Radiation and radioactivity occur naturally in the physical world.
All living beings require some kinds of radiation just to live.
Light and heat, for example, are two basic forms of radiation necessary
for all life on Earth.
Radiation is a form of energy. Radioactivity is the spontaneous
emission of energy from certain elements, and from other elements
under special conditions, in the form of particles or electromagnetic
waves.
More than 80 percent of the radiation we are exposed to comes from
such natural sources as sunlight, soil, and certain types of rocks.
Cosmic rays filtering down through the atmosphere, and radon gas
filtering up through the soil, are sources of natural radiation.
This radiation is called background radiation. It is present everywhere,
all the time and varies greatly depending on our geographical location.
In addition, people are exposed to radiation from man-made sources
such as color televisions, smoke detectors, computer monitors, and
X-rays. These sources account for less than one-fifth of our total
radiation exposure.
There is no difference between natural radiation and its effects
and man-made radiation and its effects.
The study of radioactivity begins with the atom. Tremendous amounts
of energy are stored in the center, or nucleus, of an atom. Scientists
have learned how to split atoms in a controlled process to capture
the energy stored in them. When atoms are split, heat and radioactivity
are produced. The intense heat produced when an atom is split can
be used to turn water into steam to run turbines that produce electricity.
This is the basis for nuclear power production.
The radiation produced from radioactive atoms is emitted in several
forms, most commonly, alpha and beta particles, and gamma rays.
- Alpha particles have the shortest range among these
three types of radiation. They can travel only a few inches in
the air and can be stopped easily by a sheet of paper or the outer
layer of a person’s skin. Alpha particles are harmful only
if the radioactive source material is swallowed, inhaled, or absorbed
into a wound.
- Beta particles are more penetrating than alpha particles.
They can travel through the air for several feet, but their penetrating
power, too, is limited. Although they can pass through a sheet
of paper, materials such as a thin sheet of aluminum foil or glass
can stop them. Like alpha particles, they cause their most serious
effects if swallowed or inhaled.
Some radioactive material that emits beta particles could, for
example, be attached to dust we might breathe in, or cling to
food we might eat. In such cases, some of the material would leave
the body through natural elimination processes. Some, however,
may be retained in various organs where chemicals in living cells
would be ionized and potentially damaged when the beta particles
are emitted.
- Gamma rays are electromagnetic energy. Unlike alpha
or beta particles and their relatively short ranges, gamma rays
have much greater penetration power. They are more energetic than
X-rays. This type of radiation requires shielding with such materials
as concrete, lead, steel, or water. Water is used to shield workers
from radiation emitted by spent nuclear fuel assemblies at nuclear
power plants.
A person’s exposure to radiation is measured in units called
millirem. A millirem measures the effects of radiation on the human
body much as degrees measure temperature. In the United States,
a person’s average exposure to radiation is about 360 millirem
per year. Roughly 300 millirem come from natural sources of radiation,
and 60 millirem come from man-made sources, primarily medical procedures.
Where people live, as well as their lifestyles, can play a part
in how much radiation they receive. The natural or background radiation
exposure a person receives can vary depending on how high above
sea level he or she lives and on the radioactive content of the
soil and rocks in the vicinity. People who live at higher altitudes
receive more exposure to radiation that comes from space.
Background radiation in the United States varies considerably from
state to state. For instance, a person living on the Atlantic Coast,
on average, receives about 55 millirem per year from the ground
and space. A person living on the Colorado Plateau receives, on
average, 140 millirem per year from the ground and space. The background
radiation exposure on the Colorado Plateau is higher because of
the higher altitude and radioactive particles from uranium that
occur in the soil. The atmosphere at higher altitudes is thinner,
allowing more radiation from space to penetrate.
People living in the northwest region of Washington state receive
about 240 millirem per year, on average, from natural and man-made
sources, whereas residents in the state’s northeast region
receive doses of about 1,700 millirem per year, most of it from
radon that occurs naturally in the rock and soil.
Because of other factors, it is not uncommon for a person to receive
far more than the average 360 millirem per year. Things that can
affect a particular person’s annual radiation exposure level
include airplane travel, dental and medical X-rays, and occupation.
- A person taking a cross-country flight would receive about
two to five additional millirem of radiation per roundtrip, depending
on flight altitude and shielding on the airplane. Due to the thinner
atmosphere at the altitudes involved in cross-country flights,
a traveler is exposed to more cosmic radiation. Because of their
occupations, airline pilots and flight attendants routinely are
exposed to higher levels of radiation than many other workers.
Airline crew members and frequent flyers receive annual doses
on the order of between 500 and 600 millirem.
- A person undergoing a full set of dental X-rays would receive
about 10-39 additional millirem per set.
- A person working in a nuclear power plant would receive approximately
300 additional millirem per year (the Nuclear Regulatory Commission’s
limit is 5,000 millirem per year for occupational exposures).
- A person living directly outside a nuclear power facility would
receive approximately one additional millirem per year.
- Transportation casks are constructed with multiple layers of
dense metal designed to protect public health and safety during
transportation and shield people from radiation. A person standing
30 meters (100 feet) from a vehicle transporting spent nuclear
fuel, moving 15 miles per hour, would receive a radiation dose
of approximately 0.0004 millirem. If the same person were standing
the same distance from all potential shipments (approximately
50,000 truck shipments over 24 years), he or she would receive
a total whole-body dose of about 20 millirem. During the same
24-year timeframe, that same person would receive over 7,000 millirem
from natural background radiation.
The U.S. Environmental Protection Agency is responsible for establishing
exposure limits to protect public health and safety and the environment
for a repository. Federal law has directed the agency to consider
recommendations by the National Academy of Sciences, and, through
a rulemaking process, to establish environmental standards for a
repository. The rulemaking process allows for public input into
how these environmental standards are developed and instituted.
The Nuclear Regulatory Commission will then incorporate the results
into its criteria for licensing a repository.
| All matter is made up of atoms. Atoms
are particles held together by electrical charges and internal
forces. When an atom loses the delicate balance among the forces
holding its center, or nucleus, together, it tries to regain
its balance by giving off energy — radiation — in
a process called radioactive decay. An atom that emits radiation
is called a radioisotope or radionuclide. Over time, radioactive
elements decay to gain balance: some in a few minutes, some
in a few days or years, and some in thousands or millions of
years. The length of time needed for half the atoms of a radioactive
substance to decay is called its half-life. |
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