2 Characteristics of Radioactive Wastes

2 Characteristics of Radioactive Wastes

The basic precondition for the efficient and economic processing and disposal of all types of radioactive wastes is a thorough knowledge of their amount and composition. The properties of wastes are closely related to their origins and it is therefore generally possible to characterize wastes generated by various technological operations. Basically, there are several sources of radioactive wastes, which include: - the mining and processing of radioactive raw materials, - the operation of nuclear power plants, - the reprocessing of spent fuel, - the production and use of radionuclides, - the activity of nuclear research institutions. Radioactive wastes are easily characterized from all but the last-mentioned source, i.e., nuclear research establishments where the range and composition of wastes may differ widely and vary depending on the research work currently being conducted. In their physical properties, radioactive wastes are either solid, liquid, gaseous or concentrates. Gaseous wastes make up 90 % of the total activity of radionuclides discharged into the environment from nuclear power plants. The radioactivity present in these wastes occurs in form of gases, vapours and solid and liquid aerosols. The term gaseous wastes is therefore not very accurate. Liquid wastes contain not only dissolved salts but also varying amounts of solid and colloidal substances. The most important properties of solid wastes are those which determine their further processing, viz. combustibility, compactibility or non-reducibility in volume.

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2.1 Wastes from Mining and Processing Radioactive Raw Materials The mining and processing of radioactive raw materials generate considerable amounts of wastes which, in their amount and character, do not differ basically from wastes from other raw material mining and processing. The only difference is that these wastes contain various amounts of radionuclides and thus constitute a greater potentional danger for man and the environment. The average content of uranium in the earth's crust is 4 x 10- 3 kg/t. Currently uranium is mined either separately or as a by-product of ores with a uranium content of 0.15 -400 kgjt. Most mined ores have a uranium content of 0.5 -10 kgjt. The ores are mined in open-cast or deep mines and in some instances uranium is leached at the mine or from the mine tips. The mining technology and demands on processing and safe disposal are to a certain extent influenced by the environment of the mine or dressing plant. The main factors affecting radioactive waste management are: the climate, the magnitude and frequency of floods, geography, topography, seismicity, demography, hydrology, geology, mineralogy and flora and fauna in the environs of the mine and dressing plant. Uranium and thorium mining generates the following wastes: - solid wastes: waste rock, contamined waste structural and building materials and contamined parts of equipment; - liquid wastes: mine waters and seepage waters from waste rock and ore heaps; - gaseous wastes: polluted air from the ventillation shafts of deep mines and exhaust gases from certain equipment, such as crushers and transport equipment. The amount of waste rock varies considerably. In old, favourably sited deep mines waste rock made up one tenth of the mined ore; its amount in currently operated mines is 30-fold the amount of exploited ore and this amount is bound to increase. The waste rock is mostly more varied than the ore and therefore efforts are being made to reprocess these materials. Depending on the uranium content and type of rock, the possibility is considered of leaching the uranium from heaps, using the rock to backfill worked-out spaces, for road work and building work in the mine area or outside this area. This will considerably reduce the amount of waste rock. The contamination of building and structural materials is very low in mining ore with a uranium content of less than I %. Waste materials are therefore disposed of without any special preliminary treatment. The mining of richer ores is accompanied by such contamination that some materials must be considered as radioactive wastes. Mine waters form the major part of liquid wastes. The mine waters consist of

31

surface or ground waters and in surface open-cast mining, also precipitation (rainfall and snowfall). Other major sources are waters from drilling work, drinking and utility waste water and utility water and water from ore slurry. These waters flow into the mines through underground fissures or cracks. Table 4: Characteristics of Liquid Wastes from Miningand Processing Radioactive Raw Materials

Components

a) radioactive U, 226Ra, Rn 210Pb 230-232Th 21 0pO b) non-radioactive Na Ca,Mg Ba Fe, Co, Ni Zn,Cu Mn Pb V Cr As Mo 80:HCOi"

Mine waters acidic

alkaline

x x x

x

X

x x

CO~-

PO:COFN03' Oils TBP

x x x x

x x x

x x x

x x

x x x x x x

x x

x x x x x x x x x x x x x x x x x x

x x

x

X

x

X

x x x x x x x x

Water from chemical Water from workedprocessing plants out mines acidic alkaline acidic alkaline

x x

x x

x x

x x x x x x x x x

x

x x x x

x x x

x x

x

x

Part of the mine waters may be used for various purposes, e.g., in drilling or for the removal of dust. The amount of mine water may be reduced by sealing the cracks or by diverting the flow of surface waters. Nevertheless, the major part of mine waters requires suitable treatment prior to re-use or discharge. Depending on the length of contact with the ore, the mine water may contain dissolved uranium, thorium, radium, radon and other metals. Table 4 shows that mine waters have a considerable content of various substances.

32

Gaseous wastes from deep mines contain radon and its decay products, toge.her with dust particles of ore, rocks and gases originating from blasting and the operation od diesel engines. The level of contamination is, however, very low and the air is therefore directly discharged. In some instances dust particles are arrested and the air is discharged from a stack of suitable height. The mined ore is taken to chemical processing plants where it is processed, using acidic or alkaline leaching. The techniques of uranium ore dressing are described in Fig. 2. The final product is usually the raw chemical uranium concentrate, which is then processed into a pure nuclear product. The following wastes are generated by chemical ore processing: - solid wastes: tailings and diverse contaminated structural and building materials and contaminated parts of equipment; - liquid wastes: acidic or neutralized waste solutions from acidic leaching, water from alkaline leeching, used for sluicing slurry into slurry ponds, small volumes of liquid wastes from laboratories, waters which have been used for washing floors and equipment, seepage from ore slurry dams, leachings from heaps and decantation solutions from slurry dams; - gaseous wastes: dust from conveyer belts, crushers, chemical preparations, product drying, etc., aerosols, fumes from solution and leaching processes. As the major part of 226Ra remains in insoluble form during the leaching process, the tailings contain almost the complete 226Ra content of the ore. It has been calculated that the amount of 226Ra accumulated in the tailings of the 1.5 x 106 t of uranium to be processed by 1990 will reach 18.5 PBq (5 x 10 5 Ci). The 226Ra content of oceans and seas reaches more than 37 EBq (I x 109 Ci). Despite the fact that the specific activity of radium in tailings is not higher than it was in the ore, 226Rl now occurs on the earth's surface in the form of finely crushed tailings and therefore poses a considerable environmental danger. From the tailings the radiaoctive material passes into the atmosphere in the form of radon and fine particulate matter, and it is also leached by precipitation and carried by erosion processes to contaminate water flows. The tailings will also contain other harmful substances, heavy metals, sulphates, sulphuric acid, etc. In some instances acids are formed owing to the presence of pyrites in the tailings. The oxidation of sulphides causes several chemical reactions, which may be expressed by the following equation: 4 FeS2

+ 2 H 20 +

15 O 2

-+

2 Fe2(S04h

+ 2 H 2S04

The reaction rate is influenced by light, temperature and acidity. Chemical oxidation is very slow whereas oxidation by bacteria (Thiobacillus Ferrooxidans) is much faster at pH less than 3.5 -4.0, with optimal contitions at pH 1.5- 3.5.

33

Smaller amounts of concentrated wastes containing radioactive components are generated by ore processing, mostly in the form of slurry of filter cakes with the radioactive fraction being concentrated in the solid phase. This group consists of slurries containing the Ba(Ra)S04 precipitate from the decantation solution or the filter cake with radium and lead from monazite processing. In some instances sulphuric acid is prepared from pyrites obtained from uranium slurry with a radium content. It was found that the activity of slurries in cyclones, pipings and washing columns is 1-2 orders of magnitude higher than the activity of the initialyranium slurry. Contaminated parts of the equipment mostly include ventilation equipment, clogged filters, filtration fabrics and discarded equipment used for uranium extraction, drying and packaging. The composition of waste waters from chemical processing plants using acidic and alkaline leaching is shown in Table 4. Waters from alkaline leaching are recycled, whereas waters from acidic leaching are used for sluicing the slurry into the slurry pond and may contain higher concentrations or harmful substances, such as sulphuric acid, heavy metals, nitrates, sulphates, organic solvents, amines, chlorides and radionuclides 26Ra). Decanter waters may contain 1-10 kBq/m 3 (l00-1000 pCi/l) of 226Ra and other impurities and therefore have to be treated before discharge. Seepage from slurry ponds will vary considerably during operation. In the initial stages it will tally with decanter solutions and will have a content of 10 kBq/m 3 (1000 pCi/l) of 226Ra. In the subsequent stages the proportion of decanter solutions will gradually decrease to the benefit of precipitation. Changes will occur in pH, and the content of 226Ra, sulphates and heavy metals will decrease. These changes have a long-term character. Dust particles are generated by crushing and sorting, and these particles contain ore. Another source of gaseous wastes is chemical fumes from the use of nitric, sulphuric and hydrochlc ric acids for leaching at elevated temperatures. In some instances various gases are released during acidic leaching, such as arsine, hydrogen sulphide and sulphur dioxide, which require special processing. The closure of old mines and chemical processing plants also generates radioactive wastes, because some ore always remains unexploited in the mine and in some instances tailings from chemical ore processing plants are backfilled into the worked-out mines. These materials are a source of radon and may cause contamination of ground and surface waters. Leaching of ore heaps may also generate waste material with a content of uranium, radium, sulphates and heavy metals. The disuse and dismantling of old chemical ore processing plants will generate solid wastes, such as contaminated bricks and masonry, ceilings and floors, etc. The most complex problem is that of controlling the tailings area after the closure of the processing plant.

