JOSEPH H. OXLEY, JAMES F. HANNAH, and JOHN M. BLOCHER, Jr. Battelle Memorial Institute, Columbus, Ohio IVOR E. CAMPBELL National Steel Corp., Weirton, W. Va.
Coating Uranium Dioxide Powders with M eta IIic Tungsten Metallic-coated nuclear fuel particles permit fabrication of improved fuel elements a n d offer a superior fuel material for slurry or fluidizedb e d reactors
O P E r u n o N of nuclear fuel elements at high temperatures and high power generation rates creates many severe problems, such as accelerated corrosion rates, loss of fission products, fusion a t the element center element, fuel volatilization. and catastrophic failure of the
On the basis of the experimental evidence obtained during this investigation, it was concluded that:
b
b
b
Coating of uranium dioxide powder with metallic tungsten films by hydrogen reduction of tungsten hexachloride in a fluidized bed is feasible at a reaction temperature of about 400"C. High conversions of tungsten hexachloride to deposited metal can be obtained at an excess hydrogen ratio of about 125 to 1 . Mechanical vibration of the reactor is essential for uniform and complete coverage of micron-sized uranium dioxide powder. Deposits of a p p r o x i mately 20weightyo tungsten are required for essentially complete coverage of the micron-size uranium dioxide powder.
fabricated element. Multiphase fuel elements, either metal-ceramic mixtures or mixtures of ceramic phases, have been developed to help solve such problems. A uniform dispersion involving a matrix material surrounding a uniformly distributed second phase is believed to be the optimum configuration for such elements. This can probably be obtained by using precoated material in their fabrication. I n addition, metalliccoated nuclear fuel particles should provide a n excellent fuel material for slurry and fluidized-bed reactor systems.
Experimental Materials and Equipment. A sample stream of repurified hydrogen containing metered traces of argon was continuously analyzed for detrimental impurities with a mass spectrometer during several experiments. Feed system for tungsten hexachloride consisted of a sublimation vessel with provision for introducing argon carrier gas. Metered argon was passed u p through the tungsten hexachloride bed which was maintained a t a suitable temperature by electrical resistance heaters around the outside of the sublimer. Temperature and argon flow rate were varied to obtain a given feed rate of tungsten hexachloride to the reactor. T h e expanded reactor section was mounted on top of the reactor to minimize loss of powder product by elutriation. T h e tungsten hexachloride inlet distributor was mounted about 0.5 inch above the uranium dioxide support screen with its axis perpendicular to that of the reactor. Fluidizing and reductant hydrogen was introduced below the support screen for the powder bed. Heat for the reaction was transferred from external electrical resistance heaters through the reactor walls. Reaction temperature was measured by means of a
thermocouple i n a stainless steel well immersed directly in the bed. Because sonic energy improves the fluidizability of fine powders ( Z ) , several devices were employed to add energy to the reactor. A loudspeaker was mounted several feet below the supporting screen for the fluidized bed and coupled to the reactor by rubber radiator hose. A mechanical vibrator was used during various experiments, mounted directly on the side of the reactor. A cyclone was mounted on top of the expanded reactor section after initial experiments. T h e solids-return leg extended down into t h r fluidized bed ending 0.5 inch above the supporting screen. A d r y solids trap, a water-washing solids trap, and a filter trap filled with glass wool prevented loss of powder product. Procedure. T h e reactor was charged with several hundred grams of uranium dioxide and the sublimer with approximately 100 grams of tungsten hexachloride. Sublimer. reactor, and trap systems were then coupled and leak tested. T h e equipment was then purged with argon and brought to reaction temperature. T h e mechanical vibrator was turned on, hydrogen flow was increased to somewhat above the rate for incipient fluidization, and a metered argon flow passed through the sublimer and into the reactor. After a given time, varying from 0.5 to 12 hours, argon flow through the sublimer was terminated, and the reactor and sublimer were cooled to room temperature and then purged with argon. Bed, sublimer, and material collected in the traps were weighed. T h e coated material was screened and analyzed for tungsten. I n several cases, complete analyses for impurities in the deposited metal were also performed. Conversion and material balances for the runs were determined from these data. T h e product was examined for uniVOL. 51, NO. 1 1
