The Manufacture of Uranium Dioxide Fuel in Pellet Form

THE MANUFACTURE OF URANIUM DIOXIDE FUEL IN PELLET FORM

Arnold Blum Iscar Ltd , Nahariya

Introduction

Commercial nuclear fuel for conventional pressurized water reactors (PWR) is being produced in tonnage quantities on the basis of low enriched uranium oxide (UO ), i.e., UO in which the uranium is enriched to 5% or less in the fissionable isotope U-235. The bulk of the fuel processed is of an enrichment between 2-3% U-235,

While the fuel is generally drawn from the enrichment plants in the form of UFR, there are a variety of processes by which it is converted to uranium oxide. The method of conversion has a significant bearing on the properties of the uranium oxide powder which serves as the stating material for pellet fabrication This problem is being dealt with by another, and will not ba treated here,

A fossil-fuel furnace is principally designed to generate heat, but in the process of doing so, also produces fay-products in the form of solid residues and gaseous effluents. A pressurized water reactor is ana- logous in its mode of operation. One of the solid residues of the fuel is the metal plutonium, which can be re-cycled and used in subsequent fuel cores. The fabrication of plutonium containing fuel, irrespective of its use, falls outside the scope of this paper.

The fabrication of low enriched uranium oxide pellets is in essence a process for generating ceramic bodies, but the fabrication processes are complicated by problems of nuclear safety, security, health & safety, high intrinsic value, enrichment control, scrap recovery, accountability, effluent control and environmental protection, and the high sensitivity of the fuel to even minor contaminants, which are of no consequence in - 2 - conventional ceramics. The problems peculiar to the production of nuclear fuel result in a high overhead operation, with a fixed cost unheard of in most other industrial operations.

The basic process, and some of the problems alluded to abova, will be dealt with in the following parts of this paper.

The Basic Process

1. Powder Since the mechanism by which UO^ powder is converted to fuel pellets is based on the application of pressure and heat, the required magnitude of these, depends in large measure on the intrinsic surface energy of the powder. On the National Archives in Washington, D.C. is found the inscription: "The Past is Prologue". The truth of this state- ment can nowhere's be demonstrated as effectively as in the fabrication of UO, pellets.

The impact of powder properties on the economics of fuel fabrication is overwhelming, in regard to the reguired compacting pressures, sintering temperatures and sinterinq times, and the resulting density and integrity of fuel pellets which have a direct bearing on product yield.

At the time of my last involvement in the fabrication of pellets, about one year ago, there was still raging a controversy among fuel designers as to the most desirable structure of sintered fuel.. Since in the fission process, fission gases are generated, these must be acco- modaterl inside the fuel pellets if these are not to disintegrate into dust under the influence of internal gas pressure. On one side of the argument were lined up the advocates of moderate density pellets (92 - 93% of theoretical) characterized by a microstructure with a high density matrix interspersed by well rounded, uniformly distributed pores. - 3 -

Such a structure, because of the spherical nature of the porosity, would not be expected to shrink significantly under the influence of the hiqh temperatures prevailing inside the fuel under reactor operating condi- tions .

On tho other side of the controversy were the advocates of high density pellets (96% of theoretical and higher) who argued that on high burn- up, fuel shrinkage was bound to occur even if the pores were well rounded in medium density structures, and such shrinkage would tend toward fuel clad failure by tube buckling. Some such failures had just been discovered in operating reactors, and these reactors shut-down by order of the U.S. A.E.C.

The outcome of this controversy will h»ve a decided impact on the types of powders required.

High density pellets require powders with a specific surface of 3 rn /gram and up and Fisher Sub sieve Sizes in the range of 0.8 - 1.1 microns.

Medium density pellets require lower specific surfaces, about 2 2.5 - 2.8 m /gram and Fisher Sub Sieve Sizes of 1.2 - 1.5 microns.

There are, of course, other means of affecting the density of fuel pellets, such as the magnitude and kind of the lubricant addition and the

mariner in which it is applied; the magnitude of U,0o additions, used as the preferred method of recycling sintered scrap; the compacting pressure; and the sintering temperature. But the predominant factor in achieving a desired sintered fuel pellet structure lies in the quality of the starting powder.

Attempts were made in correlating the particle size distribution with the sintering behavior of powders, using 3 different types of instruments. None of the results were successful. - 4 -

Another important quality attribute of UO powders is that of the 0/U ratio. Since UO is not the hiohest oxide of uranium, it can readily oxidize in the'finely'divided state. A certain amount of re-oxidation is almost inevitable as UO powder exits from the reduction step and is auto- natically conveyed through mechanical treatments to the weighing and blending station. Dry ice is used to provide a CO blanket over exposed UO and also to extinguish fires which sometimes start spontaneously in exposed containers.

