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Immobilisation of Radioactive Waste in Bitumen
CHAPTER 18
Immobilisation of Radioactive Waste in Bitumen Contents 18.1 Bituminisation 18.2 Composition and Properties of Bitumen 18.3 Bituminous Materials for Waste Immobilisation 18.4 Waste Loading 18.5 Bituminisation Technique 18.6 Acceptance Criteria 18.7 Bitumen Versus Cement 18.8 Immobilisation of Radioactive Waste in Polymers References
305 306 307 309 310 313 314 317 318
18.1 BITUMINISATION Embedding radioactive waste in bitumen has been used in immobilisation since the 1960s and the total volume of radioactive waste immobilised in bitumen currently exceeds 200,000 m3. In the bituminisation process, radioactive wastes are embedded in molten bitumen and encapsulated when the bitumen cools. Bituminisation combines heated bitumen and a concentrate of the waste material in either a heated thin film evaporator or extruder containing screws that mix the bitumen and waste. The waste is usually in the form of a slurry, for example, salt aqueous concentrates or wet ion exchange (IEX) resins. Water is evaporated from the mixture to about 0.5% moisture, intermixed with bitumen so that the final product is a homogeneous mixture of solids and bitumen, termed bitumen compound. Its radionuclide retention properties usually exceed those of cements at higher waste loadings (Fig. 14.7). Bituminisation is particularly suitable for water-soluble radioactive wastes such as bottom residues from evaporation treatment and spent organic ion exchangers (IAEA, 1970, 1993).
An Introduction to Nuclear Waste Immobilisation DOI: https://doi.org/10.1016/B978-0-08-102702-8.00018-2
© 2019 Elsevier Ltd. All rights reserved.
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18.2 COMPOSITION AND PROPERTIES OF BITUMEN Bitumen is a generic term used to cover a wide range of high molecular weight hydrocarbons. Bituminous materials have been widely used in the building industry for many years. As early as 3800 BC they were used in construction because of their adhesive and waterproofing properties. Obtained from naturally occurring deposits, these bitumens were used by the rulers of the Assyrian, Sumerian and Chaldean empires to waterproof their palace walls. Core samples from the natural Oklo Reactor (see Section 4.4) contain inclusions incorporating bitumen, which probably acted as a reducing buffer and/or hydrophobic water shield suppressing oxidative dissolution of the uraninite cores. Three main types of hydrocarbons occur in bituminous materials: • • •
Asphaltenes Resins Oils (aliphatic hydrocarbons)
The bitumen properties depend on the ratio of these components. Heavyweight fractions such as asphaltenes impart viscoelastic properties to bitumen at ambient temperatures (10°C 40°C). Lightweight fractions such as oils act as a carrier for the asphaltenes and resins. The viscous properties of bitumen are complex and affected by changes in its colloidal nature that occur with heating. However, when the temperature is high enough for the bitumen to be liquid it behaves as a Newtonian fluid and the rate of shear is directly proportional to the shearing stress. The most important characteristics of bituminous materials are: • • •
Penetration Softening point Flash point
The penetration characterises their hardness and is measured from the depth of penetration that a weighted needle achieves after a known time at a known temperature. The most common penetration test is carried out with a weight of 100 g applied for 5 seconds at 25°C. Typical values obtained are approximately (mm) 10 for hard coating grade asphalts, 15 40 for roofing asphalts, and up to 100 or more for certain waterproofing materials.
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The softening point characterises the temperature at which a steel ball falls a known distance through the bitumen when the test assembly is heated at a known rate. The usual test is carried out with a 19.05 mm (0.75 in.) diameter steel ball weighing 35 g sinking 25.4 mm (1 in.) through a 15.875 mm (0.625 in.) diameter 6.35 mm (0.25 in.) thick disc of bitumen held in a brass ring, with the whole assembly heated at 5°C/ min. The softening point value is used to grade bituminous materials into groups. Typical values are up to 132°C for coating grade asphalts, from 60°C to 105°C for roofing asphalts, down to approximately 45°C for waterproofing bituminous materials. The flash point characterises the temperature at which a bituminous material ignites in an open-air crucible. Flash points and flammability temperatures of bitumen are higher than 200°C 350°C, depending on the type of bitumen. Harder bituminous materials have higher flash points. The normal solubility of water in bitumen is on the order of 0.001 0.01 wt.% and in practice is considered negligible. The presence of water soluble salts in the bitumen results in a larger capacity for water absorption by osmosis.
18.3 BITUMINOUS MATERIALS FOR WASTE IMMOBILISATION Several bitumen varieties are commercially available for immobilisation of radioactive waste including: • • • •
Direct distilled Oxidised Cracked Emulsions
Direct distilled bitumen is a residue from petroleum distillation, while oxidised bitumen is created by blowing air through petroleum residues, which oxidises light fractions of the residues. Cracked bitumen is generated from thermal breakdown of heavy oil fractions in the oil refining industry and emulsions are produced by direct injection and emulsification of the bitumen in water. Bituminisation techniques used in radioactive waste immobilisation utilise different bituminous materials as shown in Table 18.1.
