A Dirty Bomb Release Example: Cesium
137
by Dr. John S. Nordin, Ph.D.
A van containing conventional explosives is detonated
causing considerable blast damage. One ounce of radioactive isotope Cesium 137 was
mixed in with the explosive, which is dispersed as a very fine dust into the atmosphere.
The dust cloud is carried downwind. Assuming a wind speed of 20 mph, what radiation
exposure might be expected if a person is caught in the dust cloud traveling downwind?
This question was submitted to AristaTek, Inc. by
someone going through hypothetical training scenarios relating to terrorist treats.
The NOVA documentary on dirty bombs aired on public television late February 2003
also covered this subject
Cesium 137
Cesium 137 is a radioactive isotope with a half-life
of 30.3 years. Each disintegration results in emission of a beta particle of maximum
energy of 1.176 MeV and gamma ray energy of 0.66164 MeV. The daughter isotope is
Barium 137, which is stable. These disintegrations and associated energies are a
signature of Cesium 137 which enable the isotope to be identified by radiation detection
equipment in case of a release to the environment. The radiation activity of Cesium
137 is 86.6912 curies per gram. The number 137 means that the cesium nucleus contains
55 protons plus 82 neutrons (55 + 82 = 137); this nucleus is unstable. One of the
neutrons disintegrates forming a proton (which remains in the nucleus) and an electron-like
particle (called a beta particle) with release of gamma ray energy. The beta particle
is ejected from the atom.
Cesium 137 is used by the food industry for food
radiation. It is also used in industrial radiography. A terrorist would probably
use cesium 137 in the form of cesium chloride which is a fine powder like talc and
is easily dispersed. The former Soviet Union is believed to have produced a considerable
quantity of cesium 137.
Modeling the Dust Cloud
We will make an assumption that all of the Cesium
137 is scooped up in the dust cloud resulting from the explosion, and that this
dust cloud travels downwind. In practice, some Cesium 137 will remain near the source,
and some will fall out onto the ground as the dust cloud travels downwind. If the
dust is fine enough, the dust will behave similar to a gas or vapor released suddenly.
Depending upon atmospheric conditions, the dust can travel a long way. Some of the
atmospheric particulates seen on the west coast of North America originated in China,
for example. Eventually the dust will settle or deposit with precipitation.
To describe the dust cloud, we will use one of the
Gaussian Puff models applicable for a “D” atmospheric stability. The basic Gaussian
puff (“instaneous”) equation calculating the centerline concentration is
C = [ Q/((2)1/2
p3/2 sx sy sz
)]
where C = dust cloud centerline concentration
sx = standard deviation of the dust cloud
concentration in the downwind direction
sy = standard deviation of the dust cloud
concentration in the cross wind direction
sz = standard deviation of the dust cloud
concentration in the vertical direction
We will express concentration in units of “curies
per cubic meter”, Cu/m3 . One ounce of Cesium 137 is equivalent to 28.35(86.6912)
= 2457.7 curies. This is the value of Q. The “sigmas” (sx
, sy ,
sz ) are empirical expressions which are a function of downwind
distance X and each have units of meters. There are several different expressions
for the sigmas in the literature, but we will use the ones listed in the following
reference:
Spicer, T.O., and J.A. Havens, “Users Guide for
DEGADIS 2.1” U.S. Environmental Protection Agency, Report EPA-450/4-89-019. ALOHA
uses the DEGADIS dense gas portion described in this user’s guide.
A plot of the centerline downwind concentration predicted from modeling is shown
in figure 1.
However it is the radiation dose that is of most
interest. We need to know the duration of the dust cloud. If concentration is plotted
against time at any given location, theoretically a “bell-shaped” curve results,
that is, the concentration increases as the dust starts to pass over a location,
reaches some maximum, and then decreases. Figure 1 is a plot of the maximum concentration
as a function of distance using a 18.3 second time average. To calculate the dust
cloud duration, we need to know centerline concentrations as the cloud approaches
and recedes. The calculation is,
C = [ Q/((2)1/2
p3/2 sx sy sz
)] F
where F = exp[0.5((X –Ut)/sx
)2 ]
X = downwind distance, meters
U = wind speed, meters/second (20 mph = 8.943 m/s)
t = time, seconds
sx = 0.068X0.9 , the sigma expression
used here
Note that when X = Ut, F = 1 which is the distance
to the center of the dust cloud as it travels downwind. When the dust cloud concentration
is one-fourth of the center concentration, F = 0.25. We will make a series of plots
for F = 0.5, F= 0.25, and F = 0.1 where the dust cloud duration is plotted as a
function of distance X downwind (figure 2). We will assume that the initial dust
cloud leaving the site of the explosion lasts 15 seconds, and because of air turbulence,
the dust cloud spreads out as it travels downwind. F = 0.1 means that this is the
cloud duration where concentrations are between 10% and 100% of the maximum value
(figure 1), F = 0.25 means the duration between 25% and 100% of the maximum concentration
value.
