Development of a Process for Incorporation of Radioactive Waste Solutions and Slurries in Emulsified Asphalt Herschel W. Godbee, Jerry H. Goode, and Raymond E. Blanco
Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830 -
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Incorporation in asphalt is a promising method for preparing alkaline solutions and slurries of intermediate-level radioactive wastes for storage or burial. The wastes are mixed with a n asphalt-water emulsion, the water is volatilized by heating the mixture to 160" C., and the residue containing the radioactive solids is drained into the final disposal container. The volume of waste to be stored is reduced by a factor of about 2. Products containing 60 weight waste solids were homogeneous and showed satisfactorily low leach rates in water. The fraction of radionuclides leached through a specific surface per day was about 4 X lo-' and 4 X 10-6 (sq. cm. per gram)-' day-', respectively, for soluble and insoluble radionuclides, with no significant increase at radiation (external 6nCo)doses as high as lo9rads. The rate for the soluble nuclide cesium was about one tenth of that, when the cesium was sorbed on grundite clay. No evolution of gases or swelling was observed in products that contained up to 5 2 curies per gallon at radiation doses up to 1 X los rads.
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he intermediate-level wastes generated in nuclear installations generally have been stored in tanks or mixed with cement and buried in specially designated land areas above the water table. These wastes include second- and third-cycle aqueous raffinates ; solutions from decladding of spent reactor fuel, solvent cleanup, and off-gas scrubbers; and slurries and solids from processes for decontaminating process waters. These wastes are characterized by modest levels of radioactivity-Le., decay heat and radiation dose levels are not serious problems-and by high salt or solids content, which prevents their efficient treatment by a conventional method such as ion exchange or precipitation. Both storage in tanks and solidification in cement have limitations. Storage in tanks is considered by many to be only a temporary measure that requires constant surveillance. The products formed by mixing the waste with cement are only moderately insoluble. The integrity of cement decreases greatly as the concentration of included soluble salts increases. If low concentrations of waste salts are used in the cement, the volume of the final product to be stored is much larger than the original waste volume. Most operations with cement are dusty, good mixing is difficult to achieve, and the operations are cumbersome. A promising recent development is the use of asphalt t o solidify and insolubilize these wastes. Immobilization of wastes by incorporation in this cheap, insoluble material before burial or other storage can reduce not only treatment costs but also the flow of radioactivity to the environment. Plants for incorporating intermediate-level radioactive waste in asphalt are already in operation at Mol, Belgium (Dejonghe, 1034 Environmental Science and Technology
Baetsle, et id., 1964; Dejonghe, Van de Voorde, et ul., 1963; Van de Voorde and Dejonghe, 1966). and Marcoule, France (Rodier, Alles, er id., 1966; Rodier, Lefillatre, et a/., 1963; Wormser, Rodier, et i t / . , 1964), while a plant similar to that in Belgium is being constructed a t Harwell, England (Burns, Clarke, et d., 1966). At Mol, a batch process is used to incorporate wastes-incinerator ashes, concentrated aqueous solutions, or filtered sludges with variable water content-in asphalt at about 250' C. by a method that requires simultaneous gradual addition and violent stirring. Most of the work at Mol has involved sludges. At Marcoule, a continuous process has been developed for mixing sludges that contain emulsifying agents with asphalt at about 125" C. The major portion (80 to 90%) of the water in the sludges is exuded and thus separated from the coated wastes. The residual water in the asphalt mixture is then volatilized, and the mixture is heated to about 130" C. Since the exuded water contains most of the soluble radionuclides, the process is applicable only to wastes in which these radionuclides are bound in the sludge solids. Soluble salts, such as sodium nitrate, also remain in the exuded water and must be stored or discharged to the environment. In the process being developed (Godbee, Blanco, e? id., 1967; Goode, 1965; Goode and Flanary, 1968; Rom, 1966. 1967; Suddath and Long, 1967) at Oak Ridge National Laboratory (ORNL), wastes are introduced into an emulsified asphalt (about 35 weight water-65 weight asphalt) at any convenient temperature below the boiling point of the solution, water is volatilized by heating, the temperature of the product is increased until the product flows freely, and, finally, the product is drained into a steel drum for preliminary storage and shipment to a disposal area. Attractive features of the process include use of emulsified asphalt, which flows readily at room temperature; evaporation at low temperatures to minimize degradation of the asphalt; relatively low agitation rates, which provide adequate mixing and keep the heated surfaces of the evaporator clean; operation in either a batch or a continuous manner; general applicability to all waste types; and retention of both soluble or insoluble solids with equal effectiveness. While retention of nonradioactive soluble salts is desirable to prevent their discharge to the environment, the retention of oxidants, such as nitrates, does produce significant operational and safety problems. Most low and intermediate-level aqueous radioactive wastes contain high concentrations of sodium nitrate and the safety aspects of producing and storing an asphalt product containing high concentrations of nitrate must be carefully assessed. Thus, the development program at ORNL has been directed almost exclusively to processing the major waste types, which contain nitrate, and assessing the safety problem. Other waste types, which d o not contain nitrate, can undoubtedly be processed by the same method.
