218 lines
13 KiB
Plaintext
218 lines
13 KiB
Plaintext
ÜÜÜÜÜÜÜÜÜÜÜÜÜ ÜÜÜ ÜÜÜÜ
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ÜÛÛÛÛÛÛÛÛßÛßßßßßÛÛÜ ÜÜßßßßÜÜÜÜ ÜÛÜ ÜÛÛÛÛÛÛÛÛÜÜÜÜÜÛßß ßÛÛ
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ßÛÛÛÛÛÛÛÛÛÛÛÛÛÛÜ ßÛÛ ÜÛÛÛÜÛÛÜÜÜ ßÛÛÛÛÜ ßÛÛÛÛÛÛÛÜÛÛÜÜÜÛÛÝ Ûß
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ßßßÛÛÛÛÛÛÛÛÛÛÜ ÞÝ ÛÛÛÛÛÛÛÛÛÛÛßßÛÜÞÛÛÛ ÛÛÛÛÛÜ ßßÛÛÛÞß
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Mo.iMP ÜÛÛÜ ßÛÛÛÛÛÛÛÝÛ ÞÛÛÛÛÛÛÛÛÛ ÞÛÛÛÛ ÞÛÛÛÛÛÝ ßÛß
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ÜÛÛÛÛÛÛÛ ÛÛÛÛÛÛÛÛÝ ÞÛÛÛÛÛÛÛÛÝ ÛÛÛ ÛÛÛÛÛÛ
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ÜÛÛÛÛÛÛÛÝ ÞÛÛÛÛÛÛÛÛ ÞÛÛÛÛÛÛÛÛ ß ÞÛÛÛÛÛÛÜ ÜÛ
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ÜÛÛÛÛÛÛÛÝ ÛÛÛÛÛÛÛÛ ÛÛÛÛÛÛÛÛÝ ÞÞÛÛÛÛÛÛÛÛÛß
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ÜÛßÛÛÛÛÛÛ ÜÜ ÛÛÛÛÛÛÛÛÝ ÛÛÞÛÛÛÛÛÝ ÞÛÛÛÛÛÛßß
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ÜÛßÛÛÛÛÛÛÜÛÛÛÛÜÞÛÛÛÛÛÛÛÛ ÞÛ ßÛÛÛÛÛ Ü ÛÝÛÛÛÛÛ Ü
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ÜÛ ÞÛÛÛÛÛÛÛÛÛÛß ÛÛÛÛÛÛÛÛÛ ßÛÜ ßÛÛÛÜÜ ÜÜÛÛÛß ÞÛ ÞÛÛÛÝ ÜÜÛÛ
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ÛÛ ÛÛÛÛÛÛÛÛß ÛÛÛÛÛÛÛÛÛÛÜ ßÛÜ ßßÛÛÛÛÛÛÛÛÛß ÜÜÜß ÛÛÛÛÜÜÜÜÜÜÜÛÛÛÛÛß
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ßÛÜ ÜÛÛÛß ßÛÛÛÛÛÛÛÛÛÛÜ ßßÜÜ ßßÜÛÛßß ßÛÛÜ ßßßÛßÛÛÛÛÛÛÛßß
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ßßßßß ßßÛÛß ßßßßß ßßßßßßßßßßßßß
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ARRoGANT CoURiERS WiTH ESSaYS
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Grade Level: Type of Work Subject/Topic is on:
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[ ]6-8 [ ]Class Notes [Essay on Nuclear Power ]
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[x]9-10 [ ]Cliff Notes [ ]
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[ ]11-12 [x]Essay/Report [ ]
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[ ]College [ ]Misc [ ]
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Dizzed: o4/95 # of Words:1837 School: ? State: ?
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ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>Chop Here>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ
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Radioactive wastes, must for the protection of mankind be stored or
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disposed in such a manner that isolation from the biosphere is assured
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until they have decayed to innocuous levels. If this is not done, the world
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could face severe physical problems to living species living on this
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planet.
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Some atoms can disintegrate spontaneously. As they do, they emit
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ionizing radiation. Atoms having this property are called radioactive. By
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far the greatest number of uses for radioactivity in Canada relate not to
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the fission, but to the decay of radioactive materials - radioisotopes.
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These are unstable atoms that emit energy for a period of time that varies
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with the isotope. During this active period, while the atoms are
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'decaying' to a stable state their energies can be used according to the
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kind of energy they emit.
