IDATION-FLUORINATION OF URANIUM E PELLETS IN A FLUIDIZED BED
I
L. J . A N A S T A S I A A N D W . J . M E C H A M z 4 r , p i n e ,\'ntzonal
Laboratory, Arfonne, Ill.
In pilot plant development studies of a fluorination process for recovery of fissionable values from spent power reactor fuels, UOz fuel pellets are fluorinated to volatile UFs by reaction with elemental fluorine. The highly exothermic fluorination reaction has been carried out in a fluidized-packed bed system which consists of two reaction zones: one for oxidation of UOz pellets to U308 fines (approximately 20-micron diameter) and one for fluorination of U308 to UFS. In the two-zone system, the less-exothermic oxidation reaction occurs in the fluidized-packed b e d and the fluorination reaction takes place in a superincumbent, fluidized bed where heat transfer coefficients are higher than in the fluidized-packed bed. Several methods of improving U308 fines transport from the oxidation zone to the fluorination zone were investigated; pulsation of the fluidizing gas was the most successful. With the pulsed system, 20-pound batches
of WOz pellets were processed in about 12 hours a t 500" C. Average production rates of 50 pounds of UFs (per hour sq. foot of reactor cross-sectional area) and fluorine utilization efficiencies of 75% were achieved with good temperature control and absence of caking.
These efficiencies included a fluorination
period a t the end of the run in which the last portions of uranium compounds were volatilized from the fluidized bed.
number of current and proposed nuclear power reactors employ a ceramic-type fuel such as uranium dioxide, which is compressed and sintered to a high density and contained in a thin metal cladding such as stainless steel or Zircaloy. These fuels are generally of low enrichment with u p to about 57, U2". A fluidized-bed fluoride volatility process for the recovery of fissionable values from spent nuclear reactor fuels of the uranium dioxide pellet type is being studied on the pilot plant scale at Argonne National Laboratory (2). This separation process utilizes fluidization and volatilization techniques to recover uranium and plutonium as the volatile hexafluorides from the spent fuel which contains radioactive fission products. A block diagram illustrating the major steps in the complete process is shown in Figure 1. This report concerns pilot plant development of the fluorination step of the flowsheet. I n this work nonirradiated uranium dioxide pellets were fluorinated in a fluidized-packed bed system. T h e packed bed was formed by a charge of dense, reactor-grade, cylindrical uranium dioxide pellets which are consideled to be rrpresentative of nuclear fuels (8). T o aid in heat reriioval for the highly exothermic reactions involved, a n iceit-granular solid was fluidized in the voids of the pellet bed a n d in the space above the bed. Some of the properties of the fluidized-packed bed system, including thc rates of radial gas mixing a n d the raws of heat transfer, have recently heen reported (9> 70, 74). Earlier work 011 thc fluorination of uianium dioxide pellets has also b r a i r c p r t e d (72j. 'l'he purpose of the work reported here was the drnionsiration of practical reaction rates and fluorine utiiizatiou iiutlcr cunditioiiv cJt satisfactory temperature control and rrlihblc c,pcra~.iori. I n p a i ~ i c u l a ~the . \+ark has i!ren directed toward evaluation of the batch fluoriIL375" C.) to ensure the formation of L-308 rather than nonstoichiometric intermediate uranium oxides. T h e U s 0 8 spalls off the surface of the pellets as fine particles (approximately 20-micron diameter) of high specific area ; consequently, the fines are highly reactive. These fines are formed in the lower zone of the reactor by oxidation and are transported by the mixing action of the bed to the upper zone, where they come in contact with fluorine and are removed as volatile uranium hexafluoride in the gas phase. Equipment and Procedure