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Table 18.1 Applications of typical bitumen varieties for waste immobilisation Bitumen type
Application
Direct distilled Oxidised Cracked Emulsion
Batch melter, thin film evaporator Batch melter, extruder, thin-film evaporator Extruder Thin-film evaporator
Table 18.2 Parameters of bitumen for radioactive waste immobilisation Type of bitumen
BN-II
BN-III
BN-IV
Fractions (wt.%)
55.5 17.5 27.0 81 120 40 200
54.4 15.0 30.5 41 80 45 200
50.0 11.0 39.0 21 40 70 230
Oils (aliphatic hydrocarbons) Resins Asphaltenes Penetration at 25°C (mm) Softening temperature (°C) Flash point (open crucible) (°C)
Advantages of bituminous materials as matrices for waste immobilisation are: • • • • • • •
Water insolubility Low diffusion of water Chemical inertness Plasticity and good rheological properties Slow ageing rates High incorporation capacity enabling high waste loadings Ready availability and low cost
However, bitumen is an organic material and therefore it has a number of inherent disadvantages such as: • • •
Combustibility, although not easily flammable Lower stability against radiation than cement Ability to react with oxidising materials such as sodium nitrate
In France bituminous wasteforms are cast at 120°C 130°C with a soft and fluid bitumen, Viatotal 70/100 has a penetration depth between 7 and 10 mm and a softening point of 45°C 51°C. Table 18.2 demonstrates typical parameters of several bituminous materials used to immobilise low and intermediate level waste in the Russian Federation. The radiation stability of bituminous materials depends on their type, although swelling .3% by volume occurs at relatively high absorbed
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doses above 0.5 2 MGy (IAEA, 1993). Irradiation produces radiolysis gases with hydrogen as the main radiolysis product. The radiation chemical yield is characterised by the G-value, which is the number of a species produced per 100 eV of energy loss. The GH2 values for radiolytic hydrogen production of gamma-irradiated bituminous materials are from 0.2 to 0.4 (compared with GOH 5 2.8 of water exposed to gamma radiation).
18.4 WASTE LOADING On bituminisation the water is evaporated and the remaining salt residue with some remnant moisture is immobilised as small particles (B100 200 μm) distributed uniformly within the bitumen matrix. On increase of waste loading above certain threshold loadings clusters may form of several salt particles which are close or in contact (Fig. 18.1; Gwinner et al., 2006; Sercombe, Gwinner, Tiffreau, Simondi-Teisseire, & Adenot, 2006). Bitumen compounds can be loaded with B50 wt.% (B25% by volume) of salt residues from nitrate-type aqueous radioactive wastes (Sobolev & Khomchik, 1983). This value is considered as a threshold level since above it, leaching of radionuclides increases stepwise (Fig. 18.2A) as a result of formation of percolating-type clusters made of contacting salt particles in the host bitumen matrix (Fig. 18.2B and C). Threshold waste loading for ion exchangers is also B50%. The waste loading in practical uses of bitumen as a wasteform is kept below the threshold level, which depends on both waste and bitumen type and is typically determined experimentally.
Figure 18.1 Scanning electron microscopy images of bitumen wasteforms containing BaSO4 NaNO3 CoS PPFeNi Na2SO4 waste sludge. Waste loadings (A) 38 wt.%, (B) 42 wt.%. PPFeNi is a nickel and potassium preformed precipitate. Courtesy Elsevier.
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Figure 18.2 The inventory fraction of leached 137Cs (X) from bituminised radioactive waste with waste loading volume fraction (θ) demonstrating a change in release mechanisms from a diffusion-controlled before to a dissolution-controlled above the threshold loading B25 vol.% (A), and a high contrast micrograph (binary image) of a percolating cluster made of (black) salt particles (bar size 100 μm) (B) and with the connectivity graph of the percolation cluster (C). Courtesy Natalie Ojovan, SIA RADON, Russia.
It was found that essentially all radioactivity of the bituminised wasteform is associated with the asphaltene fraction of the bitumen and that aging leads to an increase in asphaltene fraction content and hardening of the bituminous host material (Ojovan, Ojovan, Golubeva, Startceva, & Barinov, 2002).
18.5 BITUMINISATION TECHNIQUE Immobilisation of radioactive wastes via bituminisation can be done as a batch or continuous process. Batch processes usually involve drying the waste followed by mixing the dried material in molten bitumen. In this process waste is continuously introduced into a metered volume of molten bitumen at about 200°C 230°C (Fig. 18.3). The mixing vessel is externally heated to evaporate water and the solid residue particles are then mixed with the bitumen. When the required composition of the wasteform is reached, no more waste is added. The mix is heated and stirred for some time to evaporate the residual water and is then discharged into drums or other containers to cool and solidify. Batch processes are not widely used and continuously operating bituminisation processes are more common, generally based on either extrusion or film evaporation systems. With extrusion systems the concentrates are fed into a multiple screw extruder together with bitumen at 130°C 200°C (Fig. 18.4A and B). During passage through the heated extruder the waste and bitumen are intensively mixed with simultaneous evaporation of water.
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Stack Rotating shutter
Absolute filters
Splash head
Sludge feed Motor
Contact condenser
Cooling installation Electrostatic filter
Stirrer 10-t bitumen tank
Heating Induction heaters
OR steam Heater
Drainage Oil filter
To water processing
Figure 18.3 Batch bituminisation process at Mol, Belgium. Courtesy IAEA.
(B) Concentrates
(A) Feed throat
Pump
Bitumen
Pump
Volatiles
Screw
Compound Heater band
Condensers
Extrusion screws
Figure 18.4 Schematics of (A) an extrusion apparatus for immobilisation of radioactive waste in bitumen (e.g. bituminisation), and (B) of coating extrusion machine at Marcoule, France. Courtesy IAEA.
