Radioactive Waste Management at Nuclear Power Plants

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Radioactive waste managementat nuclear power plantsAn overview of the types of low- an d intermediate-levelwastes and how they are handledby V. M. Efremenkov

In many countries nuclear power plants are an impor-tant part of the national energy system. Nuclear poweris economically competitive and environmentally cleancompared to most other forms of energy used in electric-ity production. Used in conjunction with them, it con-tributes to the security of national electricity supplies. Itseems certain that in the medium term and beyond, agrowing contribution to national energy supplies fromnuclear energy will continue to be necessary if the stan-dard of living in industrialized countries of the world isto be maintained and the energy needs of the developingcountries are to be met.As a result of the operation of nuclear reactors, someradioactive wastes are produced. Yet compared to theamount of waste produced by coal-fired electrical gen er-

ating plants, these are of considerably smaller volume.(See table.) The wastes generated at nuclear powerplants are rather low in activity and the radionuclidescontained therein have a low radiotoxicity and usually ashort half-life. However, nuclear power plants are thelargest in number among all nuclear facilities andproduce the greatest volume of radioactive wastes.The nature and amounts of wastes produced in anuclear power plant depend on the type of reactor, itsspecific design features, its operating conditions and onthe fuel integrity. These radioactive wastes contain acti-vated radionuclides from structural, moderator, andcoolant materials; corrosion products; and fissionproduct contamination arising from the fuel. Themethods applied for the treatment and conditioning ofwaste generated at nuclear power plants now havereached a high degree of effectivity and reliability andare being further developed to improve safety andeconomy of the whole waste management system.

Wastes generated at nuclear power plantsLow- and intermediate-level radioactive waste

(LILW) at nuclear power plants is produced by contami-nation of various materials with the radionuclides gener-ated by fission and activation in the reactor or releasedMr Efremenkov is a senior staff member in the IAEA Division ofNuclear Fuel Cycle and Waste Management.

from the fuel or cladding surfaces. The radionuclides areprimarily released and collected in the reactor coolantsystem and, to a lesser extent, in the spent fuel storagepool.The main wastes arising during the operation of anuclear power plant are components which are removedduring refuelling or maintenance (mainly activatedsolids, e.g. stainless steel containing cobalt-60 andnickel-63) or operational wastes such as radioactiveliquids, filters, and ion-exchange resins which are con-taminated with fission products from circuits containingliquid coolant.In order to reduce the quantities of waste for interimstorage and to minimize disposal cost, all countries arepursuing or intend to implement measures to reduce the

volume of waste arisings where practicable. Volumereduction is particularly attractive for low-level wastewhich is generally of high volume but low radiationactivity. Significant improvem ents can be ma de throughadministrative measures, e.g. replacement of papertowels by hot air driers, introduction of reusable long-lasting protective clothing, etc., and through generalimprovements of operational implementation or"housekeeping".Liquid wastes and wet solid wastes

According to the different.types of reactors now oper-ating commercially all over the world, different wastestreams arise. These streams are different both inactivity content and in the amount of liquid waste gener-ated. Reactors cooled and moderated by water generatemore liquid waste than those cooled by gas. Thevolumes of liquid waste generated at bo iling-water reac-tors (BWRs) are significantly higher than at pressurized-water reactors (PWRs). Because the cleanup system ofheavy-water reactors (HWRs) works mainly with once-through ion-exchange techniques to recycle heavywater, virtually no liquid concentrates are generated atthem.

Active liquid wastes are generated by the cleanup ofprimary coolants (PWR, BWR), cleanup of the spentfuel storage pond, drains, wash water, and leakagewaters. Decontamination operations at reactors also

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Featuresgenerate liquid wastes resulting from maintenanceactivities on plant piping and equipment. Decontamina-tion wastes can include crud (corrosion products) and awide variety of organics, such as oxalic and citric acids.

