627 M MANAGEMENT OF RADIOACTIVE WASTES RADIOACTIVE WASTE Radioactive waste may be defined as solid, liquid, or gaseous material of negligible economic value containing radionu- clides in excess of threshold quantities. High level wastes (HLW) are produced in the first cycle of reprocessing spent nuclear material and are strongly radioactive. Intermediate level wastes (ILW) can be divided into short lived, with half lives of twenty years or less, and long lived, in which the half lives of some constituents may be thousands of years. Low level wastes (LLW) contain less than 4 GBq/ton of alpha emitters and less than 12 GBq/ton of beta and gamma emitters. Very low level waste (VLLW) contains activity concentrations less than 0.4 MBq/ton. ACTIVITY AND EXPOSURE The Becquerel (Bq) is the activity of one radionuclide having one spontaneous disintegration per second. One Curie (Ci) is defined as 3.7 ϫ 10 10 disintegrations per second. The Becquerel is the more commonly used unit. The unit of ion- izing radiation which corresponds to energy absorption of 100 ergs per gram is the rad (roentgen-absorption-dose). The newer unit is the Gray (Gy), which is equal to 100 rads. The amount of radiation which produces energy dissipation in the human body equivalent to one roentgen of X-rays is the rem (roentgen-equivalent-man). One Sievert is equal to 100 rems and is the commonly accepted unit. Philosophy The group of people engaged in management of radioac- tive wastes has evolved from a small body of operators who, originally with little or no expert knowledge, were engaged in day-to-day solution of unpleasant problems. They now form a recognized profession, extending from whole-time research scientists to field workers who in some countries are conducting a profit-making industry. The members of the profession came mainly from Health Physics and brought with them the caution and “conserva- tive” attitude to radiation hazards characteristic of Health Physicists. They regard their mission as being to ensure that members of the public, as well as workers in the field of nuclear energy, will not be harmed by the radioactive mate- rial for which they are responsible. With their Health Physics background this sometimes leads to an attitude which indus- try regards as overrestrictive, although recent controversies have tended to cast them ironically in the role of particularly dangerous polluters of the environment. It is clear that any human activity that involves conversion of something into something different must produce waste. Conversion of energy from one from to another is no excep- tion. It is sometimes possible for an industry to recycle its waste products and to convert part of them to a useful form, but there is always some minimal residue which cannot be retained within the system. This must find some place within the environment. Usually the cheapest procedure is to dis- charge it in some way that will ensure a sufficient dilution to make it innocuous. If this is impracticable for technical or political reasons it must be confined, but usually the more effective the confinement, the higher the cost. To say that a process must be conducted with now waste is equivalent to saying that the process may not be conducted at all, and to demand a certain level of confinement or restriction of wastes implies an acceptance of the cost of the waste management system as a necessary part of the cost of the process. Discharge of potentially noxious materials into the envi- ronment involves some risk, which may or may not be mea- surable. Within very broad limits research in nuclear hazards enables us to forecast the effects of exposure of large groups of people, for extended periods, to low doses of radiation. We can also estimate, with less accuracy, the probability that an individual will suffer some harm from such exposure, and we can say with much greater confidence what will happen if an individual is exposed to larger doses—say 50 rem and upwards—in a single dose. The nuclear industry, then, can provide some information on the probable consequences of environmental contamination extended over a lifetime, and C013_001_r03.indd 627C013_001_r03.indd 627 11/18/2005 10:38:18 AM11/18/2005 10:38:18 AM © 2006 by Taylor & Francis Group, LLC 628 MANAGEMENT OF RADIOACTIVE WASTES better information on the probable consequences of a major nuclear accident which leads to high radiation exposure. In other words we can, within rather broad limits, estimate the risks. The situation is different in most other industries. The consequences of acute doses of cyanide, lead, fluoride or carbon tetrachloride are well known, and there is some evi- dence for the effects from low doses received over a lifetime, but who knows what effect to expect in humans from benz- pyrene or nitrous oxide emitted from smoke stacks or from the low levels of polychlorodiphenyls and mercury com- pounds that are liberated into the environment? They affect every age group in the population and are a potential life- long hazard. But nothing is known about the probability that they will eventually do harm, and it is difficult to see how such knowledge could be obtained in a human population. Every human activity is associated with some risk, however small. Normally we do not solemnly calculate the risk, weigh it against the benefit we expect to obtain, and then decide for or against the activity. Yet to decide to do something—such as driving a car, getting up in the morn- ing, or going mountain climbing—must involve some sort of conscious or unconscious weighing of risk against benefit. In deciding upon a particular waste management system, or in deciding to license a particular kind of nuclear power station, a much more deliberate weighing of cost vs. benefit must be undertaken. There is, however, a fundamental dif- ficulty which up to now has made it impossible to express such a judgment in numbers. It is characteristic of a ratio that the numerator and the denominator must be in the same units. It should be possible to express most of the benefits of nuclear power, for example, in dollars, but if we regard part of the cost of nuclear power as an increase in the probability that people will develop cancer or that they will experience a shortened lifetime, how can that be expressed in dollars? One benefit of nuclear power is the difference between death and injury among uranium miners and processors and