T h e arrangement of the experimental equipment is shown in Figure 2. l h e nickel fluidized-bed reactor was 3 inches in inside diameter and 4l/'2 feet long; an 11-inch I . D . X 26inch disengaging section surmounted the 3-inch section of the reactor and housed the two process filters. T h e 3 X 18 inch bayonet-type filters were composed of sintered nickel with a nominal porosity of 10 microns. Automatic blowback of the filters was provided periodically by reverse gas flow through nickel gas compressor jets located in the downstream line of each filter. Blowback by a short ','2-second pulse of 30p.s.i.g. nitrogen effectively removed solids caked on the filter and returned them to the reaction zone. L r a n i u m hexafluoride product \vas collected in a nickel condenser bvith internal condensing surface provided by a n axially finned tube through which coolant circulated. TriC. chloroethylene was the coolant, chilled to about -30' by a mechanical refrigerator. T h e condenser \vas equipped with flexible lines and mounted on a remote-reading scale in order to moniwi. the CF6 collection rate. A sodium
fluoride bed provided additional removal of U F s from the condenser exit gas. 'This adsorber was mounted on flexible lines and was also Lvrighable. h diaphragm gas compressor \vas tired to recycle p'rocess off-gas to the fluorination reactor as a means of increasing fluorine economy. .I'o promote mixing and transport of the U 3 0 8 fines. the fluid bed was pulsed ivith nitrogen. which \vas introduced to the column immediatrly below the pellet support plate. T h e pulse duration and frequency Lvere controlled by a system of automatic timers. Approximately 0.02 cu. foot of nitrogen \\-as admitted to the column Lvith each pulse. Fluorine was obtained from commercial sources in 6-pound cylinders. Residual 'hydrogen fluoride in the fluorine ivas removrd by a sodium fluoride trap. T h e fluorine \vas metered to the reactor from a 17-cu. foot storage tank. Pressure drop on ihe tank was recorded and used as a measure of fluorine addition; fluorine flow rate was controlled by a system of orifices and flow control valves. Similar orifice-control valve systems xvere used to measure rates of oxygen and nitrogen flo\v to the reactor. Fluorine concentration in the reactor off-gas \\.as measured in two ways. Samples of the gas were collected intermittently and made to react \vith mercury in a calibrated volume to measure pressure change. For continuous measurement. fluorinator off-gas was passed through a sodium chloride trap at 100 cc. per minute; the resulting reaction replaced fluorine with chlorine in the sample gas. and the chlorine was determined by thermal conductivity against a reference gas composed of the process off-gas. T h e reactor grade uranium dioxide pellets used in these stitdics \vere hydrogen-fired and had densities at least 93% of theoretical. These pellets ( l '2-inch diameter, inch long, and typical of reactor-fuel material) are less reactive to fluorination than pellets sintered in a n inert atmosphere ( 3 ) . T h e inert fluidizable material was Norton's Alundum (either Type 38 or high purity Blue Label grade), which consists primarily of refractory grain, recrystallized a-alumina. I n the fluidized-packed bed configuration. the alumina ( -40 +l7O mesh) had a n incipient fluidization velocity of 0.2 foot per second. In a typical operating procedure, a weighed quantity of alumina (12 to 18 pounds) was placed in the reactor and fluidized with nitrogen. A weighed quantity of pellets (19.4 pounds) corresponding to a packed height of 12 iiiches was then added to the fluidized alumina and allowed to settle into place. Fluidization by nitrogen was continued while the system was heated to the operating temperature, a t which point simultaneous introduction of oxygen and fluorine was begun at predetermiced flow rates. Uranium hexafluoride production was monitored a t '/*-hour intervals by weight change of the condenser and sodium fluoride adsorber. Oncethrough gas flow was maintained until most of the pellet bed had reacted. T h e run was then either terminated for inspection of the bed, or was continued with partial recycle of the off-gas to achieve better utilization of fluorine while the last portion of the uranium oxide bed was fluorinated.
FUEL FABRICAT1
u o p (PUO,)
I'I
/
I
I
I
VAPORIZATION
I
! I
1
I i
I
L---l--_FP--
-_FP--t
Figure 1 . Conceptual flowsheet for fluidized-bed fluoride vola ti Iity process
TOWERS
Figure 2 . Schematic diagram of equipment for two-zone oxidation-fluorination of vanium dioxide pellets
Gas-Solid Chemical E!eactions
In the fluorination of uranium dioxide pellets, the heat of reaction ( 7 7) for the over-all reaction is given by Equation 1 :
I
Hexafluoride Production UFqlsl + F2lgl-
UFg(g1
U02FZISI + 2 FZlg)-UFglgl t 02191 UgG$sl + 9 FZ(gl3 UFg(g1 + 4 021gI
With separate oxidation and fluorination zones, the heat of reaction ( 7 7 ) is split up because the over-all reaction takes place in two steps:
3UO>(s)
+ O,l(g)
+
1!08(s);
-
U3$lSl
+
3 UOzF2Isl
2 UF6(g)-4
U02FZIs)
02191
+
+
UF~IS!
