Nuclear_Waste

What is nuclear waste' Nuclear waste is the radioactive waste produced by industries involved with nuclear production and large-scale radioisotope production. It includes low, intermediate and high level waste. Types of radioactive waste (radwaste) Low-level Waste is generated from hospitals, laboratories and industry, as well as the nuclear fuel cycle. It comprises paper, rags, tools, clothing, and filters etc. which contain small amounts of mostly short-lived radioactivity. It is not dangerous to handle, but must be disposed of more carefully than normal garbage. Usually it is buried in shallow landfill sites. To reduce its volume, it is often compacted or incinerated (in a closed container) before disposal. Worldwide it comprises 90% of the volume but only 1% of the radioactivity of all radwaste. Intermediate-level Waste contains higher amounts of radioactivity and may require special shielding. It typically comprises resins, chemical sludges and reactor components, as well as contaminated materials from reactor decommissioning. Worldwide it makes up 7% of the volume and has 4% of the radioactivity of all radwaste. It may be solidified in concrete or bitumen for disposal. Generally short-lived waste (mainly from reactors) is buried, but long-lived waste (from reprocessing nuclear fuel) will be disposed of deep underground. High-level Waste may be the used fuel itself, or the principal waste from reprocessing this. While only 3% of the volume of all radwaste, it holds 95% of the radioactivity. It contains the highly radioactive fission products and some heavy elements with long-lived radioactivity. It generates a considerable amount of heat and requires cooling, as well as special shielding during handling and transport. If the used fuel is reprocessed, the separated waste is vitrified by incorporating it into borosilicate (Pyrex) glass which is sealed inside stainless steel canisters for eventual disposal deep underground. Approaches to radioactive waste disposal Waste disposal is discarding waste with no intention of retrieval. Waste management means the entire sequence of operations starting with generation of waste and ending with disposal. Solid waste disposal, of waste such as municipal garbage, is based on three well-known methods, namely landfills, incineration and recycling. Sophisticated methods of landfills are adapted for radioactive waste also. However, during incineration of ordinary waste, fly ash, noxious gases and chemical contaminants are released into the air. If radioactive waste is treated in this manner, the emissions would contain radioactive particulate matter. Hence when adapted, one uses fine particulate filters and the gaseous effluents are diluted and released. Recycling to some extent is feasible. We have already dealt with the reprocessing approach, whereby useful radioactive elements are recovered for cyclic use. But it still leaves some waste that is a part of the high-level radioactive waste. Radioactive waste management involves minimizing radioactive residues, handling waste-packing safely, storage and safe disposal in addition to keeping sites of origin of radioactivity clean. Poor practices lead to future problems. Hence choice of sites where radioactivity is to be managed safely is equally important in addition to technical expertise and finance, to result in safe and environmentally sound solutions. The International Atomic Energy Agency (IAEA) is promoting acceptance of some basic tenets by all countries for radioactive waste management. These include: (i) Securing acceptable level of protection of human health; (ii) Provision of an acceptable level of protection of environment; (iii) while envisaging (i) and (ii), assurance of negligible effects beyond national boundaries; (iv) Acceptable impact on future generations; and (v) No undue burden on future generations. Next we review some approaches for radioactive waste disposal. To begin with, the radioactive waste management approach is to consider the nature of radioactive elements involved in terms of their half-lives and then choose the appropriate method of handling. If the concentrations of radioactive elements are largely short-lived, then one would resort to what is referred to as ‘delay and decay’ approach; that is, to hold on to such a waste for a sufficiently long time that the radioactivity will die in the meanwhile. A second approach is to ‘dilute and disperse’ so that the hazard in the environment is minimized. But when the radioactivity is long-lived, the only approach that is possible is to ‘concentrate and contain’ the activity. In order to carry out concentrating the waste (generally the sludge), chemical precipitation, ion exchange, reverse osmosis and natural or steam evaporation, centrifuging, etc. are resorted to. The resulting solids are highly concentrated in radioactivity. Low-level radioactive waste and even transuranic waste is often buried in shallow landfills. One has to pay attention to any groundwater contamination that may result due to this. The highly radioactive liquid effluents are expected to be ultimately solidified into a leach-resistant form such as borosilicate glass, which is fairly robust in the sense that it is chemically durable, resistant to radiolysis, and easy to process remotely.. One seeks to dispose-off the high-level radioactive waste packages contained in multiple metal-barrier canisters within natural or man-made barriers, to contain radioactivity for periods as long as 10,000 to 100,000 years. ‘The barrier is a mechanism