e

34

~

e:,.,

Precipitation, filtration, drying of concentrate

Fig. 2. Methods of uranium ore processing

Precipitation, filtration, and drying of concentrate

Elution of uranium from ion exchanger

Filtration

Drying of concentrate

Sorption on ion exchangers or extraction from pure solution

Sorption on pulp ion exchangers

Precipitation by sodium lye

Recycling of solution

Concentration or filtration

Separation of sand slurry

Filtration

Elution from ion exchanger or elution from extraction agent

Acidic leaching with ion exchangers or extraction

extraction of depleted solution

precipitation, filtration, drying of concentrate

Uranium elution from ion exchanger

Sorption on ion pulp exchangers

Separation of sand slurry

Acidic leaching and sorption on ion exchangers

Leaching by sulphuric acid and oxidizing agent

Crushing in water

Acidic processing

Leaching by carbonate solution and sorption on ion exchangers

Ore intake, crushing, sampling

Leaching by carbonate solution and precipitation by sodium hydroxide

Crushing in carbonate solution Hot leaching under oxidation conditions

Alkaline processing

The production of radioactive wastes from the mining and processing of radioactive raw materials in the countries of the European Economic Community for 1970 has been estimated as follows: a) gaseous wastes:10 1-1O.2 TBq (103 _104 Ci) of radon, b) liquid wastes: 5 x 106 m 3 with a total content of 370 GBq (10 Ci) isotopes of uranium, thorium and their decay products.

2.2 Wastes from Nuclear Power Plants The character and amount of radioactive wastes generated by nuclear power plants depends to a considerable extent on the type of reactor, its specific design, the trapping equipment and the processing of primary radioactive wastes. The processing of radioactive wastes determines the changes in the physical and chemical properties of the initial primary wastes and of the final waste products. In the initial stage, all wastes are present inside the reactor vessel and in the space between the reactor vessel and the biological shielding. Its further travel depends on the type of reactor, its specific parameters and the normal operation of the main and ancillary equipment of the reactor. Nuclear power plants may be operated under normal contitions and under the conditions of an accident occuring at the plant. Under normal operating conditions the wastes are in the main reactor building, in the area housing the ancillary equipment for waste processing and in the storage area. An accident occuring at the power plant may result in the uncontrolled release of radioactive substances and, depending on the nature of the accident, to the contamination of other areas or of the entire nuclear power plant site. Radioactive materials which are classified as wastes are either fission products from uranium fission or products generated during the reaction of the neutrons with the nuclear fuel, the moderator, the coolant and the structural materials, i.e., activation products. Only a very small amount of the fission products enters the wastes because almost all fission products remain bound in the spent fuel. Of the activation products, the most usual are the radionuclides of iron, cobalt, chromium and zinc. The ratio of activation to fission products depends on the operating -conditions and on the tightness of the fuel elements. The various radioactive wastes from the operation of the reactors differ widely in their physical, chemical and radiochemical properties. The most important physical properties are: a) state - solid, liquid, gaseous b) form-compact, powder, viscose liquid, etc. c) thermophysical properties -thermal conductivity, ignition temperature. boiling point, melting point, vapour pressure, liquefaction temperature, d) mechanical strength, fluidity e) homogenity.

36

Important chemical properties are chemical reactivity, chemical compactibility, chemical composition, leachability and pyrophoric quality. With regard to radioactivity, the characteristic properties are activity, half-life of radio nuclides present, specific activity, isotopic composition, maximal energy of gamma radiation, heat release and radiotoxicity. Wastes from the operation of nuclear power plants are generated by various sources, the most important being: - wastes generated by the normal operation of the reactor; various parts of the reactor, equipment, air filters, waste waters, air from the cooling and ventilation systems, etc.; - wastes from radioactive materials processing, from the maintenance of contaminated parts of the circuits and from contaminated equipment; - wastes from the decontamination of equipment, spaces and materials. In most types of reactors radioactive wastes are accumulated after processing in the following equipment: a) containers for the storage of liquid wastes, b) containers for the storage of gaseous wastes, c) containers for the transport of spent fuel, d) tanks for the storage of spent fuel, e) tanks for the storage of used sorbents, f) bunkers for the storage of solid radioactive wastes. Wastes generated by the normal operation of nuclear power plants constitute a relatively small environmental danger compared with other radioactive wastes. They are generated on a limited area and in limited amounts, contain a relatively small amount of fission products and activated corrosion products and have a very low content of toxic radionuclides. 137CS has the longest half-life, viz., 104 days. A further decline in the amount and activity of radioactive wastes from nuclear power plants may be expected in the future. and the closed cycle of cooling water or cooling gas has been introduced for this purpose. The amount of radionuclides in wastes may further be reduced by improving the design of fuel elements and by using structural materials whose irradiation does not generate activation products with a long half-life. The use of better valves, pumps and other equipment will also help to reduce the amount of wastes caused by escapes from the primary circuit or generated during maintenance or the replacement of faulty equipment. Table 5 gives the basic data on the amount and nature of solid wastes generated during the operation and processing of liquid and gaseous wastes in current nuclear power plants with PWR and BWR with an installed capacity of 1300 MW(e). For our conditions we mainly require detailed information of PWR power plants. The data given on radioactive wastes vary and are considerably influenced by different local conditions, i.e., half-life of liquid and gaseous wastes, before discharge the dilution of liquid wastes by cooling waters, etc., give us an idea of the order of volumes and activities that may be expected during normal operation.

37

<:.<:l

00

Miscellaneous

paper, clothing spare parts control rods reactor instrumentation material for maintenance

activated charcoal

powder ion exchangers dry materials

Tertiary

Waste gases filters

powder ion exchangers filter bed

dry material

filter material dry material

dry material

Concentrate from evaporator

Type of material

Reactor water

Treatment systems

Liquid waste processing

Source of wastes

200 x 1()4 200 x 1()4 400 0.003 0.001 0.15

-

7.4

-

100 550

0.7-4 15-18.5

-

40-400

185-1,850

0.9-13 7.4-93 1-18.5 4-75

0.2

0.08 0.002

0.3 0.06

-

0.03

0.01

0.2 0.03 0.36 0.1

5

1200

70000

-

1.7

9 0.9

0.2-1.3

-

1-10

0.3-7

0.2-3

0.3 0.01 0.01 0.001 0.001

0.01 0.01

-

0.01 0.01

0.4 0.05 0.2 0.04

105

0.07-7 4-400 105 105

0.4-40 0.4-40

-

4000-18500 1850---18500

0.5-48 4-400 0.07---0.7 0.4-4

GBq/m 3

m 3/d

m 3/d

volume activity GBqfm 3

volume activity

volume volume

PWR

BWR total activity GBqfd

0.02---0.2 0.02-2 1.400 85 56

0.004-0.4 0.004-0.4

-

30-150 15-150

0.02---0.2

0.2-20

total activity GBq/d

Table 5: Amount and Character of Solid Wastes from the Operation of Nuclear Power Plants and from Processing Liquid and Gaseous Wastes

in PWR and BWR Nuclear Power Plants with a Daily Output of 1300 MW(e)

The Beznau nuclear power plant in Switzerland has two units with an output of 350 MW(e) each and has been in operation since 1951. Gaseous emissions mainly contain 133Xe and are discharged in an average amount of 35 m 3js, the maximal amount of effluent being 55 m 3js, and the stack is 45 m high. The permitted annual amount of effluent is 19 PBq (5 x 105 Ci) of rare gases, 11 TBq (300 Ci) of 131 1 and an unlimited amount of tritium. The actual effluents discharged are 1.2 % of the permissible level, of rare gases and 2 x 10- 3 % of that of 131 I; the amount of tritium discharged is 14 TBq (370 Ci) per annum. Gaseous wastes are stored in interim decay tanks. Cooling water up to the volume of 3.74 m 3jd is treated on ion-exchange filters with a mixed bed, and the other liquid wastes are processed by filtration, evaporation and demineralization. The operation of ion-exchange filters generates 3 m 3 of waste ion exchangers annually with a total activity of TBq (4000 Ci). Solid waste processing includes fragmentation, compacting and incineration. The Mihama-2 power plant in Japan has an installed capacity of 500 MW(e). Gaseous emissions are discharged from two 55-m stacks, in amounts of 11.4 m' js from the containment and 500 m 3/s from the other operating areas. The annual permissible emission limit is 390 TBq (10500 Ci) of rare gases, the actually discharged amount being only 3.2 % of the permissible level. Before discharge the gaseous wastes are allowed to decay in decay tanks. Cooling water (12.74 m 3jd) is treated by filtration and demineralization, which generates an annual amount of 65 m 3 of sludges with a total activity of 8.5 TBq (230 Ci). Solid wastes are compacted. US data are generalized for one standard nuclear power with an installed capacity of 1100 MW(e). Gaseous emissions are discharged from a 60-m stack and contain an annual amount of 460TBq (13000 Ci) of radioactive effluents; of this, 85Kr makes up 19 TBq (500 Ci), 133Xe 440 TBq (12000 Ci), other rare gases 19 TBq (500 Ci), 131 1 1.9 GBq (0.05 Ci), radioactive aerosols 150 MBq (0.004 Ci), tritium 40 TBq (1000 en, 14C GBq (8 Ci) and 41 Ar 930 GBq (25 Ci). Gaseous wastes from the containment are purified using absolute filters, and gaseous wastes from the other operating areas are purified by filters with activated charcoal. Liquid wastes are treated using filtration, ion exchangers and evaporation, concentrates (0.36 m' jd) are solidified with cement and solid wastes (0.25 m' jd) are compacted. The Biblis A nuclear power plant in the GFR has an installed capacity of 1204 MW(e). Gaseous emissions are discharged from a 96-m stack in an average amount of 7.6 m' js. Purification is by aerosol filters and filters with activated charcoal. The annual permissible limits are 3.3 PBq (89 500 Ci) of rare gases, 25.7 MBq (0.695 mCi) of halogenes, 4.3 TBq (115 Ci) of radioactive aerosols with a half-life of up to 8 days and 306 GBq (8.25 Ci) of radioactive aerosols with a half-life of more than 8 days. The actual amount of effluents discharged is 2 % of the permissible level for rare gases, 1 % for 131 1 and 0.2 % for aerosols. The coolant is treated by demineralization and the other liquid wastes are treated by

