nism of timer cams, as many as eight units can be served by one recorder. We believe that a number of improvements can result from the application of automation to viscose filterability measurements. These include improved sensitivity in weighing, substantial elimination of human operation error, attainment of a more nearly instantaneous rate, very precise time intervals employed for rate measurements, and data recorded instantaneously in a form suitable for immediate interpretation without replot. Although the automatic equipment is primarily designed as a research tool, application may well be found to plant quality control work leading to mutual benefit for both pulp producer and consumer.
literature Cited
(1) Bryson, R. G., U. S. Patent 2,996,132 (Aug. 15, 1961). (2) Golben, M., Tappi 38, 507-12 (1955). (3) Gonsalves, V. E., Rec. Trau. Chlm. 69, 873-903 (1950). (4) Hermans, P. H., Bredee, H. L., J. .Yoc. Chem. 2nd. 55, 1T-4T (19 36) , (5) Herrent, P., Lunde, A,, Jnoff, G., SLlensk Papperstid. 54, 153-68 (1951). (6) Samuelson, O., Ibid., 52, 465-73 (1949). (7) Vosters. H. L.. Ibid., 53, 613-21 (1950). RECEIVED for review May 28, 1959 RESUBMITTED December 6, 1962 ACCEPTED January 25, 1963 Division of Cellulose Chemistry, 129th Meeting, ACS, Dallas, Tex., April 1956. Contribution No. 19 from the Research Division of Rayonier Inc.
MULTICYCLE REPROCESSING AND REFABRICATION EXPERIMENTS ON SINTERED UOpFISSIA PELLETS S Y D N E Y S T R A U S B E R G A N D
E. W .
M U R B A C H
Atomics International, Canoga Park, Calq.
A multicycle reprocessing and refabrication scheme was simulated (using UOn“spiked” with fissia) to evaluate the feasibility of refabricating high-burnup UO2 to a cumulative level of 105 MwdIMTU. Various powder characteristics were studied, using oxidation-reduction methods as the reprocessing scheme. Processing conditions were established for the multicycle tests. Fissia, typical fission products (mostly oxides), were added to the reprocessed powder prior to compacting operations. There should be no difficulty in refabricating pellets from high-burnup material; conversely, the sinterability of UOz containing comparatively large concentrations of fissia is even enhanced. Well-formed pellets of density near 98% of theoretical were made for simulated burnups of 60,000,80,000, and 100,000 MwdIMTU. Cesium was the only fission element removed during processing.
THOUGH high burnup can be achieved in poner reactors by using slightly enriched UOz. the spent fuel will probably still warrant reprocessing ( 8 ) for recovery of valuable fissionable material (Figure 1). To evaluate the feasibility of refabricating very high burnup UO2. urania fuel was exposed to five pseudo-reactor cycles, representing 2% burnup per cycle (Table I). The effect of increasing fissia concentrations on processing and refabricating operations was evaluated, primarily by density comparison of the sintered pellets. Pellets were fabricated by conventional cold-pressing and sintering methods. Preliminary Investigation
Initial efforts were carried out on UP,. or L 0 2 containing low fissia concentrations. to establish criteria for the subsequent multicycle tests. Published literature ( 7 , 2. 5) indicated that successive oxidation-reduction treatments on LO? powdeis could activate the urania to produce a material with more enhanced sintering characteristics. Experimental work confirmed the previous findings and permitted establishment of the operating criteria to be employed in the multicycle reprocessing tests. 228
I&EC PROCESS DESIGN A N D DEVELOPMENT
Table 1. Relative Composition of Stable Isotopes Representing Anticipated Fission Products Formed after 2% Burnup (2% tisria)
Fission Product
Fission Product Compound
Ba
BaC03
Ce
CeOz
Cs
La Mo
Nb Nd Pr
Ru Sm Sr
Y Zr
P.P.M. of Compounri.
Cs*CO,
La203 Mo
Nb2Oj Nd203
Pr6011 Ru Sm?Oa SrO y?o3 ZrO2
Total
963 2112 1470 803 1800 63 2137 81 1 961 291 881 423 3240 15,9551
a Actually parts of compound per million parts of UO,, calculated from &ion product data of Blomeke and Todd ( 4 ) . Approximately 7.6% by weight; difference (from 2 % ) would be volatile compounds and other minor components.