The temperature of the bitumen waste matrix is kept reasonably low (130°C 140°C) although at lower temperatures the viscosity would be too high to maintain the extrusion process. The final product is uniform, with mineral residue particles from waste coated with a layer of bitumen. However, some waste constituents partially dissolve in the bitumen. The bituminised product is discharged into drums and allowed
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to cool and solidify. Typical extrusion bituminisation processes used in France and in Japan are shown in Fig. 18.5A and B. In film evaporator systems liquid wastes are initially concentrated to about 60 wt.% dry matter. Preheated waste sludge and bitumen are fed separately to the top of the thin-film rotary evaporator where the two streams mix and flow down the heated wall. The water in the mixture is progressively evaporated and the remnant mineral residue from the waste is mixed with bitumen (Fig. 18.6). Rotary thin-film evaporators use wiping blades to assist mixing of waste mineral residues and bitumen. The bituminised waste material is drained near the bottom into steel containers. The vapours generated are condensed, first in the built-in condenser in the evaporator and subsequently in an external condenser. Thin-film evaporator bituminisation is used to immobilise both aqueous radioactive waste and spent organic ion exchangers (Fig. 18.7). The first bituminisation plant in Russia was put into operation at Moscow SIA ‘Radon’ in 1977 (Sobolev & Khomchik, 1983; Zakharova & Masanov, 2000). It uses an industrial steam-heated rotary evaporator as mixing unit operating at 135°C while the waste loading is up to 60 wt.%. The bitumen compound is poured into carbon steel containers. Parameters of the bituminous materials used at this plant are listed in Table 18.2, and 18.3 summarises the application of bituminisation processes worldwide as reported up to 1999 (Vanbrabant & Selucky, 1999). Although immobilisation of radioactive waste continues to be used there are few reported data on total amounts of immobilised waste to require this table to be updated. (A)
(B)
Waste
Chemicals
Sludge water Content 90%
Filter
Reaction vessel
Feeding vessel
Surface-active agents Sludge Bitumen 22% water Content 50% Preliminary coating
Water salted out after preliminary coating
Filtered water
Bitumenous composition containing 80% water
Bitumen Condenser
Drying
Extruder Drum
Condensed water
Drainage into drums
Figure 18.5 Schematic of extruder type bituminisation processes: (A) at Tokai Works in Japan, and (B) at Marcoule, France.
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Geared engine
Upper bearing of the rotor
Exit of water vapour to condenser
Mist separator Bitumen
Concentrates
Distribution rings
Exit of heating medium
Heating medium
Film layer of the product Body of the evaporator Rotor
Lower bearing of the rotor
Heating medium
Discharge of the product
Figure 18.6 Schematic of thin-film evaporator type bituminisation processes. Courtesy IAEA.
18.6 ACCEPTANCE CRITERIA Waste acceptance criteria for bitumen compounds used to immobilise radioactive waste are set into the state standard of the Russian Federation GOST 50927-96. The most important parameters are illustrated in Table 18.4. It is assumed that these requirements are met both for freshly prepared bitumen compounds and for compounds in storage and disposal.
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(A) Steam Water
Rotary evaporator
Waste
Drum
(B)
Bitumen tank
Additives
Ion exchanges Powder Beads Concentrates
Grinder
30 m3 Feed tank Condenser Cooler
Feed tank
Steam
Oil control
Oil filter
Thin silm evaporator Slurry Heating oil
To low level waste
Bitumen compound
Figure 18.7 Schematic of thin-film evaporator bituminisation processes. (A) Liquid aqueous waste bituminisation process at Moscow SIA ‘Radon’, Russia; (B) spent ion exchange resin bituminisation plant at Barsebäck NPP, Sweden. NPP, Nuclear power plant.
18.7 BITUMEN VERSUS CEMENT Immobilisation in suitable matrices significantly reduces the potential for release of radionuclides into the environment. Average hazard diminishing factors Kwf for bituminised aqueous radioactive wastes are of the order of several hundreds and those of cemented aqueous radioactive wastes are
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Table 18.3 Immobilisation of radioactive waste in bitumen Bituminisation plant (country)
Operation period reported
Marcoule and La 1966 2017 Hague, CEA (France) STE 3, La Hague, COGEMA (France) ‘Mummy’, Belgoprocess (Belgium) Dukovany NPP (Czech Republic) Temelin NPP Risφ RNL (Denmark) JNC, Tokai Works (Japan) PS-44, NPP Jaslovske Bohounice (Slovakia) LUWA, NPP Trnava (Slovakia) NPP ‘Sarri’ (USA) LUWA 210, Barseback (Sweden) Forsmark NPP (Sweden) Asea-Atom, Olkiluoto (Finland) RB-1000, Ignalina NPP (Lithuania) UBD-200, SIA ‘Radon’ (Russia) URB-8, SIA ‘Radon’ (Russia) RB-800, Kalinin NPP (Russia) TB-16, LSK ‘Radon’ (Russia) RB-1000-14, Leningrad NPP (Russia)
Amount of waste immobilised
1989 98
75,000 in 220 L drums/Marcoule produces 200 drum/a and La Hague, 800 1000 drum/aa 10,000 drum (151,000 m3 of waste)
1988 98
5000 m3 of waste
1994 2018
20,500 drumb
2002 18 1970 99 1982 97
5500 drumb 160 t of compound 30,000 drum (7500 m3 of waste)
1995 98
957 drum (508 m3 of waste)
1995 98
117 m3 salt concentrate
1991 92 1975 83
623 m3 salt concentrate 3500 drum (960 m3 compound)
?-1985
2000 drum (220 m3 compound)
1979 84
2000 drum (390 m3 compound)
1987 98
9719 m3 (15,390 m3 of liquid waste)
1977 78
200 m3 of waste
1978 88
500 m3/year
1989 98
1160 m3 (467 t of salts)
1978 98
1500 m3 (3000 m3 of waste)
1984 86
1650 m3 of compound (3000 m3 of waste)
NPP, Nuclear power plant. a Data courtesy Vincent Gorgues, CEA, France. b Data courtesy Peter Kopecky, CEZ, Czech Republic.
