COLLABORATIVE
REPORT
CHESTER PLACEK, Associate Editor in collaboration with EDWARD D. NORTH Mallinckrodt Nuclear Corp., Hematite, Mo.
Uranium Dioxide Nuclear Fuel A at the world’s still infant nuclear industry shows a myriad of QUICK GLANCE
projects in one stage of completion or another. Included among these: reactors for electric power generation, ship and submarine propulsion, research, and education. And most of these reactors are (or will be) fueled by what is today’s nuclear fuel of choice-uranium dioxide (UOZ). Although UOZ is, by far, the most widely used nuclear fuel, producers of the chemical are geared more toward the future than they are for immediate demands. Current producers of UOz fuel have a production capacity exceeding 1960’s demands, or even next year’s. In 1958, the UOz fuel market totaled around 200,000 pounds. And by the end of 1961, annual requirements will not go up by much more than another 50,000 to 80,000 pounds. These estimates ( 2 ) are based on the number of UOYfueled reactors now operating or under construction, and the time limits placed on their original fuel charges. But by 1962, the market may jump to about 500,000 pounds a year, then possibly to about 2,000,000 by the end of the sixties. These figures are only rough estimates, and military needs are not included. Nuclear fuel makers today are: Mallinckrodt Nuclear Corp., a subsidiary of Mallinckrodt Chemical Works (first in the field); W. R . Grace’s Davison Chemical Division at Erwin, Tenn. ; hluclear Materials & Equipment at Apollo, Pa.; and Spencer Chemical, whose plant is near Pittsburg Kan. Mallinckrodt Nuclear’s operation, with which this report deals, is located at Hematite, Mo.
which was used in the now almost legendary, first self-sustaining chain reaction carried out at the University of Chicago by Dr. Enrico Fermi and his associates. Chronologically, Mallinckrodt started producing major amounts of uranium oxide in July 1942. By the spring of 1943, a uranium tetrafluoride plant was in operation. That same summer, uranium metal was made. After World War 11, Mallinckrodt remained the only company to continue producing uranium materials of nuclear reactor quality. And, it was the only organization that stayed in operation after the technology became generally known and practiced by others. In 1949, the Atomic Energy Commission built another fuel material facility, located away from St. Louis. This unit is operated (near Cincinnati) by another firm. Then, when construction of still another uranium plant was decided upon in 1954, AEC selected Mallinckrodt as the operator. The new plant went up at Weldon Spring, Mo., and was completed in 1958. I n 1956, Mallinckrodt decided to go ahead as the first commercial manufacturer of nuclear fuels. The Hematite site was chosen for the plant, and first production began in September of that year. Since then, UOz capacity has been increased sevenfold, and facilities for producing nuclear fuel-grade metallic uranium have been installed. To date, the Hematite plant has produced and
Nuclear Role Since 1942
Mallinckrodt’s entry into things nuclear began in 1942, when the St. Louis firm was assigned the job of mass producing laboratory-pure uranium oxide, even before the Manhattan atom bomb project went into effect as such. And, in fact, Mallinckrodt purified the uranium
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sold more UOp nuclear fuel than the three other manufacturers combined. The Hematite plant’s major product, so far as sheer volume goes, is UOz in its various enrichments and grades. From the manufacturing point of view, this process is relatively simple. I t begins with uranium hexafluoride (UFs) obtained from the Atomic Energy Commission. This fluoride is reacted with ammonia to obtain ammonium diuranate, and the salt is transformed first into the oxide (UaOg), then reduced to UOz. But though UOz processing seems simple, process control is quite involved. This is due to : the complex chemistry of uranium, the high purity required, safety precautions, the number of enrichments being processed with associated need for avoiding contaminating one enrichment with another, and the high value of uranium. Process Portion
Mallinckrodt obtains most of its UFe from the AEC enrichment plant at Oak Ridge, Tenn., and some from plants at Paducah, Ky., and Portsmouth, Ohio. Strictly speaking, the enriched UFs raw material is not purchased. Mallinckrodt and eventual consumers in effect “rent” the material from AEC. All the uranium thus obtained must eventually be accounted for. The starting material generally comes in two different sized cylinders-the
The Hematite Plant Is Equipped to Produce @ Low enrichment uranium compounds (up to 5% U235) @ Middle enrichment uranium compounds (5 to 20% U235) @ High enrichment uranium compounds (more than 20% UZ3j) @ Uranium metal-high and middle enrichments @ High-fired and high-fired-shot grades of UOz