Wet solids are another category of waste generated atnuclear power plants. They include different kinds ofspent ion-exchange resins, filter media, and sludges.Spent resins constitute the most significant fraction ofthe wet solid waste produced at power reactors. Beadresins are used in deep demineralizers and are commonin nuclear power plants. Powdered resins are seldomused in PWRs, but are commonly used in BWRs withpre-coated filter demineralizers. In many BW Rs, a largesource of powdered resin wastes are the "condensatepolishers" used for additional cleaning of condensedwater after evaporation of liquid wastes.Pre-coated filters used at nuclear power plants toprocess liquid waste produce another type of wet solid

waste-filter slu dges. The filter aids usually diato-maccous earth or cellulose fibres and the crud that isremoved from the liquid waste together form the filtersludges. Som e filtration systems do not require filter aidmaterials. The sludges arising from such units thereforedo not contain other materials.

Treatment and conditioning of liquid/solid wasteLiquid radioactive waste generated at nuclear powerplants usually contains soluble and insoluble radioactivecomponents (fission and corrosion products) and non-radioactive substances. The general objective of wastetreatment methods is to decontaminate liquid waste tosuch an extent that the decontaminated bulk volume ofaqueous waste can be either released to the environmentor recycled. Waste concentrate is subject to further con-ditioning, storage, and disposal. Because nuclear powerplants generate almost all categories of liquid waste,

nearly all processes are applied to treat radioactiveeffluents. Standard techniques are routinely used todecontaminate liquid waste streams. Each process has aparticular effect on the radioactive content of the liquid.The extent to which these are used in combinationdepends on the amount and source of contamination.Four main technical processes are available for treat-ment of liquid waste: evaporation; chemical precipita-tion/flocculation; solid-phase separation; and ionexchange.

These treatment techniques are well established andwidely used. N evertheless, efforts to improve safety andeconomy on the basis of new technologies are under wayin many countries.The best volume reduction effect, compared with theother techniques, is achieved by evaporation. Dependingon the composition of the liquid effluents and the typesof evaporators, decontamination factors between 104

and 106 are obtained.Evaporation is a proven method for the treatment ofliquid radioactive waste providing both good decon tami-nation and volume reduction. Water is removed in thevapour phase of the process leaving behind non-volatilecomponents such as salts containing most radionuclides.Evaporation is probably the best technique for wasteshaving relatively high salt content with a wide hetero-genous chemical composition. (See accompanyingfigure.)Although it can be considered a fairly simple opera-tion which has been successfully applied in the conven-tional chemical industry for many years, its applicationin the treatment of radioactive waste can give rise tosome problems such as corrosion, scaling, or foaming.Such problems can be reduced by appropriate provi-sions. For example, the pH value can be adjusted toreduce corrosion; organics can be removed to reducefoaming or anti-foaming agents can be added; and the

Evapora tion of contam inated liquid efflue nt 104 C

8 0 CLiquid LLWeffluentsystem

Collection tank

Org. phase

Main streamBy-pass stream

vapour compressorandrecirculation pum p

^ Combustion

TreatmentDistillate p l a n t

Solidif ication

Source: KFZ, Karlsruhe, Fed. Rep. of G ermany.Concentrate

LLW-Evaporation

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Solidification of liquid radioactive waste with cementSol d/I iqu id/separator

Distillate

Borate waste.