U/0 ratios greater than 2.15 are not acceptable for pellet powder. Great care is exercised in maintaining the chemical and isotopic purity of UO- powders. Since mechanical conveyances are used in the preparation of the powder, there is the danger of contamination by abrasion of the contacting surfaces, principally by iron group metals. If is for this reason that 400 series stainless steel is preferred as material of cons- truction of contacting surfaces. This series of stainless, being magnetic, can be removed by magnetic separation.

Neutron absorbers, such as cadmium, used as plating for steel faste- ners, and silver, used as a constituent of braze joints, are carefully excluded from powder and pellet facilities. So are boron, and the rare earths. The powders are analyzed for a lona list of elements, each speci- fied as to the permissible maximum concentration allowed.

Isotopic analysis by raass-spectograph is required for each powder and pellet lot. In a plant processing a diversity of enrichments, a thorough "isotopic" cleanup between different enrichments is a major effort. Ins- pection and release by Quality Control is required before the facility may be used for a new enrichment. - 5 -

2. Press Feod Preparation I all automatic compactino presses the reproducibility of green density, weight and heiaht of a compact depends on reproducibly volume- tric die filling.

In order to assure this, the powder must be rendered flowable in a reproducible manner. This requires the addition of organic lubricants which tend to hold tooether aggregates of primary particles to form spheres or granules which flow into the pressing die. If the spheres are imperfect, or fines are present in the press feed, they tend to cause improper die filling by "bridging", and the weight of material delivered to the dies fluctuates within excessive limits.

Conventional press-feed preparation uses liaht compaction of the raw powder into low density compacts which are pushed through a screen and so broken up into granules. These granules are blended with a small percen- tage of an organic binder and rounded off into balss during the blending operation, so as to make them free-flowing. Some processors screen the blended material in order to remove fines and large balls, which impede flow.

More advanced means of accomplishiRo this parpose are by the use of a "spray-drier". In such equipment a slurry composed of raw powder, an organic vehicle and the binder is pumped t.o a drying tower, where it is sprayed upward from a nozzle and the volatile vehicle is evaporated by a counter-current flow of hot, inert gas, causing the droplets of slurry to congeal into small balls or spheres which are then removed from the tower.

Such press feed is more uniform, softer and freer of contamination, since it has not been exposed to abrasion as in the case of mechanical granulation. - 6 -

However, nuclear safety aspects of the spray-drying techniaue have to be carefully engineered, due to the exposure of the UO-to fair-sized volumes of hydrogenous vehicles.

Yet other press-feed results directly from the powder making process, in which the primary aggregates are generated in spherical form. Such aggregates are free flowing without an intermediate processing step. Lu- bricant must then be added to the dip walls during the pressino operation, since the close fitting punches cannot operate without lubrication.

3. Pellet Pressing

Si.nce a 3000 kg. lot of powdr;r is converted into several hundred thousand pellets depending on pellet size and density, the press must be capable of high productivity and the tooling must be highly abrasion resis- tant to survive a production run. Cemented tungsten carbide tooling is therefore standard for UO' pellet production.

To minimize pellet-cladding interaction, as fuel clad tubes shrink onto the fuel stack, it has been found advisable to chamfer fuel pellets. Furthermore as fuel center line temperatures tend to be far higher than periphereal temperatures, there is a non-uniform axial expansion of fuel pellets in an operating reactor. Pellets with flat end faces would therefore tend to bulge axially, so as to form convex end configurations with resultant danger of intrusion of clad wall between adjacent pellets. For this reason fuel pellets are designed with dished end faces. The resulting pellet design therefore incorporates both, dished and chamfered end faces.

This type of pellet geometry pressnts a complex stress pattern in the green compact with the chamfered and dished portions being of higher green density than the annular flat land, separating them., Such a stress pattern often relieves itself on sinterincr by a partial separation of the ends from — 7 — the body of the pellet Such a phenomenon is called "cappinq" and is one of the problems besetting the pellet manufacturer.

"Capping" can be minimized by compacting pellets at the lowest compacting pressure which will yield the desired sintered density, and by the addition of U 0 from sintered scrap. The latter, however, also de~ JO presses the sintered density of pellets and a trade-off between pellet density and pellet integrity must then be made. Chipping of pellets results from manual handlinc. and the random impingement of green pellets aqainst each other if gravity filling of furnace boats is used. Chipnina can be minimized by mechanical take-off of pellets from the press table either by vacuum or by cushioned gras- ping or pushing devices coupled with mechanical pellet stackers.