NOVEMBER 1959
139 1
Schematic diagram of coating operation with complete material and equipmeni specifications
T
Hydrogen
I
Y
burner Solids -return I ine
FI uidized- bed reactor
L
Hydrogen purif icot ion
Flowmeter
Mercury manomet ers
Materials Material Source HZ(prepurified Air Reduction electrolytic) Sales Co. Ar Linde Air Products Co. WC16 A. D. McKay, UO? (I-micron Inc. av. particle size)
Flowmeter
Material of Con-
Purpose HZpurification Ha purification
Moisture monitor
Dryer control, H? purification
Getter furnace
Ar purification
Sublimer
WCls feed
WCh bed support
Stainless steel Deposition of W on Stainless UOn powder steel Minimize powder Stainless loss steel Stainless steel
400-mesh screen
Inlet distributor
Charging WCls to Sttiinless reactor steel
0.25-in. tubing
Loudspeaker
Addn. of sonic energy to reactor
50-watt
Vibrators
Addn. of mechanical energy to reactor
Reactor Expanded reactor section UOZ support
struction
Dimensions or Specifications
Equipment Deoxo catalytic burner Lectrodryer
Remarks Contained Molecular Sieve adsorbent
Consolidated Elec- Type 26-301 trodynamics Corp. Contained 2 lb. Zr sponge WCls charged through screwed pipe cap fitting at top
Stainless 1.5411. pipe, 2-ft. steel length Stainless 1.5-in. pipe, 15steel in. length
Cyclone
1 392
Source Catalog S o . Baker & Co. Type 100-50 McGraw-Edison Co. Type BAC-25
INDUSTRIAL AND ENGINEERING CHEMISTRY
1.5-in. pipe, 20in. length 4-in. pipe, 6-in. length 400-mesh screen
Held in place at reactor base by insertion between slip-on flanges gasketed with stainless steelclad asbestos rings Tubing projected 1 in. into fluidized bed. End welded shut, six '/sp-in. and six l/,~-in. holes drilled in sides for uniform dispersion of WClb vapors Driven by oscillatoramplifier, 0-10,000 C.P.S. Syntron Co.
0.25-in. i.d.
V-15 (75 watts) V-30 (150 watts)
M E T A L L I C C O A T E D N U C L E A R FUEL Black and white prints of color photomicrographs (250 X ) show progress of coating in fluidized bed reactor
Progress from Uncoated uranium dioxide powder
to
Partially coated product
to
Fully coated product
VOL. 51, NO. 1 1
NOVEMBER 1959
1393
100
z
0 v)
a 80
W
> z 60
i9
TzEEEEl 00
Ny
Figure 1.
E X C E S S HYDROGEN RATIO
Increasing the excess hydrogen ratio did not improve conversion
formity of coating with a polarized light and a n optical microscope. Uranium dioxide is red and tungsten green or silver under polarized light. This color contrast provided a n excellent means of determining coating uniformity.
Results and Discussion Process Operability. Low-frequency mechanical vibration applied to the walls of the reactor was necessary for satisfactory fluidization of the uranium dioxide powder. Higher frequency sonic energy, however, did not significantly improve fluidizability and therefore was discontinued. Uniformity of fluidization was extremely poor when no mechanical vibration was employed. Dense uranium dioxide powder packed into a claylike mass and little solids turnover occurred, even when relatively large amounts of hydrogen were passed through the bed. Hydrogen tended to channel through the packed powder, but by using sufficient mechanical vibration to collapse these channels, the bed took on the ebullient properties of the normal fluidized bed. A threshold rate of energy addition seemed to exist. At low vibration levels gross fluidization was poor, and the powder tended to segregate according to density and particle size. When the rate of energy input was somewhat less than 150 watts, initially coated powder segregated a t the bottom of the bed and prevented uncoated powder from contacting tungsten hexachloride vapors. This resulted in a wide range of surface coverage for individual particles. However, energy input rates of 150 watts or higher gave a fairly homogeneous product.
