Radioactive wastes, must for the protection of mankind bestored or disposed in such a manner that isolation from thebiosphere is assured until they have decayed to innocuouslevels. If this is not done, the world could face severephysical problems to living species living on this planet. Some atoms can disintegrate spontaneously. As they do,they emit ionizing radiation.
Atoms having this property arecalled radioactive. By far the greatest number of uses forradioactivity in Canada relate not to the fission, but tothe decay of radioactive materials – radioisotopes. Theseare unstable atoms that emit energy for a period of timethat varies with the isotope. During this active period,while the atoms are ‘decaying’ to a stable state theirenergies can be used according to the kind of energy theyemit. Since the mid 1900’s radioactive wastes have beenstored in different manners, but since several years newways of disposing and storing these wastes have beendeveloped so they may no longer be harmful. A veryadvantageous 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 thesame equipment: evaporation of the waste solution mixed withthe ————————————————————1) borosilicate: any of several salts derived from bothboric acid and silicic acid and found in certain mineralssuch as tourmaline. additives necesary for the production of borosilicate glass, calcination and elaboration of the glass. These operationsarecarried out in a metallic pot that is heated in an induction furnace. The vitrification of one load of wastes comprisesof the following stages. The first step is ‘Feeding’.
Inthis step the vitrification receives a constant flow ofmixture of wastes and of additives until it is 80% full ofcalcine. The feeding rate and heating power are adjusted sothat an aqueous phase of several litres is permanentlymaintained at the surface of the pot. The second step is the’Calcination and glass evaporation’. In this step when thepot is practically full of calcine, the temperature isprogressively increased up to 1100 to 1500 C and then ismaintained for several hours so to allow the glass toelaborate. The third step is ‘Glass casting’. The glass iscast in a special container.
The heating of the output ofthe vitrification pot causes the glass plug to melt, thusallowing the glass to flow into containers which are thentransferred into the storage. Although part of the waste istransformed into a solid product there is still treatment ofgaseous and liquid wastes. The gases that escape from thepot during feeding and calcination are collected and sent toruthenium filters, condensers and scrubbing columns. Theruthenium filters consist of a bed of ————————————————————2) condensacate: product of condensation. glass pellets coated with ferrous oxide and maintained at a temperature of 500 C. In the treatment of liquid wastes, the condensates collected contain about 15% ruthenium.
This is then concentrated in an evaporator where nitric acid isdestroyed by formaldehyde so as to maintain low acidity. Theconcentration is then neutralized and enters thevitrification pot. Once the vitrification process is finished, thecontainers are stored in a storage pit. This pit has beendesigned so that the number of containers that may be storedis equivalent to nine years of production. Powerfulventilators provide air circulation to cool down glass.
The glass produced has the advantage of being stored assolid rather than liquid. The advantages of the solids arethat they have almost complete insolubility, chemicalinertias, absence of volatile products and good radiationresistance. The ruthenium that escapes is absorbed by afilter. The amount of ruthenium likely to be released intothe environment is minimal.
Another method that is being used today to get rid ofradioactive waste is the ‘placement and self processing radioactive wastes in deep underground cavities’. This isthe disposing of toxic wastes by incorporating them intomolten silicate rock, with low permeability. By this method,liquid wastes are injected into a deep underground cavity withmineral treatment and allowed to self-boil. The resulting steam is processed at ground level and recycled in a closedsystem.
When waste addition is terminated, the chimney isallowed to boil dry. The heat generated by the radioactivewastes then melts the surrounding rock, thus dissolving thewastes. When waste and water addition stop, the cavitytemperature would rise to the melting point of the rock. Asthe molten rock mass increases in size, so does the surfacearea.
This results in a higher rate of conductive heat lossto the surrounding rock. Concurrently the heat productionrate of radioactivity diminishes because of decay. When theheat loss rate exceeds that of input, the molten rock willbegin to cool and solidify. Finally the rock refreezes,trapping the radioactivity in an insoluble rock matrix deepunderground.
The heat surrounding the radioactivity wouldprevent the intrusion of ground water. After all, the steamand vapour are no longer released. The outlet hole would besealed. To go a little deeper into this concept, thetreatment of the wastes before injection is very important. To avoid breakdown of the rock that constitutes theformation, the acidity of he wastes has to be reduced.
Ithas been established experimentally that pH values of 6. 5 to9. 5 are the best for all receiving formations. With such apH range, breakdown of the formation rock and dissociation of the formation water are avoided.