the corresponding figure for equivalent energy production by the coal mining industry. This, again, cannot be expressed in dollars. To work out a true COST/BENEFIT ratio is thus little better than a dream, and the people responsible for approving a waste management system or a new power sta- tion are therefore faced in the last analysis with a value judg- ment, which is at least to some extent subjective. It is not a scientific decision. In the broadest sense, the decision is political. Controls The responsibility for making decisions on matters related to “dealing in”—i.e. having anything to do with—radioactive materials, machines capable of producing electromagnetic radiation (expect for medical purposes) and certain scheduled materials such as heavy water, usually rests with a national atomic energy authority. Typically, regulations are issued by the authority that have the force of law. Assistance is given to the authority in assessing hazards of reactors and other installations—including waste management systems—by an independent advisory committee which can call on the ser- vices of an expert staff. In most countries regulations lay down the maximum per- missible exposure to radiation for workers in nuclear industry and also for the general population. Maximum permissible doses (MPDs) have been recommended by the International Commission on Radiological Protection (ICRP), which have received worldwide acceptance as the fundamental basis for national regulations. The ICRP has derived from the MPDs a list of maximum permissible concentrations (MPCs) in air and water on the basis that if workers were to breathe air, or drink water, at the MPC for any particular radionuclide over a lifetime they would not suffer any unacceptable harm. “Unacceptable” means “detectable”, in the sense that it could reasonably be regarded as caused by the radiation. The ICRP has also laid down rules for calculating the MPC for mixtures of more than one radionuclide. The MPDs are constantly under review by the ICRP, which consists of people who have devoted their profes- sional lives to assessment of radiation hazards. They drawn upon the work of large numbers of scientists throughout the world, many of whom are actively engaged in research on somatic and genetic effects of radiation. Changes have been made from time to time in details of the ICRP recommenda- tions but it is remarkable that in such a rapidly developing field the necessary changes have been so few. The ICRP has consistently emphasized that the MPD and its associated MPs are maximum permissible figures. The Commission has made another recommendation equal in force and status to those on maximum permissible doses. This states that exposure to radiation must always be held down to the lowest PRACTICABLE dose. The world “prac- ticable” was carefully chosen, after considerable debate. If “possible” had been used it could have been claimed that a single contaminated rat must be buried in a platinum box. It is our mission to see that all practicable steps are taken to protect mankind from exposure to radiation, and we can do that very effectively. SOURCES OF WASTES Uranium Mining and Milling Apart from the normal hazards associated with hard-rock mining, the workers in uranium mines are exposed to radon and the decay products which arise from the radium content of the ore. These hazards can be controlled by sealing old work- ings and general “good house-keeping”, but more particularly by installation of an efficient ventilation system and, where necessary, the use of respirators. The ventilation air contains radioactive material and dust, some of which can be removed if necessary by filtration, but the radon remains. The large volume of air used for mine ventilation is ejected at high velocity from a stack, which ensures adequate dilution into the atmosphere. The end products of the mill are uranium oxide and “tail- ings”. The tailings, together with mine drainage water, contain most of the radium originally present in the ore. Radium is C013_001_r03.indd 628C013_001_r03.indd 628 11/18/2005 10:38:19 AM11/18/2005 10:38:19 AM © 2006 by Taylor & Francis Group, LLC MANAGEMENT OF RADIOACTIVE WASTES 629 one of the most toxic of all radionuclides and presents a serious potential hazard. Various methods of treatment, such as co-precipitation with barium, render most of the radium insoluble. But the water draining from tailings ponds often contains more radium than is permissible in drinking water. Proper design of outfalls into suitable bodies of water can ensure adequate dilution, but vigilance is necessary to pre- vent rupture of the tailings ponds or improper practices that will nullify or bypass the treatment system. A monitoring system for analysis of downstream water and fish is common today, but in the early days of the industry the dangers were little understood or ignored, with the result that lakes and streams in uranium mining areas became contaminated. In Canada the existence of a problem was recognized in time to avert a public hazard, but the Report of a Deputy Minister’s Committee showed that action was necessary to protect the environment in the Elliott Lake and Bancroft areas. This was particularly urgent as greatly increased activity in uranium mining was anticipated within a few years. The size of the problem can be judged from the fact that a Congressional Hearing was told that 12,000,000 gallons of water containing nearly 10 g of radium was discharged daily to the tailings ponds of American uranium mills. Processing of Uranium Oxide The crude (70%) U 3 O 8 produced by the mills may be con- verted to metal, to UO 2 or to UF 6 . The hexafluoride is used in separation of 235 U from 238 U. A serious waste problem would result from nuclear fission if a critically large amount of 235 U were to accumulate accidentally in one place. This is a rare event, but is not impossible. Otherwise, the wastes consist of uranium chips and fines, contaminated clothing and res- pirators and dust accumulated in air-cleaning systems. The uranium at this stage is practically free from radium so it is hardly a radioactive hazard. The toxicity of natural uranium or 238 U is that of a toxic metal rather than of a radionuclide. Uranium metal is produced by converting the dioxide to tetrafluoride which is then reduced to the metal at high temperature with magnesium. The waste form this process— magnesium fluoride