1 394
500
Operation of the reactor above 400" C. caused most of the coated powder to sinter and form agglomerates, resulting in an even more rapid and pronounced bed segregation. Although this difficulty was never completely eliminated, operation a t about 400' C. with relatively high levels of mechanical vibration yielded a satisfactorily uniform coating. Although 400' C. is considerably below the usual sintering temperature of tungsten powder, fine particle size of powder, purity of the deposit, and presence of a halide transfer agent accelerated the sintering process kinetics and thereby lowered sintering temperature. Disentrainment and recycling of solids were also essential for attractive yields. Because powder particles were very small and difficult to fluidize, there was a strong tendency to be elutriated from the reactor under satisfactory fluidization conditions. The cyclone with a solidsreturn leg permitted return of most of this elutriated product to the reactor for further processing. Process Variables. Considerable excesses of hydrogen were required for reduction of tungsten hexachloride at about 400' C . For convenience, the amount of hydrogen used has been expressed in terms of excess hydrogen ratio, the amount of hydrogen actually fed into the reactor divided by the stoichiometric amount required for reduction of hexachloride. Excess hydrogen ratios between 100 and 150 were necessary for efficient conversions. Higher conversion of the tungsten hexachloride to tungsten could be obtained with higher bed temperatures, but this resulted in the sintering previously discussed. At a n excess hydrogen ratio of 150, average conversions of 82 and
INDUSTRIAL AND ENGINEERING CHEMISTRY
99% were obtained a t 425' and 525' C., respectively. Greater excesses of hydrogen did not significantly increase these conversion levels. However, reduction of hexachloride in deeper beds would probably yield higher conversion levels a t a given reaction temperature and excess hydrogen ratio. Deposition rates from 0.4 to 1.7 pound of tungsten per hour per square foot of reactor cross sectional area were obtained. A deposition rate of 1.2 pound per hour per square foot appears to be a reasonable design estimate on the basis of all the reduction experiments. Product Quality. Purity of deposited metal was relatively good with the exception of contamination by iron and chlorine. All other impurities were below 200 p.p.m. for tungsten deposits of approximately 20y0(weight). Iron contamination was due to corrosion of the reactor and sublimer. A typical product contained approximately 2000 p.p.m. of iron. However. this level could be reduced by improved equipment design. Chloride contamination was especially high in products prepared a t about 400' C. The reduction reaction is thermodynamically less favorable a t low reaction temperatures, and if sintering is minimized by using reduced temperatures, precautions must be taken to avoid traces of unreacted lower chlorides in the deposited metal. Postreduction hydrogen treatment lowered chloride content from 5000 to 1900 p.p.m. Further reduction could undoubtedly be obtained by more severe clean-up rreatments. However, the relatively high chloride content might be avoided by using a greater excess of hydrogen during the deposition process. Uniformity of coverage was excellent if reduction took place a t about 400' C. and if sufficient mechanical vibration assured good solids mixing in the fluidized bed. Approximately 20 yc (weight) tungsten was required to achieve essentially complete coverage, although heavier deposits may be required for the most satisfactorv fabricated fuel element. Products which contained less extensive deposits always showed incomplete surface coverage of the particles. Coatings appeared to be adherent, and no evidence of phase separation was found.
References (1) Cline, J. E., Wulff, J., J . Electrochem. SOL.98, 384 (1951). (2) Morse, R. D., IND.ENG.CHEM.47, 1170 (19551. (3) Oxley, J. H., Campbell, I. E., J . Metals 11, 135 (1959). RECEIVED for review February 11, 1959 ACCEPTED June 8. 1959 Work performed at Battelle Memorial
Institute under AEC Contract W-7405eng-92.
