US2918717A - Self sintering of radioactive wastes - Google Patents

Description

Dec. 29, 1959 E. e. STRUXNESS ETAI- 2,918,717

SELF SINTERING 0F RADIOACTIVE WASTES Filed Dec. 12, 1956 2 Sheets-Sheet 1 IN V EN TORS Thomas N. Mc Vay BY James R. Johnson Edward G. Srruxness 0 2 4 6 e 10 12 14 16 Karl Morgan TIME, DAYS v GZ" TEMPERAT 2 Sheets-Sheet 2 gan By James R. Johnson ATTORNEY Dec. 29, 1959 E. G. sTRuxNEss ETA!- SELF SINTERING OF RADIOACTIVE WASTES Filed Dec. 12, 1956 SiiVM mam United States Patent ISELF SINTERING or RADIOACTIVE WASTES "Edward G. Struxness, Oak Ridge, Tenn., James R. Johnson, White Bear,'Minn., Karl Z. Morgan, Oak Ridge, Tenn., and Thomas N. McVay, Tuscaloosa, Ala., as-

. signors to the United States of America as represented by the United States Atomic Energy Commission Application December 12, 1956, Serial No. 627,967

'5 Claims. (Cl. 25156) The present invention relates to disposal of radioactive waste materials, and more especially to a novel method of treatment and disposal of highly radioactive waste materials such as those derived from the chemical processing of fuel elements from nuclear reactors.

A major problem posed by the rapid expansion of nuclear power production is the safe and efficient disposal 'of'rad'ioactive wastes. The intense radioactivity emitted bysuch wastes could contaminate the surrounding air, water supply, underground streams, and soil, if proper disposal methods are not devised. The most troublesome wastes are those derived from the chemical processing offuel elements, which contain significant amounts of 'the'materials in which the element is'canned, such as aluminum, zirconium, or stainless steel. Also present in the wastes are fission products, fissile materials, certain chemicals added in processing, and corrosion products. Wastes which'have an activity level at the emitting surface of about two roentgens per hour or more -may be generally classified as high level wastes, while those with a value of up to 50 milliroentgens per hour may be described as low level wastes.

The disposal of radioactive wastes is particularly difiicult because they can not be disposed of by burning, evaporation, filtering, or the transfer of the active materials from one physical or chemical state to another. At present, wastes are stored in underground metal tanks. This method isnot satisfactory in that the metal tanks "are expensive to construct and to bury, and there is the "everpresent danger that active material will be released from the tank due to corrosion of the metal, leaks in the'tank, unintentional puncture, or disruption by earthquakes. Upon occurrence of one such event serious 1 contamination-of both the community water supply and the surrounding ground surface could result. Moreover, underground storage is expensive-the cost having been reported to be from 0.37 to $2 per gallon.

*With a knowledge of the disadvantages of present rnethods of storage of high level radioactive wastes, we have as a principal object of our invention, provision of -a' method for disposal of radioactive waste materials which-minimizes theradiation hazard due to subsequent leachingof' the wastes by ground streams or sea water. Another object of our invention is to utilize the energy 'of-the radiations which make the wastes dangerous to trap the dangerous elements in a relatively durable "ceramic cake. A-further object of the invention is to vtprovide amethod of-treatment of waste which uses only sinexpen-sive, relatively abundant materials, and which :uses -aminimum amount of such materials to minimize the volume ofthe final waste product. Another ob- '-:ject of the invention is to provide a method of treatment of radioactive waste as it is received from fuel reprocess- I ing plants and which does not require any expensivehot .laboratory and radiation shield, and which requires 'a fminimum of handling and transportation from the -lplantswhere the wastes are produced.

These and other objectsof our invention will become tual radioactive materials, supplying internal heat from ulite.

"clay may constitute a suitable charge. filter mat may be disposed over the solution untilv it limestone, and 16.7 pounds-16 mesh shale. wasplaced in a stainless steel pot insulated with carbon black and with vermiculite, which was retained by an 2,918,717 Patented Dec. 29, 1959 apparent fromthe following detailed description ofthe ticing the invention,

Fig. 2 is a curve showing the temperature rise era waste mix heated electrically, and

Fig. 3 is a curve showing the temperature rise of a waste mix under conditions simulating the decay of fission products.

In one method of practicing the invention, high level radioactive liquid wastes are thoroughly mixed with clays and fluxes to form a slip or slurry mix. The mixis placed in a layer in a thermally insulated pit in the earth, the layer having a sufficiently low surface-to-volurrie ratio so that the heat generated by the radioactivity'of the wastes themselves heats the mix to a temperature high enough to drive off the liquid content. When the mix has boiled dry, it is then covered with a layer of a thermal insulator. The heat power produced by the decay of fission products in the waste then causes the insulated dry mass to rise substantially in temperature, ultimately sintering the mass into a ceramic cake with the radioactive materials relatively fixed in the cake. The cake thus formed is resistant to leaching and is suitable for burying in the earth or at sea.

