Science

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.

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.

Everything that you need to know about recent advances in Nuclear Fusion

NUCLEAR FUSION

NUCLEAR FUSION

The process in which two nuclei of light atoms (like that of hydrogen) combine to form a heavy, more stable nucleus (like that of helium), with the liberation of a large amount of energy is called nuclear fusion. It takes huge amount of energy to put together two nucleus of even a lighter element like hydrogen, which can only be generated through intense heating, so much, so that the atoms turn into plasma state of matter and experience intense acceleration, required to overcome the large force of repulsion between two nucleus. Heating the light atoms to an extremely high temperature carries out the nuclear fusion; the process is thus called thermo-nuclear reaction. There is some loss of mass during the fusion process, which produces a tremendous amount of energy. The energy produced in nuclear fusion reaction is much more than that produced in a nuclear fission reaction, because the energy that holds a nucleus together is far greater than the one that holds electrons within the atom. There are three difficult requirements for a sustained nuclear fusion reaction, and they must all be met simultaneously. Scientists must Heat a small quantity of fusion fuel to about 100 million °C, Contain and push the resulting plasma together long enough and at a high enough density for the fuel atoms to fuse, Recover enough net useful energy to make fusion profitable. The two major approaches are magnetic confinement and inertial containment. The magnetic confinement approach is called tokamak approach, while the inertial confinement is called the laser bombardment approach. An experimental reactor working on laser bombardment approach in Princeton University has been named after the Hindu God, Shiva. However most of the recent researches have been based on the tokamak approach.

ENGINEERING PROBLEMS SO FAR ENCOUNTERED IN ITS DEVELOPMENT

A critical temperature must be built and maintained and sustained. Even if the ignition and confinement problems are solved, scientists still face formidable problems in developing in a workable nuclear fusion reactor and plant. At the centre of the reactor the plasma may be 100 million °C, but only 2 meters away, around the magnets, the temperatures must be near absolute zero (—273°C) to be achieved by using liquid helium—a substance that may soon be scarce.

The entire massive chamber must also be maintained at a near perfect vacuum. The inner wall of the reactor must resist constant baths of highly reactive liquid lithium (at 1,000°C) and steady bombardment by neutrons (which destroy most known materials) for 10 to 20 years. A wall of any known metal would have to be replaced every 2 to 10 years at such an enormous cost that fusion may never be economically feasible In addition, repairs would have to be made by automatic devices since no human worker could withstand the radiation. Fusion reactors, though much less dangerous than conventional or breeder reactors do have some potential radioactivity hazards. Worst would be the release of radioactive tritium (hydrogen-3), either as a gas or as tritiated water, which in turn could enter the human body through the skin, mouth, or nostrils.

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Tritium is extremely difficult to contain, because at high temperatures and high neutron densities it can diffuse through metals. The long-term disposal of worn-out radioactive metal parts from fusion reactors could also create a problem. Worn-out lithium blankets from fusion reactors would create ten times the volume of wastes created by fission plants. In late 80s and early 90s, a new phenomena was reported called cold fusion

COLD FUSION OR HYPOTHETICAL FUSION

Two scientists, Martin Fleischmann of the University of Southampton, England, and B. Stanley Pons of the University of Utah, U.S.A., did a simple experiment by which they claimed to have found that it would be possible to duplicate in the laboratory what occurs in the sun at temperatures of millions of degrees. The reported results received wide media attention and raised hopes for a cheap and abundant source of energy. Many scientists tried to replicate the experiment with the few details available. Hopes fell with large no of negative replications, withdrawal of many positive replications, discovery of flaws and sources of experimental error in the original experiment, and finally the discovery that the scientists had not actually detected nuclear reaction by products.

THE INDIAN APPROACH

Aditya-L1 :: India's expedition to the SUN source: defencenews.in ‘ADITYA’ is a medium-sized Tokamak conceived, designed and fabricated indigenously and it is commissioned and operational at the Institute of Plasma Research, Gandhinagar. The chief scientific objectives of ADITYA are: Investigation and control of edge phenomenon for improving confinement properties; Investigation of density and current limits of a Tokamak, with special emphasis on interesting phenomena, like MARFES, detached plasma, disruptive instabilities and their control and Study of novel regimes of operation. e.g. H mode obtained using bulk/localized heating by RF.

THE RECENT BREAKTHROUGH IN CHINA

Scientists at the Hefei Institute of Physical Science produced hydrogen gas more than three times hotter than the core of the Sun. They did this inside the Experimental Advanced Superconducting Tokamak (EAST) fusion device. The scientists managed to maintain the extremely high temperature – 50 million C°– for 102 seconds, a feat not achieved anywhere in the world yet. The scientists were aiming for 100 million Kelvins for over 1,000 seconds (nearly 17 minutes).

MECHANISM OF THE NEW PROCESS

Inside the EAST device, a large metallic doughnut-shaped chamber, using magnetic confinement approach or in a tokamak reactor, hydrogen isotopes ‘deuerium’ and ‘tritium’ are collided at high speeds to produce helium. This produces a large amount of energy, akin to almost a medium-scale thermonuclear explosion. The heat produced inside the EAST device is 8,600 times that of the Earth’s core. To keep the helium gas suspended in the fusion chamber the scientists created a magnetic field using superconducting coils fitted across the structure.

THE PROBLEMS CHINESE FACE

While, theoretically the temperatures could be sustained for 102 seconds, it will take a while for the core of a device to sustain the high temperatures for a long period of time. The costs involved in building commercial power plants, which have a core that can sustain the extremely high temperatures, are very high, consequently, iy raises questions on its economic viability. The apparatus and temperatures needed to ensure high-speed collision of nuclei are not easy to put together. It is difficult to get the positively charged hydrogen isotopes to come close enough to collide. At times, it can also be difficult to contain the high amount of energy produced. Many safety layers need to be put in place, so make it radiation protected, disposal of waste, etc.