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Environment
Thermal Springs as
Vaults of Nuclear Waste
by VK
Joshi
 Nuclear
energy seems to be the inevitable alternative for a country like India,
which is forging ahead with its development plans. Generating power or
using the energy for benefit of the society, like nuclear medicine etc.
is the ultimate goals of the government. The concept is wonderful. Use
of radioactive elements for the society no doubt is one of the best
cheap and eco-friendly solutions, but a safe disposal of such elements
is vital. Even after having been used the radioactive or nuclear waste
remains active enough to harm the mankind.
In order to understand the perils of the issue, it is better to know a
bit about what is radioactivity and what are the measurement units and
permissible limits. Certain elements in nature emit particles and
radiations spontaneously. This phenomenon is called as Radioactivity.
Geologists use radioactivity to determine the age of rocks. As atoms
lose particles of elements as heavy as nuclei of helium they become
atoms of some other element. Likewise this element changes to some other
element and the series continues till it ends with a stable element. The
rate of changeover from one to another element or decay is at different
rates. This rate is measured in half lives. That is the time it takes
for one half of any given quantity of radioactive element to
disintegrate. The longest half life is that of the 'isotope' 238U of
Uranium. It is 4.5 billion years and the shortest half life of 30
seconds is that of the isotope of Rhodium 106.
Earlier the measurement of radioactivity used to be expressed in Curie (Ci).
But now it is expressed as Becquerel (Bq) units. Thus 1BQ is equal to 1
decay or disintegration per second. Compared to Ci, 37 Bq are equal to 1
nano-Ci. For measuring the health risk through radioactivity the unit
used in the USA is rem or mrem (millirem). In Europe it is measured in
Sv (Sievert) or msv (milli Sv). Ten mSVs make one rem.
Waste, sensu stricto by definition is any material that has been
or will be discarded as being of no further use. At times waste
discarded by one is wealth for another. For example, a rag picker in
India lives only on picking and selling waste for reuse. The radioactive
waste to some extent is somewhat like that only. Yet after reuse the
waste produced needs to be handled with caution and care. Some of the
ways of disposal of radioactive waste have already been dealt in the
article
How Safe is Nuclear Power?
World over the nuclear scientists, environmentalists and geologists are
trying to find out ways to safe disposal of nuclear waste. There are
wastes which may not affect the mind for another 100, 000 years, but
certainly they are a hazard for the future generations. That is why the
endeavor is to find impregnable vaults where the nuclear waste can be
parked safely without apprehension of affecting any one at any time.
R.K. Bajpai and P.K. Narayan of Bhabha Atomic Research Centre, Mumbai
have come out with one such solution of using natural thermal springs in
North Eastern India for safe disposal of nuclear waste.
It is well established that the nature is one of the biggest atomic
reactor. The atomic minerals present in the nature keep on
disintegrating as per their half life periods, yet in general the nature
keeps them under control so as to not to harm the living beings. This
philosophy gave the impetus to geologists to think in terms of using
deep geological formations as 'safe vaults' for nuclear waste.
The only risk in such vaults is that in case the radionuclides find
their way to groundwater or they are vaporized in case of geothermal
springs and travel to surface they can cause lots of harm to the biota.
But on the other hand the geothermal systems of the world are enriched
with radioactive elements yet their radioactivity remains within the
threshold values, i.e. harmless to us. The physico-chemical parameters
control rates of processes operating at nuclear waste-water-rock
interfaces say Bajpai and Narayan. These hot springs provide a workable
analogy about how the temperatures at depth, pressure and water
chemistry, effect the material composition and in what time frame. For
example, uraninite found at depths in thermal springs is considered as a
good analogue of spent fuel. It differs from the nuclear waste in the
sense that it has intense radiation effects while the later has high
amount of fission products. When the nucleus of an atom splits in to
smaller nuclei, the outcome releases energy. This fission when
controlled in a Power Plant is used for producing electricity. The
fissile element transmutes to another element. And the process goes on.
The decay of nuclear waste is a long drawn process. It takes tens of
thousand of years. Thus extrapolation of results of short term
laboratory experiments to establish the end product of a nuclear waste
within a natural vault like a geological formation is not possible.
However, a thermal spring provides all the permutation and combinations
of conditions through which a radioactive material has to undergo over a
period of time. Thus it acts as a good analogy for the scientists
involved with studies on nuclear waste disposal.
Bajpai and Narayan studied a group of thermal springs around Resubelpara
locality near Sarangkhol, East Garo Hills district, Meghalaya. These
springs have temperatures upto 50 degrees centigrade. Drawing analogy of
the radioactive material present in these springs they have tried to
project a scenario of how safe it would be to use such springs for
disposal of nuclear waste.
In Indian conditions a geological repository is envisaged in suitable
granitic formations in the depth range of 400 to 500 m. Such depth
provides an isolation of the wastes from mankind for a few tens of
thousands of years, they claim. By this time period it is anticipated
that the radioactivity of the wastes stored will be at par with natural
uranium ore. However, the stipulated conditions are that the site should
be located in a low rainfall region, with low groundwater potential and
should be devoid of deep seated faults/fractures.
The spent fuel coming out of the reactors is reprocessed and immobilized
in glass matrix in steel containers. Two or three of these containers
are then put in another steel over-pack. Before being 'pushed' in to the
nature's vaults these over-packs are first required to be cooled for
20-35 years so as to reduce their surface temperatures considerably.
In order to project the possible effect of such storage on the spent
fuel Bajpai and Narayan carried out a 3D numerical analysis. The
projected result indicates a temperature of 104 degree centigrade over
the skin of the over-pack, 35 years after disposal. Requirement for the
nature's vault at this stage is to have a temperature of 50-80 degrees
centigrade at various levels.
They chose Resubelpara Group of Thermal Springs (REPTS) as they seem to
meet the requirements for an ideal and safe repository. The choice apart
from other conditions was based on the study of movement of uranium in
the subsurface with geothermal waters to the surface. They found that
uranium in a few tens of million years had traveled only a distance of
80 m. Despite the presence of faults it was found that uranium did not
travel along these weak planes. Reason for this is attributed to the
filling of carbonates and clay along these weak planes.
Therefore Bajpai and Narayan feel that even in an eventuality of a
breach of waste container the leaked, spent fuel will remain restricted
to a small area at depth.
Well drawing analogies from nature about distant future is a tough job
and before such ventures of disposal of spent fuel are taken in the
geothermal springs further detailed studies would be required. There is
no scope for a miscalculation or an oversight in such matters. The need
for locating suitable geological structures for safe disposal of spent
fuel is becoming urgent. A delay could spell a disaster because the
material spewing out of the nuclear plant requires a constant safe
keeping.
Somebody had rightly said:
"It is
very clear
Plutonium is here to stay
Not for a year
Forever and a Day.
In time the Rockies may tumble
Yucca may crumble
They're only made of clay
But the Plutonium is here to stay".
May 26, 2008
Image under license with
Gettyimages.com
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