Operaciones Sobre Estructura De Par´entesis Balanceados

Liquid, solid and gaseous wastes are produced in the mining of or in the production of reactor fuel materials, reactor operation, processing of irradiated reactor fuels and numerous other related processes. Wastes also result from the use of radioactive materials in research laboratories, industries and medical treatment.

Based on radioactivity, the radioactive wastes can be classified as-

Mildly radioactive: residues from filter and purification processes, contaminated equipment, gloves, sewage sludge from wastewater separation.

Moderately radioactive: component parts of nuclear power stations rendered active by neutrons, radioactive residues from purification plants.

Highly radioactive: includes decaying fissile materials e.g. those of strontium (90Sr), Caesium (137Cs), Iodine (129 I.). The half-life periods of these radioactive isotopes are 26, 30 and 17,200,000 years, respectively.

For disposal purposes, nuclear wastes are separated into two groups:

High- level Radioactive wastes (HLRW) which include:

i) Spent nuclear fuel after irradiation, fission products and TRU (Trans Uranic);

ii) Trans Uranic wastes which are x-emitting TRU isotopes, with half lives of over a year;

iii) High-level wastes (HLW) which are by- products of spent-fuel reprocessing, especially to extract plutonium for warheads.

Low level Radioactive Wastes (LLRW) includes:

i) Low level wastes, defined as wastes containing less than 10 nCig-1 (nCig-1= nanocuries/gm) of trans-uranic elements;

ii) Uranium (U) and Thorium (Th) by-product materials are the tailings produced by the extraction or concentration of U or Th from processed ore.

Until recently, a criterion of 10 nCi g-1 served as a cutoff between shallow land burial and other modes of disposal for TRU high-level wastes. Proposed standards define concentration limits for specific radio-nuclides. For x-emitting TRU nuclides with half-life of over 20 years, the limit is 100 nCi g -1. All other radio-nucleides with a half-life of over 20 years have a maximum of 1 nCig-1.

Treatment of Radioactive Wastes

Approximately one-third and one-fourth of the spent fuel rods in Pressurized Water Reactors

(PWR) and Boiling Water Reactor (BWR) respectively, is removed and replaced. The main objectives of fuel reprocessing are the removal of HLW and TRU from fuel rods and the separation of plutonium. After the initial stages, a nitric solution of the fuel is put in contact with an immiscible solvent, like tributyl phosphate present in an organic diluent. This solution, Raffinate, is highly radioactive and is concentrated by evaporation. Raffinate is stored in special stainless steel containers. Approximately 10 cubic metres of concentrated waste is produced from each GW of electricity. Reprocessing produces both solid and liquid wastes. Liquids can be solidified by spray calcinations and fluidized-bed calcinations. In fluidized-bed calcinations, liquid waste is continuously fed into a calciner containing a bed of small nucleation particles. The bed is heated to 500-600° C by kerosene combustion. A stream of air is passed through the particles so that they flow like a liquid contact between the liquid and particles, causing drying and calcinations. .

In spray calcination, the liquid waste is sprayed into the top of a tower that is heated in the furnace. At about 700° C, water is driven off resulting in calcinated solids, which is collected at the bottom of the tower. Heating them to 900° C drives off the remaining nitrates, whereas, if it is to be vitrified, the powder is heated to 1000- 10000°C to form a mass of glass. The ‘supercalcine’ process produces a calcine with up to 23 per cent additional constituents like lime.

Radioactive Wastes Disposal

Storage in tanks above ground: The US has been practising this for over 20 years. There are over 200 steel and concrete tanks having over 3 million litres of highly radioactive liquid. These radioactive wastes generate heat, besides radiation and hence require constant cooling. The heat is transferred to the condenser by pipes carrying steam. Mixing of contents with compressed air ensures uniform heating and does not allow settling of solids.