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Table 18.4 Minimal waste acceptance criteria for bitumen compounds Parameter
Value
Chemical durability (leaching rate Cs-137) (g/cm2 day) Stability to swelling (volume increase after 90-day immersion in water) (%) Content of free water For salt concentrates (%) For ion-exchange resins (%) Thermal durability Flash point (°C) Ignition temperature (°C) Self-ignition temperature (°C) Radiation durability, increase of volume after 106 Gy (vol.%) Biological stability
, 1 3 10
Testing method 23
GOST 29114
,3
Change of volume
,1 3 5
Loss of mass at heating up to 110° C GOST 12.1.044
$ 200 $ 250 $ 400 #3
Change of volume
Absence of fungus
GOST 9.049
Table 18.5 Comparative features of cement and bitumen immobilising matrices Property
Cement
Compressive strength Waste loading
Excellent at correct formulations Poor to moderate
Resistance to biodegradation Thermal stability Resistance to leaching Radiation durability Gas generation Chemical compatibility
10 25 wt.% ion-exchangers Stable Good Poor to excellent
Bitumen
25 50 wt.% ion-exchangers Moderate Poor, can melt and ignite Excellent
Excellent Moderate Low Moderate Good with most materials, Good, worse for worse for boric acid, chelating solvents and oils agents
similar [see Section 3.6 and Eq. (3.10)]. A comparison of cement and bitumen matrices for immobilisation of spent IEX resins is given in Table 18.5.
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Bitumen has the advantage over cements of higher waste loading, but it also has some disadvantages, the most important being its potential fire hazard. The possibility of combustion in the case of an accidental fire has led to certain restrictions on the use of bitumen as an immobilising matrix. Several fires have occurred in different countries during the filling of final storage drums at temperatures of about 120°C. Two significant fire incidents occurred in Belgium in 1981 and in Japan in 1997 in bitumencontaining nitrates from evaporator concentrates. Bituminisation facilities must therefore include fire suppression and extinguishing systems.
18.8 IMMOBILISATION OF RADIOACTIVE WASTE IN POLYMERS Polymeric materials have been used for immobilisation of radioactive waste offering a high degree of radionuclide retention and good compatibility with waste materials such as spent IEX resins (IAEA, 1988, 2014). Polymeric materials in use are typically epoxy resins, polyesters and Table 18.6 Industrial application of polymer immobilisation process Country
Site
Radioactive waste
Argentina France
Atucha Grenoble
Liquid Polyethylene Concentrates, Polyester, sludge, IEX resin epoxy resin NPP waste Polyester, epoxy resin IEX Styrene-divinyl benzene IEX Epoxy resin IEX Styrene-divinyl benzene NPP waste Polyester
Chooz Mobile: COMETE 1, 2
Germany
Mobile: SOCODEI Mobile: FAMA, MOWA
Japan
Fukushima, Shimane, Kashiwazaki, Hamaoka Netherlands Borssele Switzerland Mobile: FAMA United Trawsfynydd Kingdom United Mobile: Dow States IEX, Ion exchange; NPP, nuclear power plant.
NPP waste IEX IEX NPP waste
Polymeric matrix
Polyethylene Styrene-divinyl benzene Vinylesterstyrene Vinylesterstyrene
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styrene. Radioactive waste such as IEX are typically dried and then mixed with liquid polymeric material before solidification promoted by additives such as hardeners for epoxy resins or lowering of temperature for polyethylene. Table 18.6 gives some data on utilisation of polymers in nuclear waste immobilisation. Composite materials based on epoxy resins (24%) and ordinary Portland cement filling (71%) have also been tested for disused sealed radioactive source immobilisation taking advantage of high mechanical strength (70 MPa), radiation durability (B108 Gy) and low radiation chemical yield GH2 1CO2 1CH3 5 0:023 (Sobolev et al., 1994).
REFERENCES Gwinner, B., Sercombe, J., Tiffreau, C., Simondi-Teisseire, B., Felines, I., & Adenot, F. (2006). Modelling of bituminized radioactive waste leaching. Part II: Experimental validation. Journal of Nuclear Materials, 349, 107 118. IAEA. (1970). Bituminization of radioactive waste, TRS-116. Vienna: IAEA. IAEA. (1988). Immobilisation of low and intermediate level radioactive wastes with polymers, TRS289. Vienna: IAEA. IAEA. (1993). Bituminization processes to condition radioactive wastes. TRS-352. Vienna: IAEA. IAEA. (2014). Mobile processing systems for radioactive waste management, Nuclear energy series NW-T-1.8. Vienna: IAEA. Ojovan, M. I., Ojovan, N. V., Golubeva, Z. I., Startceva, I. V., & Barinov, A. S. (2002). Aging of the bitumen waste form in wet repository conditions. Materials Research Society Symposia Proceedings, 713, 713 718. Sercombe, J., Gwinner, B., Tiffreau, C., Simondi-Teisseire, B., & Adenot, F. (2006). Modelling of bituminized radioactive waste leaching. Part I: Constitutive equations. Journal of Nuclear Materials, 349, 96 106. Sobolev, I. A., & Khomchik, L. M. (1983). Rendering harmless radioactive waste at centralised facilities. Moscow: Energoatomizdat. Sobolev, I. A., Ozhovan, M. I., Barinov, A. S., Timofeev, E. M., Minigaliev, R. M., & Kachalov, M. B. (1994). Polymer composite matrices for disposal of radionuclide sources of ionizing radiation. Atomic Energy, 76(2), 101 104. Vanbrabant, R., & Selucky, P. (1999). Proceedings of the international workshop on the safety and performance evaluation of bituminization processes for radioactive waste. Rez, Czech Republic: Nuclear Research Institute Rez. Zakharova, K. P., & Masanov, O. L. (2000). Bituminization of liquid radioactive wastes. Safety assessment and operational experience. Atomic Energy, 89, 135 139.