39

filtration and evaporation. The annual amount of concentrates generated has a radioactivity of 1.1 TBq (30 Ci). Solid wastes (0.33 m 3{d) are crushed and compacted. Nuclear power plants with reactors of the WWER-440 type have been in operation for a long time in the USSR and have become the basic type for the development of nuclear power production in the socialist countries. Table 6 shows the composition of waste waters at the Novovoronyezh nuclear power plant in normal operation. The first column shows waste waters containing solutions from decontamination equipment and spaces, unplanned releases of coolant and water from the hydraulic sluicing of saturated sorbents into waste tanks, and the second column shows the content of washout water from active washeries and sprays (4). Table 6: Characteristics of Process Waste Water from the Novovoronyezh Power Plant Waste waters

Composition

Water from washery

I

hardness (kgequiv.jrn") hydrocarbons (kg/m") sulphates (kg/m") chlorides (kg/m") nitrates (kg/m'') oxalates (kg/rrr') pH

oxygen consumption (kg 02/m3) solid particles (kg/m 3) surfactans (kg/m") 137CS (kBq/m 3) 106Ru (kBq/m 3) 0 9 Sr (kBq/m 3)

I

1.4 x 10- 3 0.854 0.182 0.032 0.001 0.264 10 0.089 0.032 8.2 x 10- 3 20 x 103 1.6 x 103 I.l x 103

0.002 1.769 0.124 35.5 x 10- 3

0.770 10 0.224 1.550 9.4 X 10- 3 103 410 56

In Czechoslovakia WWER 400 power plants with PWR reactors with an installed capacity of 440 MW(e) are in construction and the installation of 500 and 1000 MW(e) units is being planned. The estimated amounts and activities of solid wastes and concentrates from liquid waste processing are given in Table 7. High-level sorbents are generated during the treatment of the cooling water from the primary circuit and of cooling water from the spent fuel storage tank. Major radionuclides are mainly 88Rb, 89Rb, 89Sr, 90Sr, 91Sr, 95Zr, 95Nb, 97Zr, 97Nb, 99Mo, 131 1, 132 1, 133 1, 134 1, 135 1, 137 1, 137CS, 138CS, 140Ba, 140La, 24Na, 51Cr, 56Mn, 59Fe and 60Co. Only radionuclides with a half-life of more than I day contribute to the total activity. Corrosion products include 60Co, 59Fe and 51Cr, and fission products 90Sr, 95Zr, 95Nb, 99Mo, 131 1, 137CS, 140Ba and 140La. Low-level sorbents and concentrates from evaporators have the same composition.

40

The activity of concentrates from evaporators is initially determined by the activity of 131 I; after 2 months of storage 95 Zr + 95Nb will predominate and after a further 6 months the dominant radionuclides will be 60Co which, together with 137CS, will continue to be the main carrier of the activity of the wastes. During storage for 12 months the total specific activity will decrease from the initial 437 GBq/m 3 (1.18 x 10- 2 Ci/I) to 107 MBq/m 3 (0.9 x 10- 6 Ci/I). Table 7: Amount and Character of Solid Wastes and Concentrates Generated per day by WWERtype Nuclear Power Plants Volume (m3/d) Type of material

Solid Wastes paper and textiles hard materials reactor parts aerosol filters filters with activated charcoal Total

440 MW(e) or 500 MW(e)

12 x 440 or 500 MW(e) 1000 MW(e)

0.041 0.027 0.014 0,110

I 2 x 1000 MW(e)

0.082

0.082

0.055

0.055

0.022 0.200

0.049 0.200

0.200

0.300

0.310

0.392

0.659

0.696 Volume (m 3/d)

Type of material

Concentrates low-level sorbents high-level sorbents concentrates Total

Activity (MB/m 3 )

370-0.37 37 x 103 610 x 106 37 x 1()4

440 MW(e) 500 MW(e)

2 x 440 MW(e) 2 x 500 MW(e) 2 x 1000 MW(e) 1000 MW(e)

0.500

0.10 0.14 0.82

0.14 0.20 1.10

0.637

1.06

1.44

0.055

0.082

It is estimated that from the WWER plant, gases and aerosols will be discharged

from a 100-m stack in daily amounts of 16 TBq (430 Ci) of radioactive gases and 4 GBq (0.1 Ci) of radioactive aerosols, which are roughly 10 % of the permissible limit. Compared with PWR nuclear, power plants in other countries the Czechoslovakian nuclear power plant generates a larger amount of concentrates from evaporators, because concentration is carried out only to a salt content of 200 kg/m 3 and a considerable stand-by capacity is retained for disposal of wastes from the removal of operating failures. There is also a difference of several orders of rnagni-

41

tude in the amount of sorbent wastes because current projects do not include the regeneration of non-exchange packing material. The major part of the activity will accumulate on sorbents. When radionuclides with a short half-life are neglected, a nuclear power plant with two WWER·440 units will generate an annual volume of 62 m 3 of waste sorbents with a total activity of 3.3 TBq (90 Ci), mostly consisting of 137CS, 60Co and 90Sr. Therefore, new projects at Czechoslovakian nuclear plants also include the regeneration of sorbents. Another very widespread type are BWR reactor nuclear power plants. The MUhlenberg nuclear power plant in Switzerland has an installed capacity of 306 MW(e). Gaseous wastes are discharged from a 125-m stack at an average rate of 100 m 3/d. The permissible annual limit is 330 PBq (9 x 106 Ci) of xenon and 1.9 TBq (50 Ci) of halogens; the actual amounts discharged are 15 PBq (4 x lOs Ci) of xenon and 22 GBq per year (0.6 Ci) of 1311. Gaseous wastes are purified using filters with activated charcoal and absolute filters, and liquid wastes (76.7 m 3/d) are demineralized, filtered and decanted. Sorbents (0.05 m 3/d) are stored in the dry state in tanks. This generates an annual amount of 30 m 3 of wastes which are incinerated and 5 m 3 of wastes which are not. The Tarapur power plant in India has an installed capacity of 380 MW(e). Gaseous wastes are discharged from a 11I-m stack at an average rate of 56 m3/s. The permissible annual limit is 670 PBq (18 x 106 Ci) of rare gases and 3 TBq (82 Ci) of halogens and aerosols, the actual amount of rare gas effluent discharged being 14 % of the permissible level and that of halogens and aerosols 8 %. Liquid wastes (247 m 3/d) are processed by filtration, chemical precipitation, evaporation and demineralization, generating annual amounts of 73 m 3 of concentrates and sludges and 10 m 3 of ion exchangers. Solid wastes are compacted and gaseous wastes are purified by absolute filters and filters with activated charcoal. The Fukushima-I power plant in Japan has an installed capacity of 480 MW(e). Gaseous wastes are discharged from a 120 m stack and have an annual activity of 180 TBq (4900 Ci). The activity of the wastes is reduced before discharge by absolute filters, filters with activated charcoal and partial decomposition. Liquid wastes consist of water that has leaked through the equipment (49 m 3/d), washout water (13 m 3/d), chemical water (19 m 3/d) and water from active washeries. Cooling waters are treated by filtration and demineralization and the other liquid wastes by a combination of filtration, evaporation and demineralization. These processes generate 100 m 3 of sludges, 45 m 3 of concentrates and 25 m 3 of ion exchangers annually. The concentrates are fixed in cement and solid wastes (0.7 m 3/d) are compacted. In the USA a 1000 MW(e) nuclear power plant will have annual activity of 150 TBq (4000 Ci). Gaseous wastes contain 11 TBq (300 Ci) of 8sKr, 96 TBq (2600 Ci) of 133Xe, 11 GBq (0.3 Ci) of 131X, 40 GBq (I Ci) of 133 1, 1.5 GBq (0.04 Ci) of radioactive aerosols, 1.5 TBq (40 Ci) of tritium, 350 GBq (9.5 Ci) of