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Descriptioi~of Ei,iulsi$ed Asphalt Process
A primary advantage of the emulsified asphalt process is that it can be operated in either a batch or a continuous manner. The batch process, as carried out in the laboratory, consists in adding the waste directly to the asphalt-water emulsion in an evaporator, which contains a stirrer and a bottom outlet. The mixing is performed at any convenient temperature up to about 100" C . , with a low stirring rate (100 to 300 r.p.m.); the water is evaporated; the temperature is increased to 130" to 160" C. ; and the product is drained into a disposal container. In the continuous process (Figure l), the waste and asphalt are introduced at the top of a wipedfilm evaporator and the mixture flows down the walls of the evaporator at about 160" C. Agitator paddles sweep the walls continuously at about 300 r.p.m. and provide effective mixing and satisfactor!, heat transfer. This process has been demonstrated in the laboratory with nonradioactive waste and with radioactive waste containing up t o 52 curies per gallon in a 4-inch-diameter x 6l/?-inch-high evaporator. It has also been demonstrated on a pilot-plant scale (Suddath and Long, 1967) with nonradioactive waste in a 12-inch-diameter X 16-inchlong Pfaudler wiped-film evaporator with 4 sq. feet of heattransfer surface. The latter device processed about 11 gallons of waste per hour. The water bapor from the evaporator in either mode of operation is subsequently condensed and collected in a suitable receiier. In practice, this condensate would be sampled and sent to the low-level-waste treatment system for the plant. Insert gases from the condenser would be heated and passed through high-efficiency filters before discharge.
CONDENSER
Figure 1. Flow sheet for incorporation of intermediatelevel waste in emulsified asphalt
DE ENTRAIhlMEYT SECTICh
Experimental Appara (us The laborator>,-scale apparatus consisted, basically, of a stirred evaporator in a controlled-temperature furnace with ancillarq equipment such as a pump for transferring the waste and a condenser for condensing the steam from the evaporator. Figure 2 is a cutaway sketch of the stainless steel evaporator. A schematic diagram of the evaporator in place in the controlled-temperature furnace (Figure 3) shows a layout of the apparatus. including the stirred feed vessel, feed pump, evaporator with stirrer motor and controls, 1.5-kw. furnace, watercooled condenser. condensate catch vessel, and product catch vessel. Details of the apparatus are given by Godbee, Blanco, et cil. (1967).
THERMGCGLPLE CCNhECTIGI\.
TEFLCh W P E R ELSDE 4 incn I D X 6 5 i n c t CON T41 N ER
Figure 2. Diametric drawing of stirred evaporator used in laboratory studies of incorporation of waste in emulsified asphalt
Charucteri:ririoti of Mciteriuls The three types of simulated intermediate-level waste and the kinds of emulsified asphalt used at ORNL in waste studies thus far are described below. Waste. One type of intermediate-level waste (ILW) produced at nuclear installations is the alkaline concentrate from evaporating a mixture of various waste streams-for example, from cell and equipment decontamination, solvent cleanup, and off-gas scrubbers. This concentrate is principally a nitrate solution of sodium, potassium, aluminum, and iron, and often contains sulfate, chloride, fluoride, and phosphate. ORNL evaporator concentrate (ILW) has the composition shown in Table I. A second type, aluminum cladding solution (ACS), is formed when the aluminum cladding from uranium metal fuel is dissolved in NaOH-NaN03 (Table I). A third type consists of the aqueous raffinate from the second Purex solvent extraction cycle (2CW) for recovering the unused uranium. plutonium, or thorium in irradiated nuclear fuels. Since acid solutions are not compatible with asphalt, this acidic waste is neutralized to form an alkaline waste slurry