In practicing the invention, a suitable pit is dngin the earth and lined with a thermal insulator, such "as concrete silo blocks 1, resting upon a concrete pad 2. An inner tank 3 of concrete is disposed on foam glass blocks 4. The space between the tanks is filled with a thermal insulator 5 such as vermiculite. The waste solution is thoroughly mixed with the additives above mentioned and disposed in the bottom of the pit in a layer 6. After the liquid is boiled off, the dry mass is covered by a thermal insulator 7, which may be a layer of vermic- Suitable covers 8, 9 serve to retain the tank 3 and insulation 5 in place. A suitable pit may be 6 feet in diameter and 10 feet deep, for example. A mixture of 720 gallons of nitrate waste solution with 720 pounds of 70 mesh limestone, 720 pounds of powdered soda ash, and 2405 pounds of 16 mesh Conasauga shale A glass fiber boils dry, and depending upon the wastes present, the

gases given off may be scrubbed, filtered, or vented 'activity, and in pits of difierent sizes, correspondingly different amounts of clays and fluxes need be provided.

Moreover, the elaborate double wall and full foam glass block bottom should not be required in actual disposal pits for high level wastes, and other better insulating materials could be substituted for those mentioned for lower level wastes Tests may be run to demonstrate the practicability of the novel'method described above without utilizing acanother source. In one such test, a slip was made'by adding to 5 gallons of a waste solution containing 1.6 moles aluminum nitrate and 0.2 mole of nitric acid per gallon, 5 pounds sodium carbonate, 5 pounds 70 mesh The mixture outer container. Referring now to Fig. 2, heat wassu'pplied during the first few hours up to point A at"20 Watts, then raised to watts, and the slurry became comparatively liquid. After two'days the heat powerinure, noticed after 9 days, 21% hours. .dueed was hard and durable.

, gallon of dry cake.

and 0.001 l.b/gal. Si. 1850 B.t.u/gal. is required to raise the 27.5 poundsmixture from l212 F., assuming a specific heat of 0.6

. 3 put was increased to 200 watts at pointB, the liquid level began to drop, and mild boiling occurred, with the mass finally becoming dry on the sixth day of the heating. Vermiculite thermal insulation wasthen poured over the dried mass at point C to fill the inner container. A maximum equilibrium temperature of 850 C. was reached with a specific energy input of about 0.02 watt per cubic centimeter. Upon removal of the insulation and subsequent cooling, the mass was solid, free of cracks, and well sintered. p

A Tests of sintered cakes by leaching with tap water indicate that the most dangerous of all the elements in the radioactive Wastes, strontium is the element most completely fixedin the ceramic cake.

Further tests were run under conditions simulating the production of heat by decay of the fission products. The results of representative tests are shown in Fig. 3. Power was applied in steps (curve A) to correspond to the theoretical power distribution of the radioactive heating (curve B). The temperature of the mix was monitored by three thermocouples. Temperatures /2" above the bottom center are indicated by circles, (curve C), 3" above the bottom at the edge by +s (curve D), and 3" above the bottom at the center by Xs (curve B). The mix consisted of 3.34 gallons of waste solution containing 1.6 moles aluminum nitrate and 0.2 mole nitric'acid, 3.34 pounds ground limestone (passing 120 mesh), 11.15 pounds ground shale (80-120 mesh), and 3.34 pounds soda ash. The solids were added slowly and mixed for about 15 minutes, until the mix became soupy in char- I acter.

applied through calrod heaters in the mix, beginning at a power level of 285 watts. The power level was changed as shown in curve A from an initial 285 watts to 250 watts, 230 watts, 210 watts, and 200 watts. Heat was applied for 6 days, 22 hours, 10 minutes until the cake was dry. Then vermiculite insulation was added atop the 4" dry cake. Heating was continued until heater fail- The cake pro- Analysis of the data indicate a power applied to the 1.96 gallons of cake of 66,247 B.t.u or 33,800 B.t.u. per The average maximum temperature was 796 C. This experimental result compares favorably with the calculated power input required for selfsintering of one gallon of dry cake from 1.7 gallons (18.4) pounds of solution, 5.7 pounds clay, 1.7 pounds .limestone, and 1.7 pounds soda ash, where the solution comprises 1.67 MAI, 0.167 MHNO3, 0.02 MH2S04 Calculations show that about (1) B.t.u./lb./F.; (2) 8600 B.t.u/gal. is required to evaporate the 8.84 pounds H O formed, at 773 B.t.u./lb.; (3)

. disintegrations in the waste. 1

Experiments with heated ceramic cakes indicate that the heat losses will increase as the surface-to-volume ratio of the cake increases. For sintering at 900 C. to

,occur, the radioactive heat level and decay rate at the time of mixing should be such that the requiredheat I (34,000-44,000 B.t.u. is the range suggested'by experiments and calculations) will be emitted by the, waste before theheat emission drops to a level corresponding n ity needed 1n the waste.