42

14C, 930 GBq (25 Ci) of 41 Ar and 41 TBq (1100 Ci) of other rare gases. Gaseous wastes are purified using filters with activated charcoal and absolute filters and distilled at low temperatures. Liquid wastes (90 m 3jd) are processed by filtration, sorption on ion exchangers and evaporation. The concentrate (0.41 m 3jd) is solidified with cement and solid wastes (0.32 m3jd) are compacted. In the USSR nuclear power plants with BWR reactors are also in operation and the construction of reactors with an output of 1000 MW(e) is envisaged. The waste waters are sorted and collected separately according to their salt content and level of contamination with radionuclides. The characteristic composition of the wastes (5) is as follows: sodium (0.6-15) x 10- 3 kgjm 3 iron (0-1.5) x 10- 3 kg/rrr' corrosion products Cr, Ni, Co hardness(0-0.3) x 1O- 3kgjm 3 salt content (3-50) x 10- 3 kg/rrr' The total amount of wastes generated per day is 2760 m? of liquid wastes, of which 360 rn' has a salt content higher than 7 kgjm 3 (regeneration solutions, desorption solutions, laboratory wastes, etc.), and 2400 m 3 has a very low salt content, i.e., up to 10- 2 kgjm 3 (washout water, water from the spent fuel elements cooling tank and leaks from the reactor). The processing of these wastes generates an annual 1100 m 3 of concentrates from the evaporator with a salt content of 500 kg/rrr", 180 m 3 of sludges with perlite and 150 m' of ion-exchange resins. The 1000 MW(e) reactor discharges a daily effluent containing the following radionuclides: rare gases (Kr, Xe and Ar isotopes) 15- 19 TBq (400 - 500 Ci) 131 1 190 MBq (5 x 10- 3 Ci) aerosols with a half-life of less than 24 h 400 MBq (10- 2 Ci) 89Sr and 90Sr 1.9 MBq (5 x 10- 5 Ci) Table 5 and other data show that the BWR generates a greater amount of waste-saturated sorbents and an increased amount of concentrates from liquid waste processing, whereas PWR reactors generate larger amounts of solid wastes. There are no major differences between the other types of wastes. The third widespread type of nuclear power plant is that with the CANDU heavy-water reactor. The characteristic wastes from the CANDU reactor are given in Table 8. The Rajastan nuclear power plant in India, with an 220 MW(e) reactor, has an annual discharge of 2 PBq (53 000 Ci) from an 89-m stack. The annual permissible limit is 74 PBq (2 x 106 Ci) ofrare gases, 3.4 TBq (91 Ci) of halogens and aerosols and 900 PBq (24 x 106 Ci) of tritium in the form of water. The actual amounts of effluent discharged reach 2.5 % of the limit set for rare gases, and 0.01 % of the limit set for the other radionuclides. Heavy water serving as the moderator and coolant is returned to the reactor following treatment. Liquid wastes consist of

43

cooling water from the storage of spent fuel and various evaporation solutions. Concentrates are solidified with cement and solid wastes are compacted. The Atucha nuclear power plant in Argentina has a 320 MW(e) reactor that has been in operation since 1974. Gaseous wastes are discharged from a 40-m stack and the annual effluent limits are 410 TBq" (1.1 x 104 Ci) of 8sKr, 480 TBq (1.3 x 104 Ci) of 133Xe, 56 TBq (1.5 x 103 Ci) of 41 Ar, 36 GBq (0.96 Ci) of 131 I, 4 GBq (0.1 Ci) of aerosols and 240 TBq (6500 Ci) of tritium. The actual amount discharged is 10 %of the permissible limit for rare gases, I %for 131 1, 3.5 % tritium and 0.06 % for aerosols. Liquid wastes are checked for radioactivity and are then either discharged or are processed in evaporators and by ion exchangers. The used ion exchangers are stored in tanks, and solid wastes are compacted into drums with a volume of 0.1 m 3 (100 I). The largest heavy-water reactors are in operation in Canada, where the Pickering nuclear power plant has an installed capacity of 508 MW(e). The annual discharge from a 45-m stack is 930 TBq (2.5 x 104 Ci) of tritium, 160 TBq (4.4 x 10J Ci) of rare gases, 150 MBq (4 mCi) of 131 1 and 1.3 GBq (34 mCi) of radioactive aerosols. Liquid wastes from this power plant may be classified into three groups: - conditionally active wastes with a volume activity of less than 40 kBq (10- 6 Ci/m"): the non-active laundry, laboratories, showers, the reactor building; - active wastes with a volume activity of up to 400 MBq (10- 2 Ci/m J); the decontamination unit, laboratories, washout waters from areas in which radioactive materials are handled, the active laundry, safety showers; - chemical radioactive waters with a volume activity of up to 4 GBqjm J (0.1 Ci/m"): from the decontamination unit, laboratories, spent fuel storage area. Waste waters are checked for volume activity and are then either discharged with non-active waste water or are stored for a period of time needed for their volume activity to decay to a safe level, and are then discharged with the cooling water. Wastes generated by heavy-water treatment contain 6 m J of ion exchangers and 2 m" of filtration materials per annum. Solid wastes are sorted into combustible (0.54 mJ jd) and non-combustible (0.04 m J jd). They are compacted and incinerated in the central incinerating unit. Tables 5 and 8 show that, compared with PWR and BWR reactors, heavy-water reactors generate a considerably larger amount of tritium, which is discharged in gaseous and liquid wastes. Heavy water is used as moderator and coolant and its total amount is returned to the reactor. After treatment, the amount of liquid wastes and hence also the amount of radioactive concentrates are lower. The most recent type of nuclear power plants have gas-cooled reactors and are mainly in operation in Great Britain. The Tokai nuclear power plant in Japan has an installed capacity of 166 MW(e) and waste effluents are discharged from an 80-m stack. The total annual activity of the effluents is 230 TBq (6300 Ci) at an annual average volume flow-rate of

44

~

I:Jl

Solid wastes

Liquid wastes

Discharged liquid wastes

Gaseous emissions

Source of waste

64 x

incinerable non-burnable 10- 3

19 x 10- 3

27 12 x 10- 3

27

16

41

33 X 10- 3 2.7 X 10- 3

2.6 x 10- 8 _ - 3.9 x 10- 5 (C/kg/h)

10- 3

0.55 X 10- 3

X

5.5 x 10- 3

-

-

685

m 3jd

103

0.51

180

0.51

4.1 x 102

X

GBqjd

activity

2.9

Canada 500 MW(e) volume

5.5 X 10- 3

19 16 X 10- 3

0.10

9.1

2 x 102

GBqjd

activity

1.3 x 10- 5 _ -5.16 X 10- 4 (Cjkg/h)

2.58 X 10- 7 _ -12.9 x 10- 4 (C/kgjh)

2.5 x 10- 3

19

50---70 m 3js

5.4 x 103

3.7 m 3js 26

m 3jd

GBqjd

volume

activity

m 3jd

Argentina 320 MW(e)

volume

India 220 MW(e)

ion exchangers

concentrates filters

waste waters sludges

other radionuclides

tritium

Type of waste

Table 8: Characteristics of Wastes from Nuclear Power Plant with Heavy-water Reactors

80 m 3/s. Gaseous wastes mainly contain 41 Ar and are purified using filtration. Liquid wastes may be classified into two groups: chemical wastes (0.65 m 3/d) and waste waters from laundries and showers (15.4 m 3 jd). The processing of liquid wastes generates an annual amount of 0.2 m 3 of concentrates and I m 3 of ion exchangers. Solid wastes (0.01 m 3 jd) mainly contain paper, textiles and graphite residues. In Great Britain, modern nuclear power plants have an installed capacity of 820 MW(e), using two 410 MW(e) reactors. Gaseous wastes mainly contain 41 Ar generated by the activation of natural argon. Purification of the coolant gas is carried out with glass-fibre filters, ceramic filters and absolute filters. Ion exchangers (0.004 m 3/d) are stored in a lagoon and solid wastes (0.27 m 3 jd) are incinerated. Compared with other types of reactors, gas-cooled reactors have a high level of gaseous wastes, radionuc1ide 41 Ar being the major component. On the other hand, they have the smallest amount of concentrates from liquid waste processing. Solid wastes contain graphite and their content is higher than with a PWR of the same output. In the EEC the production of radioactive wastes from nuclear power plants for 1970 was estimated as follows: a) liquid wastes: 106 m" of tritium with an activity of 400 TBq and 70 TBq (2 x 103 Ci) of activation products, b) gaseous wastes: 70 PBq (2 x 106 Ci) of activation products with a short half-life, 400 TBq (104 Ci) of fission products of rare gases, 4 TBq (100 Ci) of halogens and 40 GBq (I Ci) of radioactive aerosols. Considerations of the long-term development of nuclear power projects envisage the use of fast reactors. In view of the non-existence of an industrial nuclear power plant with a fast reactor, considerations on wastes from such power plants will have to rely on experience with experimental fast reactors. The economic operation of a fast reactor nuclear power plant is closely linked with the reprocessing of the spent fuel from such a plant. Considering that this will be high-level waste and with regard to attempts made to process the fuel as soon as it has been removed from the reactor, projects have been designed for onsite reprocessing of spent fuel. Wastes generated by the reprocessing of spent fuel from fast reactors are described in Section 2.3. The operation of fast reactors generates radionuclides by fission or by neutron activation. If there is no fuel element leak, all activity of the primary circuit is determined by radionuclides generated by the neutron activation of sodium, the impurities and argon present therein. In the Russian BOR reactors the steady-state volume activities in the primary circuit and in the argon blanket reach the following values: 24Na (half-life 900 s) 1.70 PBq/m 3 (46 Ci/O 7 22Na (half-life 8 x 10 s) 150 GBq/m 3 (4 mCijl)