slag and uranium metal fines from trim- ming the ingots—is a normal slag disposal problem since it is sparingly soluble in water. Fuel Fabrication There are many different kinds of fuel elements, but their manufacture produces little waste beyond dust and faulty pellets or fuel pins. This material is usually recycled, par- ticularly if it contains added 235 U. Reactor Wastes An operating reactor contains a very large inventory of fission products. A 500 MW (thermal) reactor, after operating for 180 days, contains four hundred million curies for fission products, measured one day after shutdown. This is equivalent to the activity of about 400 metric tons of radium. The fission products decay rapidly at first, leaving 80 million curies at the end of a week, and more slowly later. After a month, the inventory is reduced to about 8 million curies. Nuclear power stations rated at 1000 MW (electrical)— i.e. 3000 to 5000 MW thermal—are not unusual. At first sight it would seem that these plants would be enormous potential sources of radioactive wastes, but in practice this is not the case (Figure 1). In an operating power reactor the fuel is contained within a non-corrodible cladding—usually zirconium or stainless steel—and the fission products cannot get out unless the cladding is ruptured. It is possible to operate the reactor with defects in a few fuel elements, but these sources of leakage make the primary cooling circuit radioactive. It is impracticable to operate a station in the presence of high radiation fields, so the primary coolant is continually purified by ion exchangers. Again, it is SECONDARY CIRCUIT TURBINE CONDENSER WATER COLD WATER PRIMARY CIRCUIT REACTOR CORE HOT WATER STEAM HEAT EXCHANGER FIGURE 1 Schematic diagram of processes in nuclear power station. Nearly all radioactivity remains inside the fuel, which is inside the core, which is inside the primary circuit. C013_001_r03.indd 629C013_001_r03.indd 629 11/18/2005 10:38:19 AM11/18/2005 10:38:19 AM © 2006 by Taylor & Francis Group, LLC 630 MANAGEMENT OF RADIOACTIVE WASTES a practical necessity to renew the ion exchangers after they have developed a certain level of radiation. The net result of these considerations is that for reasons of operator safety and economics the presence of more than a small proportion of ruptured fuel in a reactor will require its removal. Fuel removed from the reactor is normally stored on site for a considerable time to permit decay of shortlived radioac- tivity. Storage facilities are usually deep tanks filled with water, which acts simultaneously as coolant and radiation shield. If defective fuel is present the water will rapidly become con- taminated, but even if there are no defects in the cladding the water in cooling ponds does not remain free from radioactive material. This is because the cladding and the reactor structure contribute neutron activation products (or corrosion products) to the cooling water and the cladding itself always contains minute traces of uranium, which undergoes fission in the reactor. Hence, the pond water must be purified, usually by resin ion exchangers, so these resins also become a waste. If resins are regenerated, the regenerants (acids, alkalis, or salts) will appear as a liquid waste for disposal. Otherwise, the resin will be handled within its original container or as a powder or slurry. The radioactive content of gaseous effluents from reactors depends upon the design of the reactor. If air passes through the core very large amounts of argon-41 may be emitted from the stack. Although 41 Ar is a hard gamma emitter it has a short half-life (about two hours) so its effects are only noticeable within or very near to the plant. Radioactive iso- topes of nitrogen and oxygen decay so rapidly that they do not reach the stack in appreciable amount and the long-lived carbon-14 is not produced in sufficient amount to be hazard- ous at the present scale of nuclear power generation. Some concern has, however, been expressed that by the end of this century the buildup of 14 C in the atmosphere might become a significant source of radiation within the biosphere. More concern attaches to radioactive krypton, 85 Kr, with a half-life of 10.4 years. This, in contrast with 41 Ar and 14 C, is a fission product. It is liberated via fuel defects and by diffu- sion through fuel cladding. It is not a hazard from any single plant, but with increasing numbers of nuclear power stations it might become an ubiquitous source of low-level radiation, though the source of most of the 85 Kr would be spent fuel processing plants rather than power stations. Similar concern has been expressed regarding tritium, the radioactive isotope of hydrogen, which is produced within the fuel and by neutron activation of the heavy hydro- gen in ordinary water or the D 2 O coolant and moderator of heavy-water reactors. It is also formed by neutron activation of lithium, sometimes used as a neutralising agent in reactor coolants, or of boron which functions as a “poison” in some reactor control systems. Sometimes the significance of a “source” of radioactive waste depends on whether one is considering the safety of people within the plant, or the public outside. For example, ruptured fuel elements or ordinary day-to-day type mechani- cal failures can produce air-borne radioactive iodines and other fission products which are a nuisance to operators because they have to work in plastic suits and respirators. The ventilation filtration system and the high dispersion capability of the atmosphere combine to make sources of this kind insignificant beyond the boundary of the exclusion area. However, they may reduce efficiency and disrupt work schedules within the station very seriously, and give rise to significant disposals in the form of clean-up solutions, con- taminated clothing, mopheads and metal scrap. A noteworthy source of this nature is the tritium which builds up in the coolant and moderator of heavy-water reactors. In a 1000 MW (electrical) power station the equilibrium tritium concentration in the moderator is about 50 Ci/litre. This leads to stack discharges which are quite negligible, but any leaks in pump seals, valves or pipe joints within the station would produce operating problems for those respon- sible for the radiation safety of the staff. On the other hand, material sent for waste disposal would be no problem, partly because heavy water is