The stability of waste containing metal cations which becomehydrolysed in acid can be guaranteed only by complexingagents which form ‘water-soluble complexes’ with cations inthe relevant pH range. The importance of complexing in thepreparation of wastes increases because raising of the wastesolution pH to neutrality, or slight alkalinity results inincreased sorption by the formation rock of radioisotopespresent in the form of free cations. The incorporation ofsuch cations causes a pronounced change in theirdistribution between the liquid and solid phases and weakensthe bonds between isotopes and formation rock. Nowpreparation of the formation is as equally important.
To reduce the possibilityof chemical interaction between the waste and the formation,the waste is first flushed with acid solutions. Thisoperation removes the principal minerals likely to becomeinvolved in exchange reactions and the soluble rockparticles, thereby creating a porous zone capable ofaccommodating the waste. In this case the required acidityof the flushing solution is established experimentally,while the required amount of radial dispersion is determinedusing the formula:R = Qt 2 mn R is the waste dispersion radius (metres)Q is the flow rate (m/day)t is the solution pumping time (days)m is the effective thickness of the formation (metres)n is the effective porosity of the formation (%)In this concept, the storage and processing areminimized. There is no surface storage of wastes required.
The permanent binding of radioactive wastes in rock matrixgives assurance of its permanent elimination in theenvironment. This is a method of disposal safe from the effects ofearthquakes, floods or sabotages. With the development of new ion exchangers and theadvances made in ion technology, the field of application ofthese materials in waste treatment continues to grow. Decontamination factors achieved in ion exchange treatmentof waste solutions vary with the type and composition of thewaste stream, the radionuclides in the solution and the typeof exchanger.
Waste solution to be processed by ion exchange shouldhave a low suspended solids concentration, less than 4ppm,since this material will interfere with the process bycoating the exchanger surface. Generally the waste solutionsshould contain less than 2500mg/l total solids. Most of thedissolved solids would be ionized and would compete with theradionuclides for the exchange sites. In the event where thewaste can meet these specifications, two principaltechniques are used: batch operation and column operation.
The batch operation consists of placing a givenquantity of waste solution and a predetermined amount of exchanger ina vessel, mixing them well and permitting them to stay incontact until equilibrium is reached. The solution is thenfiltered. The extent of the exchange is limited by theselectivity of the resin. Therefore, unless the selectivityfor the radioactive ion is very favourable, the efficiencyof removal will be low.
Column application is essentially a large number ofbatch operations in series. Column operations become morepractical. In many waste solutions, the radioactive ions arecations and a single column or series of columns of cationexchanger will provide decontamination. High capacityorganic resins are often used because of their good flowrate and rapid rate of exchange. Monobed or mixed bed columns contain cation and anionexchangers in the same vessel. Synthetic organic resins, ofthe strong acid and strong base type are usually used.
During operation of mixed bed columns, cation and anionexchangers are mixed to ensure that the acis formed aftercontact with the H-form cation resins immediatelyneutralized by the OH-form anion resin. The monobed or mixedbed systems are normally more economical to process wastesolutions. Against background of growing concern over the exposureof the population or any portion of it to any level of radiation, however small, the methods which have beensuccessfully used in the past to dispose of radioactivewastes must be reexamined. There are two commonly usedmethods, the storage of highly active liquid wastes and thedisposal of low activity liquid wastes to a naturalenvironment: sea, river or ground. In the case of thestorage of highly active wastes, no absolute guarantee canever be given.
This is because of a possible vesseldeterioration or catastrophe which would cause a release ofradioactivity. The only alternative to dilution and dispersion is that of concentration and storage. This isimplied for the low activity wastes disposed into theenvironment. The alternative may be to evaporate off thebulk of the waste to obtain a small concentrated volume. Theaim is to develop more efficient types of evaporators.
Atthe same time the decontamination factors obtained inevaporation must be high to ensure that the activity of thecondensate is negligible, though there remains the problemof accidental dispersion. Much effort is current in manycountries on the establishment of the ultimate disposalmethods. These are defined to those who fix the fissionproduct activity in a non-leakable solid state, so that thegeneral dispersion can never occur. The most promisingoutlines in the near future are; ‘the absorbtion ofmontmorillonite clay’ which is comprised of natural claysthat have a good capacity for chemical exchange of cationsand can store radioactive wastes, ‘fused salt calcination’which will neutralize the wastes and ‘high temperatureprocessing’.
Even though man has made many breakthroughs inthe processing, storage and disintegration of radioactivewastes, there is still much work ahead to render the wastesabsolutely harmless.