46

23Ne (half-life 38 s) 44 TBq/m 3 (1.2 Ci/l) 3 41 Ar (half-life 6.5 x 10 s) 11 GBq/m 3 (0.3 mCi/l) Other radionuclides may be formed from impurities present in sodium, the most frequently occurring being iron, nickel, chromium, and hydrogen. The only radionuclides important from the point of view of radioactive wastes are 54Mn and 58CO. A leak from the fuel elements must be considered even in the normal operation of fast reactors. This will cause the release of fission products, uranium and transuranium elements into the sodium circuit. It is assumed that in the BaR reactor a leak from one fuel element will cause the sudden release of 93 (GBq «2.5 Ci) of 85Kr, 330 GBq (9 Ci) of 87Kr, 700 GBq (19 Ci) of S8Kr, 110 GBq (3 Ci) of 131Xe, 9 TBq (240 Ci) of 133Xe, 3 TBq (90 Ci) of 135Xe, 560 GBq (IS Ci) of 131 1 and 63 GBq (1.7 Ci) of 137CS. These fission products mostly pass into the gas blanket. The activity of the released fission products will increase considerably when sodium penetrates into the fuel element and will radically increase when it melts. The other fission products remain in the primary circuit and the cold trap will trap most fission products together with the non-active impurities. The amount and nature of the radioactive substances trapped by the cold trap depend on the system of traps and on the frequency of fuel element leaks. Deactivation waters are another source of waste. Considerable amounts are generated by the decontamination of spent fuel elements using a deactivated solution, hot water or steam. The removal of spent fuel elements will contaminate handling equipment and the respective spaces, whose deactivation will generate intermediate- and low-level wastes. The amount and composition of these wastes cannot yet be estimated.

2.3 Wastes from Spent Fuel Reprocessing Spent fuel from nuclear power plants contains economically important amounts of unused nuclear fuel and newly generated fuel. For this reason, the reprocessing of spent fuel has been included in the fuel cycle, the aim of which is to isolate the components of unused fission products. The reprocessing consits in the mechanical cutting of fuel elements into smaller parts, which are then dissolved in acidic solutions and the individual basic components are separated. The isolation of uranium, plutonium and certain transuranium elements and fission products takes place. This method is known as aqueous or liquid extraction. The most important industrially used method is the Purex system, which is used by most reprocessing plants. Dry methods, such as the distillation of fluorides, are being developed and are often considered for spent fuel from fast reactors.

47

Reprocessing plants are the source of the greatest amount of a wide range of high-level radiotoxic materials. The reprocessing technology will then depend on the composition and form of the wastes. Table 9: Specific Activities of the Most Important Fission Products in High-Level Wastes Radionuclide

90Sr 90y 93Zr 95Zr 95Nb 99Tc 103Ru 106Ru 106Rh 125Sb 125Te 127mTe 1291 134CS 135CS 137CS 144Ce 144Pr 147Pm 151Sm 152Eu 154Eu 155Eu

Half-life (s)

8.8 2.3 4.7 5.6 3.0 6.6 3.5 3.2 3.0 8.5 5.0 8.6 5.4 6.3 9.5 9.5 2.4 1.0 8.2 2.8 3.9 5.0 5.7

x X x x x x x x x x x X x X X X x x x x x x x

Total specific activity of fission products

108 105 1013 106 106 1012 106 107 100 107 106 106 1013 107 1013 108 107 103 107 109 8

10 108 107

Activity in Bq per kg of reprocessed fuel for a period of 1 year 2.8 2.8 7.0 2.1 4.8 5.2 5.6 7.8 7.8 2.3 2.6 2.2 1.4 5.6 1.1 3.7 1.2 1.2 2.8 4.0 4.0 2.4 1.6

X 1012 X 1012 X 107 X 1011 X 1011 X 018 X 109 X 1012 X 1012 X 1011 X 1010 X 1010 X 106 X X X X X X X X X X

1012 107 1012 1013 1013 1012 lOtO 108 1011 1011

6.3 x 1013

I

I

10 years 2.2 X 1012 2.2 X 1012 7.0 X 107

I

100 years

I

1000 years

-

2.4 X 1011 2.4 X 1011 7.0 X 107

-

-

7.0 X 107

5.2 X 108

5.2 X 108

5.2 X 108

-

-

-

-

-

-

1.6 1.6 2.3 9.3

X X X X

1010 1010 1010 109

1.4 2.7 1.1 3.1 3.7 3.7 2.6 4.0 2.4 1.6 5.2

X X X X X X X X X X X

106 1011 107 1012 109 109 1011 1010 108 1011 109

-

1.1 X 1013

-

-

1.4 X 106 -

1.1 X 107 3.7 X 1011

-

-

-

-

-

-

1.4 X 106

-

1.1 X 107

-

-

1.9 X 1010 1.3 X 106 3.3 X 107

5.2 X 109

1.3 X 1012

7.8 X 108

-

-

-

The wastes from reprocessing plants contain fission products resulting from the fission of the nuclear fuel, transuranium elements resulting from nuclear reections and from the gradual decay of mother isotopes during the operation of the nuclear reactor, and radionuclides generated by nuclear reactions of structural and other materials with neutrons. In the proportion of the total activity, fission products are the most important component. The generated nuclides of fission products differ considerably in half-life, in the character and energy of released radiation and in their chemical nature. A list of the most important fission products

48

contained in high-level wastes from reprocessing I kg of spent fuel from fast reactors (from burning up 2850 kJ/kg (33 MWd/t) with a specific output of the reactor 50 kJ/s kg (50 MWjt» is given in Table 9. Table 10: Specific Activities of Uranium, Important Transuranium Elements and their Products in High-Level Wastes

Radionuclide

234U 23SU 236U 238U 237Np 239Np 236pU 238PU 239pU 240pU 241PU 141PU 24lAm 242Am 242mAm 243Cm 242Cm 243Cm 244Cm

Total specific activity

Half-life (s)

7.6 2.2 7.3 1.4 6.5 2.0 9.2 2.8 7.6 2.1 4.4 1.2 1.4 58 4.7 2.3 1.4 9.5 5.7

x 10 12 X io» X 1014 X 1017 x 10 13 x lOs X 10 7 X 10 9 X io» X 1011 X 108 X 10 13 x 10 10 x 10 4 X 10 9 x 1011 x 107 x 108 x 108

Activity in Bq per kg of reprocessed fuel per year J.5 3.2 5.2 5.9 1.3 6.7 4.8 3.3 5.9 9.6 1.8 2.6 5.6 1.5 1.5 6.8 1.5 2.0 8.9

X X X X X X X X X X X x X X X X X X X

lOS 103 104 104 107 108 104 109 107 107 1010 lOs 109 108 108 108 1011 108 1010

2.7 x 1011

Concerning long-term disposal, the nuclides of Cs and Sr, nuclides with a high yield fission reaction and a relatively long half-life and relatively high radiotoxicity, are especially important. Nuclides generated by reactions other than fission reactions mainly belong among the transuranium elements. The amount of which is considerably smaller than that of fission products. Owing to their long half-lives and the nature of their radioactivity (alpha emitters) they comprise a special category from the point of view of waste disposal. Here are mainly concerned with the nuclides a neptunium, plutonium, americium and curium, whose activity in the wastes decreases considerably with increasing atomic number of the element. Table 10 shows the characteristics of the most important transuranium elements in high-level wastes on the condition that in the reprocessing the losses of uranium

49

and plutonium will reach 0.5 %. Induced radionuclides include the nuclides of iron, cobalt, chromium, nickel, manganese, etc., i.e., nuclides generated by the activation of elements contained in stainless steels. Depending on the type of fuel element can, the nuclides also include those of zirconium, niobium, titanium aluminium and certain other elements which are part of the fission products (after Nb and Zr this group includes Mo, Tc, Ru, Sr, Y, etc.). During spent fuel reprocessing the groups of radioactive materials are involved in different technological processes and concentrate in different forms. Depending on the nature of the thus generated wastes, these fractions should be concentrated, isolated or mixed. Three types of wastes are always processed separately, viz., solid wastes, liquid wastes, gaseous wastes. Solid wastes from reprocessing plants include: a) insoluble remnants of fuel elements, b) solid materials used in the technology, c) structural parts of the equipment. Depending on the type of nuclear fuel and technology, solid wastes from the first stage of the production process may include parts of fuel element cans manufactured from various metall alloys or sintered materials and structural parts of elements that have been mechanically separated. In the process of solution an undissolved residue may remain, which will become part of the high-level solid wastes. In the process of separation, ion exchangers (organic or inorganic) or sorbents are used, which become depleted after a certain period of time and must be removed. This also applies to packings of filtration units (activated charcoal, metal filters silica-gels) which are used for the purification of discharged gases and for arresting aerosols. Parts of the operating equipment become radioactive wastes, i.e., when faulty parts are replaced, whole equipment is dismantled or as a result of an accident. With these parts deactivation is always considered a necessity and their resulting activity depends on the process. Liquid wastes create the greatest problems with regard to both processing and waste disposal. Plants reprocessing spent fuel using the process of liquid extraction generate approximately 0.5 -I m" of high-level liquid wastes per 1000 kg of spent fuel from light-water reactors. This amount includes 35 -40 kg of oxides of fission products and more than double this amount of oxides of non-active materials, mostly products of corrosion. Depending on the reprocessing technology, the composition of these wastes may differ considerably owing to the proportion of dissolved parts of the fuel element can or chemical additives used in the process. The composition of liquid high-level wastes is varied and depends on many factors. The most important include:

