Radioactive but to the decay ofradioactive materials –
Radioactive wastes, must for the protection of mankind be stored or disposed insuch a manner that isolation from the biosphere is assured until they havedecayed to innocuous levels. If this is not done, the world could face severephysical problems to living species living on this planet.
Some atoms candisintegrate spontaneously. As they do, they emit ionizing radiation. Atomshaving this property are called radioactive. By far the greatest number of usesfor radioactivity in Canada relate not to the fission, but to the decay ofradioactive materials – radioisotopes. These are unstable atoms that emit energyfor a period of time that varies with the isotope. During this active period,while the atoms are ‘decaying’ to a stable state their energies can be usedaccording to the kind of energy they emit. Since the mid 1900’s radioactivewastes have been stored in different manners, but since several years new waysof disposing and storing these wastes have been developed so they may no longerbe harmful.
A very advantageous way of storing radioactive wastes is by aprocess called ‘vitrification’. Vitrification is a semi-continuous process thatenables the following operations to be carried out with the same equipment:evaporation of the waste solution mixed with the borosilicate: any of severalsalts derived from both boric acid and silicic acid and found in certainminerals such as tourmaline. additives necesary for the production ofborosilicate glass, calcination and elaboration of the glass.
These operationsare carried out in a metallic pot that is heated in an induction furnace. Thevitrification of one load of wastes comprises of the following stages. The firststep is ‘Feeding’. In this step the vitrification receives a constant flow ofmixture of wastes and of additives until it is 80% full of calcine.
The feedingrate and heating power are adjusted so that an aqueous phase of several litresis permanently maintained at the surface of the pot. The second step is the ‘Calcinationand glass evaporation’. In this step when the pot is practically full of calcine,the temperature is progressively increased up to 1100 to 1500 C and then ismaintained for several hours so to allow the glass to elaborate.
The third stepis ‘Glass casting’. The glass is cast in a special container. The heating of theoutput of the vitrification pot causes the glass plug to melt, thus allowing theglass to flow into containers which are then transferred into the storage.
Although part of the waste is transformed into a solid product there is stilltreatment of gaseous and liquid wastes. The gases that escape from the potduring feeding and calcination are collected and sent to ruthenium filters,condensers and scrubbing columns. The ruthenium filters consist of a bed ofcondensacate: product of condensation. glass pellets coated with ferrous oxideand maintained at a temperature of 500 C. In the treatment of liquid wastes, thecondensates collected contain about 15% ruthenium.
This is then concentrated inan evaporator where nitric acid is destroyed by formaldehyde so as to maintainlow acidity. The concentration is then neutralized and enters the vitrificationpot. Once the vitrification process is finished, the containers are stored in astorage pit. This pit has been designed so that the number of containers thatmay be stored is equivalent to nine years of production. Powerful ventilatorsprovide air circulation to cool down glass. The glass produced has the advantageof being stored as solid rather than liquid. The advantages of the solids arethat they have almost complete insolubility, chemical inertias, absence ofvolatile products and good radiation resistance.
The ruthenium that escapes isabsorbed by a filter. The amount of ruthenium likely to be released into theenvironment is minimal. Another method that is being used today to get rid ofradioactive waste is the ‘placement and self processing radioactive wastes indeep underground cavities’.
This is the disposing of toxic wastes byincorporating them into molten silicate rock, with low permeability. By thismethod, liquid wastes are injected into a deep underground cavity with mineraltreatment and allowed to self-boil. The resulting steam is processed at groundlevel and recycled in a closed system. When waste addition is terminated, thechimney is allowed to boil dry.
The heat generated by the radioactive wastesthen melts the surrounding rock, thus dissolving the wastes. When waste andwater addition stop, the cavity temperature would rise to the melting point ofthe rock. As the molten rock mass increases in size, so does the surface area.This results in a higher rate of conductive heat loss to the surrounding rock.
Concurrently the heat production rate of radioactivity diminishes because ofdecay. When the heat loss rate exceeds that of input, the molten rock will beginto cool and solidify. Finally the rock refreezes, trapping the radioactivity inan insoluble rock matrix deep underground. The heat surrounding theradioactivity would prevent the intrusion of ground water.
After all, the steamand vapour are no longer released. The outlet hole would be sealed. To go alittle deeper into this concept, the treatment of the wastes before injection isvery important. To avoid breakdown of the rock that constitutes the formation,the acidity of he wastes has to be reduced. It has been establishedexperimentally that pH values of 6.5 to 9.