Chemicals

Pretreatmenttank

LSource: JGCCorp. Mixer

Concentrator

evaporator system can be cleaned by nitric acid toeliminate scaling and subsequent passivation of con-struction material.Up till now, volume reduction byevaporation of low-level radioactive effluents has always been so effectivethat the clean condensate could be discharged to theenvironment without further treatment.Chemical precipitation methods based on thecoagulation-flocculation separation principle aremostlyused innuclear power plants for the treatment of liquid .effluents with low activity and high salt and mud con-tents. Their effectiveness depends largely on thechemi-cal and radiochemical composition of the liquid waste.Most radionuclides can beprecipitated, co-precipitated,and adsorbed by insoluble compounds, e.g. hydroxides,carbonates, phosphates, and ferrocyanides, and so beremoved from the solution. Theprecipitates also carrydown suspended particles from the solution by physicalentrainment. However, the separation isnever completefor several reasons, and the decontamination factorsachieved can be relatively low. For this reason, chemi-cal treatment is usually used in combination with other

more efficient methods.Solid-phase separation is carried out to remove sus-pended and settled solid matters from the liquid waste.There are several types of separation equipment avail-able, all based onthose which have been regularly usedin theconventional water andeffluent treatment plants inthe industries. Themost popular types are filters, cen-trifuges, and hydrocyclones. Particle separation is awell-established technology. Almost allnuclear facilitiesuse mechanical devices to separate suspended solidsfrom liquid waste streams. Generally, separation equip-ment isneeded toremove particles which could interferewith subsequent liquid waste treatment processes, e.g.ion exchange, or with the re-use of the water.

Typical filters can remove particles down to sub-micron sizes, particularly when a precoat isused. Once

exhausted, the filter is either "backwashed" to yield asludge of around 20-40% solids, or in the case ofcartridge types, the entire unit is replaced.Ion-exchange methods have extensive application inthe treatment of liquid effluents atnuclear power plants.Examples of these include the cleanup of primary andsecondary coolant circuits in water reactors, treatmentof fuel storage pond water, andpolishing of condensatesafter evaporation.Liquid radioactive wastes usually have to satisfy thefollowing criteria to be suitable for ion-exchange treat-ment: theconcentration of suspended solids in thewasteshould be low; the waste should have low (usually lessthan 1gram per litre) total salt content; and the radio-nuclides should be present in suitable ionic form.(Filters pre-coated with powdered resin can be used toremove colloids.)In most technical systems, ion-exchange processesare applied using a fixed bed of ion-exchange materialfilled in a column which is passed through by the con-taminated effluent either from top to bottom or vice-versa. The ion-exchange material may be regeneratedafter having reached saturation of the active groups(break through capacity). Some types of ion exchangersare also removed as waste concentrate to be solidified.Therefore the ion-exchange process represents a semi-continuous process and requires major efforts in main-tenance like flushing, regeneration, rinsing, and refillingoperations.Wet solids resulting from liquid waste treatment muststill betransformed into solid products for final disposal.Immobilization processes involve the conversion of thewastes to chemically and physically stable forms thatreduce thepotential for migration ordispersion ofradio-nuclides by processes that could occur during storage,transport, and disposal. If possible, waste conditioningshould also achieve a volume reduction.

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FeaturesThe most frequently applied methods for cond itioningwet solids are cementation, bituminization, or incorpo-ration into polymers. Immobilization of radioactivewaste using cement has been practised widely for manyyears in many countries. (See accompanying figure.)Cement has a number of advan tages, notably its low cost

and the use of relatively simple process plant. Its rela-tively high density provides the waste forms with a con-siderable degree of self-shielding thereby reducingrequirements for additional package shielding. In certaincases, in order to achieve a product of acceptable qual-ity, chemical or physical pre-treatment steps may beemployed. Sometimes additional alternative materials,such as pulverized fuel ash and blast furnace slag, canbe used. These behave in a similar way to simplecement.Bitumenization also has been used for a number ofyears in various countries for solidification of wet

solids. Bitumenization is a hot process which allows thewet stream to be dried off before being imm obilized andpackaged. This greatly reduces the volume of condi-tioned waste requiring disposal w ith a consequent savingin cost. However, bitumen is potentially flammablerequiring special precautions to prevent its accidentalignition. Nevertheless, bitumenization has found grow-ing acceptance with waste produ cers and is used for con-ditioning of radioactive waste at nuclear power plants inthe USA, Japan, Sweden, USSR, Switzerland, and othercountries.Incorporation of wet solids into plastics or polymersis a relatively new immobilization process when com-