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T4CHOHETER
STIRRED WASTE FEE0 VESSEL STIRRER MOTOR
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MOTOR SPEED CONTROL
ULSIFIED 4SPH4LT
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THERMOCOUPLES
PROOUCT VALVE
CONDENSATE VESSEL
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THERMOCOUPLE
PRODUCT VESSEL
Figure 3. Schematic diagram of batch equipment used in laboratory studies of incorporation of waste in emulsified asphalt
Volume 2, Number 11, November 1968 1035
(2CW1) before incorporation in asphalt (Table I). Synthetic waste solutions were “spiked” with 1 to 26 curies of aged mixed fission products per gallon to simulate actual wastes (Table 11). Emulsified Asphalt. The two emulsified asphalts (anionic emulsions) used in most of the studies are described (U. S. Government Federal Specification, 1962) as follows : “Type RS-2-Rapid-setting, high-viscosity, emulsified asphalt for surface treatment; and Type SS-I-Slow-setting, low-viscosity, emulsified asphalt for fine aggregate mixes, in which a substantial quantity of aggregate passes a 1/8-inch sieve and a portion may pass a No. 200 (74-micron) sieve.” A limited number of experiments were made with a cationic emulsion, which is described as follows: “Type RS-2K-
Rapid-setting, low-viscosity grade for surface treatments or bituminous macadam penetration that is particularly advantageous for use with gravels or hydrophilic aggregates.” Experimental Procedure For convenience and consistency of operation, a nominal 150-ml. batch of emulsified asphalt was charged to the evaporator in each experiment. The asphalt was usually stirred at rates of 150 to 300 r.p.m., and its temperature was increased to about 70” C . , at which point the addition of feed to the evaporator was started. The feed rates ranged from 1 to 3 ml. per minute, and the total volumes of feed added ranged from about 20 ml. to slightly more than a liter, depending on the concentration of waste solids desired in the final product.
Table I. Compositions of Simulated Wastes and Salts-Asphalt Products in Emulsified Asphalt Incorporation Studies at ORNL Waste Type Product Type ILWQ, ACSb, 2cwq 2c\vid, ILW, ACS, Component M M M M wt. wt. Na+ 6.61 4.16 0.64 6.64 18.5 16.6 NH4+ 0.19 ... ... ... ... ... ... ... H’ , . . 6.2 ... ... ... A I3+ 0.22 , . . ... 0.7 ... Fe3+ ... ... 0.54 0.37 ... ... NO14.64 2.2 6.2 4.26 34.9 23.7 OH2.06 0.06 ... 1.94 3.8 0.2 AlOz1.9 ... ... ... 19.5 c10.056 , . . ... ... 0.2 ... so420.35 , . . 1.13 0.78 4.1 ... Density at 25” C., grams per ml. 1.34 1.21 1.31 1.33 ... ... Total solids in waste, wt. 39.1 28.5 38.8 39.4 ... ... Asphalt ... , . . ... ... 37.8 40.0 Volume reduction‘ ... ... ... . . . 1.8 2.5
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... 2.4 29.6 3.1 ...
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39.0 1.7
ORNL intermediate-level waste solution (evaporator concentrates), Solution from dissolution of aluminum cladding with NaOH-NaN03. Purex second-plutonium-cycle wasts. Acid solution made basic before incorporation i n asphalt. d Purex second-plutonium-cycle waste with 455 ml. of 51.5% N a O H added t o each liter of 2CW. e Volume reduction defined as volumes of waste processed per unit volume of product obtained.