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Since the mid 1900's radioactive wastes have been stored in different
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manners, but since several years new ways of disposing and storing these
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wastes have been developed so they may no longer be harmful. A very
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advantageous way of storing radioactive wastes is by a process called
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'vitrification'.
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Vitrification is a semi-continuous process that enables the following
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operations to be carried out with the same equipment: evaporation of the
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waste solution mixed with the
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------------------------------------------------------------
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1) borosilicate: any of several salts derived from both boric acid and
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silicic acid and found in certain minerals such as tourmaline. additives
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necesary for the production of borosilicate glass, calcination and
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elaboration of the glass. These operations are carried out in a metallic
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pot that is heated in an induction furnace. The vitrification of one load
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of wastes comprises of the following stages. The first step is 'Feeding'.
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In this step the vitrification receives a constant flow of mixture of
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wastes and of additives until it is 80% full of calcine. The feeding rate
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and heating power are adjusted so that an aqueous phase of several litres
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is permanently maintained at the surface of the pot. The second step is the
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'Calcination and glass evaporation'. In this step when the pot is
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practically full of calcine, the temperature is progressively increased up
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to 1100 to 1500 C and then is maintained for several hours so to allow the
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glass to elaborate. The third step is 'Glass casting'. The glass is cast in
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a special container. The heating of the output of the vitrification pot
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causes the glass plug to melt, thus allowing the glass to flow into
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containers which are then transferred into the storage. Although part of
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the waste is transformed into a solid product there is still treatment of
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gaseous and liquid wastes. The gases that escape from the pot during
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feeding and calcination are collected and sent to ruthenium filters,
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condensers and scrubbing columns. The ruthenium consists of
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------------------------------------------------------------
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2) condensacate: product of condensation.
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glass pellets coated with ferrous oxide and maintained at a temperature of
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500 C. In the treatment of liquid wastes, the condensates collected contain
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about 15% ruthenium. This is then concentrated in an evaporator where
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nitric acid is destroyed by formaldehyde so as to maintain low acidity. The
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concentration is then neutralized and enters the vitrification pot.
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Once the vitrification process is finished, the containers are stored
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in a storage pit. This pit has been designed so that the number of
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containers that may be stored is equivalent to nine years of production.
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Powerful ventilators provide air circulation to cool down glass.
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The glass produced has the advantage of being stored as solid rather
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than liquid. The advantages of the solids are that they have almost
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complete insolubility, chemical inertias, absence of volatile products and
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good radiation resistance. The ruthenium that escapes is absorbed by a
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filter. The amount of ruthenium likely to be released into the environment
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is minimal.
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Another method that is being used today to get rid of radioactive
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waste is the 'placement and self processing radioactive wastes in deep
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underground cavities'. This is the disposing of toxic wastes by
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incorporating them into molten silicate rock, with low permeability. By
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this method, liquid wastes are injected into a deep underground cavity with
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mineral treatment and allowed to self-boil. The resulting steam is
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processed at ground level and recycled in a closed system. When waste
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addition is terminated, the chimney is allowed to boil dry. The heat
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generated by the radioactive wastes then melts the surrounding rock, thus
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dissolving the wastes. When waste and water addition stop, the cavity
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temperature would rise to the melting point of the rock. As the molten rock
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mass increases in size, so does the surface area. This results in a higher
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rate of conductive heat loss to the surrounding rock. Concurrently the heat
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production rate of radioactivity diminishes because of decay. When the heat
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loss rate exceeds that of input, the molten rock will begin to cool and
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solidify. Finally the rock refreezes, trapping the radioactivity in an
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insoluble rock matrix deep underground. The heat surrounding the
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radioactivity would prevent the intrusion of ground water. After all, the
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steam and vapour are no longer released. The outlet hole would be sealed.