or medium by which the movement of emplaced radioactive materials is stopped or retarded significantly or access to the radioactive materials is restricted or prevented’. It is obvious that recourse to multiple barriers may assure safety of emplaced radioactivity over long periods of time. Ocean-dumping For many years the industrialized countries of the world (e.g. USA, France, Great Britain, etc.) opted for the least expensive method for disposal of the wastes by dumping them into the oceans. Though this practice has been banned by most of the countries with nuclear programmes, the problem still persists. Russia, which currently controls sixty per cent of the world’s nuclear reactors, continues to dispose of its nuclear wastes into the oceans. According to Russia’s Minister of Ecology, it will continue to dump its wastes into the oceans because it has no other alternative method. It will continue to do so until it receives enough international aid to create proper storage facilities. In response, the United States has pledged money to help Russia, but the problem continues. Sub-seabed disposal Seabed disposal is different from sea-dumping which does not involve isolation of low-level radioactive waste within geological strata. The floor of deep oceans is a part of a large tectonic plate situated some 5 km below the sea surface, covered by hundreds of metres of thick sedimentary soft clay. These regions are desert-like, supporting virtually no life. The Seabed Burial Proposal envisages drilling these ‘mud-flats’ to depths of the order of hundreds of metres, such boreholes being spaced apart several hundreds of metres. The high-level radioactive waste contained in canisters, would be lowered into these holes and stacked vertically one above the other interspersed by 20 m or more of mud pumped in. The proposal to use basement-rock in oceans for radioactive waste disposal is met with some problems: variability of the rock and high local permeability. Oceanic water has a mixing time of the order of a few thousand years which does not serve as a good barrier for long-lived radionuclides. Subductive waste disposal method This method is the state-of-the-art in nuclear waste disposal technology. It is the single viable means of disposing radioactive waste that ensures non return of the relegated material to the biosphere. At the same time, it affords inaccessibility to eliminated weapons material. The principle involved is the removal of the material from the biosphere faster than it can return. It is considered that ‘the safest, the most sensible, the most economical, the most stable long-term, the most environmentally benign, the most utterly obvious places to get rid of nuclear waste, high-level waste or low-level waste is in the deep oceans that cover 70% of the planet’. Subduction is a process whereby one tectonic plate slides beneath another and is eventually reabsorbed into the mantle. The subductive waste disposal method forms a high-level radioactive waste repository in a subducting plate, so that the waste will be carried beneath the Earth’s crust where it will be diluted and dispersed through the mantle. The rate of subduction of a plate in one of the world’s slowest subduction zones is 2.1 cm annually. This is faster than the rate (1 mm annually) of diffusion of radionuclide through the turbidite sediments that would overlay a repository constructed in accordance with this method. The subducting plate is naturally predestined for consumption in the Earth’s mantle. The subducting plate is constantly renewed at its originating oceanic ridge. The slow movement of the plate would seal any vertical fractures over a repository at the interface between the subducting plate and the overriding plate. Transmutation of high-level radioactive waste This route of high-level radioactive waste envisages that one may use transmutation devices, consisting of a hybrid of a sub critical nuclear reactor and an accelerator of charged particles to ‘destroy’ radioactivity by neutrons. ‘Destroy’ may not be the proper word; what is effected is that the fission fragments can be transmuted by neutron capture and beta decay, to produce stable nuclides. Transmutation of actinides involves several competing processes, namely neutron-induced fission, neutron capture and radioactive decay. The large numbers of neutrons produced in the spallation reaction by the accelerator are used for ‘destroying’ the radioactive material kept in the sub critical reactor. The scheme has not yet been demonstrated to be practical and cost-effective. Solar option It is proposed that ‘surplus weapons’ plutonium and other highly concentrated waste might be placed in the Earth orbit and then accelerated so that waste would drop into the Sun. Although theoretically possible, it involves vast technical development and extremely high cost compared to other means of waste disposal. Robust containment would be required to ensure that no waste would be released in the event of failure of the ‘space transport system’. Conclusion The problems associated with radioactive waste management on a long-term are major ones that humanity has not been able to come to terms with so far. The problem of radioactive waste management has been compared to a Gordian knot. The Gordian knot should not be just sliced through quick and deftly. As American Ambassador Rich III put it, ‘The obstacles cannot be over soon or ignored. We must untie the Gordian knot carefully and painstakingly, using all of our resources and democratic institutions wisely and well’.