ILW ACS 2cw1
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Table 11. Radionuclide Contents of Aqueous Waste Solutions Prior to Fixation in Aspha!to Radionuclide Content, Disintegrations &fin.-*Ml.-’ Total Activity, Total rare Sr Ru Gross -1 Curies/Gal. earths p 13 Y Gross p Run I 1.07 1 . 2 8 x lo* 0 . 5 4 X IO9 0 . 3 1 X IO9 2 . 6 2 X IO7 0.99 x lo* 0.57 0.61 X IO5 0.26 X lo9 0.15 X IO9 1.26 X IO7 0.78 X IO* 1.15 1 . 6 3 X IO* 0 . 6 9 X lo9 0.39 X lo9 3.33 X 10’ 1 . 2 5 X IO8
cs 0.98 X lo8 0 . 4 7 X los 1.24 X IO8
ILW ACS 2cw1
1.65 2.35 3.22
2.40 X lo8 2.68 X IOs 4 . 0 7 X IO8
1.01 X IO9 1 . 1 3 x 109 1 . 7 2 X IOQ
Run I1 0 . 5 6 X lo9 0.63 x 109 0 . 9 6 X 109
4.88 X 10’ 5 . 4 5 x 107 8.30 X 10’
1 . 8 3 X lo* 2.05 x IO* 3.12 X IO8
1 . 8 3 X lo8 2.04 X IO6 3.11 x IO8
ILW ACS 2cw1
4.10 3.72 3.43
4 . 6 1 X IO8 4 . 2 8 X lo* 5.05 X lo8
1 . 9 4 X IO9 1 . 8 1 x 109 2.13 X l o y
Run I11 1 . 1 8 X IOg 1.01 x 109 1 . 1 8 X IO9
9.36 X 10‘ 8.77 x 10’ 10.30 X 10‘
3.53 X IOg 3.28 x IO* 3.86 X lo8
3.51 X IOe 3.77 x 108 3.84 X IO8
ILW ACS 2cw1
26.13 15.00 16.70
21.30 X IO8 16.80 X lo8 20.30 X lo*
9 . 0 0 X IO9 7.10 X lo9 8 . 6 0 X lo9
Run IV 5.01 X IO9 3.95 X IO9 4.78 X I O 9
43.50 X 10‘ 34.40 X 10’ 41.60 X IO’
16.30 X los 13.15 X IO6 15.50 X lo8
16.24 X lo8 12.84 X IO8 15.45 X lo8
Concentrations were increased by a factor of 2 when solutions were incorporated in asphalt to form a product containing 60 wt. VG solids.
1036 Environmental Science and Technology
The temperature of the evaporator was increased in such a manner that water was evaporated at a rate about equal to that at which the feed was added. Essentially all the water had evaporated when the temperature of the evaporator reached 125" to 130" C. The products were collected at three different temperatures-some at 130" C., most at 160" C., and a few at 190" C. All radioactive products were collected at 160" C. Stoichionwrry and Parameters of the Process solids from Asphalt products containing 10 to 80 weight waste have been prepared with ORNL intermediate-level waste, an aluminum cladding solution, and a Purex secondplutonium-cycle waste with added caustic. No apparent difference, other than hardness of the product, was observed in the mixture obtained with a given waste and any of the emulsified asphalts. The higher the penetration (American Society for Testing Materials. 1965) of the base asphalt, the softer the salts-asphalt mixture seemed to be. No oil or other low-boiling fractions were observed in the condensate in the laboratory or engineering-scale experiments. This can probably be attributed to the low wall temperature in these experiments (about 160" C.).The condensates collected during the evaporation of water from the asphalt products containing from 2 to 42 curies per gallon of product were slightly radioactive (9 microcuries to 0.7 millicurie per gallon) because of entrained fission products. The apparatus had no de-entrainment devices. The decontamination factors-defined as activity (curies) of feed divided by activity (curies) of condensatewere 6.4 x l o 3to 2.5 X 105 and showed some dependence on the type of waste and the fission product content of the waste. Essentially all the radioactivity in the condensates could be attributed to I3iCs; the ruthenium content was almost negligible, contributing less than 10% of the gross gamma activity found in the condensate of the most radioactive test. The amount of solids from waste contained by the asphalt products through the experimental series ranged from 10 to 80 weight % in increments of 10 weight %. The products that contained 10 to 60 weight % solids from any of the wastes studied were homogeneous and free flowing at 130" C. The mixtures that contained 70 to 80 weight solids from ILW and 2CW1 were not completely homogeneous and free flowing, while the solids from ACS, mixtures containing 70 to 80 weight though homogeneous and free flowing, were readily leached with water. The products containing 60 weight solids represented a good compromise of properties-volume reduction, homogeneity, viscosity, leachability, etc. The compositions of the products that were prepared most often, studied in greatest detail, and used for design and economic studies (Rom, 1966, 1967) of waste disposal at ORNL are given in Table I. In one series of experiments with the asphalt product containing 60 weight solids from ILW, the temperature of the product in the evaporator was increased to 160" C. and held for periods of time ranging from 15 minutes to 1.5 hours in 15-minute increments. The water content of the product varied from about 1.5 to 0 weight %, as determined by heating samples in a drying oven at 110" C. for several days. In so far as could be determined by visual observations and irradiation and leaching studies, this amount of water had no deleterious effect. No discernible difference was noted in solids) made by using stirring rates of products (60 weight 150 to 750 r.p.m., or in products prepared at 130", 160°, or 190" C. The viscosity of a product containing 60 weight solids from ILW was measured, using a modified-sphere apparatus (Fitzgerald, 1965), and was 58.4 and 45.2 poises at 150" and 170" C., respectively.
