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
    centigrade.
    *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
    nuclides.

    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
    University, Islamabad