recovered for economic reasons and partly because the maximum permissible concentrations of tritium in air and water are much higher than those of most other radionuclides. In summary, in spite of the enormous potential source of radionuclides within an operating power station the amount of waste generated is small compared with that arising from a research and development establishment, and minute in comparison with a plant fuel processing plant. This statement covers normal operation, including the ordinary accidents and malfunctions expected in any well-designed plant. It does not include the consequences of the “Maximum Credible Accident” which is, in fact, so improbable that designers of waste management systems do not normally make provision for it. However, the accident at the Chernobyl Nuclear Power Station in 1986 was particularly sensational. A reactor exploded and caught fire, releasing an estimated 30 million Curies. Half of the resulting fallout was within 30 kilometers of the plant. The remainder spread over much of Europe. There was great economic loss and many cancer deaths were attributed to the incident. Spent Fuel Processing Wastes arising from processing of spent fuel account for more than 99.9% of the “waste disposal problem”. Fuel which has been enriched with 235 U must be treated for recovery of unburned 235 U because the fission product load of spent fuel reduces its efficiency as a source of energy. It ceases to be economic as fuel long before the expensive 235 U is exhausted. After removal from the reactor, and storage for sufficient time for decay of short-lived fission products, the fuel is de-sheathed and dissolved, usually in strong nitric acid (Figure 2).Uranium and plutonium are extracted into an organic solvent, and the acid solution of fission products left behind forms the high level or primary waste. Washing of the organic extractant produces Medium Level wastes, whereas Low Level waste consists of further washings, cooling water, scrubber water and liquids from other sources too numerous to catalogue. C013_001_r03.indd 630C013_001_r03.indd 630 11/18/2005 10:38:19 AM11/18/2005 10:38:19 AM © 2006 by Taylor & Francis Group, LLC MANAGEMENT OF RADIOACTIVE WASTES 631 As long ago as 1959 fifty million gallons of High Level wastes were stored in stainless steel tanks at Hanford (USA) alone. The radionuclides in solution generate so much decay heat that many of the tanks boil, making the provi- sion of elaborate off-gas cleaning systems necessary. Some high level waste tanks have ruptured, but since they are constructed on a cup-and-saucer principle, with adequate monitoring for spills, and spare tankage is kept available, no unexpected contamination problems have arisen. Gases from the dissolvers and storage tanks contain tritium, bromides, iodines, xenon, krypton and smaller amounts of less volatile elements such as ruthenium and cesium. After storage for decay, scrubbing and filtration, off-gases can be liberated from a tall stack. As mentioned in the section on reactors, proliferation of fuel processing plants in the future might conceivably lead to local or even eventual world-wide atmospheric contamination if improved containment is not provided in time at spent fuel processing sites. Solid waste may include glasses or ceramics, used as a means for fixing the activity in high-level liquid wastes, and bitumen or concrete blocks containing less active material. Products of waste processing such as sludges, evaporator bottoms, incinerator ash, absorbers, filters and scrap fuel cladding are usually in the medium level category. Worn and failed equipment such as pipes, tanks and valves, unservice- able protective clothing, cleanup material and even whole buildings may have a variety of levels of contamination, by numerous different radionuclides, which defies quantitative assessment. This is not a serious difficulty, except for admin- istrative and recording purposes when quantitative reports have to be made, because most of these wastes have to be contained in some way and none of them are dumped into the environment. The most difficult problem for the fuel processing industry is not high or medium level waste, offgases or heterogeneous contaminated scrap. The real problem is very low level liquid waste, because it arises in such enormous volume. Coming from numerous different sources—e.g. cooling and final wash waters, laundry and decontamination center effluents, floor drainage from cleanup operations, personnel shower drainage and effluent from the final stages of liquid waste purification plants—low level and “essentially uncontaminated but sus- pect” waste adds up to billions of gallons per year. Although some countries (Sweden and Japan, for example) evaporate such effluents on a large scale they are usually discharged by some route into the environment. Research and Development A wide variety of wastes arises in such research establishments as Brookhaven (USA), Chalk River (Canada) or Harwell (UK) and the include many of the types mentioned under the head- ing of fuel processing. In addition the research reactors usually produce very large quantities of radioisotopes which may be processed onsite. However, the quantities involved are very much lower, especially in the high level category, and elaborate waste processing systems are seldom needed even at large research centers unless they are situated in built-up areas or immediately over important aquifers. Hospitals and Biological Laboratories Organic material and excreta makes wastes from these insti- tutions difficult to handle. The radioactive content is usu- ally small, and limited to a restricted list of radionuclides. Those used as sealed sources seldom appear as waste, and the rest are practically confined to 131 I, 32 P, 59 Fe, 51 Cr, 35 S and 24 Na. Other nuclides may be used in small amounts for spe- cial purposes such as specific location in certain organs. The nature and amount of radionuclides used in these institutions are such that a high proportion of the waste can be handled safely by the municipal sewage and garbage systems. SPENT FUEL STACK PURIFICATION OFF-GASES NITRIC ACID DISSOLVER HIGH LEVEL WASTE MEDIUM LEVEL WASTE LOW LEVEL WASTE SEPARATION OF PLUTONIUM AND URANIUM WASH WITH ACID ORGANIC LAYER EXTRACTS Pu plus U AQUEOUS LAYER ORGANIC SOLVENT FIGURE 2 Schematic diagram of fuel processing plant. Showing origins of main waste