Immobilisation of Radioactive Waste in Bitumen Contents 18.1 Bituminisation 18.2 Composition and Properties of Bitumen 18.3 Bituminous Materials for Waste Immobilisation 18.4 Waste Loading 18.5 Bituminisation Technique 18.6 Acceptance Criteria 18.7 Bitumen Versus Cement 18.8 Immobilisation of Radioactive Waste in Polymers References
305 306 307 309 310 313 314 317 318
18.1 BITUMINISATION Embedding radioactive waste in bitumen has been used in immobilisation since the 1960s and the total volume of radioactive waste immobilised in bitumen currently exceeds 200,000 m3. In the bituminisation process, radioactive wastes are embedded in molten bitumen and encapsulated when the bitumen cools. Bituminisation combines heated bitumen and a concentrate of the waste material in either a heated thin film evaporator or extruder containing screws that mix the bitumen and waste. The waste is usually in the form of a slurry, for example, salt aqueous concentrates or wet ion exchange (IEX) resins. Water is evaporated from the mixture to about 0.5% moisture, intermixed with bitumen so that the final product is a homogeneous mixture of solids and bitumen, termed bitumen compound. Its radionuclide retention properties usually exceed those of cements at higher waste loadings (Fig. 14.7). Bituminisation is particularly suitable for water-soluble radioactive wastes such as bottom residues from evaporation treatment and spent organic ion exchangers (IAEA, 1970, 1993).
An Introduction to Nuclear Waste Immobilisation DOI: https://doi.org/10.1016/B978-0-08-102702-8.00018-2
© 2019 Elsevier Ltd. All rights reserved.
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18.2 COMPOSITION AND PROPERTIES OF BITUMEN Bitumen is a generic term used to cover a wide range of high molecular weight hydrocarbons. Bituminous materials have been widely used in the building industry for many years. As early as 3800 BC they were used in construction because of their adhesive and waterproofing properties. Obtained from naturally occurring deposits, these bitumens were used by the rulers of the Assyrian, Sumerian and Chaldean empires to waterproof their palace walls. Core samples from the natural Oklo Reactor (see Section 4.4) contain inclusions incorporating bitumen, which probably acted as a reducing buffer and/or hydrophobic water shield suppressing oxidative dissolution of the uraninite cores. Three main types of hydrocarbons occur in bituminous materials: • • •
Asphaltenes Resins Oils (aliphatic hydrocarbons)
The bitumen properties depend on the ratio of these components. Heavyweight fractions such as asphaltenes impart viscoelastic properties to bitumen at ambient temperatures (10°C 40°C). Lightweight fractions such as oils act as a carrier for the asphaltenes and resins. The viscous properties of bitumen are complex and affected by changes in its colloidal nature that occur with heating. However, when the temperature is high enough for the bitumen to be liquid it behaves as a Newtonian fluid and the rate of shear is directly proportional to the shearing stress. The most important characteristics of bituminous materials are: • • •
Penetration Softening point Flash point
The penetration characterises their hardness and is measured from the depth of penetration that a weighted needle achieves after a known time at a known temperature. The most common penetration test is carried out with a weight of 100 g applied for 5 seconds at 25°C. Typical values obtained are approximately (mm) 10 for hard coating grade asphalts, 15 40 for roofing asphalts, and up to 100 or more for certain waterproofing materials.
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The softening point characterises the temperature at which a steel ball falls a known distance through the bitumen when the test assembly is heated at a known rate. The usual test is carried out with a 19.05 mm (0.75 in.) diameter steel ball weighing 35 g sinking 25.4 mm (1 in.) through a 15.875 mm (0.625 in.) diameter 6.35 mm (0.25 in.) thick disc of bitumen held in a brass ring, with the whole assembly heated at 5°C/ min. The softening point value is used to grade bituminous materials into groups. Typical values are up to 132°C for coating grade asphalts, from 60°C to 105°C for roofing asphalts, down to approximately 45°C for waterproofing bituminous materials. The flash point characterises the temperature at which a bituminous material ignites in an open-air crucible. Flash points and flammability temperatures of bitumen are higher than 200°C 350°C, depending on the type of bitumen. Harder bituminous materials have higher flash points. The normal solubility of water in bitumen is on the order of 0.001 0.01 wt.% and in practice is considered negligible. The presence of water soluble salts in the bitumen results in a larger capacity for water absorption by osmosis.
18.3 BITUMINOUS MATERIALS FOR WASTE IMMOBILISATION Several bitumen varieties are commercially available for immobilisation of radioactive waste including: • • • •
Direct distilled Oxidised Cracked Emulsions
Direct distilled bitumen is a residue from petroleum distillation, while oxidised bitumen is created by blowing air through petroleum residues, which oxidises light fractions of the residues. Cracked bitumen is generated from thermal breakdown of heavy oil fractions in the oil refining industry and emulsions are produced by direct injection and emulsification of the bitumen in water. Bituminisation techniques used in radioactive waste immobilisation utilise different bituminous materials as shown in Table 18.1.