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To go a little deeper into this concept, the treatment of the wastes before
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injection is very important. To avoid breakdown of the rock that
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constitutes the formation, the acidity of he wastes has to be reduced. It
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has been established experimentally that pH values of 6.5 to 9.5 are the
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best for all receiving formations. With such a pH range, breakdown of the
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formation rock and dissociation of the formation water are avoided. The
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stability of waste containing metal cations which become hydrolysed in acid
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can be guaranteed only by complexing agents which form 'water-soluble
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complexes' with cations in the relevant pH range. The importance of
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complexing in the preparation of wastes increases because raising of the
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waste solution pH to neutrality, or slight alkalinity results in increased
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sorption by the formation rock of radioisotopes present in the form of free
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cations. The incorporation of such cations causes a pronounced change in
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their distribution between the liquid and solid phases and weakens the
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bonds between isotopes and formation rock. Now preparation of the formation
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is as equally important. To reduce the possibility of chemical interaction
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between the waste and the formation, the waste is first flushed with acid
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solutions. This operation removes the principal minerals likely to become
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involved in exchange reactions and the soluble rock particles, thereby
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creating a porous zone capable of accommodating the waste. In this case the
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required acidity of the flushing solution is established experimentally,
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while the required amount of radial dispersion is determined using the
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formula:
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R = Qt
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2 mn
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R is the waste dispersion radius (metres)
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Q is the flow rate (m/day)
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t is the solution pumping time (days)
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m is the effective thickness of the formation (metres)
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n is the effective porosity of the formation (%)
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In this concept, the storage and processing are minimized. There
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is no surface storage of wastes required. The permanent binding of
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radioactive wastes in rock matrix gives assurance of its permanent
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elimination in the environment.
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This is a method of disposal safe from the effects of earthquakes,
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floods or sabotages.
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With the development of new ion exchangers and the advances made in
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ion technology, the field of application of these materials in waste
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treatment continues to grow. Decontamination factors achieved in ion
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exchange treatment of waste solutions vary with the type and composition of
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the waste stream, the radionuclides in the solution and the type of
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exchanger.
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Waste solution to be processed by ion exchange should have a low
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suspended solids concentration, less than 4ppm, since this material will
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interfere with the process by coating the exchanger surface. Generally the
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waste solutions should contain less than 2500mg/l total solids. Most of the
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dissolved solids would be ionized and would compete with the radionuclides
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for the exchange sites. In the event where the waste can meet these
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specifications, two principal techniques are used: batch operation and
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column operation.
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The batch operation consists of placing a given quantity of waste
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solution and a predetermined amount of exchanger in a vessel, mixing them
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well and permitting them to stay in contact until equilibrium is reached.
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The solution is then filtered. The extent of the exchange is limited by the
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selectivity of the resin. Therefore, unless the selectivity for the
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radioactive ion is very favourable, the efficiency of removal will be low.
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Column application is essentially a large number of batch operations
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in series. Column operations become more practical. In many waste
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solutions, the radioactive ions are cations and a single column or series
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of columns of cation exchanger will provide decontamination. High capacity
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organic resins are often used because of their good flow rate and rapid
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rate of exchange.
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Monobed or mixed bed columns contain cation and anion exchangers in
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the same vessel. Synthetic organic resins, of the strong acid and strong
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base type are usually used. During operation of mixed bed columns, cation
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and anion exchangers are mixed to ensure that the acis formed after contact
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with the H-form cation resins immediately neutralized by the OH- form anion
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resin. The monobed or mixed bed systems are normally more economical to
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process waste solutions.
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Against background of growing concern over the exposure of the
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population or any portion of it to any level of radiation, however small,
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the methods which have been successfully used in the past to dispose of
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radioactive wastes must be reexamined. There are two commonly used methods,
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the storage of highly active liquid wastes and the disposal of low activity
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liquid wastes to a natural environment: sea, river or ground. In the case
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of the storage of highly active wastes, no absolute guarantee can ever be
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given. This is because of a possible vessel deterioration or catastrophe
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which would cause a release of radioactivity. The only alternative to
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dilution and dispersion is that of concentration and storage. This is
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implied for the low activity wastes disposed into the environment. The
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alternative may be to evaporate off the bulk of the waste to obtain a small
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concentrated volume. The aim is to develop more efficient types of
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evaporators. At the same time the decontamination factors obtained in
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evaporation must be high to ensure that the activity of the condensate is
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negligible, though there remains the problem of accidental dispersion. Much
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effort is current in many countries on the establishment of the ultimate
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disposal methods. These are defined to those who fix the fission product
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activity in a non-leakable solid state, so that the general dispersion can
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never occur. The most promising outlines in the near future are; 'the
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absorbtion of montmorillonite clay' which is comprised of natural clays
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that have a good capacity for chemical exchange of cations and can store
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radioactive wastes, 'fused salt calcination' which will neutralize the
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wastes and 'high temperature processing'. Even though man has made many
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breakthroughs in the processing, storage and disintegration of radioactive
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wastes, there is still much work ahead to render the wastes absolutely
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harmless.
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