5 are the best for all receivingformations. With such a pH range, breakdown of the formation rock anddissociation of the formation water are avoided. The stability of wastecontaining metal cations which become hydrolysed in acid can be guaranteed onlyby complexing agents which form ‘water-soluble complexes’ with cations in therelevant pH range.
The importance of complexing in the preparation of wastesincreases because raising of the waste solution pH to neutrality, or slightalkalinity results in increased sorption by the formation rock of radioisotopespresent in the form of free cations. The incorporation of such cations causes apronounced change in their distribution between the liquid and solid phases andweakens the bonds between isotopes and formation rock. Now preparation of theformation is as equally important.
To reduce the possibility of chemicalinteraction between the waste and the formation, the waste is first flushed withacid solutions. This operation removes the principal minerals likely to becomeinvolved in exchange reactions and the soluble rock particles, thereby creatinga porous zone capable of accommodating the waste. In this case the equiredacidity of the flushing solution is established experimentally, while therequired amount of radial dispersion is determined using the formula: R = Qt 2mn R is the waste dispersion radius (metres) Q is the flow rate (m/day) t is thesolution pumping time (days) m is the effective thickness of the formation (metres)n is the effective porosity of the formation (%) In this concept, the storageand processing are minimized. There is no surface storage of wastes required.The permanent binding of radioactive wastes in rock matrix gives assurance ofits permanent elimination in the environment.
This is a method of disposal safefrom the effects of earthquakes, floods or sabotages. With the development ofnew ion exchangers and the advances made in ion technology, the field ofapplication of these materials in waste treatment continues to grow.Decontamination factors achieved in ion exchange treatment of waste solutionsvary with the type and composition of the waste stream, the radionuclides in thesolution and the type of exchanger.
Waste solution to be processed by ionexchange should have a low suspended solids concentration, less than 4ppm, sincethis material will interfere with the process by coating the exchanger surface.Generally the waste solutions should contain less than 2500mg/l total solids.Most of the dissolved solids would be ionized and would compete with theradionuclides for the exchange sites. In the event where the waste can meetthese specifications, two principal techniques are used: batch operation andcolumn operation.
The batch operation consists of placing a given quantity ofwaste solution and a predetermined amount of exchanger in a vessel, mixing themwell and permitting them to stay in contact until equilibrium is reached. Thesolution is then filtered. The extent of the exchange is limited by theselectivity of the resin. Therefore, unless the selectivity for the radioactiveion is very favourable, the efficiency of removal will be low. Columnapplication is essentially a large number of batch operations in series. Columnoperations become more practical.
In many waste solutions, the radioactive ionsare cations and a single column or series of columns of cation exchanger willprovide decontamination. High capacity organic resins are often used because oftheir good flow rate and rapid rate of exchange. Monobed or mixed bed columnscontain cation and anion exchangers in the same vessel. Synthetic organicresins, of the strong acid and strong base type are usually used.
Duringoperation of mixed bed columns, cation and anion exchangers are mixed to ensurethat the acis formed after contact with the H-form cation resins immediatelyneutralized by the OH-form anion resin. The monobed or mixed bed systems arenormally more economical to process waste solutions. Against background ofgrowing concern over the exposure of the population or any portion of it to anylevel of radiation, however small, the methods which have been successfully usedin the past to dispose of radioactive wastes must be reexamined.
There are twocommonly used methods, the storage of highly active liquid wastes and thedisposal of low activity liquid wastes to a natural environment: sea, river orground. In the case of the storage of highly active wastes, no absoluteguarantee can ever be given. This is because of a possible vessel deteriorationor catastrophe which would cause a release of radioactivity.
The onlyalternative to dilution and dispersion is that of concentration and storage.This is implied for the low activity wastes disposed into the environment. Thealternative may be to evaporate off the bulk of the waste to obtain a smallconcentrated volume. The aim is to develop more efficient types of evaporators.At the same time the decontamination factors obtained in evaporation must behigh to ensure that the activity of the condensate is negligible, though thereremains the problem of accidental dispersion. Much effort is current in manycountries on the establishment of the ultimate disposal methods.
These aredefined to those who fix the fission product activity in a non-leakable solidstate, so that the general dispersion can never occur. The most promisingoutlines in the near future are; ‘the absorbtion of montmorillonite clay’ whichis comprised of natural clays that have a good capacity for chemical exchange ofcations and can store radioactive wastes, ‘fused salt calcination’ which willneutralize the wastes and ‘high temperature processing’. Even though man hasmade many breakthroughs in the processing, storage and disintegration ofradioactive wastes, there is still much work ahead to render the wastesabsolutely harmless.
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