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Table 18.1 Applications of typical bitumen varieties for waste immobilisation Bitumen type
Application
Direct distilled Oxidised Cracked Emulsion
Batch melter, thin film evaporator Batch melter, extruder, thin-film evaporator Extruder Thin-film evaporator
Table 18.2 Parameters of bitumen for radioactive waste immobilisation Type of bitumen
BN-II
BN-III
BN-IV
Fractions (wt.%)
55.5 17.5 27.0 81 120 40 200
54.4 15.0 30.5 41 80 45 200
50.0 11.0 39.0 21 40 70 230
Oils (aliphatic hydrocarbons) Resins Asphaltenes Penetration at 25°C (mm) Softening temperature (°C) Flash point (open crucible) (°C)
Advantages of bituminous materials as matrices for waste immobilisation are: • • • • • • •
Water insolubility Low diffusion of water Chemical inertness Plasticity and good rheological properties Slow ageing rates High incorporation capacity enabling high waste loadings Ready availability and low cost
However, bitumen is an organic material and therefore it has a number of inherent disadvantages such as: • • •
Combustibility, although not easily flammable Lower stability against radiation than cement Ability to react with oxidising materials such as sodium nitrate
In France bituminous wasteforms are cast at 120°C 130°C with a soft and fluid bitumen, Viatotal 70/100 has a penetration depth between 7 and 10 mm and a softening point of 45°C 51°C. Table 18.2 demonstrates typical parameters of several bituminous materials used to immobilise low and intermediate level waste in the Russian Federation. The radiation stability of bituminous materials depends on their type, although swelling .3% by volume occurs at relatively high absorbed
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doses above 0.5 2 MGy (IAEA, 1993). Irradiation produces radiolysis gases with hydrogen as the main radiolysis product. The radiation chemical yield is characterised by the G-value, which is the number of a species produced per 100 eV of energy loss. The GH2 values for radiolytic hydrogen production of gamma-irradiated bituminous materials are from 0.2 to 0.4 (compared with GOH 5 2.8 of water exposed to gamma radiation).
18.4 WASTE LOADING On bituminisation the water is evaporated and the remaining salt residue with some remnant moisture is immobilised as small particles (B100 200 μm) distributed uniformly within the bitumen matrix. On increase of waste loading above certain threshold loadings clusters may form of several salt particles which are close or in contact (Fig. 18.1; Gwinner et al., 2006; Sercombe, Gwinner, Tiffreau, Simondi-Teisseire, & Adenot, 2006). Bitumen compounds can be loaded with B50 wt.% (B25% by volume) of salt residues from nitrate-type aqueous radioactive wastes (Sobolev & Khomchik, 1983). This value is considered as a threshold level since above it, leaching of radionuclides increases stepwise (Fig. 18.2A) as a result of formation of percolating-type clusters made of contacting salt particles in the host bitumen matrix (Fig. 18.2B and C). Threshold waste loading for ion exchangers is also B50%. The waste loading in practical uses of bitumen as a wasteform is kept below the threshold level, which depends on both waste and bitumen type and is typically determined experimentally.
Figure 18.1 Scanning electron microscopy images of bitumen wasteforms containing BaSO4 NaNO3 CoS PPFeNi Na2SO4 waste sludge. Waste loadings (A) 38 wt.%, (B) 42 wt.%. PPFeNi is a nickel and potassium preformed precipitate. Courtesy Elsevier.
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Figure 18.2 The inventory fraction of leached 137Cs (X) from bituminised radioactive waste with waste loading volume fraction (θ) demonstrating a change in release mechanisms from a diffusion-controlled before to a dissolution-controlled above the threshold loading B25 vol.% (A), and a high contrast micrograph (binary image) of a percolating cluster made of (black) salt particles (bar size 100 μm) (B) and with the connectivity graph of the percolation cluster (C). Courtesy Natalie Ojovan, SIA RADON, Russia.
It was found that essentially all radioactivity of the bituminised wasteform is associated with the asphaltene fraction of the bitumen and that aging leads to an increase in asphaltene fraction content and hardening of the bituminous host material (Ojovan, Ojovan, Golubeva, Startceva, & Barinov, 2002).
18.5 BITUMINISATION TECHNIQUE Immobilisation of radioactive wastes via bituminisation can be done as a batch or continuous process. Batch processes usually involve drying the waste followed by mixing the dried material in molten bitumen. In this process waste is continuously introduced into a metered volume of molten bitumen at about 200°C 230°C (Fig. 18.3). The mixing vessel is externally heated to evaporate water and the solid residue particles are then mixed with the bitumen. When the required composition of the wasteform is reached, no more waste is added. The mix is heated and stirred for some time to evaporate the residual water and is then discharged into drums or other containers to cool and solidify. Batch processes are not widely used and continuously operating bituminisation processes are more common, generally based on either extrusion or film evaporation systems. With extrusion systems the concentrates are fed into a multiple screw extruder together with bitumen at 130°C 200°C (Fig. 18.4A and B). During passage through the heated extruder the waste and bitumen are intensively mixed with simultaneous evaporation of water.
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Stack Rotating shutter
Absolute filters
Splash head
Sludge feed Motor
Contact condenser
Cooling installation Electrostatic filter
Stirrer 10-t bitumen tank
Heating Induction heaters
OR steam Heater
Drainage Oil filter
To water processing
Figure 18.3 Batch bituminisation process at Mol, Belgium. Courtesy IAEA.
(B) Concentrates
(A) Feed throat
Pump
Bitumen
Pump
Volatiles
Screw
Compound Heater band
Condensers
Extrusion screws
Figure 18.4 Schematics of (A) an extrusion apparatus for immobilisation of radioactive waste in bitumen (e.g. bituminisation), and (B) of coating extrusion machine at Marcoule, France. Courtesy IAEA.