@ Special density UOz
UOz pellets and other pressed and sintered shapes @Soluble uranium compounds such as UO2SO4 and UOzFz @ Scrap recovery plants for high and low enrichment materials
*
larger (12 inches in diameter) contains 420 pounds of UF6, and is used for all 3.7y0 or lower U235 enrichments. For UFB containing more U2as than that amount, a smaller cylinder ( 5 inches in diameter) containing a maximum of 55 pounds is used. UFO is solid at room temperature and melts at 65’ C. Its boiling point is only 2’ higher at 1 atm. pressure. The UFB obtained from AEC is used in one of three processing streams. Low enrichment material is worked in the “green room,’’ intermediates in the “blue room,” and high enrichments are processed in the “red room.” The process is identical in each room except for scale of equipment. For the process, a uF6 cylinder is heated by a steam or hot water jacket. UFO vapor flows from the cylinder through a dry, stainless steel line into a stainless steel tank (9E) containing aqueous ammonia. Flow of UFe into tank is measured on a rotameter (4E). Size of the tank varies with each of the three processing lines. For example, large volume production done in the green room uses a 1000-gallon tank. By contrast, the red room utilizes a 10-gallon container. As the fluoride reacts with ammonia, a yellow precipitate of ammonium diuranate (ADU) forms. Precipitation rates and particle size are the two variables most rigidly controlled at this step. Control of the two depends upon subsequent processing steps to be taken. Because most UOz is utilized as ceramic grade powder, particle size of the ADU is important. For example: In ceramically active grades, particle size determines activity of the finished product. Without size control, the product may range from being too inert to overactive. Particle size control methods are undisclosed. Individual particle size is in the range of one micron, and the particles accumulate into agglomerates, which can be as large as several hundred microns. During subsequent pelleting operations, crystals grow to an even larger size.
UFa-containing cylinders come from AEC plants at Oak Ridge, Paducah, and Portsmouth in these bird cages. Containers ensure maintaining a “safe geometry” between cylinders
cylinder i s placed at head end of process. to warm the cylinder
Either steam or water i s used
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Heat of reaction during the ADU precipitation is removed with a heat exchanger (3E). Source of the heat is predominantly the reaction of byproduct hydrogen fluoride and ammonium hydroxide. Six moles of H F are produced for every atom of uranium. A half hour to 90 minutes is needed to precipitate one full batch of ADUtime depends on the size of the batch. The green room is equipped to process over 1000 pounds per day. From the precipitation tank, the ADU slurry is transferred to a holding tank (7E) of the same size. From here, the slurry is sent to a plate-and-frame filter ( 7 7E); ammonium fluoride solution is piped to storage tanks for eventual disposal and the yellow ADU is spread out on drying trays. The trays are put into two electric ovens (6E),and the precipitate is dried at 350’ F. Exhausts from this hood go through a scrubber (2E) before being released into the atmosphere. The dry ADU cake is then portioned into steel “reactor” boxes measuring 2 X 2 X 5 feet. These are placed into electrically heated furnaces (72E) which are heated a t 800’ C. for 6.5 hours. At this temperature, ADU decomposes to form U30s, ammonia, and water. Reduction of U308 to U O Zis done in the same furnace in the last portion of the cycle. During reduction, ammonia is blown into the furnace and cracked to give hydrogen. This ammonia cracking is done continuously. Maximum amount of oxide in the furnace a t any one time is 150 pounds (for a low enrichment only). If U236 enrichment is in the 3.9y0region, only half as much compound is reduced at one time due to criticality considerations. After reduction, UOz is cooled in a cooling box (fabricated by Mallinckrodt Chemical Works) under nitrogen. The cooled oxide is milled in a micropulverizer (TOE)to make powder. Powder from several batches is blended. The mixing technique calls for taking one tenth of the contents of ten different drums and blending these fractions into individual drums of product. Mechanical Processing
The blended U O Zpowder is processed into several different grades, depending upon customer requirements. These are: cermet grade (high-fired), highfired shot, and pellets. T o make cermet grade fuel, U O Zis heated at 1600’ C. to obtain optimum crystal size. After heating, the powder is milled and sized to the desired range; 40 to 80 microns, for example. To make pellets, UOZpowder is mixed with a binder (like carboxymethylcellulose or camphor waxes) and a