U € z ~ c= . Zone
-25 kcal.jmole U and
F2
Intermediate Uranium Fluoride Formation U30glsl + 3 Fzlgl-
3UO21Sl + o ~ l g l - u ~ o * ~ s l
(2) Pulse
N2
c
9.N2 Figure 3. Schematic diagram of two-zone oxidationfluorination of uranium dioxide pellets VOL. 4
NO. 3
JULY
1965
339
Therefore, a significant advantage of the two-zone operation over single-zone fluorination is a lowering of heat transfer requirements in the packed bed of pellets, since in the former case oxidation rather than fluorination takes place in the pellet bed Heat transport in the fluidized-packed bed is relatively poorer than in a n unhindered fluidized bed (70). T h e mechanism of oxidation of the uranium dioxide is thought to be one of diffusion of oxygen to the tetragonal lattice U 3 0 j , followed by nucleation and growth of U30, to orthorhombic Us08 ( 7 , 6 , 7 , 73). A 307, change in volume occurs with the phase change during oxidation with the result that the U308 tends to spa11 off the surface of the pellets; the spalling is aided by the turbulent motion of the fluid bed. I n the two-zone system, the powdered U3Os must be physically transported to the fluorination zone, if the fluorination is to proceed. This process is shown schematically in Figure 3. The oxide powder must be transported in a manner which prevents formation and accumulation of intermediate uranium fluorides. These intermediates are produced as fine powders by the following reactions:
U3Os(s) 4- 3F2(g) U308(s)
-+
f 2UFs(g)
~ U O ~ F ~f( SO?(g) )
-+
+ CF4(s)
4U02F2(s)
(4)
(5)
Fluidized beds with large concentrations (above approximately 207,) of the intermediate uranium fluoride powders are difficult to maintain in a fluidized condition. Channeling and packing with reduced heat transfer in the presence of the fluorine reactant can cause caking of the bed. Removal of fines in the fluid bed takes place according to Equations 3, 6, and 7.
+ 2Fz(g) cF6(g) i- Os(g) UF,(s) + Fi(g) LF6(g)
UO,F2(s)
+
+
(6)
(7)
Accumulation of intermediate fluorides was avoided by maintaining a slight excess of flLiorine in the off-gas. Fluorine requirements were ascertained by monitoring uranium heuafluoride collection in the product condenser and fludrine concentration in the reactor effiuent stream. I n this system, the UFc production rate depends on the rate of oxidation and the rate of L-308 transport to the fluorination zone, as well as fluorination rates. The rate of pellet oxidation exhibited a first-order dependence upon oxygen concentration and was ,also temperature-dependent. \vith very rapid rates a t 500' C . ( 3 ) . The fluorination reaction was also rapid, increasing as a function of temperature ( 3 ) . Therefore, the major requirement for practical hexafluoride production rates was the proper balancing of the oxidation and fluorination rates and control over the transport of powder from the oxidation zone to the fluorination zone. Control of po\vder transport prevented accumulation of Us08 fines in the oxidation zone. thereby reducing the possibility- that large amounts of these powdered solids would enter the fluorination zone. Rapid introduction of powders to the fluorination zone Lzould result in intermediate uranium fluoride formation and elutriation of this powder to cooler parts of the reactor. Several methods of promoting fine porvder transport \vere investigated : increased fluidization velocities. column vibration, and the use of gas pulses introduced to the fluidizedpacked bed. I n addition, a longitudinal--i.e., axialtemperature gradient was maintained in the pellet bed in some runs in an effort to promote preferential oxidation from the upper layers of the pellets charge. In carrying out this study of the fluorination of uranium dioxide pellets, a number of runs were made. The experimental conditions and results of the most important of these runs are reported in Table I and discussed below.