The temperature of the bitumen waste matrix is kept reasonably low (130°C 140°C) although at lower temperatures the viscosity would be too high to maintain the extrusion process. The final product is uniform, with mineral residue particles from waste coated with a layer of bitumen. However, some waste constituents partially dissolve in the bitumen. The bituminised product is discharged into drums and allowed
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to cool and solidify. Typical extrusion bituminisation processes used in France and in Japan are shown in Fig. 18.5A and B. In film evaporator systems liquid wastes are initially concentrated to about 60 wt.% dry matter. Preheated waste sludge and bitumen are fed separately to the top of the thin-film rotary evaporator where the two streams mix and flow down the heated wall. The water in the mixture is progressively evaporated and the remnant mineral residue from the waste is mixed with bitumen (Fig. 18.6). Rotary thin-film evaporators use wiping blades to assist mixing of waste mineral residues and bitumen. The bituminised waste material is drained near the bottom into steel containers. The vapours generated are condensed, first in the built-in condenser in the evaporator and subsequently in an external condenser. Thin-film evaporator bituminisation is used to immobilise both aqueous radioactive waste and spent organic ion exchangers (Fig. 18.7). The first bituminisation plant in Russia was put into operation at Moscow SIA ‘Radon’ in 1977 (Sobolev & Khomchik, 1983; Zakharova & Masanov, 2000). It uses an industrial steam-heated rotary evaporator as mixing unit operating at 135°C while the waste loading is up to 60 wt.%. The bitumen compound is poured into carbon steel containers. Parameters of the bituminous materials used at this plant are listed in Table 18.2, and 18.3 summarises the application of bituminisation processes worldwide as reported up to 1999 (Vanbrabant & Selucky, 1999). Although immobilisation of radioactive waste continues to be used there are few reported data on total amounts of immobilised waste to require this table to be updated. (A)
(B)
Waste
Chemicals
Sludge water Content 90%
Filter
Reaction vessel
Feeding vessel
Surface-active agents Sludge Bitumen 22% water Content 50% Preliminary coating
Water salted out after preliminary coating
Filtered water
Bitumenous composition containing 80% water
Bitumen Condenser
Drying
Extruder Drum
Condensed water
Drainage into drums
Figure 18.5 Schematic of extruder type bituminisation processes: (A) at Tokai Works in Japan, and (B) at Marcoule, France.
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Geared engine
Upper bearing of the rotor
Exit of water vapour to condenser
Mist separator Bitumen
Concentrates
Distribution rings
Exit of heating medium
Heating medium
Film layer of the product Body of the evaporator Rotor
Lower bearing of the rotor
Heating medium
Discharge of the product
Figure 18.6 Schematic of thin-film evaporator type bituminisation processes. Courtesy IAEA.
18.6 ACCEPTANCE CRITERIA Waste acceptance criteria for bitumen compounds used to immobilise radioactive waste are set into the state standard of the Russian Federation GOST 50927-96. The most important parameters are illustrated in Table 18.4. It is assumed that these requirements are met both for freshly prepared bitumen compounds and for compounds in storage and disposal.
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(A) Steam Water
Rotary evaporator
Waste
Drum
(B)
Bitumen tank
Additives
Ion exchanges Powder Beads Concentrates
Grinder
30 m3 Feed tank Condenser Cooler
Feed tank
Steam
Oil control
Oil filter
Thin silm evaporator Slurry Heating oil
To low level waste
Bitumen compound
Figure 18.7 Schematic of thin-film evaporator bituminisation processes. (A) Liquid aqueous waste bituminisation process at Moscow SIA ‘Radon’, Russia; (B) spent ion exchange resin bituminisation plant at Barsebäck NPP, Sweden. NPP, Nuclear power plant.
18.7 BITUMEN VERSUS CEMENT Immobilisation in suitable matrices significantly reduces the potential for release of radionuclides into the environment. Average hazard diminishing factors Kwf for bituminised aqueous radioactive wastes are of the order of several hundreds and those of cemented aqueous radioactive wastes are
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Table 18.3 Immobilisation of radioactive waste in bitumen Bituminisation plant (country)
Operation period reported
Marcoule and La 1966 2017 Hague, CEA (France) STE 3, La Hague, COGEMA (France) ‘Mummy’, Belgoprocess (Belgium) Dukovany NPP (Czech Republic) Temelin NPP Risφ RNL (Denmark) JNC, Tokai Works (Japan) PS-44, NPP Jaslovske Bohounice (Slovakia) LUWA, NPP Trnava (Slovakia) NPP ‘Sarri’ (USA) LUWA 210, Barseback (Sweden) Forsmark NPP (Sweden) Asea-Atom, Olkiluoto (Finland) RB-1000, Ignalina NPP (Lithuania) UBD-200, SIA ‘Radon’ (Russia) URB-8, SIA ‘Radon’ (Russia) RB-800, Kalinin NPP (Russia) TB-16, LSK ‘Radon’ (Russia) RB-1000-14, Leningrad NPP (Russia)
Amount of waste immobilised
1989 98
75,000 in 220 L drums/Marcoule produces 200 drum/a and La Hague, 800 1000 drum/aa 10,000 drum (151,000 m3 of waste)
1988 98
5000 m3 of waste
1994 2018
20,500 drumb
2002 18 1970 99 1982 97
5500 drumb 160 t of compound 30,000 drum (7500 m3 of waste)
1995 98
957 drum (508 m3 of waste)
1995 98
117 m3 salt concentrate
1991 92 1975 83
623 m3 salt concentrate 3500 drum (960 m3 compound)
?-1985
2000 drum (220 m3 compound)
1979 84
2000 drum (390 m3 compound)
1987 98
9719 m3 (15,390 m3 of liquid waste)
1977 78
200 m3 of waste
1978 88
500 m3/year
1989 98
1160 m3 (467 t of salts)
1978 98
1500 m3 (3000 m3 of waste)
1984 86
1650 m3 of compound (3000 m3 of waste)
NPP, Nuclear power plant. a Data courtesy Vincent Gorgues, CEA, France. b Data courtesy Peter Kopecky, CEZ, Czech Republic.