The type of press best suited for pellet pressing is still a large question. There is one school which advocates rotary presses which can be tooled up with 16 stations or more. These presses are mechanical, coupled with an hydraulic eaualizer by which the pressing pressure is controlled. The punch movement is controlled by cylindrical cams. "Overfill" and "underfill" features are available. Hold-down can be exerted on the pellet being ejected by adjustable springs or hydraulic devices. Rotary presses are capable of pressing 160 pellets per minute when operating at 10 rpm and 16 stations, They are of simple construc- tion, easily maintained and can be equipped with automatic inspection devices with accept - reject features. Rotary presses are ralatively small, and since they press only one pellet at a time, require only small motors. They do lack some of the sophistication built into hydro- lie presses, however, such as variable rate of punch advance, and cannot operate on the withdrawal principle, since their die-table is verti- cally fixed. They also have limited control over dwell in the press posi- tion, as is available in hydraulics.

The other school favors multi-cavity hydraulics or combination, mechanical-hydraulic presses which operate on the withdrawal system. Such presses tend to be far larger than rotaries for equivalent or smaller production rates, since a multiplicity of pellets is pressed at a given stroke - 8 - of the press. They combine the advantaaes mentioned above, with the problem of uniform distibution • " press-tsec to the various die cavities ,ind use rotary devices in their feed shoes to level off the quantity of pross- feed delivered to the cavities. This further complicates the presses and tends to mash back into powder the aranules of the feed.

Though good pellets have been made on both types my own experienco favors the rotary press.

4, Pellet Sintering

As indicated earlier, the typa of powdex available has a large impact on the economics of pellet manufacture, and this can be seen easily in pellet sintering.

U0_ pellets are sintered in hydxoqen atmosphere furnaces between 1625 and 1780 C, depending on the sinterabiiity of the starting powder. Times at temperature vary between i \>'i hour and 9 hours, and production rates - when furnace output is limiting - are inversely proportional to time at temperature.

Furnace element life is inversely proportional to fuxnace temperature- Thus poorly sinterable powder will not only result in poor productivity, but also more frequent replacement of expensive molyb-denum resistance elements and the associated downtime of the furnaces.

Since for a fixed furnace cross-section the throughput varies directly witn boat loading and boats randomly loaded tend to contain only 80% of the number of peliets which can be stacked into a boat, mechanical boat stacking tends not only to reduce chipping, but increase furnace throughput. Molybdenum is the common boat material. Other materials which have been successfully applied are TZM, a titanium, zirconium, molybdenum alloy, and composites of ceramics and molybdenum or TZM.

Boat design depends on the method of boat advance in the furnace. Where hydraulic or mechanical stokers, are used, the bottoms and side- walls of the boats have to resist the thrust of the stoker, thus requiring heavier sections. Such boats arc made of 2.5 - 3 mm. gage material - 9 - with the boats ends folded over and riveted. Inteqral boats from deep- drawn, heavy molybdenum or TZH sheet are also available. They have the advantage of being easier tc clean for isotopic changes.

In addition to stoker furnaces the "walking-beam" type of furnace is gaining acceptance in the industry. This furnace type has the advantage of not requiring contact between successive boats, thus permitting the use of thinner walled boats, and longer hot zones, the stoker type of furnace being limited in length by the stoking force which would induce buckling of a column of boats, tending to generate -jam-ups.

J. Pellet Grinding

Sintered fuel pellets do not meet the tight dimensional requirements imposed by the fuel designer and must therefore be ground to fall within the acceptable tolerances. Centerless grinders equipped with SiC or diamond wheels are used. Automatic feeders and discharge mechanisms are em- ployed. Inspection is on a sampling basis, since the adherence of grinding swarf makes impossible dimensional checking without prior cleaning.

Diamond wheels are preferred over silicon carbide wheels from considerations of recycling the swarf, since such wheels abrade to a much lesser extent and their contribution to the swarf does not require wet chemical reprocessing. Silicon carbide wheels, on the other hand, tend to yield better surface finishes.

In order to avoid extensive cleaning procedures, only pure water is used as a coolant during grinding. To permit the use of water without rust inhibitor, the exposed parts of the grinder are made of stainless steel or are protected by chrome plating.

6. Final Inspection

Final inspection of fuel pellets comprises the following quality cha- racteristics: Chemistry, isotopic content, density, dimensions, end profile, compressive strength in the axial direction, O-235/unit length, surface finish,

U/0 ratio, cracks, chips and cleanliness. /1n - 10 -

7, Packing and Shipping

Pellets are loaded into the troughs of corrugated stainless steel trays from automatic pellet loaders. The trays are stacked, interleafe

The carboard boxes are stapped to wooden planks with heavy glass fibre tape in a single layer. The planks are then loaded into the cavities of special steel drums provided with water tight closures. The center line distance between adjacent drums is calculated to be criti- cally safe for the enrichment being shipped.