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Leaching of Asphalt Products The uniformity and the integrity of the asphalt coatings on waste particles were evaluated by determining the leach rate of various elements from the salts-asphalt product. The term "leach rate" rather than "dissolution rate" is used, since the asphalt matrix does not dissolve. The leach rate, expressed as the fraction of the original activity (or element) in the sample leached per day through unit specific surface area, was calculated from the relation: Activity leached/total activity in sample - -(surface area of sample/mass of sample)(time) fraction leached
This leach rate has the same numerical value as the penetration rate (gram/'sq. cm./day) used by other workers. Asphalt products containing 20, 40, and 60 weight 7; solids from ORNL ILW were leached with water for periods of up to 16 months. The simulated wastes in these studies contained tracer levels of radioactivity-that is, 0.1 microcurie of 137Cs or Io6Ruper ml. The sodium and I37Csare representative of radionuclides that are soluble in alkaline wastes which can contain carbonate, sulfate, or phosphate, whereas the lo6Ru is representative of insoluble radionuclides. The latter include strontium and rare earths. The fraction of the original 13iCs or sodium leached per day through a unit specific surface, expressed in square centimeters per day, reached steady-state values of 6.6 X IO+, 1.0 X and 3.0 X for the 20, 40, and 60 weight % solids products, respectively, in tests with stagnant water. In tests with flowing water, the fraction of 1 3 T s or sodium leached from a 60 weight product (sq. cm. per gram)-' reached a steady-state value of 5.0 X day-', a value slightly higher than that with stagnant water (Figure 4). No difference in the leach rates with stagnant and Volume 2, Number 11, November 1968 1037
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40
80
120 160 200 DAYS OF L E A C H ' Y G
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Figure 5. Leaching of sodium and fission products from ILW-asphalt product containing 60 weight solids and 52 curies of fission products per gallon of asphalt product
flowing water was observed for the product containing 20 weight solids. The 40 weight product was not tested under dynamic conditions. The fraction of the sodium and 1 3 T sleached from products containing 20, 40, and 60 weight solids from ACS and 2CW1 was nearly the same as that obtained from products containing salts from ILW. The fraction of l06Ru (0.1 microcurie per ml. of waste) leached solids from ILW from a product containing 60 weight reached a steady-state value of 7 x (sq. cm. per gram)-' day-l (Figure 4), about one hundredth of that for the soluble solids-Le., sodium and 13'Cs. Generally, the leach rates of asphalt products were lower, by a factor of 50 to 100, than those of cement products containing similar amounts of waste salts (Godbee, Blanco, et a/., 1967). The asphalt products containing from 2 to 52 curies per gallon were leached with water and with water containing 100 p.p.m. of NaCI. No significant difference in leach rates was observed with these two leachants. In general, the rates at which sodium and the gross gamma activity-Le., 13'Cswere leached from an asphalt product made from ORNL ILW containing 26 curies per gallon of solution (Figure 5 ) were similar to those (Figure 4) obtained with nonradioactive waste and tracer levels of radioactivity. Even though the asphalt product contained 52 curies per gallon and had undergone exposure to doses greater than 4 X 10' rads, the average sodium-cesium leach rates were essentially the same-that is, a steady-state value of about 4 x lo-< fraction leached (sq. cm. per gram)-' day-', as those of cmirradiated products. The rates at which the insoluble solids,gOSr,rare earths, and la6Ru were leached from the asphalt product containing 52 curies per gallon were lower by one to two orders of magnitude. The absorbed dose has now reached about 1 X lo8 rads, and the leach rates have remained essentially the same. The absorbed doses for the products containing fission products (Table 11) were calculated by assuming that the average energy (one third of the maximum) of the beta radiation from each isotope was absorbed by the product and by assuming that
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Environmental Science and Technology
