Features Radioactive waste management at nuclear power plants An overview of the types of low- and intermediate-level wastes and how they are handled by V. M. Efremenkov In many countries nuclear power plants are an impor- from the fuel or cladding surfaces. The radionuclides are tant part of the national energy system. Nuclear power primarily released and collected in the reactor coolant is economically competitive and environmentally clean system and, to a lesser extent, in the spent fuel storage compared to most other forms of energy used in electric- pool. ity production. Used in conjunction with them, it con- The main wastes arising during the operation of a tributes to the security of national electricity supplies. It nuclear power plant are components which are removed seems certain that in the medium term and beyond, a during refuelling or maintenance (mainly activated growing contribution to national energy supplies from solids, e.g. stainless steel containing cobalt-60 and nuclear energy will continue to be necessary if the stan- nickel-63) or operational wastes such as radioactive dard of living in industrialized countries of the world is liquids, filters, and ion-exchange resins which are con- to be maintained and the energy needs of the developing taminated with fission products from circuits containing countries are to be met. liquid coolant. As a result of the operation of nuclear reactors, some In order to reduce the quantities of waste for interim radioactive wastes are produced. Yet compared to the storage and to minimize disposal cost, all countries are amount of waste produced by coal-fired electrical gener- pursuing or intend to implement measures to reduce the ating plants, these are of considerably smaller volume. volume of waste arisings where practicable. Volume (See table.) The wastes generated at nuclear power reduction is particularly attractive for low-level waste plants are rather low in activity and the radionuclides which is generally of high volume but low radiation contained therein have a low radiotoxicity and usually a activity. Significant improvements can be made through short half-life. However, nuclear power plants are the administrative measures, e.g. replacement of paper largest in number among all nuclear facilities and towels by hot air driers, introduction of reusable long- produce the greatest volume of radioactive wastes. lasting protective clothing, etc., and through general The nature and amounts of wastes produced in a improvements of operational implementation or nuclear power plant depend on the type of reactor, its "housekeeping". specific design features, its operating conditions and on the fuel integrity. These radioactive wastes contain acti- Liquid wastes and wet solid wastes vated radionuclides from structural, moderator, and coolant materials; corrosion products; and fission According to the different.types of reactors now oper- product contamination arising from the fuel. The ating commercially all over the world, different waste methods applied for the treatment and conditioning of streams arise. These streams are different both in waste generated at nuclear power plants now have activity content and in the amount of liquid waste gener- reached a high degree of effectivity and reliability and ated. Reactors cooled and moderated by water generate are being further developed to improve safety and more liquid waste than those cooled by gas. The economy of the whole waste management system. volumes of liquid waste generated at boiling-water reac- tors (BWRs) are significantly higher than at pressurized- water reactors (PWRs). Because the cleanup system of Wastes generated at nuclear power plants heavy-water reactors (HWRs) works mainly with once- Low- and intermediate-level radioactive waste through ion-exchange techniques to recycle heavy (LILW) at nuclear power plants is produced by contami- water, virtually no liquid concentrates are generated at nation of various materials with the radionuclides gener- them. ated by fission and activation in the reactor or released Active liquid wastes are generated by the cleanup of primary coolants (PWR, BWR), cleanup of the spent Mr Efremenkov is a senior staff member in the IAEA Division of fuel storage pond, drains, wash water, and leakage Nuclear Fuel Cycle and Waste Management. waters. Decontamination operations at reactors also IAEA BULLETIN, 4/1989 37 Features generate liquid wastes resulting from maintenance nearly all processes are applied to treat radioactive activities on plant piping and equipment. Decontamina- effluents. Standard techniques are routinely used to tion wastes can include crud (corrosion products) and a decontaminate liquid waste streams. Each process has a wide variety of organics, such as oxalic and citric acids. particular effect on the radioactive content of the liquid. Wet solids are another category of waste generated at The extent to which these are used in combination nuclear power plants. They include different kinds of depends on the amount and source of contamination. spent ion-exchange resins, filter media, and sludges. Four main technical processes are available for treat- Spent resins constitute the most significant fraction of ment of liquid waste: evaporation; chemical precipita- the wet solid waste produced at power reactors. Bead tion/flocculation; solid-phase separation; and ion resins are used in deep demineralizers and are common exchange. in nuclear power plants. Powdered resins are seldom These treatment techniques are well established and used in PWRs, but are commonly used in BWRs with widely used. Nevertheless, efforts to improve safety and pre-coated filter demineralizers. In many BWRs, a large economy on the basis of new technologies are under way source of powdered resin wastes are the "condensate in many countries. polishers" used for additional cleaning of condensed The best volume reduction effect, compared with the water after evaporation of liquid wastes. other techniques, is achieved by evaporation. Depending Pre-coated filters used at nuclear power plants to on the composition of the liquid effluents and the types process liquid waste produce another type of wet solid of evaporators, decontamination factors between 104 waste-filter sludges. The filter aids — usually diato- and 106 are obtained. maccous earth or cellulose fibres — and the crud that is Evaporation is a proven method for the treatment of removed from the liquid waste together form the filter liquid radioactive waste providing both good decontami- sludges. Some filtration systems do not require filter aid nation and volume reduction. Water is removed in the materials. The sludges arising from such units therefore vapour phase of the process leaving behind non-volatile do not contain other materials. components such as salts containing most radionuclides. Evaporation is probably the best technique for wastes having relatively high salt content with a wide hetero- Treatment and conditioning of liquid/solid waste genous chemical composition. (See accompanying Liquid radioactive waste generated at nuclear power figure.) plants usually contains soluble and insoluble radioactive Although it can be considered a fairly simple opera- components (fission and corrosion products) and non- tion which has been successfully applied in the conven- radioactive substances. The general objective of waste tional chemical industry for many years, its application treatment methods is to decontaminate liquid waste to in the treatment of radioactive waste can give rise to such an extent that the decontaminated bulk volume of some problems such as corrosion, scaling, or foaming. aqueous waste can be either released to the environment Such problems can be reduced by appropriate provi- or recycled. Waste concentrate is subject to further con- sions. For example, the pH value can be adjusted to ditioning, storage, and disposal. Because nuclear power reduce corrosion; organics can be removed to reduce plants generate almost all categories of liquid waste, foaming or anti-foaming agents can be added; and the Evaporation of contaminated liquid effluent 104° C 80° C Liquid LLW Org. phase effluent ^ Combustion system Treatment Distillate plant Collection tank vapour compressor Main stream and recirculation pump By-pass stream Solidification Concentrate Source: KFZ, Karlsruhe, Fed. Rep. of Germany. LLW-Evaporation 38 IAEA BULLETIN, 4/1989 Features Solidification of liquid radioactive waste with cement Distillate Sol id/I iqu id/separator Borate waste. Chemicals Concentrator Pretreatment tank L Source: JGC Corp. Mixer evaporator system can be cleaned by nitric acid to exhausted, the filter is either "backwashed" to yield a eliminate scaling and subsequent passivation of con- sludge of around 20-40% solids, or in the case of struction material. cartridge types, the entire unit is replaced. Up till now, volume reduction by evaporation of low- Ion-exchange methods have extensive application in level radioactive effluents has always been so effective the treatment of liquid effluents at nuclear power plants. that the clean condensate could be discharged to the Examples of these include the cleanup of primary and environment without further treatment. secondary coolant circuits in water reactors, treatment Chemical precipitation methods based on the of fuel storage pond water, and polishing of condensates coagulation-flocculation separation principle are mostly after evaporation. used in nuclear power plants for the treatment of liquid