The equipment employed and oxidation-reduction methods used for chemical pulverization of pellets and powders have been described (6, 7 ) . To determine the powder product most desirable for pellet refabrication, urania pellets were exposed to five oxidation-reduction treatments. The particle size of the powder was measured with a Fisher Sub-Sieve Sizer and sample pellets were fabricated after each treatment. The results (Figure 2) indicated an optimum condition for the fabrication of high density pellets-Le., three oxidationreduction cycles on pellets resulting in a UO? powder with a particle size of about 1 micron. iVhile increased compaction pressures ( m i . = tons per square inch) are helpful for coarse powder (one- or two-cycle material), their effect becomes negligible with the powders of better grade (particle sizes of 1 micron or less). The optimum powder condition can probably be explained by the sponge-like structure (5) characteristic of such powd.ers, which increases with additional recycle-Le., powders become more difficult to compact because of greater quantities of entrapped gases, while simultaneously becoming more sintarable because of their increased surface areas. The UOz particle size decreased during cycling to about ' / 5 of its first cycle value, indicating a fivefold increase in specific surface area (square meters per gram). This effect was confirmed by BET surface area measurements, and a satisfactory correlation (Figure 3) was obtained with particle size data, determined with a Fisher Sub-Sieve Sizer. (The Fisher apparatus is generally understood to provide data on the agglomerate size, rather than on the discrete crystallite size of the powder.) Comparison of the BET areas with those calculated from the particle size data indicated a powder shape factor: k: of about 3-i.e., from the equation S.A. = 6 k;'pD, wherein the specific surface area, S.A., is inversely proportional to the powder crystal density and particle diameter. The least squares slope of the line (-1,086) agreed well kvith the theoretical value ( - 1.COO) expected from the above equation. During oxidation operations, recorded temperature data (taken during constant power input to the furnace) perinitted estimation of an oxidation-ignition temperature, as that a t which a significant increase occurred in the slope of the heating curve. I t appears that with decreasing particle size (Figure 4) the surface area of the U O powder ~ increased sufficiently to permit lower oxidation-reaction temperatures, and these data Ivere used as the basis .:or determining the maximum number of oxidation-reduction cycles which ivere carried out. Defining a pyrophoric po\vdtr as one \vhich ignites spontaneously in air a t about 20' C.. we can see that such a phenomenon would probably occur a t an average L O P particle size of about 0.5 micron, or after another (sixth) oxidation-reduction cycle.
REACTOR
1 -
*
OX1 DATION
BINDER
.(
MIXING
1 I PRESSING
I
1 FEJECTS
i
Figure 1.
100
LOADING
I
Fuel cycle for
UOz reactor fuels
r
POWDER- I % FISSIA, COMPACTED WITH 2 % CARBOWAX AS BINDER SiNTERlNG CONDITIONS12 h r , 1800°C, IN HYDROGEN
U
1 I
a
25tst i COMPACTION PRESSURES
88,
03
'50
0.5
50
3.0
1.0
~
PARTICLE SIZE OF POWDER ( p )
Figure 2. Effect of powder particle size on (water displacement) pellet density
e-.
Results and Discussion
CII \
N
E
1
hfulticycle refabricaiion tests simulating various burnups were carried out to a cumulative level of 105 Mwd, M T U (megawatt days per metric ton of uranium) using the optimum conditions established--i.e., about three oxidation-reduction treatments on pellets. Attempts were made to control the particle size of the -400-mesh product a t about 1.2 microns, so that variations in reprocessing or refabrication, if existing, could then be evaluated for the several burnup materials. Experiments \?ere carried out a t the 1- to 2-kg. level, using near prototype equipment where practical. An induction furnace was employed for the air-oxidation, hydrogen-reduction treatment, and a double-acting automatic press was used for compacting operations. Poly(viny1 alcohol) was found to be a more suitable binder for use with automatic pressing
a w
a
a
I .o
W
0
2 a 2
a
0.1
1
1 0. I
1.0
10 0
PARTICLE SIZE ( p )
Figure 3. Correlation o f BET surface area with particle size for UOk powder containing 1 % fissia VOL. 2
NO. 3 JULY 1 9 6 3
229
Table II. Summary of Multicyde, Reprocessing and Refabrimtion Results
93.2 f 1 . 6
ETB-IV 4% B.U. 91.0 5 1 . 2
EI-B-V 670 B.U. 98.4 f 0 . 7
57.4 r t 0 . 6 0.442 0.443
61.8 f 0 . 7 ~ 0.443 0.434
56.1 1 0 . 8 0.441 0.461
57.0 1 0 . 6 0 : 142 0: 124
93.2 f 1.6 0.372 0.384 2.03f0.03 2 % B.U. 1.8
91.0 11 . 2: 0.346 0.349 8.6310.34c 4% B.U. 1.5
98.4 f 0 . 7 0,361 0.390 2.59f0.14 6Y0 B.U. 1.3
98.8
Run number Identity Chargepellets,%T.D.b
ETB-I MCW UOz 94.2 z t O . 7
ETB-I1 Refab. UO? 95.7 f 1 . 4
Green compacts Density,%T.D.d Dia., inch Ht., inch
58.2 r t 0 . 5 0.442 0.443
95.7 5 ~ 1 . 4 0.371 0.353 2.06zt0.04 Refab. UOZ 2.1
Density, 70 T.D.a Dia., inch Ht., inch Sintg. wt. loss, % Identity Product wt., kg. a
ETB-111
2yoB.U.O
yo of
Simulnted burnup. d
% sf
thoretical dewily, datermined by iuoter-displocement mathod. theoretical dcnrity, determined by geometric method.