none of the energy of the gamma radiation was absorbed (since at least 92% of it escapes). The other types of waste (ACS and 2CW1) containing from 1 to 26 curies of radionuclides per gallon of solution gave asphalt products whose leach rates were similar to those for products made with ORNL ILW. The steady-state rates for sodium and gross gamma ( 13'Cs)-expressed as fraction for leached (sq. cm. per gram)-' day-1 - were: 4 x ORNL ILW at an absorbed dose of 7 x 10' rads. 2 x for ACS at an absorbed dose of 7 x 109 rads, and 1 x for the 2CW1 a t an absorbed dose of 6 X 10' rads. The leach rates for the insoluble radionuclides were not determined from these wastes but they are undoubtedly significantly lower. A single analysis for Io6Ruafter 220 days showed a rate about one tenth as high. These rates appear to be independent of the radionuclide concentration of the asphalt product; however, they vary with the type of waste, probably because of flocculating agents or cesium scavengers that are present in the wastes--e.g., Al(OH), in the ACS. and Fe(OH)3 in the 2CW1; the ILW has essentially none. Recent experiments with tracer-Ie~el~~'Cs has shown that the addition of about 2 weight grundite clay to ORNL IWL can decrease the leach rate of cesium by a factor of about 10; thus, the addition of this material, which selectively sorbs cesium, could reduce the leach rate of cesium to a value comparable to that for ruthenium and strontium. In other tests with external irradiation from W o (see radiation stability tests below), the leach rate was not affected at a dose of lo8 rads. Thus, it is apparent that either external or internal irradiation to at least 108 rads does not affect the leach rate of the asphalt products. Radiation Stability of Asphalt Products Conservative calculations (Godbee, Blanco, er ul., 1967) show that, generally, a n absorbed dose of about lo9 rads will not be exceeded for intermediate- and low-level wastes. The radiation stability of asphalt products was determined by external irradiation of nonradioactive samples with €OCo, and internal irradiation of samples containing from 2 to 52 curies per gallon of aged mixed fission products. External Irradiation. Products containing 60 weight solids from ORNL ILW were prepared from a hard-base asphalt (about 150 penetration) and irradiated to doses of lo6to lo8 rads with a W o source. A dose of l o Erads showed only a negligible effect, a dose of lo7 rads caused slight swelling, a dose of 108 rads caused a sample to swell about 3 6 z in volume (Figure 6), and a dose of lo9 rads caused a sample t o swell 7 0 z in volume. In contrast, similar samples prepared from a soft-base asphalt (about 200 penetration) and irradiated to a dose of lo8 rads showed only a slight increase in volume (Figure 6). Presumably, gaseous products, if formed, were released by the softer product. The results obtained from samples containing 60 weight salts from ACS and 2CW1 were about the same as those for ILW at similar dose levels. In addition to swelling, the samples became harder the longer they were irradiated; however, the samples that were irradiated to the highest doses were still pliable at room temperature. Internal Irradiation. Products containing 60 weight solids from ORNL ILW, ACS, and 2CW1 and containing from 2 to 52 curies of aged mixed fission products per gallon were prepared from a hard-base asphalt (about 150 penetration). An ORNL ILW-asphalt product that contained 52 curies per gallon of product showed no gross volume increase
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Figure 6. Cross sections of asphalt products containing 60 weight solids from simulated ORNL intermediate-level waste Uppwkfl. Upper right.
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Unirradiated.
Hard asphalt irradiated to 10' rads Lower left. Hard asphalt irradiated to 109 rads Lowerright. Soft asphalt irradiated to 108rads Irradiation by externa16DCo
Fiaure 7. Asphalt products after exposure to radiation Upper. ILW-asphalt product after exposure to 4.2 X IO' rads. Product contained 52.6 curies per gallon Cenier. ACS-asphalt product after exposure to 2.3 X 1O'rads. Ploduct contained 30 curies per zallon Lower. 2CW-asphalt product after exposure to 1.8 X 10' rads. Product contained 33.4 curies per gallon
or large gas bubbles after an exposure of 4.2 X lo7 rads (Figure 7, upper); however, bubbles 1 mm. or less in diameter were observed. After an exposure of 2.3 X IO1 rads, the ACSasphalt product was soft and rubbery but contained no bubbles (Figure 7, center). A few '/Anch-diameter bubbles could be seen (Figure 7, lower) in the hard 2CW1-asphalt product after exposure to 1.8 X 107 rads. No net evolution of gases or swelling has been measured in these internal irradiation tests with hard asphalt which have now reached a dose of 1 X lo8 rads (over 838 days) as contrasted to an average 36% volume increase observed with external irradiation at 1 X IO8 rads (over 10 days). The difference in irradiation time may be significant-for example, the gases, if formed, may recombine with the asphalt over an 838-day period. It was concluded that asphalt products show satisfactory physical and chemical stability at irradiation doses of lo8 to los rads. Net gas formation and product swelling may occur but can be controlled by using a softer base asphalt t o permit release of gases. These results compare favorably with those reported by others. Almost a decade ago, workers (Watson, Hoiberg, et al., 1958) at ORNL concluded that, in general, asphalts and tars can be used successfully in radiation environments as long as the total exposure does not exceed about lo9rads. Also, Belgian workers (Van de Voorde and Dejonghe, 1966) found that irradiation up to a dose of loprads had no significant effect on the elution rates of sludge-asphalt mixtures.