streams. Reactor fuel contains over 99.95% of the total radionuclides eventually disposed of as waste . C013_001_r03.indd 631C013_001_r03.indd 631 11/18/2005 10:38:19 AM11/18/2005 10:38:19 AM © 2006 by Taylor & Francis Group, LLC 632 MANAGEMENT OF RADIOACTIVE WASTES Sealed sources are, however, a very difficult matter. While they remain sealed they are usually within heavy shielding in teletherapy machines, which are only operated by compe- tent people, or they are in the form of needles and plaques for implantation, or instrumental standard sources used by specialists. However, the time comes when such sources have decayed to the point where they are no longer useful. Sufficient activity remains for them to be highly dangerous to the unwary, so they are dealt with in special ways, usually after return to the supplier. Isotope Production Plants These facilities are often associated with large reactors, and wastes are similar to those generated in Research and Development plants. Processing of very large sources of volatile elements such as iodine and tellurium necessitates an elaborate ventilation cleaning system. Manufacture of large sources of 90 Sr, 137 Cs or the trans-uranic elements as power sources may call for sophisticated remote handling equipment in heavily shielded cells. But the waste prob- lems are difficult only in scale from those encountered in an R and D plant. Some people have considered the separation of 90 Sr and 137 Cs from fuel processing wastes as a helpful step in their management. Removal of these nuclides leaves a mixture which, during 20 years’ storage, would decrease in activity by a factor of about 30,000. However, an industry handling the fission products from 50 tons of 235 U burned in one year would have to deal with 500,000,000 Curies of separated 90 Sr and about the same amount of 137 Cs. It might be difficult to find a market for sources of this scale unless they were cheap, and it must be remembered that they would eventu- ally come back as “waste.” Industrial Applications Use of radioisotopes in industry is not a significant source of wastes. Most industrial sources are sealed, and nearly all unsealed sources are short-lived. Transportation Ships are the only form of transportation using nuclear reactors as a source of power. They include naval ships, ice breakers and merchant vessels. They contain large amounts of fission products within the reactors, but as a source of waste they are not important, except possibly in some har- bours and inshore waters. During start-up of the reactor the secondary coolant expands and the limited space in submarines necessitates the dumping of this expansion water. In common with landbased reactor coolant it contains radioactive corrosion products and tritium. The coolant is maintained at a low level of activity by means of ion exchangers, which become waste eventually. Normally this material is disposed of on land, although it has been shown by the Brynielsson Panel of the International Atomic Energy Agency that resin from a fleet of as many as 300 nuclear ships could be dumped safely if this were done only on the high seas. Apart from these sources wastes from nuclear shipping consist of clean-up solutions, laboratory wastes, laundry effluent and other minor sources common to all reactor operations. Except in submarines, practically all wastes can if necessary be retained on board for disposal ashore. DISPOSAL PRINCIPLES There are two main procedures available for disposal— Concentration and Confinement: or Dilution and Dispersion. a) If wastes are truly confined, in the sense that in no credible circumstances could they be liberated into the environment, then the only additional requirement is “perpetual custody” to ensure that the confinement is never broken. This is easier said than done. In the field of high level wastes when we say “perpetual” we are speaking in terms of thousands of years. Few private firms go back for 100 years, political regimes have seldom lasted for as long as 500 years, and there are few civilizations that have survived for 2000 years. In our own day forecasters tend to regard dates beyond 2000 AD as being in the distant future. What, then, can we do about “perpetual custody” of wastes containing, for example, plutonium with a half-life of 24,000 years? This is not a fanciful dilemma. A story from Chalk River will illustrate the point. When the Canadians decided to concentrate on natural ura- nium heavy water reactors for power production it became apparent that processing of spent fuel would be uneconomic until the price of uranium or plutonium rose considerably. Processing was there- fore stopped, but the wastes accumulated during the pilot plant operation had to be disposed of. A considerable volume of medium level waste was mixed with cement in steel drums and enclosed within solid concrete monoliths below ground in the waste management area (Figure 3).The ques- tion then arose “What if some archeologist digs this structure up 1000 years from now and thinks it is an ancient temple or tomb?” Eventually some- one suggested that its true nature should be inlaid in non-corrodible metal on the top of the monolith. Dr. A. J. Cipriani, who had listened to the debate in silence, then asked “In what language?” The implications of this question are profound. Some of the wastes for which we are responsible will still be radioactive after our present civilization has disappeared and perhaps been forgotten. So far as we know there is no practicable solution to the problem. The best we can do is ensure that the nature, amount and location of all major disposals C013_001_r03.indd 632C013_001_r03.indd 632 11/18/2005 10:38:19 AM11/18/2005 10:38:19 AM © 2006 by Taylor & Francis Group, LLC MANAGEMENT OF RADIOACTIVE WASTES 633 are recorded in the nearest approximation we have to a perpetual repository of archives—a government department. Beyond that we can only rely on folk memory. After all, farmers in Europe have been ploughing around Neolithic tumuli and prehistoric roads for thousands of years for no good reason known to them, except that it was accepted to be the right thing to do. b) Dilution and dispersion is the traditional method that men have always used for dealing with their wastes. Until recently it seemed to work fairly well unless populations became very concentrated, but it is now becoming clear that there are so many people that the system