pared to the use of cement or bitumen. The use of poly-mers such as polyester, vinylester, or epoxide resins isgenerally limited to those applications where cement orbitumen are technically unsuitable. Such polymers areconsiderably more expensive and a relatively complexprocessing plant is needed. Polymers have the advan-tages of offering greater leak resistance to radionuclidesand of being generally chemically inert.There has recently been increased interest in the useof mobile units to condition radioactive waste fromnuclear power plants. This has arisen mainly becausethey provide saving in capital cost where on -site arisings

of waste are small. M obile immobilization units for con-ditioning of radioactive waste of nuclear power plantsare used, for example, in the USA, Federal Republic ofGermany, and France. Most of them utilize the cementa-tion process, although several designs for utilizingpolymers have been developed.

Gaseous waste and radioactive aerosolsIn normal operation of nuclear power plants, someairborne radioactive wastes are generated in either par-ticulate or aerosol of gaseous form. Paniculate radioac-tive aerosols can be generated in a wide range of particle

sizes in either liquid or solid form, po ssibly in combina-tion with non-radioactive aerosols. Three main sourcesof aerosols are generated by emission of activated corro-

sion products and fission products; radioactive decay ofgases to involatile elements; and adsorption of volatileradionuclides formed in the fission process on existingsuspended material.The most important volatile radionuclides, whichform gaseous radioactive waste generated during normal

operation of nuclear power plants, are halogens, noblegases, tritium, and carbon-14. The composition and theamount of radioactivity present in the various airbornewaste streams largely depend on the reactor type and therelease pathway.All gaseous effluents at nuclear power plants aretreated before discharge to the atmosphere to removemost of the radioactive components from the effluence.

Treatment of gaseous effluentsIt is common practice at all nuclear power plants forcontaminated gases and building ventilation air to be

first passed through filters to remove paniculate activitybefore discharge to the atmosphere via stacks. Ventila-tion and air cleaning system usually employ coarsepre-filters followed by high-efficiency-particulate-air(HEPA ) filters. These have typical particle removal effi-ciencies of 99.9% or better for 0.3 mm particles.Radioactive iodine arising from power plant opera-tion is routinely removed by impregnated charcoalfilters, used in combination with paniculate filters.Impregnation is required to trap the organic iodine com-pounds from gas effluents.Because noble radioactive gases released from fuelelements in a small amount are mainly short-lived,delaying their release will allow radioactive decayprocesses to greatly reduce the quan tities finally releasedto the environment. Two delay techniques are used forthis purpose: storage in special tanks or passage throug hcharcoal delay beds.For decay storage, the noble gases and their carriergas are first pumped into gas tanks which are thensealed. After a storage time between 30 and 60 da ys, thecontent of the tanks is ventilated to the atmospherethrough a ventilation system. If release is not permissi-ble, the storage period is extended as necessary.Delay beds consist of a number of vessels filled with

charcoal, which relatively retards the passage of noblegases in relation to the carrier gas and allows radioactivedecay to take effect.Treatment and conditioning of solid waste

During the operation of a nuclear power plant, vari-ous types of dry solid wastes containing radioactivematerials are generated. The nature of these wastes varyconsiderably from facility to facility and can includeredundant items of the reactor plant, ventilation systemfilters, floor coverings, contaminated tools, etc. Anothersource of solid waste is the accumulation of miscellane-ous paper, plastic, rubber, rags, clothing, small metallicor glass objects, used during the operation and main-tenance of the nuclear power plant. Depending on the



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Compaction of solid radioactive waste Hoist

Drum dumper Feed conveyor Meta l box Metal boxhandling table Source: JGCCorp.