Table 1. Two-Zone Oxidation-Fluorination of UO? Pellets UOZpellet charge 19.4 FVeight. lb. 12 Bed height, inches '4lumina charge \Veight, lb. 11 9 to 17 8 -40 +170 Size limits, mesh Oxygen diluent Nz Fluorine diluent Sone Runs Z1-Z6 Process off-gas Runs 2 7 - 2 9 Fluorination zone 500-525 Temperature, C. 1.1-1 6 Gas rate, cu. ft./min. 7-1 7 Fluorine. %
uo
Oxidation Zone T$mp.
~
C. Intermittent column vibration 500 400 Continuous column vibration 5400 Thermal gradientsb 5 400 Thermal gradientsb 400 Pulsed gas and thermal gradients Pulsed gas and thermal gradients 400 Pulsed gas, subcooled pellets 340-480 340-480 Pulsed gas? subcooled pellets 340-480 Pulsed gas. subcooled pellets 2-9 100 [3X (moles CTF6 produced)!(moles F Zfed)].
Run 2-1 2-2 2-3 2-4 Z-5 2-6 2-7 2-8
M o d e of Operation
Oxygen,
G a s rate, cu. f t . / m i n .
3tO 8
1.4 1.3 1.1 1.2 1.3 1. o 0.9 0.9 1.o
%
4 t o 12.5 18 to 31 26 22 20 4.5 6
8
Processing Time, Hr.
15.5 13 0 13.8 16.3(18.3)~ 13.5 1.5 13.5(15.0)~ lO.O(l2.5)~ 9 . 2 (1 2 . 5 ) ~
%
AD. Production Rate. L b . UF6/ ( H r . ) ( S q .Ft.)
88 82 95 92(97)~ 46
32.0 32.4 35 6 31 . O 17.6
58 53 49 56 49
09(iOo)c 97(100)c 94(
37' 9
66 5(64 5 ) ~ 74 9(72 5 ) c -8 5(74 9)c
2
Reacted to [IF+
50.0 52 0
A L . Fluorine C'tiliration:
cc
6 6 0 8(53 0 ) ~ 5
Temperature of G O 2pellet bed Lcas lower at bottom, to promote oxidation preferentially near upper surface of bed. Values in parenthses are final results of two-zone processing plus additional Period of uranium cleanup in which 50 to 80c7, Ff is recycled through reactor at 500" C. b
c
340
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
Control o f Fines Transport from Oxidation Zone
High gas rates aid fines transport from the oxidation zone but also tend to eluviate a portion of unreacted and partially rracted U:$& fines from the fluorination zone, even though an excrss of fluorine is maintained. Most of the elutriated poivder accumulates on the cone joining the column to the discngaging section, arid the rest is collected by the metal filters. Reaction of the uranium oxide with fluorine in the upper parts of the column is slow because of the reduced temperature (200" to 300' C.) in this region. I n the runs described below. it was found that the major problem was to control the rate of entr) of oxide fines to the fluorination zone. T h e first run in the development program which utilized the t\vo-zone oxidation-fluorination technique was run Z-1 ; this run utilized high superficial fluidization velocities (1.2 to 1.4 feet per second). 'The effect of external column vibration \vas investigated during a 2.2-hour period in this run. External vibration was applied by a n automatic hammer device \\hich struck the reactor a t a rate of 20 blows per minute. During this period of operation, the external vibration increased the UFe production rate from 27 to 7 0 lb. UF6! (hr.) (sq. ft.) ; however, it \vas not evident whether the increased production rate causecl by vibration \vas due to return of eluiriated fines from the disengaging section to the fluorination zone or to transport of fines from the oxidation zone. (From separate laboratory te:%ts, it was known that the vibration effrctively returned poin.der from the disengaging section to the fluorination zone.) R u n 2 - 2 was, therefore, carried out under operating conditions similar to run Z-1; however, the superficial fluidization velocity was lowered to 0.9 foot per second in the oxidation zone and mechanical vibration was used for the entire run. R u n 2 - 2 was purposely halted before completion. It was observed that 697' of the uranium compounds (3.1 pounds) remaining in the oxidation zone had reacted to LT~Og powder and that no fines had accumulated in the column disengaging section. Hence, it was concluded that column vibration was effective for returning powder to the fluorination zone but inadequate for fines transport from the oxidation zone, and ):hat with higher gas velocities, excessive elutriation of fines wou!d occur. At loiver gas velocities. it appeared that powder transport from the