operations than Carbowax which was previously used for powder compaction with a manual press. T h e press was operated a t its slowest speed and compacts were made a t the rate of 12 per minute; '/winch dies were used in an attempt to form 3/s-inch cylindrical, sintered pellets of equal height and diameter. A continuously operated, molybdenum-wound furnace capable of batch-handling 300 compacts was used for the sintering treatment. Pellets (and powders) were finally oxidized a t about 400° C . , then reduced a t about 650' C. in flowing air and hydrogen, respectively. The screened -400-mesh UOa powders were blended with fissia, mixed with 1%poly(vinyl alcohol) (binder)
Calculated Density of UOz Containing Varying Firria Concentrations
Tabla 111.
Firsio Concentrotion, % Of uoz iJsrio
Run
Of UOs
No. (nominal) 0 ET€-I 1.6 ETB-I1 ETB-111 3.2 ETB-IV 4.8 ETB-V 6.4 ETB-VI 8.0
(actually added) 0
1.37 3.18
4.65 6.15 7.53
Simulated Bzrnup, A t . yo U , M w d I M T U 0 0 2 20,000
4 6 8 10
40,000 60,000 80,000 100,000
Cnlcd. Denrity, G./Cc.
10.97 10.85 10.73 10.61 10.49 10.41
ETB-VI 8% B.U. 98.8 1 0 . 3
60.3 1 0 . 3 0.442 0.432
:k 0 . 3 100.3 rt 0.3 0.368 0 . 363 0.366 0 . 360 2,7810.02 3.9310.09 8 % B.U. 10% B.U. 1 .o 0.9 P p p h o r i c product of three oxy-reduction treatments re-
and 20% distilled water, dried overnight, and finally granulated to -16 100 mesh size; a die lubricant (0.5Oj, zinc stearate) was added to the dried granules, prior to the pressing operation. T h e compacts were sintered a t 1750' C., for 12 hours, in a flowing hydrogen atmosphere.
+
The results of the multicycle test8 are shown in Table 11. Data on fabrication (300 pellets initially and about 130 in the last test) were averaged from a minimum of 20 values and include variability expressed in standard deviation form. Theoretical density calculations (Table 111) assume the presence of all added constituents in their original form, with the fissia behaving as one material (6.4 grams per cc.) which occupied its own volume when sintered with the UOa. Since some fissia may have been removed during processing and some solid solution of fissia components may have occurred, some of the theoretical density values may be biased low. I n the first test (ETB-I), Mallinckradt (MCW) pellets were processed and then refabricated as pure UOz to check out the equipment and methods; the run was also intended to provide reference control data for comparison with the subsequent multicycle U0,fissia tests. Reprocessing and refabrication methods for UO, containing fissia were no different from those required for pure UO,. The results indicated that the sinterability of UOz containing large concentrations of fissia-Le.,
COMPLETED OXIDATION-REDUCTION CYCLES
0.1
10
100
AVERAGE PARTICLE SIZE ( p )
Figure 4. Effect of porticle size or number of oxidationreduction cycles on oxidation reaction temperature ond t o p density 230
l&EC PROCESS DESIGN AND DEVELOPMENT
Figure 5 . Simulated 10% burnup pellets, and others refabricated during multicycle reprocessing and refobricotion experiments
F
E
Figure 6.