50

a) the type of fuel and reactor, fuel element burn-up, the duration of the storage of spent fuel in the interim storage area (cooling of spent fuel elements), b) the type of process including the used chemicals, structural materials, equipment, etc. The reactor type and its operation affect the ratio of fission products in the generated wastes. This ratio is also affected by the time that elapses between the removal of the spent fuel from the reactor and its processing. In general, it may be said that the chemical composition of the fission products is not affected by burn-up and cooling mostly changes only the ratio of radioactive to stable nuclides. The energy spectrum of the neutrons of the fission reaction has only a minor effect on the ratio of certain nuclides. This mainly applies to fast reactors, where fission takes place by neutrons with a higher mean energy and where the mixture of fission products is enriched with nuclides of medium fission yield, i.e., radionuclides of Ru, Rh, Pd, Ag, Cd, In, Sn and Sb. All of these deviations from the rule have only a slight effect on the chemical composition of the fission products. The composition of the waste products is much more affected by the reprocessing technology, The most widely used extraction method is the Purex technique with modifications to certain stages. The fuel element is treated with high-molar nitric acid, which converts uranium, transuranium elements, fission products and part of the structural materials into a solution. The substances contained in this solution are then separated in a series of extraction processes, purified and isolated for purposes ensuing from the logic of the fuel cycle. Radioactive substances with different specific activies may then be processed as wastes, either as a mixture or separately. Volatile fission products containing rare gases, iodine, part of the ruthenium and part of the fission products are released in the first stage of the process and are processed as gaseous wastes. Following the first extraction cycle, most of the other fission products and radionuclides generated in the reactor by nuclear reactions involving structural materials become concentrated high-level wastes. These wastes should be processed separately because their composition has a fundamental influence on the technology selected for processing the highleve I wastes. Substances forming this group of high-level wastes may be classified as follows: a) fission products, b) radionuclides from the structural components of the fuel element, c) non-active dissolved parts of the fuel element, d) chemical additives used in the reprocessing: - nitric acid, - salting-out agents (NaN0 3 , AI(N0 3 h , etc.), - oxidizing agents, reducing agents and catalysts, - substances with a highly efficient neutron cross-section (gadolinium), e) non-active corrosion products, f) products of radiolysis.

51

The proportions of fission product components are relatively stable whereas the specific activity and concentration of the other components is variable. Following suitable fixation, those waste products are important which form a considerable part of the total amount of generated wastes and thus significantly affect their nature. Significant concentrations are formed of residues of nitric acid, salting-out agents, dissolved non-active corrosion products and remnants of the structural materials of fuel elements. The concentration of nitric acid may be up to 6 - 7 M. For the interim storage of such wastes it is necessary to use high-grade chromium-nickel steel. It is also possible to neutralize the nitric acid, which will considerably increase the content of ballast (sodium) in the wastes designed for reprocessing. It will be mentioned in Section 4.1 on high-level waste processing that other methods of interim storage assume the decomposition of nitric acid prior to waste solidification. Salting-out agents, namely sodium nitrate and aluminium nitrate, may reach high concentrations, but on the other hand may be totally absent from some wastes. Gadolinium, which in view of the high concentration of rare earths in fission products does not affect the qualitative composition of the wastes, is used as a neutron scavenger to prevent the critical level of fission materials in radioactive wastes being exceeded. One of the common components of the wastes is dissolved parts of stainless-steel equipment used for the reprocessing of spent fuel elements. This consists mainly of iron, which in most wastes occurs in concentrations an order of magnitude higher than the concentrations of nickel and chromium. Radiolysis wastes consist of various compounds of phosphorus, i.e., acids in higher oxidation states, chlorides, etc. Various dissolved metals, such as stainless steel, zirconium, aluminium, magnesium and molybdenum, also pass into liquid wastes from the remnants of spent fuel elements. Considering other types of extraction methods of reprocessing, the Thorex technique will generate non-active impurities, such as fluorides, sulphates, phosphates, thorium and aluminium, the Redox technique will generate aluminium nitrate and the Darex technique will generate chlorides, etc. Several typical compositions of hig-level waste solutions are given in Table 11. Next to the high-level wastes from the first extraction cycle, other liquid wastes from spent fuel reprocessing also contain a large amount of radioactive wastes. A wide range of components of fission products pass into other technological flows and the wastes therefrom should therefore be considered as high-level wastes, even though the specific activities of the radioactive substances will be several orders of magnitude less than those mentioned abcve. Special attention should be devoted to wastes with a high content of transuranium elements, which should be processed separately because most of the nuclides are long-lived. The action of radiation on the solvent extraction medium used causes its radiolytic decomposition. With the commonly used tributylphosphate, a mixture of carboxylic acids, nitro compounds and organic nitrates is generated, together with

52

Table 11: Typical Composition of Solutions of High-Level Wastes (6) A-

Concentration (rnol/m") Na NH4 Mn Zn Hg AI Fe Cr Ni Mo Th U Pu HN0 3 N03" SO~-

PO:FBO:-

Non- Fission Products

ICPP Purex I I ICPP I I Thorex I Purex I Tarapur I Handford acidic stainless Darex I Marcoule 0.2

1.0

0.001 0.3 0.06 0.05

0.1 0.7 0.02 0.01

0.03 0.003 0.003 0.06 0.01

0.1 0.01

0.97 0.02

0.05

0.012 1.6 0.05

0.07 0.02 0.01

1.3 0.4 0.02

0.01 0.09 0.02 0.03

0.005 5 x 10- 4

0.003

8.4x 10- 4

0.017 8.4 X 10- 6 7.0 2.49

0.D7 0.05 0.03

0.5

1.1

H+/1.0 5.8 0.01

H+/3.2 2.6 0.6

H+/0.9 6.4

1.96 4.43 0.01

0.01 0.012

CI-

0.003

I B - Fission Products

Se Rb Sr y Zr Mo Tc Ru Rh Pd Tc Cs Ba La Ce Pr Nd Rare earth

0.2 2.0 6.0 1.3 20.0 20.0 4.0 7.6 1.0 1.0 1.7 13.0 5.0

3.5 5.5 5.0 10.0 10.0 10.0 4.8

22.0 45.0 44.0 9.0 16.0

5.9

25.0

4.3 5.0 10.0 34.0

2.3 1.3 4.8 4.7

3.6 1.72 1.6 3.7 1.7 3.8

81.0

53

a wide range of other types of organic compounds in lower concentrations. These compounds often have complex-forming properties and cause the retention of a number of metal cations in the organic phase. The thus degraded extraction medium has to be regenerated. This is done by vacuum distillation, which generates intermediate-level liquid wastes, sorption by organic sorbents, which generates solid wastes (AI20 3 , Si0 2 , Mn0 2 , Ti0 2 , etc.), or by chemical decomposition, e.g., by hydrolysis using mineral acids, pyrolysis, dealkylation, etc. AIl technologies that have so far been developed result in the generation of solid or liquid radioactive wastes, with an activity higher than that of wastes from the first extraction cycle. The situation is completely different with the so-called dry technologies of spent fuel reprocessing, where the fluoride volatility process deserves special attention. Following the fluorination of mechanically treated fuel and the distiIIation of volatile fluorides, there usually remains a residue containing the less volatile fluorides of fission products (Cs, Sr, rare earths, Y, etc.), in a relatively pure state or mixed with partially fluorinated aluminium oxide, which forms the bed of the most commonly installed fluidized bed reactor. Volatile fluorides of the other fission products are adsorbed on sorption columns with a packing of fluorides of alkali metals or alkaline earths metals, which in the process of chemisorption form complex fluorides or addition compound. The wastes from the process have an extremely high specific activity owing to the small amount of baIlast materials and the high burn-up of the fuel of fast reactors. This type of waste is solid, but with Table 12: Gaseous Radioactive Wastes from Spent Fuel Reprocessing Reactor Fuel burn-up (fJjkg) Capacity of reprocessing plant" (kgjd) Cooling period (d) Nuclide

PWR

I 2.6 4,800 150

1

1

3H

8SKr

130 x 103 1.3 x 106

270 X 103 2.7 X 107

7.5 73

550 5.4 x 103

Radiation dose·· exposure (pGyjday) (mradjyear)

2 77

9 328

520 1.9 x 104

2,350 8.6 X 104

Required decontamination factor···

20

50

2000

Discharged effluent (GBqjday) (Ci/year)

129

131

;

2000

• is related to the reprocessing of spent fuel from the fuel cycle of nuclear power plants with an output of 50 000 MW(e) .. with 8sKr, related to the skin exposure from external irradiation; with 3H, related to wholebody exposure from internal contamination by inhalation and ingestion of tritium; with 131 1 and 1291, related to the exposure of the thyroid gland owing to iodine ingestion.