is showing signs of break- ing down. It depends upon the capacity of the environment to dilute or detoxify the wastes to a level that is innocuous to man and to organisms of interest to man. We are still a very long way from contaminating our environment with radioactivity to a point where radiation effects are observable, even in close proximity to nuclear enterprises, but we must maintain vigilance to ensure that slow and subtle changes do not occur which escape our notice until it is too late. Safety in discharge to the environment depends upon three factors—(1) Dispersion by such means as atmospheric dilution, mixing into big bodies of water, or spreading through large volumes of soil. (2) Fixation of radionuclides on soil minerals and organic detritus. (3) Decay of radionuclides, dis- persed or fixed, before they are able to affect man. The principle of dispersion has one logical trap into which regulatory bodies have sometimes fallen. In some countries the discharge of liquid and gaseous wastes is limited by the concentra- tion in the effluent pipe or the concentration at the stack mouth. This is based upon the assumption that if the concentration is limited to the maxi- mum permissible value, all will be well. However, the “dilution capacity” of a river is a function of the number of Curies per day put into the river, divided by the daily flow of water. If an operator wishes to dispose of double the amount of waste, and he is limited only by the concentration in the effluent pipe, he need simply double the amount of water flowing in the pipe. But the downstream effect will be a doubling in the concentration, unless he has doubled the flow in the river. FIGURE 3 Pouring a concrete monolith. Steel drums filled with waste, solidified by mixing with cement, were stacked on concrete slabs surrounded with forms. The forms were filled with concrete. The monoliths were about 2 m below ground level. C013_001_r03.indd 633C013_001_r03.indd 633 11/18/2005 10:38:19 AM11/18/2005 10:38:19 AM © 2006 by Taylor & Francis Group, LLC 634 MANAGEMENT OF RADIOACTIVE WASTES For this reason, limitations must be made in Curies per unit time, not in micro-curies per mil- lilitre, and account must be taken of volume of river flow if this is seasonally variable. Regulations set on the basis of concentration at the point of discharge only protect people close to the discharge point. DISPOSAL PRACTICES Gases Radioactive gases arise mainly in reactors, spent fuel pro- cessing, isotope production, and research and development facilities. The general principles are the same for all procedures that depend upon dispersion into the atmosphere. If we have a stack that is emitting Q Curies/sec., the con- centration C at a given distance downwind will be KQ. The parameter K is a very complex function which depends upon wind speed and direction, weather conditions, stack height, topographical features, variability of temperature with height, velocity and buoyancy of the effluent and other conditions. Values of K for a range of conditions can be calculated from equations proposed by Sutton (1947), Pasquill (1961) and Holland (1953). These equations have been used to calculate the permissible emissions from stacks by inserting appropri- ate numbers and parameters applying to unfavourable weather conditions likely to obtain at the site. The permissible emission rate has been set at a value which would ensure that popula- tions downwind would not be exposed to more than an agreed maximum radiation dose rate. The classical equations have been based on statistical theory with empirical values for the diffusion parameters being obtained from experimental work which has some- times had little relation to real emissions from actual stacks. Returning to the superficially simple equation C ϭ KQ, it is apparent that if we could observe, over a long period of time, the maximum value of C ever attained per unit emission rate, we could define a figure K max which was not likely to be exceeded. With a sufficient number of obser- vations of C and Q, extended over a sufficient variety of weather conditions, we could estimate the probability that our value K max could ever be exceeded. When a maximum permissible concentration is set for a noxious substance the decision really depends upon a belief that the probability of damage is so low that it is acceptable. If, then, C is set at the MPC at a given distance from the stack, and K max is known for that distance, then Q p , the max- imum permissible release rate, is determined. It has been shown by Barry that K max is not very depen- dent upon topography or climate, because it depends mainly on rather large-scale behaviour of the atmosphere, and the frequency of most adverse conditions normally experienced do not vary grossly from one place to another. The maximum permissible emission rate—or in some cases the MPC at the stack mouth—is given in the regula- tions governing the plant or laboratory. It is then the respon- sibility of the operator to ensure that emissions are kept as far below the permissible level as may be practicable. Numerous methods are available, other than variation of stack height, for achieving this end (Figure 4). Filtration It is advisable to filter contaminated air near to the source of the activity. This reduces the amount of air to be filtered and also cuts down the “plating-out” of radionu- clides on the duct-work, which can be a source of radiation fields with the plant. Filters must be suitable for the job they are supposed to do. They should be made of non-flammable material such as glass or other fibre and should be tested before and after installation. If fine (e.g. “Absolute”) filters are used it is often necessary to precede them with a coarse filter to avoid rapid clogging with dust. Filters must be very efficient to be adequate for fuel processing plants and incinerators burning highly active waste. For example, a sand filter at Hanford capable of pass- ing 10,000 m 3 /min had an efficiency of more than 99.5%, but this was inadequate. The necessary efficiency of 99.99% was attained with a bed of glass fibers 100 cm thick. Electrostatic Precipitators Small airborne particles are usually electrically charged. The charge can be increased by passing the air through a corona discharge, or through a charged fabric screen. The particles are attracted to a sur- face carrying the opposite charge, from which they