A word of caution is required. If the initial explosion
is big or if there is a fire, the initial dust cloud behavior will be different.
The dust cloud duration would be much greater but downwind concentrations might
be less. Tests at the Nevada Test Center (the “Kit Fox Series performed in 1995)
where carbon dioxide was released in a series of 15-second puffs showed that local
micrometeorology greatly influenced the cloud behavior is it traveled downwind,
and that the cloud had a trailing end (i.e., it took longer for the cloud to clear
out than what models predicted). It is our opinion that the model underpredicts
the dust cloud duration.
Radiation Dose
We will consider the radiation dose from three sources:
- Inhaling the dust as the dust cloud passes by
- Whole body radiation from the dust cloud as it passes by
- Radiation from dust that may adhere to the body and clothing
We will not consider radiation from ingestion of
food or water.
1. Inhalation:
Inhalation is particularly insidious because cesium
137 continues to undergo radioactive disintegration producing energetic beta particles
and gamma radiation within the human body. These beta particles and gamma radiation
ionize body tissues; if severe enough, death can occur within months from radiation
burns. Even low doses of radiation have the potential for causing cancer later in
life.
The U.S. Nuclear Regulatory Commission has established
that inhalation of 200 microcuries of Cesium 137 results in 5 rem exposure. Numbers
are also published for other radioactive isotopes in 10 CFR Part 20 Appendix B.
The 5 rem exposure is the maximum annual exposure allowed for a worker who may be
in contact with radioactive isotope. Lower limits are recommended for the young
and for pregnant women. Assuming that the rem exposure is proportional to the microcuries
inhaled, a radiation dose by inhalation might be estimated from figures 1 and 2.
A normal resting breathing rate is 20 liters/minute.
Table 1. Estimated Radiation Dose from Inhaling
Cesium 137 in Dust Cloud
Distance Downwind, meters
|
Peak Concentration in Cloud, mCu/m3
|
Cesium 137 inhaled, mCu
|
Radiation dose, Rem
|
100
|
3619000
|
21700
|
543
|
200
|
572000
|
3600
|
90
|
50
|
53400
|
360
|
9
|
1000
|
8900
|
100
|
2.5
|
2000
|
1550
|
20
|
0.5
|
5000
|
163
|
3.8
|
0.1
|
10000
|
30.5
|
1
|
0.025
|
The radiation dose by inhalation start to become
significant for distances less than 1000 meters. The calculations suggest that a
person located less than about 150 or 200 meters away might receive a fatal dose,
but remember, that these are idealized calculations. Some of the Cesium 137 might
remain at the source or a person may be lucky and escape breathing in most of the
radioactive dust.
2. Whole body radiation from the dust cloud as
it passes by:
There are two sources of ionizing radiation. One
is from penetrating beta particles and the other is gamma radiation. Each disintegration
of a Cesium 137 atom produces one beta particle of kinetic energy of 1.176 MeV and
one gamma ray of energy 0.66164 MeV. A beta particle with this energy can travel
a distance of 1.12 meters in air or can pass through 0.2 inches of body tissue,
even if protected by clothing. Gamma rays can theoretically travel an infinite distance
but gamma radiation drops off according to the square of the distance of the source.
We will make several assumptions here to come up
with a rough estimate of the dose a standing adult might receive as the dust cloud
passes, excluding inhalation. We will ignore quality factor and modifying factor
adjustments so that the absorbed dose expressed in “rads” is the same as the dose
equivalent expressed in “rems”. Only the dust cloud that is within one meter of
the person contributes significantly to the absorbed dose. Beta particles outside
this one meter envelope can’t travel to the person and penetrate the person’s skin,
and gamma radiation from Cesium 137 more than 1 meter away will be less than from
the gamma radiation up close. We will imagine the person standing in the center
of a dust cloud cylinder 2 meters in diameter and 2.8 meters high. Roughly 9 % of
the beta emissions within this cylinder will be impact the human (the other 91%
fly out of the cylinder envelope or impact the ground).
The calculation for gamma radiation is rather complex.
It involves calculation of a photon flux (units: photons/m2-hr) and converting
to a radiation exposure at the photon energy of 0.662 MeV, and calculation of the
dose for the duration of the cloud. The results are summarized in table 2.
