Environmental Engineering Reference
In-Depth Information
by an (n,γ) reaction on
238
U and is separated for use in nuclear weapons. Fallout plutonium consists
of
239
Pu (t
1/2
= 2.411 × 10
4
year) and
240
Pu (t
1/2
= 6.563 × 10
3
year) in a 240/239 atom ratio of 0.18. For
convenience, the pair will be denoted as Pu. A third Pu isotope,
238
Pu (t
1/2
= 87.7 year), was intro-
duced into the atmosphere in the Southern Hemisphere stratosphere when a satellite which included
a Systems for Nuclear Auxiliary Power Generator (SNAP 9A) failed and reentered the atmosphere.
Harley estimated that in 1970, in the Northern Hemisphere <4% of atmospheric Pu was due to
238
Pu
from weapons plus SNAP-9A (Harley 1980). His
238
Pu estimate for the Southern Hemisphere was
18% of the total Pu in that hemisphere. The Pu injection into the atmosphere was about 400 kCi
(1.5 × 10
16
Bq), some of which remained on or close to the test sites. Because the majority of the
weapons tests were conducted in the Northern Hemisphere, ∼80% of the Pu fallout occurred there.
22.2 SIMILARITIES AND DIFFERENCES
As noted earlier, the principle source of U and Th in the atmosphere is derived from soil and rock.
The base chemical composition is usually silicate or carbonate. The U in atmospheric aerosols
exists primarily in the +6 valence state, while Th resides in the +4 state. Fallout Pu deposits on the
earth's surface as the oxide and is principally in the +4 valence state.
Once aerosolized these actinides become of interest as part of the total human exposure to radia-
tion. Unlike U, the principle exposure route for Th and Pu is inhalation rather than ingestion. The
International Commission of Radiological Protection (ICRP) suggests that for insoluble compounds
of Th and Pu, the gastrointestional uptake (f
1
) is on the order of 5 × 10
−4
(ICRP 1993). The ICRP has
adopted an f
1
value of 2 × 10
−2
for U, assuming equal absorption from diet and water (ICRP 1993).
However, Spencer et al (1990), in the only controlled study of uptake of U in humans, showed that
the principal source of uptake of U was from drinking water to the extent of about 5%.
The air concentrations of U and Th are inluenced by climate and land mass while fallout Pu is
dependent on the latitude of the initial injection, primarily the Northern Hemisphere.
22.3 SAMPLING
The collection of atmospheric aerosols is almost exclusively that of total suspended particulates
(TSP), that is, particles <500 μm in diameter. More important are inhalable particulates, <10 μm in
diameter, which may enter the nose and throat. Samplers which are designed to collect this class of
particulates are referred to as PM10 samplers. Respirable particulates, <2.5 μm in diameter (PM
2.5
),
may reach the functional areas of the lungs.
For fallout radionuclides, high volume pumps were and are used to draw air through ilters made
of material such as polystyrene. The large air volumes, usually in excess of 25,000 m
3
, were neces-
sary to collect suficient samples for wet radiochemical analyses. The naturally occurring radionu-
clides were also present in the total aerosol but from terrestrial sources.
There are remarkably few radionuclide measurements of environmental respirable particulates.
Two such studies will be described later. Resuspension of deposited material has attracted little
attention except for studies in the environs of nuclear weapons production facilities. There is one
study of the resuspension of uranium from environmental sources. This will also be discussed later.
The sites for air collection to determine the concentration of these long-lived α emitters are
almost exclusively land based in the mid-latitudes of the Northern Hemisphere. A few measure-
ments of U have been made from collections at remote sites, including the Atlantic Ocean and
Antarctica. Measurements of the Th concentration in air are rare. The U.S. Department of Energy
Environmental Measurements Laboratory's (EML) Surface Air Sampling Program (SASP) was the
most geographically comprehensive monitoring program for fallout Pu. Air samples were collected
and analyzed from stations at Nord, Greenland (80°N, 17°W) to South Pole Station, Antarctica
(90°S, 0°W), roughly along the 80th meridian. The EML database for SASP can be accessed at their
website: http://www.nustl.st.dhs.gov/databases/
Search WWH ::
Custom Search