54

respect to its origin and composition (containing most of the fission products) and to the technology employed for its disposal it is very similar to high-level liquid wastes from the first cycle of spent fuel reprocessing. Spent fuel reprocessing generates not only high-level wastes but also larger amounts of intermediate-level liquid wastes. These are mainly used decontamination solutions, rinsing solutions, residues from the generation of the organic phase, etc. These solutions are collected and processed using various solidification methods. Reprocessing spent fuel is by far the largest source of gaseous radioactive wastes in the fuel cycle of nuclear power plants, causing the quantitative release of all gaseous radionuclides that accumulate in the fuel element during fission. The gaseous radionuclides are released in the first stage of spent fuel reprocessing, i.e. during the removal of the can from the fuel rods and the destruction of the fuel itself by dissolution in acid or by the dry method, i.e. pyrometallurgical technologies. The principal radionuclides which may be present in the gaseous wastes of the reprocessing plant are the nuclides of iodine, xenon, krypton and tritium. With regard to the short half-life of 131 1 (8.05 days) and 133Xe (5.27 days), the amounts of these radionuclides in the gaseous wastes from the reprocessing plant depend on the time which elapses between the removal of the spent fuel from the reactor and its reprocessing. After 150-180 days from the removal of the fuel from the reactor, the activity

High-temperature

3H

8.6 1000 150 8'Kr

160 x 103 1.6 1.6 X 106 1.6

X X

106 107

Fast

I I

1291 4 40

131 1

4.3

3H

8'Kr

440 15 x 10 3 1.6 X 10 3 1.5 X 106 1.6

X X

106 107

2.5 95

5 193

1700 275 1.0 x 104 6.1 X 104

2.5 90

5.5 200

100

100

3000

20

100

for critical group of population for dispersion coefficient = 10- 7 s/m 3 ... calculated on assumption that D (30 mrad/year)

=

D

6.9 1400 90 1291 6 58

1.5

131 1

133Xe

120 X 103 900 1.2 x 106 8.9 X 104

410 550 x 103 7 X 104 2.0 X 10 5 x 10'

eH) + D (8'Kr) + 1/3 D ( 1291 + 1311)

0.1 3.8

-

0.8 p.Gy/d

55

of the gaseous wastes from the reprocessing plant is determined by the content of long-lived gaseous radionuclides in the wastes, i.e., 85Kr, 129 1 and tritium. The variation of the activity of the individual gaseous radionuclides which are released from reprocessing spent fuel from the fuel cycle of a nuclear power plant with an output of 50000 MW(e) using light-water, fast or high-temperature reactors with the degree of the fuel burn-up, the cooling period and the capacity of the reprocessing plant is shown in Table 12.

Fig. 3. Dependence of 1311 decontamination factor on cooling time of spent fuel

The technologies used in reprocessing plants assume a cooling period of ISO to 180 days. Under such circumstances the activity of 133Xe in the spent fuel and thereby also in the gaseous wastes is negligible and allows their discharge into the atmosphere. The amount of 131 1 which passes into the gaseous wastes during the reprocessing of thus cooled fuel is not so high as to become a limiting factor for the discharge of these gaseous wastes into the atmosphere, provided that a suitable technology is used for removing 131 1 from the wastes. The situation is completely different when the spent fuel cooling period is shortened to less than 100 days, i.e., as is required by the fuel cycle of fast reactors, Table 12 shows that in such circumstances both 133Xeand especially 131 1 contribute significantly to the total activity of the gaseous radioactive wastes and thereby also to the potential contamination of the environment of the reprocessing plant. The values of the decontamination factor for gaseous wastes for reducing the dose commitments to 0.82 Gy(d (30 mrad/year) will reach values for 131 1 with which currently used technologies are unable to cope. This situation is illustrated in

56

Fi g. 3, which represents the dependence of the decontamination factor for 131 1 on the time needed for cooling spent fuel from a light-water reactor with a specific output of 40 kJjkg s (40 MWjt) at an average burn-up of3.5 TJjkg (40000 MWdjt). The required effectiveness of the decontamination process is given for reprocessing plants with capacities of 300 and 1,500 t of fuel per annum given the highest permissible discharge of 131 1 effluents into the atmosphere at 628 kBqjs (1.67 Cijs) or 5 GBqjd (50 Cijyear). Of the gasous radionuclides, 85Kr, 129 1 and tritium are very special. The cooling period of the spent fuel is so short compared with their half-life that it does not affect their accumulated activity in the fuel elements, i.e., their activity, which may potentiaJly escape into the atmosphere during reprocessing. Owing to the vertical and horizontal circulation of air currents in the atmosphere, these radionuclides represent a continuous source of atmospheric contamination not only in the area of the reprocessing plant but practically on a global scale. I

I I

--151.& I

10

I -]222 I

I I

~I ~

I --1/f.81 I 1,

t;::.

I

1~0.5/f

""~ i

I I~ I I I

10

I

i

1970

1980

1990

2 '00

2/f

353

1660

't500

--- year -OWe

Fig. 4a. Growth of total world production of • 5Kr

On the basis of the technological and economic interdependence of nuclear power plants and reprocessing plants, and on the basis of demands on power sources and the ensuing indispensable growth of nuclear power, production estimates may be made of the amounts of 8sKr, 129 1 and tritium generated and potentially released from the fuel cycle of installed nuclear power plants. Many analyses have been made in this situation (8). The annual and total amounts of 8sKr, 3H, 1291

57

generated comply with the envisaged growth of the installed capacity of nuclear power plants (Fig. 4). The bottom curve in Fig. 4 shows the annual production of these radionuclides and the upper curve their total amount discharged into the atmosphere at normal pressure and temperature if all of the 85Kr, 129 1 and tritium generated during spent fuel reprocessing were to be discharged into the atmosphere. All such forecasts are only approximate, yet they do provide basic data which should be used for evaluating the potential hazard caused by these long-lived radio nuclides on both a national and an international scale. This is so even though the model used does not adequately consider the fact that approximately 75 % of the discharged effluents will be released in the northern hemisphere and that the circulation of the air currents in the atmosphere does not guarantee the even distribution of these effluents in the atmosphere on a global scale. With regard to the considerably varied physical properties of krypton, iodine and tritiated water vapour the concentration of iodine 129 vapour and tritium vapour in the atmosphere will be considerably less than the values given in Fig. 4. owing to their washout by precipitation or deposition in solid or liquid aerosols. The fact that plants for reprocessing fuel with a short cooling period use equipment for removing 131 1 from gaseous wastes and that 80-90 % of the tritium from the reprocessing of spent fuel passes into liquid wastes makes 85Kr the dominant component of gaseous wastes from the reprocessing of spent fuel and long-lived fission products discharged into the atmosphere. The' fact that radiation exposure of the body organs of the world's inhabitants to 85Kr will not in the near future reach the maximal permissible dose does not justify its discharge into the atmosphere. It also appears that the contamination with 85Kr of krypton and other rare gases separated from the atmosphere may become a serious limiting factor for their practical application. It is therefore desirable that appropriate separation methods be applied in all spent fuel reprocessing plants.

2.4 Wastes from Research Centres and from the Production and Use of Radionuclides Radioactive wastes from research and development work, the production of radioisotopes and their use in the national economy are generated in small amounts and are characteristic of the wide range of materials and radionuclides present in these materials. They mostly originate from contact between various materials and radionuclides and their nature and amount are highly dependent on the operations conducted. Radioactive wastes of this type consist of solid materials, such as filter-paper, textiles, plastics, laboratory glass and injection syringes, carcasses of experimental animals and litter from biological experiments. Liquid wastes mostly

58

--year Fig. 4b. Growth of total world production of 3H

Fig.4c. Growth of total world production of

129

1

59

include materials used for washing and cleaning solid materials, remnants of radioactive solutions and animal excreta. Gaseous wastes may contain aerosols and gases generated from the evaporation of radioactive solutions or the incineration of solid materials. According to the methods used for the sorting, processing and disposal of these materials, and their properties, types and radionuclide content, the wastes may be classified into the following groups: 1. the content of radionuclides, into high-level, low-level and with a conditional activity level, 2. the type of radionuclides, into long-lived and short-lived, 3. the type ofradionuclides, into radionuclides with a high level ofradiotoxicity, a medium level of radiotoxicity and a low level of radiotoxicity, 4. chemical properties, into acidic, neutral and alkaline aqueous solutions organic liquids, aggressive substances and non-aggressive substances, 5. their physical properties, into solid, liquid and gaseous, compactible and non-compactible, combustible and non-combustible, 6. according to safety level, into sharp, brittle, explosive and flammable, 7. according to disposal method, into wastes disposable on site, on heap or dump, by pipeline and by central off-site burial. Regarding the stages of development of the use of radionuclides, most countries undergo two stages, which differ considerably in the amount and character of the radioactive wastes involved. In the first stage only small amounts of relatively short-lived radionuclides are used, in health care, agriculture, industry and applied research. The amount of wastes generated is small and their disposal does not pose any difficulties. In the second stage of development, nuclear research institutes are established with research reactors, the production of radionuclides and other similar activities which require the appropriate processing and disposal of wastes. Such installations may be used to advantage for processing wastes from small workplaces. There are two widely differing methods of applying radionuclides, i.e., the open emitter and safely sealed emitters. The wastes from these two types of emitters differ considerably. In industry, various types of instruments are used with built-in emitters for measuring the thickness, density and level, the removal of electrostatic charges, the sterilization of foodstuffs and products, etc. The use of these instruments with built-in emitters usually does not generate any radioactive wastes. The emitters do, however, require special handling. If the service life of the equipment is shorter than that of the emitter, the producer will have to remove the emitter and will re-use it. In other cases, the service life of the equipment is much longer than that of the emitter. If the decay of the radionuclides causes the radiation abundance of the emitter to decrease below the permissible limit, the emitter will have to be replaced. The old emitter will be disposed of. Only a leak in the emitter due to corrosion,