.sults than when used alone. cause of its availability and low cost. At increased cost,

to the losses at 900 C. for the particular size of cake. For example, to dispose of a waste solution having a known decay curve, the volume of cake formed is estimated, and the surface-to-volume ratio desired is determined. The total volume of cake is multiplied by 40,000 to get the estimated B.t.utor cake formation. The power losses from a cake of the size selected at 900 C. are determined. This gives a base power. The area under the decay curve is integrated from a level corresponding to the base power to a point sufiiciently far up on the curve to obtain the required amount of heat. This point will indicate the strength of the activity required for a pit of the size chosen to form a self-sintered mass. A curve may be plotted of surface-to-volume ratio vs. power density in the cake to obtain the estimated heat losses at the selected temperature. One such curve for a heavily insulated cake at 900 C. shows about .003 watt/cc. for a pit 6' in diameter and 2' high; .0065 watts/cc. for a pit 2' in diameter and 4" high; .014 watts/cc. for a pit 1' in diameter and 4" high; and .054 watts/cc. for a pit 6" in diameter and 2" high.

All clays should be effective, the only requirement being that the clay form a hard ceramic cake at temperatures which can be achieved with the wastes at hand. Certain clays are more effective at removing certain materials than others, as is known in the art, so that they may be chosen according to the type of wastes to be treated. Conasauga shale, kaolinite, and bentonite are among clays, which are readily available. A large number of experimental compositions were made with aluminum nitrate wastes and various proportions of limestone, dolomite, sodium carbonate, local shale, fly ash, Portland cement stack dust, and phosphate ,tailings. Limestone is preferred because of its availability, and in combination with sodium carbonate gives better re- Shale is also preferred beclays having greater surface area can be obtained and these, such as bentonite, will allow a reduction in the volume of clay required, hence a reduction in the activ- It will be apparent to those versed in the art that the novel method described above is characterized by several important advantages over methods now known to the art: First, the waste is disposed of in a simple process which minimizes dangerous handling of highly contaminated material; second, the waste is fixed in comparatively inexpensive material; third, the waste is processed completely inside an earth pit which serves as a radiation shield; and four and most important, the self-heating of the wastes through its own radioactivity is utilized to fix the wastes in the ceramic body.

Having described our invention, what is claimed as novel is: r

1. A method for disposal of radioactive liquid waste materials which comprises the steps of incorporating said wastes into a ceramic slip, disposing said slip in a thermally insulated container in a layer, elevating the temperature of said layer by conversion therein of the energy of radioactivity to heat to form a dry mass, covering said mass with thermal insulation, and sintering said mass into a ceramic cake solely by further conversion of said energy to heat. 1

2. A method for treatment of radioactive liquid waste material which comprises the steps of disposing said wastes in a layer in a thermally insulated container in an earthen pit, mixing with said wastes a selected amount of clay to form a slip, boiling said slip to a dry mass by convertingto heat in said layer the energy emitted by said waste material, covering said dry mass with a thermal insulator, further elevating the temperature of said mass by conversion of said energy to heat, and sinter- .ing said mass by further conversion of said energy to heat to form a ceramic body having fixed thereinradioactive components of said wastes. i

3. A method for treatment of radioactive liquid waste material which comprises the steps of admixing with said wastes clay, sodium carbonate, and limestone to form a slip, disposing said slip in a layer in a thermally insulated earthen pit, elevating the temperature of said layer to the boiling temperature by conversion to heat therein of the energy generated by said radioactive wastes, evaporating said slip to a dry mass, thermally insulating said dry mass, further elevating said temperature by conversion of said energy to heat, and sintering said mass by further conversion to heat therein of said energy to form a ceramic body having substantially fixed therein radioactive components of said waste.

4. A method for treatment of radioactive waste materials including aluminum nitrate, nitric acid, and fission products which comprises the steps of admixing with each gallon of said waste solution substantially 3.34 pounds clay, 1 pound sodium carbonate, and 1 pound limestone to form a slip, disposing said slip in a layer in a thermally insulated earthen pit, evaporating said slip to a dry mass solely by conversion of the energy generated in said wastes to heat in said layer, covering said dry mass with vermiculite, elevating the temperature of said mass by further conversion of said energy to the sintering temperature, and sintering said mass by further conversion of said energy, to form a ceramic body having fixed therein radioactive components of said wastes.

5. A method for treatment of radioactive liquid waste materials generating more than about 3400 B.t.u. per gallon which comprises the steps of mixing with said wastes a selected amount of clay to form a slurry, disposing said slurry in a layer having a predetermined ratio of surface to volume in an earthen pit lined with concrete, boiling said slurry to a dry mass by conversion of the energy of radioactivity of said waste to heat, covering said dry mass with a thermal insulator, further elevating the temperature of said dry mass by conversion of said energy to heat, and firing said dry mass into a ceramic cake by further conversion therein of said energy to heat.

References Cited in the file of this patent Sewage and Industrial Wastes, vol. 28, No. 6, June 1956, page 791, lines 42-47. Article entitled, Problems of Radioactive Waste Disposal by C. P. Stroub.

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