oxidation to the fluorination zone might be improved by preferential oxidaticmn in the upper part of the oxidation zone. Longitudinal (axial) heat transfer tests with fluidizedpacked beds (5) had indicated that a longitudinal temperature gradient could be imposed within the oxidation zone by allowing heat to be transferred from the fluidized alumina in the fluorination zone to the upper surface of the fluidized-packed bed-i.e., the oxidation zone. Thus, with the upper portion of the fluidized-packed bed a t a temperature high enough for rapid oxidation rates, the lower parts of the bed would be a t lower temperatures anti oxidation rates would be reduced. This effect was important because, with uniformly high temperatures. oxidation occurred throughout the pellet bed ; under these conditions 1 5 3 0 8 formed a t the bottom of the bed had to be transported to the fluorination zone through tortuous paths in the voids of the pellet charge. A typical temperature gradient ranged from about 400' C. a t the top to 100' C. at the bottom of the pellet-containing bed. This gradient was established by maintaining the fluorination zone a t a temperature of 500' C., thereby conducting heat downward through the fluidized-packed bed constituting the oxidation zone, and by cooling the bottom flange of the reactor by natural convection of ambient air. Since the gradient Iobvered the general temperature level of the pellets,
the effect of temperature on the oxidation rate was partially offset by use-of 18 to 31% oxygen instead of the 3 to 12.5yo used previously. T h e internal fluidized-packed bed temperatures were monitored by five thermocouples welded in place at various elevations within a single longitudinal thermowell and were continuously recorded during a run. Temperature profiles obtained during run 2 - 3 are shown in Figure 4. .4s the uppermost pellets reacted, the relatively cool pellets surrounding a given thermocouple gradually disappeared and were replaced by the heated fluidized bed. This effect is evidenced in Figure 4 by the increase in temperature at thermocouple positions 3, 4, and 5. Temperatures a t the bottom of the fluidized-packed bed did not reach 400' C. because of heat losses through the bottom flange of the reactor. T h e use of thermal gradients gave promising results in two runs (2-3 and 2-4) with sustained production rates of about 31 to 36 lb. UF6/(hr.)(sq. ft.) and with processing times of 14 to 18 hours for 9570 complete fluorination. I n one run temporary stoppage of the fluidizing gas occurred (due to external equipment failure) and the established temperature gradient was disrupted. T h e resulting drop in bed temperature is shown in Figure 4 a t the 5-hour mark. T h e effect of such a disruption is to interrupt fluorination until proper temperatures can be re-established. Gas pulsing was considered promising as an alternative method of promoting fines transport to the fluorination zone and was investigated in Lucite tubes containing fluidized; packed beds (5). I n these preliminary tests it was observed that a pulse of gas (20 p.s.i.g.) imparted movement to pellet beds 6 and 12 inches deep a t superficial fluidization velocities of 0.5 to 1 . 5 feet per second. T h e sudden expansion of the pellet bed constituted a momentary increase in the void space and changed the path of gas flow through the voids of the stacked pellets. I n addition? after several pulses, the pellets
THERMOCOUPLE NO.
HEIGHT ABOVE SUPPORT P L A T E (in.)
POSITION AT START-UP
-4
BELOW SUPPORT PLATE
0
BOTTOM OF P E L L E T BED
4
IN PELLET BED
8
IN PELLET BED
I2
TOP OF PELLET BED ALUMINA FLUID BED
26
\
500
0 1
Figure 4.
I
1
Q
k
I I PROCESSING TIME, hr.
'
10
'
'
12
'
Bed temperatures during run 2 - 3
Two-zone oxidation-fluorinotion of uranium dioxide pellets temperature gradients through pellet zone
VOL.
4
NO. 3
JULY
1965
with
341
were rearranged in a packing configuration which offered minimum pressure drop to the fluidizing gas. I n t\vo experiments, runs Z-5 and Z-6, both the gas-pulsing and the temperature gradient techniques \
