Management of Commercially Generated Radioactive Waste - U.S. ...

3.3.1.1 Cosmic Radiation
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Cosmic radiation refers both to primary energetic particles of extraterrestrial origin
that strike the earth's atmosphere and to secondary particles generated by the interaction
of primary particles with the atmosphere (radionuclides produced by cosmic radiation are
discussed later). The primary cosmic radiation consists of particles produced outside the
solar system and particles emitted by the sun. The cosmic ray dose rate to the population
living at sea level is about 26 mrem per year, taking into account shielding from struc-
tures. Considering the altitude distribution of the U.S. population, the average dose rate
is 28 mrem per year. In Denver, which is the largest city at a relatively high altitude
(1600 meters) in the United States, the average dose rate from cosmic rays is about 50 mrem
per year. In Leadville, Colorado (3200 meters), which has a population of about 10,000,
the average cosmic ray dose rate amounts to 125 mrem per year. High altitude airplane
flights add a small fraction to the population dose from cosmic rays at ground level. For
example, a jet flight of 5 hours duration (e.g., transcontinental or transatlantic at 12 km
altitude) at mid-latitudes would result in a dose of approximately 2.5 mrem to the whole
body. An extreme case would be a 10-hr polar route flight from, for example, California to
Europe where the long flight time and the higher cosmic ray intensities at high latitudes
would result in a passenger dose of approximately 10 mrem (or 20 mrem for a round trip).
3.3.1.2 Terrestrial Radioactivity
Terrestrial radioactive material is present in the environment because naturally
radioactive isotopes are constituents of a number of elements in the earth's crust. The
nuclear interaction of cosmic rays with constituents of the atmosphere, soil, and water
also produce a number of different radionuclides. These naturally occurring radionuclides
give rise to both external and internal irradiation of man.
Cosmogenic Radionuclides
Cosmogenic radionuclides are produced through interaction of cosmic rays with atoms in
the atmosphere and in the outermost layer of the earth's crust. The entire geosphere con-
tains radionuclides produced in this fashion. The four cosmogenic radionuclides that con-
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tribute measurable dose to man are hydrogen-3 (tritium) ( H), beryllium-7 ( Be), carbon-14
(14C), and sodium-22 (22Na), all produced in the atmosphere. The total contribution to
the average dose rate (in addition to direct cosmic radiation) by these four nuclides is
less than 1 mrem/yr.
Primordial Radionuclides
Several dozen naturally occurring nuclides are radioactive with half-lives of at least
the same order of magnitude as the estimated age of the earth (4.5 x 10 9 yr), and are con-
sequently assumed to represent a primordial inventory (that is, some radionuclides are
remaining since the formation of the world). There are three chains or series radionu-
clides headed by thorium-232 (232Th), uranium-235 (235U), and uranium-238 ( 238 U). These
radionuclides decay ultimately to a stable isotope of lead through a chain of decaying
nuclides of wide ranging half-lives. These chains contain the, perhaps more familiar,