of more than 10,000 millirem over a short period of time can cause a slight increase in a person's risk of developing some types of cancer years after exposure. The risk that radiation-induced cancer will develop during the person's lifetime is estimated by the National Academy of Sciences to be about 1/20 of one percent for every 1,000 millirem of short-term exposure greater than 10,000 millirem. In other words, if you were to receive a short term exposure of 11,000 millirem (11 rem), your estimated risk of developing some type of cancer would increase by 0.05 percent.

Both the federal government and the State of California have established radiation dose limits for the public from a nuclear power plant at 100 millirem/year. For doses expected to be greater than 1,000 millirem, protective actions such as evacuations or sheltering would be required. The purpose of these actions would be to keep people away from the radioactive materials that might be released during a severe accident.

Diablo Canyon's design makes a large release of radioactive materials extremely unlikely. The safety systems at the plant are designed to control and contain the release of radioactive materials under accident conditions. The federal and state limits on radiation exposure established for the public, guide the emergency planning for public protective actions. The emergency plans for the Diablo Canyon Power Plant are designed to minimize the exposure to the public by using the protective actions of sheltering and evacuation.