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".