Thursday 23 June 2016

NUCLEAR WASTE DISPOSAL

NUCLEAR WASTE DISPOSAL

A very small amount of nuclear fuel generates wastes when put through the fusion pro­cess. Since the half life periods of these wastes are much much longer compared to human time scale, they have a potential to emit radiation for a much longer time.
The radiation, emitted in the form of  a. b or g rays need to be stored in such "containers" which cannot be penetrated by these rays and in case of even an accident, there is no damage' to the environment.

A number of proposals advocating va­rious methods have been mooted to store nuclear wastes. But the efficacies of such methods remain questionable.

A 1,000 megawatt conventional fission reactor will produce 27,000 kilograms of 238 U, 340 kilograms of 235 U, 700 kilo­grams of a multitude of different fission products, and 230 kilograms of actinides (mostly plutonium, its daughter products and chemical relatives) as wastes in one year. Fission products include hundreds of isotopes with different chemical and/or radiologic properties, most of which lose their radioactivity in several hundred years. The greatest difficulty in disposing of high level fission products wastes is dealing with chemically different prob­lems. Actinides are relatively immobile in multitude of chemical terms (that is they generally have low solubilities in water), but may take thousands of years to lose their radioactivity.

THREE methods are currently being used to dispose of radioactive wastes:  

Dilute and disperse

Delay and decay

Concentration and containment

In dilution and dispersion, low level wastes are released into the air, water or ground to be diluted to presumably safe levels. As wastes proliferate, this dangerous practice will begin to add significantly to artificial radiation levels in the environ­ment particularly from hydrogen­3C (tritium) and krypton-85 which are difficult and fairly expensive to contain and remove.

Delay and decay can be used for radioactive wastes with relatively short half lives. They are stored as liquid, slurries in tanks. After 10-20 times their half lives, they decay to harmless levels at which they can be diluted and dispersed into the environment.

Concentration and containment is used for highly radioactive wastes with long half lives. They are not only radioactive but also thermally hot (primarily from caesium-137 and strontium 90).

The objective of all high level waste disposal is to isolate toxic or radioactive waste from the biosphere through a sys­tem that is free of the risk of sabotage, theft and leakage. The system must prevent the accumulation of an explosive critical mass of the waste, it must dissipate the heat of the wastes, and it must be effective for thousands of years, till the radiation is reduced to a very low level.

Present disposal systems meet few of these criteria. Hundreds of millions of liters of radioactive defence wastes are being stored in stainless steel tanks as sludge or liquid and 8,000 tonnes of spent fuel rods from power plants are being held in liquid filled tanks, often on the premises of the plant itself. Many waste holding facilities are reaching the end of their useful lives. With the potential for an on-going growth in high level wastes in the future, permanent methods for dis­posing them are obviously badly needed.

Periods and phases of disposal

Four periods are expected in the life of a permanent waste repository.
The period of testing and excava­tion when geologic tests determine the acceptability of local geologic conditions.
The operational period when the waste is placed in the repository but can be retrieved should any geologic criteria fail.
The thermal period, which is the first 1,000 years after the repository is sealed. During this period heat generated by the wastes will increase and then gradually dissipate. Physical and chemical changes in the waste, the waste container and between the waste and the repository will proceed at the highest rate during this period.
The post thermal period, a storage time of thousands of years during which the radioactive actinide waste lose radia­tion.
Once a repository is sealed, waste can escape isolation to the biosphere only if it is exposed by either geologic processes or by humans or if it is dissolved or otherwise transported by water. The concept of isolating a hazardous waste from exposure of contact with water for thousands of years requires predicting reactions over time periods far beyond any human observation. Very long term predictions of waste and geologic behaviour thus will have to be based upon very short term (in a geologic sense) tests.

To prevent exposure, burial of wastes deep within the earth (300 to 900 metres) in an area of very low geologic activity is proposed. To prevent transport by ground water from such a site, a multiple barrier approach has become the accepted alternative, in which a succession of inde­pendent barrriers to stop wastes move­ment are established.
Conversion of the waste to a form that can withstand intense heat, is impermeable to water, and is unleachable. Vitrification (incorporating the waste in borosilicate glass) is the most widely accepted approach.
Enclosing the waste in sealed canisters made of alloys that are resistant to corrosion.
Backfilling the repository with material that is impermeable to ground­water, that strongly binds wastes, and that neutralises any leaching capabilities of groundwater. Thus, backfilling would protect, surround, and isolate the cannisters.
Choosing a host rock with high strength that conducts heat rather than absorbs it and that does not expand too much upon being heated. A high degree of ability to bind any free waste is also an essential host rock's characteristic that minimise unfavour­able chemical reactions with waste pro­ducts should they reach a liquid form.
Employing geologic factors with other natural barriers to groundwater flow. This means rock with few or no fractures, few seismic faults, low per­meability to water and low porosity for water flow that is isolated from groundwa­ter - by an impermeable barrier.
MOST experts agree that a repository that meets most of the qualifica­tions of the multiple barrier system could probably isolate wastes for as long a period of time as humans can envision. However, the cost of a risk-free waste storage system will be high adding to the already capital intensive nature of the nuclear industry. Morever, no such re­pository has been developed. Arguments continue.

Initially it was proposed that the wastes should be surrounded with concrete and stored in surface warehouses until a better solution was found. But there were grave dangers associated with these proposals.

In 1977 the American Science Congress came with a novel proposal. The proposal was to solidify wastes, encapsulate them in glass or ceramic and place it in metal containers and bury the containers deep inside in earthquake and flood-free geological formations such as dug out salt or granite deposit. But the proposal if implemented would have created more problems than it would have solved because long-term occurrence of natural disasters cannot be predicted. Moreover, heat from radioactive decay would have cracked  the glass containers and fractured salt or granite formations allowing ground water to enter the depository contaminating the ground water supplies.

Transportaion of deadly radioactive wastes to repository sites would have caused additional problems and if inspite of all the project failed, wastes would have been difficult to retrieve.

Jakino and Bupp in 1978 proposed to bury the nuclear wastes in an underground hole created by a nuclear bomb so that the wastes eventually melted and fused with surrounding rocks into a glassy ball. But for such an unknown thing the effects being unknown and unpredict­able-the danger of failure of the project was always there and such a failure would have necessarily contaminated the groundwater.
• Earnest E Angina in his paper published in Nature under the heading ‘High Level and Long Level Radioactive Waste Disposal' came out with a rather novel proposal. His proposal essentially envisaged changing harmful isotopes into harmless ones by using high level neutron bombardment lasers or nuclear fusion. The proposal, however, was too novel in its approach for technological feasibility to be established. Even if technology could make it feasible there was every chance that process would have created materials which would have required disposal. So such a proposal necessarily had to be dispensed with.

Researchers working on nuclear waste's disposal have recently shown that once a suitable container was designed, the nuc­lear wastes could either be dropped in ocean or the oceanic sediments and floor that are going to subduct in the mantle.
The problem was essentially related to designing the container free from any fault. If such a container was dropped to be subducted, the fear that these wastes might spread out somewhere else by volcanic activity would linger. This fear, scientists feel, is largely unfounded since the subduction zone is believed to be the final destroyer, and the crust which is destroyed in the mantle itself, contains radioactive elements. A 'number of specific technological problems also remain to be resolved.
Political decisions about irretrievable burial or allowance for access in the future also must be made. These technical problems can be solved through research efforts.
Meanwhile, the ethical question whether we have the right to leave potent toxins as a legacy to civilisations that hundreds or thousands of years from now may or may not know of their existence, is a value judgement that societies must face as they decide the fate of the nuclear industry.

No comments:

Post a Comment