Nuclear Safety Nuclear safety considerations provide the overridina design crite- rion for the production process. Wherever possible "safe geometry" is resorted to. This concept makes the generation of an unintended ciiticality impossible, as long as there is no tampering with the approved processina equipment and equipment lay out. Where for reasons of production feasibility of efficiency "safe geometry" cannot be used, "administrative control" is resorted to which restricts the mass of nuclear material in a given location to pre- determined limits, using sign-m and sign-out procedures under tight su- pervision. Roving inspectors, and criticality monitors and alarms further implement these procedures.

Security While low enriched U0- fuel would require the diversion of relati- vely large quantities of material for the illicit generation of a nuclear device, the danger of terrorizing the population upon even a minor diversion becoming known, must be reckoned with. Any nuclear facility must therefore be monitored at all possible means of ingress and egress, and the effectiveness of the control procedures must be frequently checked. Details of security measures used fall them- selves into a classified area and are not treated here. The same holds

.../ll - 11 - true for security measures applied to the transport of nuclear maceriaLs.

Health and Safety While in countries outside Israel increasing attention is being paid to industrial safety the nuclear fuel processor, for humanitarian and au- rely selfish motives, has to pay particular attention to die health and safety of his employees and the qeneral public. The inhalation and ingestion of nuclear materials leads to body burdans, particularly in the lungs and bone marrow which can generate leukemia and other forms of cancer, "'ven minor cuts are not to bs taken lightly. The use of filtered exhausts at work stations, special protective clothing, including gloves, shoes, shoe covers, hats and under clothing, the firm requirement for showers prior to leaving the facility, perso- nel monitoring, periodic nose smears, urine analysis, fecal counts and full body counts are all parts of the arsenal the health physicist uses to protect the employess of a nuclear fuel processing plant. The general public is being protected by the monitoring of plant effluents, the use of absolute filters and the inspection to extremely tight specifications of all containers, vehicles and all materials leaving the processing plant.

High intrinsic Value The high intrinsic value of enriched uranium is a great incentive to careful handling. The financial exposure of a fuel processor by far exceeds any profits tc be derived from toll-conversion. Careless handling of nuclear fuel will therefore be intolerable not only from the points of view of r.acleax safety, security, health and safety, and environmental protection, but will also lead very quickly to the financial death of the processor.

Enrichment Control Since the various parts of a single reactor core are generally nom- posed of more than, one enrichment, it follows that a fuel processor must know how to deal with different enrichments of fuel to be handled succes- sively in the same equipment. This requires tight control over the cleanliness of each piece of equipment and the plant befce a new enrichment

.../12 - 12 - can be processed. This consideration dictates that ail equipment and the plant be designed for ready cleaniation of enrichment is always costly.

•Scrap Recovery The recovery of cold, i.e non-irradiated, scrap is an integral part of the fabrication of nuclear fuel. While sintered scrap can normally be recycled by oxidation to U,0 , followed by comminution and meterina back into the 110 powder stream as U30Q or, upon reduction to UO , contaminated scrap roust be recycled through dissolution, and solvent extraction. This is A costly process beset by its own peculiar set of problems of chemistry and nucloar safety.

Considerations of security (ability to rapidly discover diversion of sipecial nuclear materials); nuc3ear safety; enrichment control-, financial responsability; effluent control and environmental protection require that an accurate, perpetual inventory be maintained by the processor on both a local and plant-wide basis of all special nuclear materials under his control. This implies that a fuel processina plant be administratively sub- divided into material balance areas (MBA's) each of which is treated as a reciver, a repository and a shipper of special nuclear materials, and that each movement in and out of this MBA be documented and approved by a control center. Since there may be hundreds, or even thousands, of such transactions in a given week, it must be clear that computer assistance is required if such transactions are to be monitored with any degree of tiraelijiess an-1 accu- racy. Depending on enrichment the physical ihventori.es in each MBA are com- pared periodically with the. book inventories maintained by computer and the difference between them determined..This difference, called MUF (material unaccounted for) must fall within pr^ "stermined narrow limits and so must the limits of error of tnis quantity (LEMUF), which are the result of .in elaborate statistical ©valuation. The MUF must always be less than the LEMUF an<3 must be a fraction of a

-\:l\ ' , : .;••' • '. • ••"••• - .-••• .•.-•••'.. • • ,../i3 - 13 - percent of the weight in notion durinq the inventory period. Exceeding these Jimits will triqqer an investiaation and lead to the presumption of illicit diversion.

Summary I^ The fabrication of UO^ fuel pellets w-as discussed as a process for the consolidation of ceramic powders, having special aspects of isotope control, scrap recovery, nuclear safety, security, health and safety, high intrinsic value, effluent and environmental control, high sensitivity to contamination, and accountability.