physical nature andfurther treatment methods, dry solidwaste usually isclassified andsegregated into four maincategories: combustible, non-combustible, compactible,and non-compactible waste. However, each facilityusually has its own level of classification according tothe prevailing conditions.On e of the essential aims in the treatment of solidwaste is to reduce asmuch aspossible thewaste volumesto be stored and disposed of and to concentrate andimmobilize as much as possible the radioactivity con-tained in the waste.As solid radioactive waste at nuclear power plantsconsists of a broad spectrum of materials and forms, nosingle technique can adequately treat this waste; a com-bination of processing techniques isgenerally used. Thebasic andmost common technique used for processingmost voluminous portions of solid waste hasbeen basedon compaction. T his method reduces thestorage and dis-posal volume requirements by a reasonable amount, but

achieves little in terms of improvement of the wasteproperties from the viewpoint of longer termmanagement.Experience hasshown that between 50% and80 % ofsolid radioactive waste produced atnuclear power plantscan beclassified asburnable waste. Incineration of thiswaste represents a substantial improvement from a num-ber of viewpoints over simple compaction. Very highvolume reduction and mass reduction can be achieved.The final product is an homogeneous ash which can bepackaged without further conditioning into containersfor storage and disposal. While incineration is onlysuitable for combustible waste, it has the advantage ofbeing capable of destroying organic liquids, e.g. oils,greases or solvents, which otherwise are difficult totreat. (See accompanying figures.)Incineration of small quantities of solid waste isroutinely carried out in relatively simple units. Suchincineration facilities have now been installed at the

Incineration of combustible radioactive waste ExhaustTert iarychamber

PrimarychamberWeight registration

Waste heat Filterrecovery boiler baghouse

kz>

Off-gasscrubber

Source: SEG, nc., Oak Ridge, USA.

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Featuresnuclear power plants in the USA, Japan, Canada, andother countries. More advanced incineration facilitiesthat can incinerate waste with relatively high specificactivity are installed at centralized waste treatment facil-ities that can accept waste from many plants in the coun-try and from abroad. Such facilities are operating inSweden, Belgium, France, and other countries.

As a pretreatment step for compaction or incinera-tion, cutting, shredding, and crushing are used to reducethe physical size of individual waste items. Paper,plastic, cloth, cardboard, wood, and metals can beshredded into ribbon-like pieces, while brittle materialssuch as glass or concrete blocks can be crushed intosmaller fragments. These techniques can also be used asstand-alone processes for volume reduction of solidwaste.New developments

Most treatment and conditioning processes for LILWhave now reached an advanced industrial scale.Although these processes and technologies are sufficientfor effective management of radioactive waste at nuclearpower plants, further improvements in this technologyare still possible and desirable. The increasing cost ofradioactive waste disposal provides an incentive to adoptprocedures and techniques to minimize waste quantitiesand to develop new techniques to minimize volumes atthe treatment and conditioning step. It is not possiblehere to summarize all new developments and improve-ments which are being made in this direction in MemberStates.Some examples of such new developments includethe use of specific inorganic sorbents to improve liquidwaste treatment; use of membrane techniques for liquidwaste treatment; dewatermg and drying of bead resinand filters slurries; incineration of spent ion-exchangeresins; dry cleaning of protective cloth to reduce quan-tity of laundry drains; use of high integrity containersfor packaged dried filter sludges; vitrification of someintermediate-level waste to reduce volumes of waste tobe disposed of; and supercompaction of unburnablewaste.

Perhaps not all of these new developments will findbroad implementation in waste management technolo-gies, in particular at nuclear power plants. However, theresearch and development reflects the fact that thenuclear industry and utilities take great care in a safe andeconomic management of radioactive waste at nuclearpower plants, and that improvements in existingtechnology are forseen.

At the Marcoule facility in France, containers of vitrified high-levelwaste are transferred to air-cooled shafts for interim storage.(Credit: ANDRA)


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Radioactive Waste Management at Nuclear Power Plants