can be removed mechanically. It is possible to use the same prin- ciple by imposing a charge on filters. Steam Ejector Nozzles The most efficient air clean- ing device other than “Absolute” filters consists of a nozzle in which the air is mixed with steam and expelled into an expansion chamber where the steam condenses on the par- ticles. After passing through a second construction into another expansion chamber, where the air is scrubbed with water jets, removal efficiency for 0.3 micron particles is 99.9%. Incinerator Off-gases The hot gas from an incinerator carriers with it fly ash, tars and water vapour as well as particles. Tars may be removed and the gases cooled by water scrubbing devices. Water droplets must then be eliminated by reheating or passage through a “cyclone”. This is a cylinder with a conical bottom. Gas injected tangentially at the top sets up a vortex which causes deposition of particles on the sides. In smaller incinerators the gases are cooled and some fly ash is removed by passage through a cooling chamber fitted with baffles. After this stage a roughing or “bag” filter is used, followed if necessary by Absolute or charcoal filters. Processing Plant Gases The devices required for clean- ing gaseous effluent depend on the nature of the process. Off-gas from boiling high level wastes must be passed through condensers and scrubbers to recover nitric acid as well as to remove volatile radionuclides. However, these and other air cleaning equipment previously mentioned will not remove gases such as 85 Kr, nor hold back all of the radioactive halogens. Radioactive iodine in molecular form is fairly easily absorbed by alkaline scrubbers and copper or silver mesh filters, but in the form of methyl iodine it can only be arrested by an activated charcoal filter. These filters have to be kept cool, not only to remove the decay-heat of adsorbed halogens C013_001_r03.indd 634C013_001_r03.indd 634 11/18/2005 10:38:20 AM11/18/2005 10:38:20 AM © 2006 by Taylor & Francis Group, LLC MANAGEMENT OF RADIOACTIVE WASTES 635 but also because 85 Kr is absorbed much more powerfully by cold charcoal. This is the only practical means we have for removal of radioactive noble gases. The very large dispersive capacity of a high stack usually makes it unnecessary to remove 14 C (as 14 CO 2 ) or tritium (mainly 3 H 1 HO) because their toxicity is very low. However, the coolant CO 2 in a gas-graphite reactor does contain enough 14 C to require alkaline scrubbing, which removes radioiodine as well. Liquids Storage The necessity for long-term storage of very large quantities (many millions of gallons) of high level, strongly acid waste has led to the development of tankage and pipeline sys- tems which have stood up to severe conditions for many years. Failures have occurred, but good design and carefully selected materials have prevented environmental contamination. Tanks are constructed from material, often stainless steel, which will not be corroded by the solutions to be stored. Secondary containment is provided by catch tanks or drip trays and sufficient spare tankage is kept available for rapid emptying of a ruptured tank. Leakage is detected by a monitoring system which alarms immediately if radioac- tive liquid appears in the catch tank (Figure 5). Movement of active liquid is effected by pumping rather than by gravity to ensure that it is the result of deliberate action rather than accident. Evaporation The most straight-forward and apparently the simplest method of treatment for radioactive liquid wastes is evaporation. In a carefully designed evaporator with an efficient droplet de-entrainment system the radio- nuclide content of the distillate can be about one millionth of that in the pot. There is little about the design that is spe- cifically related to radioactivity except that shielding may have to be provided for the operator, and off-gases must be monitored and possibly treated in some way. Unfortunately, evaporation is expensive because it consumes a large amount of energy and the end product—the concentrate—is still a radioactive liquid waste. Evaporation to dryness or to the point of crystallization has been practised, by the residue is so soluble in water that without further processing it is not suitable for disposal. Where discharge of a large volume of low-level waste into the environment is unacceptable the cost of evapora- tion may be justified by its many advantages. Practically all liquid wastes are treated by evaporation in Denmark and Sweden, and it is also widely used in Japan. Residues from evaporation may be mixed with cement, fused with glass frit or various ceramic mixtures, or incor- porated with melted bitumen. The product is then handled as a solid waste. CONTAINMENT: IN CLADDING IN PRIMARY CONTAINMENT IN SECONDARY CONTAINMENT IN EXCLUSION AREA EMERGENCY COOLING FILTER–ADSORBER SYSTEMS ORNL– DWG 70– 9869 FIGURE 4 Reactor containment system. Any leakage from fuel must pass through the cladding, the primary containment, and either the secondary containment or the stack filters. Contamination within the building can be removed by sprays and/or filters. C013_001_r03.indd 635C013_001_r03.indd 635 11/18/2005 10:38:20 AM11/18/2005 10:38:20 AM © 2006 by Taylor & Francis Group, LLC 636 MANAGEMENT OF RADIOACTIVE WASTES Flocculation and Precipitation The cheapest and sim- plest process for treatment of radioactive liquids is removal of the activity on some kind of precipitate, either as an integral part of the precipitated material, or adsorbed on its surface. In most waste tanks a sludge settles out which may con- tain up to 90% of the activity, and a copious precipitate of metallic hydroxides is formed on neutralization which may carry down up to 90% of the remainder. Further purification of the clear effluent after separation of these sludges can be achieved by addition of lime and sodium carbonate. Up to 99% of the remaining activity can sometimes be removed by this treatment. Treatment with lime and sodium phosphate is also very effective (Figure 6). The treatment used depends upon the particular radio- nuclides present in the waste, and also its gross composition for example, the pH and salt content of the solution. In some cases ferric chloride, clay or other additives are introduced at carefully chosen points in the process. The selection of the process, and modifications introduced as the composition of the waste