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Table 18.4 Minimal waste acceptance criteria for bitumen compounds Parameter
Value
Chemical durability (leaching rate Cs-137) (g/cm2 day) Stability to swelling (volume increase after 90-day immersion in water) (%) Content of free water For salt concentrates (%) For ion-exchange resins (%) Thermal durability Flash point (°C) Ignition temperature (°C) Self-ignition temperature (°C) Radiation durability, increase of volume after 106 Gy (vol.%) Biological stability
, 1 3 10
Testing method 23
GOST 29114
,3
Change of volume
,1 3 5
Loss of mass at heating up to 110° C GOST 12.1.044
$ 200 $ 250 $ 400 #3
Change of volume
Absence of fungus
GOST 9.049
Table 18.5 Comparative features of cement and bitumen immobilising matrices Property
Cement
Compressive strength Waste loading
Excellent at correct formulations Poor to moderate
Resistance to biodegradation Thermal stability Resistance to leaching Radiation durability Gas generation Chemical compatibility
10 25 wt.% ion-exchangers Stable Good Poor to excellent
Bitumen
25 50 wt.% ion-exchangers Moderate Poor, can melt and ignite Excellent
Excellent Moderate Low Moderate Good with most materials, Good, worse for worse for boric acid, chelating solvents and oils agents
similar [see Section 3.6 and Eq. (3.10)]. A comparison of cement and bitumen matrices for immobilisation of spent IEX resins is given in Table 18.5.
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Bitumen has the advantage over cements of higher waste loading, but it also has some disadvantages, the most important being its potential fire hazard. The possibility of combustion in the case of an accidental fire has led to certain restrictions on the use of bitumen as an immobilising matrix. Several fires have occurred in different countries during the filling of final storage drums at temperatures of about 120°C. Two significant fire incidents occurred in Belgium in 1981 and in Japan in 1997 in bitumencontaining nitrates from evaporator concentrates. Bituminisation facilities must therefore include fire suppression and extinguishing systems.
18.8 IMMOBILISATION OF RADIOACTIVE WASTE IN POLYMERS Polymeric materials have been used for immobilisation of radioactive waste offering a high degree of radionuclide retention and good compatibility with waste materials such as spent IEX resins (IAEA, 1988, 2014). Polymeric materials in use are typically epoxy resins, polyesters and Table 18.6 Industrial application of polymer immobilisation process Country
Site
Radioactive waste
Argentina France
Atucha Grenoble
Liquid Polyethylene Concentrates, Polyester, sludge, IEX resin epoxy resin NPP waste Polyester, epoxy resin IEX Styrene-divinyl benzene IEX Epoxy resin IEX Styrene-divinyl benzene NPP waste Polyester
Chooz Mobile: COMETE 1, 2
Germany
Mobile: SOCODEI Mobile: FAMA, MOWA
Japan
Fukushima, Shimane, Kashiwazaki, Hamaoka Netherlands Borssele Switzerland Mobile: FAMA United Trawsfynydd Kingdom United Mobile: Dow States IEX, Ion exchange; NPP, nuclear power plant.
NPP waste IEX IEX NPP waste
Polymeric matrix
Polyethylene Styrene-divinyl benzene Vinylesterstyrene Vinylesterstyrene
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styrene. Radioactive waste such as IEX are typically dried and then mixed with liquid polymeric material before solidification promoted by additives such as hardeners for epoxy resins or lowering of temperature for polyethylene. Table 18.6 gives some data on utilisation of polymers in nuclear waste immobilisation. Composite materials based on epoxy resins (24%) and ordinary Portland cement filling (71%) have also been tested for disused sealed radioactive source immobilisation taking advantage of high mechanical strength (70 MPa), radiation durability (B108 Gy) and low radiation chemical yield GH2 1CO2 1CH3 5 0:023 (Sobolev et al., 1994).
REFERENCES Gwinner, B., Sercombe, J., Tiffreau, C., Simondi-Teisseire, B., Felines, I., & Adenot, F. (2006). Modelling of bituminized radioactive waste leaching. Part II: Experimental validation. Journal of Nuclear Materials, 349, 107 118. IAEA. (1970). Bituminization of radioactive waste, TRS-116. Vienna: IAEA. IAEA. (1988). Immobilisation of low and intermediate level radioactive wastes with polymers, TRS289. Vienna: IAEA. IAEA. (1993). Bituminization processes to condition radioactive wastes. TRS-352. Vienna: IAEA. IAEA. (2014). Mobile processing systems for radioactive waste management, Nuclear energy series NW-T-1.8. Vienna: IAEA. Ojovan, M. I., Ojovan, N. V., Golubeva, Z. I., Startceva, I. V., & Barinov, A. S. (2002). Aging of the bitumen waste form in wet repository conditions. Materials Research Society Symposia Proceedings, 713, 713 718. Sercombe, J., Gwinner, B., Tiffreau, C., Simondi-Teisseire, B., & Adenot, F. (2006). Modelling of bituminized radioactive waste leaching. Part I: Constitutive equations. Journal of Nuclear Materials, 349, 96 106. Sobolev, I. A., & Khomchik, L. M. (1983). Rendering harmless radioactive waste at centralised facilities. Moscow: Energoatomizdat. Sobolev, I. A., Ozhovan, M. I., Barinov, A. S., Timofeev, E. M., Minigaliev, R. M., & Kachalov, M. B. (1994). Polymer composite matrices for disposal of radionuclide sources of ionizing radiation. Atomic Energy, 76(2), 101 104. Vanbrabant, R., & Selucky, P. (1999). Proceedings of the international workshop on the safety and performance evaluation of bituminization processes for radioactive waste. Rez, Czech Republic: Nuclear Research Institute Rez. Zakharova, K. P., & Masanov, O. L. (2000). Bituminization of liquid radioactive wastes. Safety assessment and operational experience. Atomic Energy, 89, 135 139.