60 © The Containment of Underground Nuclear Explosions
Figure 4-1—The Typical Bimodal Curvefor
Fission-Product Yieid
Table 4-1—Common Radionuclides Invoived ina
Nuclear Explosion
Radionucide
Half-Life
Uranium-238 ..............-0 00.0005
4,500,000.000 years
Plutonium-239 .......-. 00.0... 0000000 cee 24,300 years
Cardon-14 200.0 eee . 5.800 years
Radium-226 20000. ec nee 1,620 years
Caesium-137 ...00
16. eee
....
.. 30 years
Strontium-90 20... ce ae 28 years
Percent fission yleid
Tritum oo. cee
... 12.3 years
Krypton-ۤ 6000 ee eee
.... 10.9 years
lodine-131 20... eee. eo. 2... , 8 days
K@NON-133 2. ee eee . 5.2 days
lodine-132 2... eee ee
. 2.4 hours
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The type of release is also importantin predicting
what radionuclides will be present. For example.
atmospheric tests release al] radionuclides created.
Prompt, massive ventings have released a nonnegligible fraction of the radionuclides created. Latetime, minorseeps,like those since 1970, release only
the most volatile radionuclides. In an underground
explosion, radionuclides also separate (called *' fractionation”’) according to their chemical or physical
characteristics. Refractory particles (particles that
do not vaporize during the nuclear explosion) settle
out fast underground, while more volatile elements
that vaporize easily condenselater. This has a strong
16
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30
142
94
!
166
Mass number
Products of a nuclear explosion occur predominantly in two major
groups of nuclides.
SOURCE: Modified from Lapp and Andrews, Prentice-Hall, Inc., 1972.
An individual radioactive species follows the
half-life rule of decay—that is. half of the nuclei
disintegrate in a characteristic time. called a ‘*halflife.’” However, a mixture of fission products has a
more complicated decay pattern. The general rule of
thumb for a nuclear explosion is that the total
activity decreases by a factor of 10 for every
sevenfold increase in time. In other words, if the
gammaradiation | hour after an explosion has an
intensity of 100 units, then 7 hourslater it will have
an intensity of 10. Consequently. the time after the
explosion has a dramatic effect on the amount of
radioactivity. A 1 kiloton explosion in the atmosphere will produce 41 billion curies | minute after
determination, but this will decrease to 10 million
cunes in just 12 hours.
effect on radioactive gases that seep slowly through
the soil from an underground explosion. In an
underground explosion, nearly all the reactive materials are filtered out through the soil column. and the
only elements that come up through the soil to the
atmosphere are the noble gases. primarily krypton
and xenon.
CRITERIA FOR CONDUCTING
A TEST
Although every attempt is made to prevent the
accidental release of radioactive material to the
atmosphere, several safety programs are carmed out
for each test. These programs are designed to
minimize the likelihood and extent of radiation
exposure offsite and to reducerisks to people should
an accidental release of radioactive material occur.
The Environmental Protection Agency monitors the
population around the test site and has established
plans to protect people should an accident occur.
EPA's preparations are aimed toward reducing the
whole-body exposure of the off-site populace and to
minimizing thyroid doseto offsite residents, particu-