changes, require constant analysis and control by specialized chemists. One problem common to all flocculation processes is how to deal with the sludge. The floc settles very slowly and after it has been drained through filters or separated by centrifugation it is in the form of a thick cheese-like solid which, in spite of its appearance, still contains 80 to 90% of water. In a successful British process the sludge is repeat- edly frozen and thawed. The separation of pure ice crystals leaves behind a concentrated salt solution which coagulates the small particles of floc into a form which settles more rapidly and is less likely to clog vacuum filters. Ion Exchange The effluent from a flocculation process may still contain too much activity for discharge to public waters. It can then be passed through ion exchangers, which are expensive but very efficient. They cannot be used eco- nomically on a solution with a high salt content because their ion-exchange capacity would rapidly be exhausted by absorbing the dissolved salts. The effluent from a well-controlled flocculation process has a low total-solids content and after filtration to remove traces of floc it can be passed through a cation exchanger or mixed-bed resin suitable for removal of the radioactive COOLING COIL RISER INSTRUMENT RISER CONDENSER FILTER (FIBERGLASS) NOTE: ALL WELDS ARE RADIOGRAPHED STEEL WASTE TANK SHOTCRETE GROUT STEEL PAN CONCRETE SLAB WATERPROOF MEMBRANE CEMENT PLASTER SUPPORT COLUMN VERTICAL COOLING COIL HORIZONTAL COOLING COILS STEEL WASTE TANK INLET FIGURE 5 Structure of high level waste tank at Savannah River. C013_001_r03.indd 636C013_001_r03.indd 636 11/18/2005 10:38:20 AM11/18/2005 10:38:20 AM © 2006 by Taylor & Francis Group, LLC [...]... R.L., et al., Report ORNL-4584, 1970, pp 16–36 Cerre, P., Disposal of Radioactive Wastes, 1, pp 226 to 234 International Atomic Energy Agency, Technical Reports Series No 116, Bituminization of Radioactive Wastes, IAEA, Vienna, 1970 Mawson, C.A., Management of Radioactive Wastes, pp 117–124, Van Nostrand, Princeton, N.J., 1965 Beard, S.S and W L Godfrey, Disposal of Radioactive Wastes into the Ground,... P.J., Ind Eng Chem., 43, 1532, 1956 DeBruyn, J and K W Pearce, Report AERE-M 713, 1960 McElroy, J.L., J.N Hartley and K.J Schneider, Report BNWL-1185, 1970 Elliott, M.N and J.R Grover, Report AERE-R 4844, 1965 Watson, L.C., A.M Aikin and A.R Bancroft, Disposal of Radioactive Wastes, Vol 1, pp 373–399, IARA, Vienna, 1960 Merritt, W.F., Disposal of Radioactive Wastes into the Ground, pp 403–408, IAEA, Vienna,.. .MANAGEMENT OF RADIOACTIVE WASTES 100 637 100 pH-12 D A pH-11 pH-10 B 90 C E F G 80 pH-9 80 H 70 60 60 REMOVAL, % STRONTIUM REMOVED, % I 50 40 40 30 20 20 10 0 0 5 15 10 20 PHOSPHATE/CALCIUM RATIO 25 30 FIGURE 6 Decontamination with lime and phosphate—effect of pH and lime/phosphate ratio on removal of strontium-90 contaminants If properly chosen such a resin will remove 99.9% of most radionuclides... only 1/25 to 1/100 of the rate of movement of the ground water If the site of the waste management area is selected with care in relation to potable water supplies, so that the time of transit between the point of disposal and the point of human consumption is prolonged in relation to the half-life of the critical radionuclides, direct ground disposal of low level waste is effective and safe There are... Agency, Safety Series No 12, Management of Radioactive Wastes Produced by Radioisotope Users, IAEA, Vienna, 1965 Nuclear Accidents—Harmonization of the Public Health Response WHO, Geneva, 1989 Krauskopf, K.B., Radioactive Waste Disposal and Geology, Chapman and Hall, London, 1988 Chapman, N.A and I.G McKinley, The Geological Disposal of Nuclear Waste, Wiley, Chichester, 1987 Office of Technology Assessment,... encourage movement of oil to gas through a formation towards a well This process has been adapted to disposal of medium-level wastes A horizontally bedded formation—shale has been used up to now—is drilled to several thousand feet A high pressure jet of sand and water cuts through the well casing and penetrates between the strata near the bottom of the hole The well is then sealed and water forced down... to prevent the ingress of water, so joints in FIGURE 9 Concrete trench Double trench, for medium-level solid wastes, is covered with a light roof when in use The filled trench is levelled with sand and a concrete roof is poured Note galvanized steel seals for joint © 2006 by Taylor & Francis Group, LLC C013_001_r03.indd 639 11/18/2005 10:38:21 AM 640 MANAGEMENT OF RADIOACTIVE WASTES CONCRETE TILE HOLES... European Office sets forth not only the effects of Chernobyl but also presents an excellent discussion of the © 2006 by Taylor & Francis Group, LLC C013_001_r03.indd 640 11/18/2005 10:38:21 AM MANAGEMENT OF RADIOACTIVE WASTES foundations for international cooperation in case of a future accident With the reduction of tensions between the major nuclear powers the questions of nuclear weapons destruction and. .. different versions of the types of disposal facility just described Some are in the open, some within buildings, but all are designed to prevent access of water to the contents It is often convenient to delay the passage of radionuclides contained in high volume low-level wastes before discharge into the environment in order to take advantage of radioactive decay If the local soil and ground water regime... legal circumstances restrict the possibility of burial of radioactive material in the ground Elsewhere, ground burial is regarded favourably In the latter case bales and non-combustible waste are likely to be buried in sparsely populated regions Where land is cheap, low-level wastes may be buried without any volume reducing process Conditioning Pre-treatment of waste before final disposal is called “conditioning” . LLC 636 MANAGEMENT OF RADIOACTIVE WASTES Flocculation and Precipitation The cheapest and sim- plest process for treatment of radioactive liquids is removal of the activity on some kind of precipitate,. conducted at all, and to demand a certain level of confinement or restriction of wastes implies an acceptance of the cost of the waste management system as a necessary part of the cost of the process Radioactive Wastes, IAEA, Vienna, 1970. Mawson, C.A., Management of Radioactive Wastes, pp. 117–124, Van Nostrand, Princeton, N.J., 1965. Beard, S.S. and W. L. Godfrey, Disposal of Radioactive Wastes