A few serious loopholes in this method are listed below:

1. Strong radiation from wastes might lead to corrosion of the tanks and a consequent leakage of radioactive wastes e.g. Hanford, Washington, the prime deposit site in the US experienced seepage of 4,90, 000 litres

of radioactive waste;

2. Fuming wastes require constant refrigeration. They produce 9 kW/m3 of highly radioactive waste. In case of a refrigeration failure, temperatures can easily shoot over 1000°C resulting in the explosion of the tank and a calamity. The University of California has estimated that if a storage tank containing 3 million litres of highly radioactive liquid were to explode, an area twice that of Switzerland would be rendered uninhabitable for several decades. Radiolytic water disintegration produces H2 and O₂ at a fast rate. If proper ventilation is not provided, the H₂ produced, in the absence of dilution, will reach the lower explosion limit of 4 per cent in a few hours, resulting in an explosion by combining with the O₂.

Packaging of spent fuel: If spent fuel were the primary form of waste, the anticipated packaged waste through the year 2000 would be 2.2x106 cubic feet (68,000 cubic metres). If this were stacked as a solid cube, each side would measure nearly 40 metres. About 38,000 megacuries and 175 MW of heat would be produced by this mass of spent fuel.

According to a Swedish project (KBS 1977), spent fuel should be stored in a water pool in a granite cavern 30 metres below the surface.

After 40 years of storage to dissipate heat, bundles of 500 fuel rods would be loaded in copper canisters with lead and copper covers. Each canister, weighing 18 tonnes, will be transferred to the granite cavern 500 metres below the ground in holes 7.7 metres deep and 1.5 metres in diameter, lined with 40 centimetres of isostatically compressed bentonite.

Radioactive wastes reveal radical changes after few hundred years. First, the heat generation rate decreases by an order of magnitude in the period of 10-100 years and by another order of magnitude in 100-1000 years (the decrease in the heat generation rate depends on the half-life period of the particular radioactive waste). Secondly, the toxicity of HLW needed for 1 GW per year electricity decreases by about three orders of magnitude in the first 300-400 years due to the decay of short-lived fission products (1 Giga Watt=109 watts; a measure of consumption of electricity). Toxic levels drop to the level of average ores of toxic elements. After this time, toxicity diminishes slowly, a million years

being required for another two orders of magnitude. Thus, the first 300-400 years represent the most critical phase of disposal.

Use of salt mines: The idea originated in West Germany, as salt mines have very little connections with groundwater, thus conferring a high degree of storage security. Asse II, Germany, stores many small and large caverns filled and sealed with mild radioactive wastes. By AD 200, Asse II is expected to store upto 2, 50,000 cubic metres of mildly radioactive wastes. Recent reports of ground water contamination questions the vulnerability of the system. Besides salt; granite, basalt and shale have been extensively studied. As a repository should contain and isolate these wastes, site selection involves the consideration of the properties of the host rock, the hydrologic properties of the site, its tectonic stability, its resource potential and the capability of the site geohydrology, to provide natural barriers to the movement of the waste.

Turning to the sea: UK deposits 80,000- 90,000 Ci of LLRW into the ocean each year,

which constitutes 90 per cent of the total waste deposited by Europe. Although US abandoned this method in the 1960s, it had deposited about 1, 00,000 Ci by then.

Sub-seabed geologic disposal: The abyssal hill regions in the centres of sub-ocean tectonic plates underlying large ocean surface currents, are vastly remote from human settlements, biologically unproductive, have weak and variable bottom currents and are covered with red clays to a depth of 50-100 metres. The clay has a high cation retention capacity, low permeability, vertical and lateral uniformity; and it becomes increasingly rigid and impermeable with depth. Only about 0.006 per cent of the area of central North Pacific would enable the disposal of HLW by the US till 2040.

Conclusion

Hazardous wastes should be disposed of as early as possible and with as little damage to the environment as possible. Currently cost-efficient technology for handling a large number of hazardous wastes is lacking.