Table 2. Estimated Radiation Dose from External
Exposure to Cesium 137 as Dust Cloud Passes By
Distance Downwind, meters
|
Peak Concentration in Cloud, mCu/m3
|
Estimated Beta Radiation Dose, Rem
|
Estimated Gamma Radiation Dose, Rem
|
100
|
3619000
|
0.03
|
5.6
|
200
|
572000
|
0.005
|
0.9
|
500
|
53400
|
<0.005
|
0.1
|
1000
|
8900
|
<0.005
|
0.02
|
2000
|
1550
|
<0.005
|
0.005
|
5000
|
163
|
<0.005
|
<0.005
|
10000
|
30.5
|
<0.005
|
<0.005
|
The passage of the dust cloud at 20 mph does not
impart as much of a radiation dose because the exposure time is short. This is in
contrast to the exposure resulting from inhaling cesium 137, which has a half-life
of 30.2 years.
3. Radiation from dust that adheres to skin and
clothing:
This is the most difficult of all to estimate. In
addition, Cesium 137 dust clinging moving vehicles and people will contaminate otherwise
clean areas. The dust that clings to a person’s skin and clothing will continue
to radiate beta particles and gamma radiation. In addition, the person may breathe
some of the dust. Food and water may become contaminated. Even only 0.01 grams (10
milligrams) of Cesium 137 dust clinging to a person’s skin and clothing might result
in say 60 rem/hr of radiation exposure to the person.
What Radiation Dose is Safe?
The dose from normal background radiation for a
non-smoker is about 0.15 to 0.2 rem/year. A person living at a high elevation (7000
to 10000 feet) might add another 0.06 to 0.12 rem/year. Excessive radon gas in the
home will also boost this number (up to perhaps 0.4 rem/year). Smoking increases
radiation exposure to target organs (up to 8 rem/year to bronchial epithelium of
the respiratory tract).
The threshold for lethality from radiation exposure
depends whether the risk for developing cancer later in life is considered. If cancer
is not considered, the threshold of lethality for acute radiation exposure for the
more sensitive individuals appears to be about 200 rems whole body radiation. Death
is almost certain at 1000 rem dose for all individuals, even though the person may
not initially feel any initial discomfort (death occurs perhaps a week or two later).
The lowest radiation dose that will result in cancer many years later in life is
a subject of dispute, but the NOVA documentary on dirty bombs aired on public television
in February 2003 suggested an eight times normal background (15 rem dose) results
in an increased one in five chance of developing cancer later in life.
The U.S. Nuclear Regulatory Commission recommends
(see 10 CFR Part 20) a maximum radiation exposure of 5 rem/year for adult radiation
workers and 0.1 rem/year for the general public including children. These radiation
exposure numbers are above normal background.
The U.S. National Council on Radiation Protection
recommends a maximum dose of 100 rem to an older person (45 years or older) engaged
in emergency lifesaving operations. This radiation dose is accumulative; the person
can’t engage in another activity which results in a dose of 100 rem in an incident
months later.
The logic of permitting a higher radiation dose
for older people is that their cell division rate is lower and less likely to develop
cancer from the incident during their remaining lifetime. This is also a debatable
topic.
What can we learn from this analysis?
- An analysis of this type is difficult to do because of the many
unknowns (such as dust cloud duration and how much dust will settle). There are
really no good experimental tests (at least not available in the public domain)
where models can be calibrated and assumptions verified.
- Major radiation exposure concerns are confined to within one kilometer
of the explosion, at least for this hypothetical explosion.
- We do not want to breathe the radioactive dust and we don’t want
the dust to settle on our person or clothing. This is how the greatest exposure
occurs.
- We don’t want to track the dust around.
What action should be taken if an explosion occurs?
There is no way of knowing whether a suspected terrorist
explosion has been seeded with radioactive isotopes. If remnants of lead shielding
are seen at the site, radioactive isotopes probably are present. Use of radiation
detection equipment is essential before approaching any site where an incident has
occurred.
If an explosion has occurred, efforts must be made
to avoid breathing in dust. Respirators designed to screen out fine particulates
is essential. Unless there is danger of fire or collapsing buildings, probably the
general public should be sheltered in place (inside homes and buildings). If excessive
radiation is present in the dust cloud, the public should remain in place until
the dust cloud has passed and an orderly evacuation can occur. Efforts must be made
to keep the dust outside the buildings and homes by avoiding traffic in and out
and sealing up doors and vents. The media has made fun of “duct tape and plastic
sheeting”, but this is serious business. When the public is evacuated, it may be
necessary to send people through a decontamination station to remove dust on skin
and clothing.
The original one ounce container of cesium 137 would
also be very dangerous. One ounce of unshielded cesium 137 would impart a gamma
radiation dose of approximately 1000 rem/hr to a person only one meters away. At
10 meters away, the unshielded radiation dose would be about 10 rem/hr. A terrorist
setting this material up would be “fried” unless the material were adequately shielded
with lead.