lubricant-usually soft waxes like paraffin or hydrocarbon waxes, or hydrogenated fats. This mixing stage, called the agglomerating process, is done with some water to wet the powder prior to incorporating the various ingredients. The agglomerate is dried a t about 100’ C., and flowed into die cavities. The pelleting mixture is pressed into pellets at a pressure of about 50 tons per square inch (and sometimes as high as 100 tons per square inch). The compacting machine (5E) discharges the pellets onto a belt which feeds into a dewaxing furnace (7E) maintained at 800’ C. Here, the various organic ingredients of the pelletizing mixture are burned off, and the dewaxed pellets next go to firing furnaces (8E). The pellets are charged into small molybdenum boats and covered with aluminum oxide to cut thermal shock. Firing temperature depends upon density desired. Temperatures can range from 1450’ to 1750’ C.; firing time is 4 hours, and temperatures are held within 10’ of optimum. After cooling, the fired pellets are ground to *0.5 mil of required diameter. Pellets can be fired to 1 2 to 3 mils without grinding. Safety
Although the making of uranium dioxide as such is straightforward where unit operations and processes are concerned, it is distinctly not so from the safety point of view. Safety measures, to be adequate, have to cover both chemical and nuclear properties of uranium. Chemical considerations, briefly, are guided (7) by these characteristics. UF6 is highly reactive, reacting with water, ether, and alcohol to form soluble reaction products. It also reacts with most organic compounds and with many metals. I t does not, however, react with oxygen, nitrogen, and dry air. Reactions with hydrocarbons and many other organics, if held under confinement, produce explosive pressures. Although not flammable, UFe is highly corrosive and can seriously burn body tissue. Nuclear safety concerns itself with two factors. The primary consideration is criticality, a term applied to the possibility of having a sufficiently large mass of fissionable material such as U235 to initiate a chain reaction. The second is radiation, which is the primary concern of the health physics program. Regarding criticality, the major consideration, especially in low enrichment, is the control of mass. In the intermediate and high enrichments, as the limited safe mass is quite low and restricts the batch size considerably, nuclear safety is achieved by the use of safe geometry equipment. Throughout
the Mallinckrodt plant, from storage of UFe to the shipping of processed uranium (except where safe geometry equipment is used), the maximum amount of material held in any one container is less than the limited safe mass of that enrichment. Containers are separated from each other by spacing or by placing “bird cages” around them. All except the smallest shipments are transported in bird cages to ensure separation of containers. The bird cages have been designed to provide adequate spacing and to have sufficient strength to prevent collapse of the cage in the event of a maximum credible accident. The maximum credible accident for this type has been deemed the head-on collison of two truckloads of uranium moving at 60 miles per hour over a bridge. These cages have been designed in such a way that the drums, even when immersed in water following such an accident, will not produce a chain reaction. I n the intermediate and high enrichment areas, uranium is processed in vessels having safe geometry dimensions. Examples of safe geometry are five-inch cylinders, slabs of not more than 1 1 / 2 inch thickness, and containers limited to 293 cubic inches. The use of equipment limited by these factors permits processing of the batches containing more than the limited safe mass for the enrichment being processed. This is extremely important when dealing with high enrichments for which the limited safe mass is approximately one pound of uranium. Uranium is processed wherever possible in enclosed hoods which are provided with exhaust ventilation. In this way, the uranium is kept inside the hood. Personnel, insofar as possible, stay outside. A sufficient face velocity is incorporated in the hood design to provide adequate containment of material. The air exhaust from these hoods is cleaned by high efficiency air filters. Dusty operations such as grinding and screening are carried out in equipment supplied with dust control ventilation. Operations of this type involving highly enriched materials are performed in totally enclosed dry boxes designed by Mallinckrodt. Operator access to such equipment is through gloves fastened in the face of the box.