60

incorrect handling or damage may cause contamination of the equipment and of the workplace environment. The contamination is, however, restricted to a small area and decontamination will generate only a small amount of waste. Similar considerations apply to fire detectors containing 226Ra or 241 Am. In contrast to the above equipment they are used on a much wider scale and may not only be affected by failure, leakage, etc., but may also be damaged or destroyed in a fire. It is usually possible to detect and remove the emitter using simple instruments; when this is not possible, the presence of the emitter must be taken into consideration in the removal of the whole equipment. Another group of radioactive objects are various luminescent signs or advertisements on buildings. In the past radium activation was used for these purposes, but nowaday activation by tritium and 147Pm is used exclusively. No special precautions need be taken in the demolition of buildings and the remnants of the colours from the signs may be removed and disposed of with the building material. Sealed emitters which are not connected to the equipment are a much greater hazard. These consist of emitters used in hospitals, for gamma radiography in field tests and neutron sources used for geological surveying. Originally radium emitters were used in medicine, but became a hazard owing to leakages and radon escape. This problem was solved with the introduction of artificial radionuclides for these purpose. The use of sealed emitters does not generate any wastes under normal operating conditions. Only if there is a considerable decrease in the activity of the emitter due to decay will it be necessary to remove and dispose of the emitter as radioactive waste material. These solid wastes are characterized by their small volume and considerable activity. Only very rarely do these sealed emitters cause environmental contamination which results from the leakage of the emitter or its surface contamination during manufacture. A relatively large amount of radionuclides in the form of sealed emitters is used in the radionuclide sources of electric power. Terrestrial sources contain 90Sr in ceramic form and plutonium is often used in space research. The technique for the removal of the radioactive fuel from the source following shut-down is usually determined before the assembly of the source. This radioactive material is usually re-used. The application of radionuclides in open emitters usually generates radioactive wastes. The removal and disposal of these wastes is usually concurrent with the application and the amount of waste is therefore minimal and disposal is easy. Radionuclides are sometimes used directly in the environment itself, e.g., in monitoring the movement of ground and surface waters using tritium, monitoring the movement of sand and mud using glass particles labelled with 46SC or 198Au and monitoring the distribution of phosphorus in fertilizers. These applications do not generate radioactive wastes, but they do require careful environmental control. The remaining radionuclides should either be allowed to decay in one place, e.g., radionuclides used for monitoring the environmental hazards of fertilizer applica-

61

tion, or should quickly be dispersed to such an extent as to prevent their becoming a hazard to man and the environment. Another frequent application is the monitoring of the movement of materials in various industrial processes. The most frequent applications in industry are monitoring the process and efficiency of mixing materials, the duration of material retention in equipment, the effectiveness of filtration, the distribution of salts in crystallization, etc., i.e. applications which serve to optimize the technological process. Negligible amounts of radio nuclides suffice for large amounts of tested materials, which means that the radionuclides will be sufficiently diluted in the equipment and no radioactive wastes will be generated. Radionuclides are also used for materials testing, i.e., to check craks in materials and to study the movement of the medium in pipes. In gas pipelines the most frequently used radionuclides are 85Kr and the liquid phase of 24Na. The latter is short-lived so that not even the use of a relatively high activity will generate radioactive wastes; the low radiotoxicity of 85 Kr allows the discharge of large amounts into the environment. Only very low activities will be used for research on and the development of new industrial applications. In most instances the behaviour of one element or component is monitored and for this purpose a large number of diverse radionuclides are sometimes used. In biological research mainly tritium and substances labelled with 14C are used; the radionuclides characteristic of other research fields are difficult to determine. Higher activities are used in research involving irradiated samples, such as activation analysis or the investigation of materials fatigue. The irradiated material should be disposed of as solid radioactive waste because it contains induced activity, i.e., 60Co and 55Fe. The use of open emitters is of long standing in medicine, 131 1, 198Au, 32p and 90y are used for therapy and a wide range of small amounts of radionuclides are used for diagnostic purposes. The radioactive wastes generated (liquid and solid) are mostly short-lived and decay during short-term storage, Liquid wastes are then checked for radioactivity and discharged; solid wastes are either incinerated or disposed of using other appropriate technologies. Luminescent colours are a very widespread form of the use of radionuclides, and are used on the dials of various equipment. The initially used 226Ra has been replaced with 90Sr. Equipment and instruments with such dials show a considerable intensity of radiation above the surface and are therefore disposed of as solid radioactive waste. Recently, tritium and promethium have been used to excite luminescence in these colours. These elements are characterized by low-intensity radiation and a much lower radiotoxicity. The problems of radioactive wastes from workplaces involved in the development of reactors and of technological processes of the fuel cycle and workplaces producing radionuclides are completely different.

62

Radionuclides are produced either at independent workplaces or within larger radiochemical research complexes. Currently more than 100 radionuclides are produced and the amount increases by 20 % annually. The most important radionuclides produced are tritium, 32p, 60Co, 90Sr, 131 I, 137CS, 1921r, 198Au and isotopes of transuranium elements. The production of radionuclides generates solid, liquid and gaseous radioactive wastes contaminated with almost all radionuclides produced. The amount and nature of the wastes depend on the amount of the radionuclide produced, its physical and chemical properties and the production technology. It is therefore very difficult, if not impossible, to characterize the amount and nature of the wastes. The separate collection and disposal of concentrated wastes containing the radio nuclides will be highly significant for waste processing and disposal. Gaseous wastes from radio nuclide production mainly contain tritium, radioactive iodine, activation and fission products and certain alpha emitters. The development of nuclear power production and its fuel cycle, namely the production of fuel and spent fuel reprocessing, is under way in various nuclear research institutions. These institutions tackle such problems as improved design and operation of existing reactors, the development of new and better types of nuclear power plants, the development of nuclear fuels and structural materials, spent fuel reprocessing and radioactive waste processing. In most instances nonradioactive materials are used, but in some instances radionuclides, often highlevel, have to be used to test the methods developed. This will generate a considerable amount of various radioactive wastes containing a wide range of radionuclides in various concentrations. Most of these are low-activity liquid and solid wastes. The volume and activity of wastes from research institutions vary with the scope and objective of the research work; the method of waste collection and sorting has a decisive importance for low-level wastes. In some instances cooling and waste waters from non-active workplaces are not separated at all or are only partly separated, which in turn generates large volumes of low-level wastes. The largest amounts of radioactive wastes are from research institutions with a high proportion of radiochemical laboratories, whereas theoretical research institutes obviously generate smaller amounts of radioactive wastes. The operation of many research institutes makes it possible to specify the amount of radioactive wastes generated. Those workplaces in which cooling water and waters from non-active workplaces are separated generate annually 10 - 100 m ' of low-level liquid wastes per worker handling radioactive substances, which corresponds to 2 - 30 m' annually per employee of the research institution. Given adequate waste sorting, only 5 - 10 % of these wastes will require processing. At workplaces in which cooling waters and water from non-active workplaces are not separated, the volume of low-level wastes will reach 200-300 rrr' per worker handling radioactive substances, corresponding to 50-200 m' of wastes per employee of research institution.

63

As for low-level solid wastes, the annual amount is 0.30-0.8 m' per worker handling radioactive substances. Of these wastes, 40 - 60 %are various combustible materials, such as paper, wood, textile, rubber, plastics and biological materials, and the remainder are non-combustible materials, of which 30 - 60 % may be reduced in volume by crushing and compacting. In addition to these wastes from routine operation, larger amounts of solid wastes of a different nature are sometimes also generated by the reconstruction of active areas, replacement of equipment, dismantling of laboratory and pilot plant equipment, etc. These solid wastes usually contain contaminated building and structural materials and large equipment, such as fume chambers, dust-tight cupboards, containers, and laboratory equipment, whose deactivation and volume reduction are difficult. Gaseous wastes from research centres mostly contain 41 Ar, other rare gases, tritium and other activation and fission products. The amount and composition of intermediate level-wastes depend entirely on the nature of the research work conducted at the particular institution. The main sources of such wastes include metallurgical research on the properties of irradiated fuel from which the major part of the radioactive material remains in the solid state, the development of spent fuel reprocessing and the separation of rare products which generates liquid wastes. In the EEC the production of radioactive wastes in for 1970 has been estimated as follows: (9) a) liquid wastes; - production of radionuclides: total volume of liquid wastes 1000 m", total activity 40 TBq (1000 Ci) of tritium, 4 TBq (100 Ci) of other radionuclides, - use of radionuclides in medicine: total volume of liquid wastes 100 m' total activity, 40 TBq (1000 Ci) of short-lived beta emitters, - other uses of radionuclides: total activity 4 TBq (100 Ci) of tritium, 3 TBq (100 Ci) of other beta emitters, - nuclear research centres: total volume of liquid wastes 106 m", total activity 400 TBq (104 Ci) of tritium, 400 TBq (104 Ci) of other beta emitters, 40 GBq (I Ci) of alpha emitters, b) gaseous wastes; - production of radio nuclides 70 TBq (2 x 103 Ci) of tritium, 190 GBq (5 Ci) of activation and fission products, 10 GBq (I Ci) of iodine isotopes, 10 GBq (I Ci) of alpha emitters, - research reactors: 4 PBq (10 5 Ci) of 41 Ar, 4 PBq (10 5 Ci) of other rare gases, 40 TBq (10 3 Ci) of tritium, 4 TBq (100 Ci) of other activation and fission products, - research nuclear centres: 40 TBq (104 Ci) of tritium, 400 GBq (10 Ci) of rare gases, 400 GBq (10 Ci) of other activation and fission products.

64