Safety Evaluation of Salts-Asphalt Products The incorporation of inert solids in asphalt does not appear to present any hazard beyond that normally present when an organic material with a high flash point is being processed, stored, or shipped. In fact, the flash point of an asphalt product should increase as the amount of inert solids increases. The hazards caused by incorporating an oxidant (nitrate salts) in asphalt were studied intensively. Tests on the ILW product (60 weight % solids, including 35% nitrate) by the Bureau of Explosives (Association of American Railroads, New York, N. Y.)indicated that the material did not fall within any of the classes of dangerous goods as defined by Interstate Commerce Commission regulations. In laboratory tests at ORNL, small (0.5 to 1.5 grams) samples were heated on a hot plate. A Nichrome wire was embedded in the samples to provide a higher-temperature ignition source for the vapors being formed. Samples were heated to about 220" C., and the igniter wire was then heated rapidly until it was bright red. With base asphalt (plus emulsifying agent) the temperature (average of ten tests) at which a flame was self-sustaining was about 280" C.; with samples containing 60 weight % solids from ILW, this temperature (average of 11 tests) was about 330" C. The flame was easily extinguished when. the air was replaced by nitrogen. In other tests, similar samples, containing 60 weight % solids from ILW, ignited spontaneously at -330" C. Samples containing 65 weight % solids, made from a solution containing 2.7M NaN03, 2.7M NaNO1, and 2.1M NaOH, spontaneously ignited at a temperature 55" lower-Le., 215" C. However, base asphalt alone did not spontaneously ignite at temperatures up to 450" C. The facts that NaNOa and NaNO, melt at 308" C. and 271" C., respectively, and that the solids tend to settle to the bottom as the asphalt mass liquefies at higher temperatures, may be significant. The burning rate in all cases was much faster for the simulated waste products than for pureasphalt. A subcontractor (Herickes and Schicker, 1966) made standard drop-weight, autoignition, shock sensitivity, and flameVolume 2, Number 11, November 1968 1039
propagation tests of small samples containing 20, 40, and 60 weight solids from ILW. These tests did not show any explosive potential. In standard burning tests (Herickes and Schicker, 1966) with 30-gallon-drum samples, the saltsasphalt mixtures burned vigorously, as would be expected, but did not explode or show a significant increase in burning rate over that for pure asphalt. However, the internal temperature in the asphalt mass was not monitored in these tests. Conclusions Ignition tests on small samples of asphalt product containing large amounts of nitrate or nitrite salts showed a sharply enhanced burning rate but no explosive tendency or ability to support combustion internally in an inert atmosphere. Nitrite samples were not evaluated in the standard explosive tests. The burning rate of 30-gallon-drum samples containing 60% solids (35% nitrate) was not significantly faster than that for drums containing pure asphalt. Further work is planned to define more precisely the effect of impurities, such as nitrate or nitrite, and the probability of fire or explosion. A careful evaluation of the fire hazard is required for the storage of the asphalt products containing oxidants either in drums prior to burial, or in massive quantities in tanks. Acknowledgment The authors acknowledge the assistance and advice of many ORNL personnel: J. R. Flanary, G. D . Davis, J. M. Holmes, and E. J. Frederick. Many experiments were performed by G. D . Davis, L. A. Byrd, 0. L. Kirkland, D. 0. Rester, R. L. Taylor, and R. C. Runowski. Gratitude is also expressed to experts in asphalt technology for advice and samples: B. R. Mogul, Vulcan Materials Co., Chattanooga, Tenn. ; Boyd Bassett, Vulcan Materials