From deep under to everywhere
By Dr A.H. Nayyar
LAHORE: An underground nuclear test is a contained explosion. Since it
is carried out down under at a sufficient depth, a major part of the
effects of the explosion does not appear on the surface, which makes it
look safe for the environment.
For this reason the Test Ban Treaty prohibits atmospheric and under
water nuclear tests, but allows underground tests. All the tests
conducted in India and Pakistan so far have been underground.
The tests, as in the case of those carried out at Chaghi, use a nuclear
fission process, in which a large nucleus (that of either uranium or
plutonium ) breaks up into two smaller nuclei (the fission product
nuclides) releasing a large amount of energy and particles like
neutrons, helium nuclei and the fission products. The energy relased is
in the form of gamma rays, X-rays and heat.
A nuclear explosion has four major effects:
*An intense light which is a result of the released gamma rays, ionzing
the air molecules.
*Intense heat with temperature on the order of a million degrees
*The shock wave due to an over-pressure that is over a million times the
atomospheric pressure, and
* Particulate radioactivity in the form of neutrons and fission product
In an atmospheric burst of a nuclear weapon, the first radiance of light
is so intense that an eye pupil exposed to it is blinded irreparably,
the expanding shock wave razes all structures to the ground, the intense
heat burns all combustibles and produces the mushroom cloud that rises 7 to 10 km
up, or even higher.
In an underground explosion, much of this effect is confined under the
surface. What is most strongly felt on the surface is the quake caused
by the shock wave. And of course, a glow on the surface is also observed which is
due to gamma rays that manage to escape out and produce ionization in the atmospheric
level close to the surface.
Is it true that no harmful particulate radioactivity leaks out of the
surface in an underground nuclear explosion? Not quite. A small amount
always leaks out to the atmosphere, though not as intensely as in the
case of an atmospheric test. It, in fact, slowly seeps out of the porous
soil, and once out, it can drift from place to place with the wind.
Every underground nuclear test (and there have been close to a thousand) was followed
by such relased of radioactivity. No wonder then that
after the recent Pokhran tests there were reports of radiation sickness
among the nearby population. The immediate effects of such radiation
exposures are nausea, vomiting, giddiness, etc. The long term effects
could possibly be increased occurrences of cancers of various kinds.
How does the particulate radioactivity manage to seep out from
underground tests? To get an idea, let us see what happens deep down
under the earth when a nuclear device explodes.
The tremendous energy of a nuclear explosion is relased in two forms:
radiation and heat. The heat produces the shock wave. In the first
instants of an underground explosion, the heat relased is almost
completely absorbed by the surrounding rocks and soil. It is so huge and
is relased so fast that it has no time to spread out. Consequently, the
surrounding rocks and soil melt and vapourise, creating a cavity around
the point of detonation. The pressure in the cavity rises to many
million times the atmospheric pressure. This pushes the cavity walls in
all directions. A spherical shock wave is generated, the lateral part of
which travels thousands of miles and is observed as seismic waves. The
part travelling upwards from a deep enough detonation is reflected back
inward from the surface. On the way back it stretches the rocks,
loosening them and making them fall in the cavity. This is how a crater
is formed on the surface. How deep is a detonation deep enough for the
upward shock wave not to break through the surface? It depends upon the yield of
the nuclear weapon to be tested. If the shaft is at least as
deep as a hundred times the yield to the power one thirds, the shock
wave will be reflected back . That is to say, a 300 meter deep
detonation of a 27 kiloton weapon will remain contained under the
surface while explosion of a slightly larger weapon wil break the
surface spewing out much of the contamination into the environment.
It is how easy to understand how the highly toxic radiocativity emerges
out of an underground test also. The soil that has been loosened up by
the reflected shock wave, that falls into the cavity to produce a crater
on the surface, is porous enough to allow seepage of nuclides to the
surface. Some of the release is prompt and some takes days and weeks.
The fission product nuclides buried deep inside must not be regarded as
non-contaminant. True that a large part of them get frozen in the
solidified molten rocks, but much is out in the free state also. These
can at times get into underground water reservoirs and streams, and in
gas and petroleum wells. Since they are very long lived therefore if
they get into the cycle of human consumption, they can prove as
dangerous as those relased in the atmosphere.
From the television pictures of the Chaghai explosion, it appears that
the tests were conducted in barren mountains. The initial glow was
clearly visible, as was the subsequent landslide. As expected, the
prompt contamination must have seeped out to the surface, and delayed
seepage may also be coming out. Winds will spread them out to larger
areas in the surroundings, carrying with them the loosened soil on the
mountain slopes. Rains, if there are any in this region, will also wash
out more soil than usual, again, spreading the contamination far and
wide. It will therefore be necessary to continuously monitor the region
and prevent the nearby habitations from ingesting the contamination.
The writer is a prominent physicist and lectures at the Quaid-i-Azam