G
Photomicrographs of UOz pellets containing varying amounts of fissia. to simulate burnup from Photographed 01
A.
B. C.
D.
Original Mallinckrodt UOnpellet Refobricoted UOn pellet Simulated 2% burnup pellet Simulated 4% burnup pellet
simulated high-burnup fuel-was greater than that of pure UOz. Some slight but inconclusive evidence indicated that UOZwith small amounts of fissia (2% burnup material) may be less sinterable than pure UOa or UOZwith the larger concentrations of fissia. The densities of the highcr burnup materials may be biased high (1 to 2%); nevertheless, even a n a pure UOn basis (10.97 grams per cc.), the density of the 10% burnup pellets was 95.2y0 of theoretical. The particle size (Table 11) of the powders was controlled by adjusting the number of oxidation-reduction treatments (three or four) on the original pellets. Some difficulty was encountered in the third run (ETB-III), wherein a pyrophoric product was formed, and the reason for this phenomenon is still not understood. Though fabrication of the resulting 7 3 3 0 8 material produced satisfaactary-appearing pellets (Figure S ) , the low pellet densities confirm the disadvantage of sintering UaOs compacts. I n addition, a significant amount of uranium was lost (ca. 2.5%, expressed as UOJ during the sintering operation. In subsequent runs, the particle size was measured after each pulverization cycle, in lieu of determining the powder size only after the third oxy-reduction treatment, and no further difficulties were encountered. Product weight differences from the first to the last test (2.1 us. 0.9 kg.) were primarily due to the large number of samples taken. Photomicrographs (Figure 6) indicated the presence af fissia (one or more of the components) as white spheroids which tend to congregate a t the grain boundaries; void areas also concentrate a t the grain boundaries. I t is apparent that some of the calculated densities were high, especially those of pellets which had indicated densities approaching 100% of theoretical; this is probably due to the low-biased theoretical density calculations discussed previously. However, the bias is considered to be less than about 1 to 2% of theoretical density, based upon comparison with similar UOI photomicrographs
0 to 10%
500X E.
F. G.
Simulated 6% burnup pellet Simulated 8yo burnup pellet Simulated 10% burnup pellet
published by Belle and Lustman (3). The grain size of the high-hurnup pellets, measured about 15 microns, would correspond to UOz densities of 98 to 9&S% of theoretical, according to thc same report. No attempt was made to identify the white material in the pellets, although the techniques of x-ray spectrography were employed for the rapid analysis of the simulated fission products remaining after processing. Analytical methods were based upon comparison of process samples with those of prepared standards containing fissia in the amounts originally added. Sintered pellets simulating each burnup condition were analyzed and the results indicated the removal of cesium from each of the pellets. The remaining constituents (Sr, Y , Zr, Nb, Mo, Ru, Ba, La, and the rare earths) were still detected in the amounts originally added. In actual reprocessing operations the spent fuel will be oxidized and reduced prior to the sintering step, and it is probable that some ofthe cesium may be removed during the former treatments. Literature Cited
(1) Bard, R. J., U. S . At. Energy Comm., Rept. LA-1952 (October 1955). (2) Bard, R. J., Bertino, . I . P., Bunker, D. L., Ind. Enp. Chem. 53, i n n 3 (1761). (3) Belle, J., Lustman, B., U. S . At. Energy Comm., Rept. WAPD184 (September 1957). (4) Blomeke, J. O., Todd, M. F., Ibid., ORNL-2127, Part I, Vol. II ana and ,I I1 (1'3,). (1957). (5) Stenquist, :nquist, D. R., Anicetti, R. J., Zbid., HW-51748 (December I"L7, "42,.
1.
amberg, S., Ibid., (6) Strausberg, Ibrd.,NAA-SR-3911 (May 1960). .allshrrr S T E., F. Ibid., lh;,i N A A - S R - W l n I(August Aimist (7) Strausberg, S., Tnphhpn Luebben, T. NAA-SR-3910 lY,Y,.
(8) Strausberg, S., Luebben, T. E., Rosen, F. D., Gum, J., Murbacb, E. W., Ind. Ens. Chem. 52, 45 (1960).
AEC Technical Meeting, Oak Ridge, Tenn., December 1761