Co., Knoxville, Tenn.; K. E. McConnaughay and John Spahr, K. E. McConnoughay, Inc., Lafayette, Ind.; and W. T. Ratliff, Tennessee Asphalt Co., Knoxville, Tenn. Literature Cited American Society of Testing Materials, “1967 Book of ASTM Standards, Part 11. Bituminous Materials for Highway Construction, Waterproofing, and Roofing; Soils; Skid Resistance,” Test ASTM D-5-65 (1965). Burns, R. H., Clarke, J. H., Wright, T. D., Myatt, J. H., “Present Practices in the Treatment of Liquid Wastes at the Atomic Energy Research Establishment, Harwell,” SM-71/58, pp. 17-29, in “Practices in the Treatment of Low- and Intermediate-Level Radioactive Wastes,” IAEA, STIjPUBjll6, Vienna, 1966. Dejonghe, P., Baetsle, L., Van de Voorde, N., Maes, W., Staner, P., Pyck, J., Souffriau, J., “Asphalt Conditioning and Underground Storage of Concentrates of Medium
1040 Environmental Science and Technology
Activity,” in “Third United Nations International Conference on the Peaceful Uses of Atomic Energy,” A/CONF. 281P1774 (Mav 1964). Dejonhe, P., Van de Voorde, N., Pyck, J., Stynen, A., “Insolubilization of Radioactive Concentrates by Asphalt Coating,” Final Report 2, 1st Part, Concerning Proposal 167, EURAEC-695, April 1, 1961, to March 31, 1963. Fitzgerald, C. L., “Waste Treatment and Disposal SemiAnnual Progress ReDort, July-December, 1964.” . -DD. _ 54-57, ORNE-TM-1081 (May 1965). Godbee. H. W.. Blanco. R. E.. Frederick. E. J.. Clark. W. E.. Rajan, N. S. S., “Laboratory Development o f a Process for Incorporation of Radioactive Solutions and Slurries in Emulsified Asphalt,” ORNL-4003 (April 1967). Goode, J. H., “Fixation of Intermediate-Level Radioactive Waste in Asphalt : Hot-Cell Tests,” ORNL-TM-1343 (Nov. 18,1965). Goode, J. H., Flanary, J. R., “Fixation of Three IntermediateLevel Radioactive Waste Concentrates in Asphalt: Hot Cell Evaluation,” ORNL-4059 (January 1968). Herickes, J. A., Schicker, W., “Safety Characteristics of Waste-Asphalt Products,” Summary Rept. TR-PL-9278; (work performed under Subcontract 2064 with Atlantic Research Corporation, Alexandria, Va.) (Dec. 28, 1966). Rodier, J., Alles, M., Auchapt, P., Lefillatre, G., “Solidification of Radioactive Sludges Using Asphalt,” SM-71/52, pp. 713-29 in “Practices in the Treatment of Low- and Intermediate-Level Radioactive Wastes,” IAEA, STI/PUB/ll6 (ORNL-Tr-1432), Vienna, 1966. Rodier, J., Lefillatre, G., Scheidhauer, J., “Bitumen Coating of the Radioactive Sludges from the Effluent Treatment Plant at the Marcoule Center,” Review of Progress Reports 1 , 2 , 3, and 4, CEA Rept. 2331 (ORNL-Tr-202), 1963. Rom, A. M., “Development of the Waste-Asphalt Process on a Semiworks Scale: Design and Installation of Evaporator Eauipment in Building- 4505. ORNL-TM-1637 6. e p-tembe; 1966). Rom. A. M.. “Incorooration of Intermediate-Level Waste in Asphalt: Preliminiry Design and Cost Estimate of a FullScale Plant for ORNL,” ORNL-TM-1697 (January 1967). Suddath, J. C., Long, J. T., Pilot Plant Tests, Chemical Technology Division Annual Progress Rept. ORNL-4145. 115-19(May31,1967). United States Government. Federal Soecification SS-A00674C (1962). Van de Voorde, N., Dejonghe, P., “Insolubilization of Radioactive Concentrates by Incorporation into Asphalt,” SM-71/4, pp. 469-600 I’n “Praciices in the Treatment of Low- and Intermediate-Level Radioactive Wastes,” IAEA, STIJPUBI116 (ORNL-Tr-1431), Vienna, 1966. Watson, C. D., Hoiberg, A. J. West, G. A., Ind. Eng. Chem. 50, NO, 8, 87A-90A (1958). Wormser, G., Rodier, J., de Robien, E., “Improvements in the Treatment of Radioactive Residues,” “Third United Nations International Conference on the Peaceful Uses of Atomic Energy,” A/CONF,28/P/86 (May 1964). Receiced for reciew May 1, 1968. Accepted September 3, 1968. Diuision of Petroleum Chemistry, 156th Meeting, ACS, Atlantic City, N . J., September 1968. Research sponsored by the U.S . Atomic Energj Commission under contract with the Union Carbide Corp.