Health
The industrial hygiene department has the responsibility of evaluating the uranium processing operations with respect to personnel exposures. Routine monitoring of the plant environment to determine airborne uranium concentrations and radiation intensities is perVOL. 52, NO. 6
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A ADU (ammonium diuranate) is precipitated in this tank. Particle size i s especially important at this stage of the process
b This fllter press separates ADU from the aqueous ammonia solution, which i s recycled to the precipitation tank
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A Firing of high enrichment cermet UOz i s done in this furnace. Temperatures here can range from 1450" to 1750" C.
4 UOZ i s milled in a micropulverizer prior to blending A continuous dewaxing furnace burns off organic binders and lubricants used in the pelletizing stage. Here, the furnace i s discharging dewaxed UOz pellets
b
In the blending operation, several batches are blended by mixing one tenth of the contents of ten different drums into individual drums of product. The step guarantees drum-to-drum uniformity
A After sintering, UOZ pellets look like this
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formed. This information, complemented by data from personnel dosimeters, is employed in the determination of individual exposures. From a toxicity point of view, UFG reacts with moisture in air to form a white fog consisting of U O Z F (highly ~ toxic), and hydrogen fluoride, also toxic as well as corrosive. Toxicity of uranium compounds varies with how these reach the interior of the body. Inhalation is usually a more significant problem than ingestion. The primary action of uranium chemical poisoning is on kidneys. From the health physics point of view, Mallinckrodt has strict rules to guarantee personnel safety. These rules are stringently enforced, even for minor infractions. The health physics program includes such things as weekly reports of dust exposures, monthly reports on radiation exposures, urine analysis, and annual physical examinations. Another safety measure is in washing a worker’s clothing. Uniforms are pre-
A button of highly enriched uranium metal, another product of the Hematite plant
washed by the company before being sent out for cleaning. Other Nuclear Fuels
Mallinckrodt Nuclear’s products are not limited to uranium dioxide. T h e
Hematite plant is equipped to make any potential uranium-containing fuel. Research projects are under way in several areas which appear to have future value. A recently completed project provides an improved process for the manufacture of high enrichment uranium metal and uranium dioxide. Uranium hexafluoride is reacted with a reducing agent to produce uranium tetrafluoride directly. Uranium tetrafluoride is then reduced to uranium metal or pyrohydrolyzed to UOZ. Chemical purity of both products was increased by a factor of three. Also made by iMallinckrodt Nuclear on a commercial scale: uranium salts (tetrafluoride, carbide, nitrate, nitride, and sulfate), uranium specialties, and uranium oxide-thorium oxide pellets.
literature Cited (1) Arendt, J. W., Powell, E. W., Saylor, H. W., “A Brief Guide to UF8Handling,”
Atomic Energy Commission Research and Development, Rept. K-1323, p. 10
11957). \ - - I
(2) IND.ENG.CHEM.51, No. 4, pp. 23A25.4 (April 1959).
Processing Equipment (1E) Alpha Tank Co., Long Idand City,
N. Y . , ADU slurry holding tank. (2E) American Air Filter Co., Inc., Louisville, Ky., exhaust scrubber. (3E) American-Standard Industrial Divi-
sion, Detroit. bfich., heat exchanger.
(4E) Brooks Rotameter Co., Lansdale,
Pa., rotameter.
(5E) Decison Engineering Division, American Brake Shoe Co., Columbus, Ohio, UOzpellet press. (6E) Despatch Oven Co., Minneapolis,
This i s a portion of the “red room,” Mallinckrodt‘s high enrichment processing facility
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Minn., ADU electric drying oven. (7E) Harper Electric Furnace Corp., Buffalo, N. Y . ,dewaxing furnace. (8E) Zbid.,pellet firing furnace. (9E) Lee Metal, Inc., Philipsburg, Pa., precipitating tank. (10E) Pulverizing Machine Division, Metal Disintegrating Co., Summit, N. J., No. 1-SH, micropulverizer. (11E) Shriver, T., & Co., Harrison, N. .J., ADU filter press. (1 2F,) Westinghouse Electric Corp., Pittsburgh. Pa., electric furnace.
