I
SYDNEY STRAUSBERG, T.
E. LUEBBEN, F.
D. ROSEN, JEROLD GUON, and
E. W. MURBACH
Atomics International, Canoga Park, Calif.
l o w Decontamination-Processing of Uranium Dioxide By Oxidation and Reduction Spent uranium dioxide fuels, pulverized by oxidation-reduction and sintered in hydrogen, can be returned to the reactor
PYROMETALLURGICAL
METHODS which have been developed for metallic reactor fuels are generally not applicable to uranium dioxide because of its high melting point and chemical stability. To develop a simplified process, oxidation and reduction of urania by gaseous reactants were studied. Because a 300/, volume increase occurs when uranium dioxide is oxidized, the oxidation and reduction of fuel shapes from this material yields a finely divided powder which can be enriched by blending with uranium-235 oxide, refabricated into new fuel shapes, and then returned to the reactor. Some decontamination of fission products occurs; however, high decontamination is not a major consideration in high burn-up fuels because the major poisons, xenon and samarium, reach equilibrium early in the reactor cycle ( 4 ) . With this process, most of the cesium, ruthenium, tellurium, iodine, and probably rare gases can be removed. Also, similar fuels, which have been subjected to high burn-up and temperatures above their melting point, probably can be treated successfully.
Chemical Pulverization Uranium dioxide pellets, obtained from Bettis Atomic Power Division, REACTOR
DECLADDING
I
FILTER-PFIOS
HYDROGEN OXYGEN
-
Oxidation reduction of irradiated uranium dioxide. Radioactivity of the exit gas was continuously recorded by a counterrate recorder
Westinghouse Electric Corp., were placed in nickel or alumina crucibles and heated by an induction or resistance furnace. Pellets were pulverized (5, 6), using either oxygen or air as the oxidizing medium. In some tests only oxidation was used, but in others, both oxidation and reduction were carried out. Sometimes, oxygen and hydrogen were alternately admitted for a 10-minute period until up to 10 cycles were completed. I n other instances, only one cycle of oxidation-reduction was used, but these cycles were for longer periods of time to ensure completion of the chemical reaction. The reducing medium was always hydrogen. Experiments were conducted either in a static media or with gases flowing; gas flow was especially helpful in removing water which resulted from reduction.
Table 1.
+- 4400 + 325 - 325
LOADING REJECTS
I
' I
This process reduces fuel shapes of uranium dioxide to a fine powder which can be reformed and then returned to the reactor
When both oxidation and reduction were used, a pumping and purging procedure between each operation prevented explosive mixtures of hydrogen and oxygen from forming in the furnace.
Resuits and D~~~~~~~~~ Usually small particle sizes-i.e., high surface area-are most conducive to sintering high density objects (2, 7). Electron photomicrographs indicated that the average size of the -400 fraction (Table I) was about 0.1 micron, but light microscope and surface area measurements suggested about 1 micron. Most of the pellets used averaged 93% theoretical density. T o simulate used fuel which might be discharged from a power reactor, sintered fissia pellets were made from a mixture of uranium
lower Temperatures Plus Oxidation in Air Rather Than Oxygen Reduce Sintered Pellets of Uranium Dioxide to Finer Powder (Screen analyses of product powders," w t . %)
Oxidation temp., C. Reduction temp., O C. Mesh size
I
ARGON
1100 1100
900 900
700
700
600
500
700
None
None
None
77.6
65.4 28.6 6.0
13.7 67.1 19.5
14.4 66.6 19.0
24.5 27.5 47.Qd C
23.3 20.1 56.6d C
19.8 2.3
375 650
Ob 2.QC 97.0d
Conditions A A A B D A = 10 static oxidation-reduction cycles, each including 10-min. exposures to 02 and then Ha; press., 1 atm. B E 2-hr. exposure to flowing 0 2 ; press., 1 atm. C = 6-hr. exposure to flowing OB;press., 1 mm. Hg. D = 3-hr. exposure to flowing air plus 31/2-hr. exposure to flowing HI: press., 1 atm. a 10-min. shake time in Tyler sieve shaker. +50mesh. -50 400 mesh. -400 mesh.
+
VOL. 52, NO. 1
JANUARY 1960
45
dioxide and representative fission products in the form of stable isotopes; this fissia material represented 1o/o total burn-up. Because the core of pellets can melt in power reactors, portions of uranium dioxide and fissia pellets were melted by resistance heating. Sintered fissia pellets, melted uranium dioxide, and melted fissia were separately pulverized by the oxidation-reduction process; sieve analyses of the resulting powders were similar to those obtained from uranium dioxide pellets. The optimum conditions for producing fine particles from 60 grams of sintered uranium dioxide pellets were a 3- to 4hour oxidation at 375' + 20' C., in flowing air at 1 atm. of pressure. Conditions for reduction were not optimized; however, a 4- to 5-hOur treatment at 650' C., in flowing hydrogen at 1 atm. of pressure was used. Under these conditions, little sintering occurred, and
Table II. Powders Prepared b y Oxidation-Reduction o f Sintered Uranium Dioxide or Fissia Shapes Were Reformed into New Pellets" 0 2 = 111 Compac% Theoretical Densityb tion Treatments, Press., rnt./Vol. Water Displacement No. TSI EO* 1% fissia UO* 1% fissia 1 1 2 2
50 75 50 75
88.9 91.2 94.8 96.1
i 0.3. i 1.0 f 0.8 f 0.3
92.4 93.7 96.7 97.1
zt 0.4 zt 0.4 i: 0.3 & 0.4
91.4 zt 0.1 92.5 i: 1.0 96.1 zt 0.8 97.2 i 0.1
93.1 i: 0.4 94.6 i 0.3 97.8 & 0.4 98.4 i 0.4
a Cold pressed and sintered for 12 hr. in Hz at 1800' C. 10.90 and 10.97 g./cc. for 1% fissia and Eon. respectively. Tolerances represent typical rar.ge oi values for pellets; 3-4 size, about 3/8-in.in both diameter and height. ~
Table 111.
Oxid.
Reduct.
No.
Temp.,
' C.
Temp., ' C.
Time,
Cycles
Min.=
Ce
Cs
REb
Ru
2 4
600 600 1250 600
900 900 1250 600
60 15 15 10
1.1 1.0 1.1 0.9
3.8 3.7 16 1.9
1.1 0.9 0.9 0.9
7 11 13 16
Id
10
~
~~~
Oxidation-Reduction of Irradiated Uranium Dioxide Removes Most of the Cesium, Ruthenium, and Tellurium
Decontamination Factors Sr 1.0 1.0 1.0 1.0
Te 5 5 26 4
Zr 0.9 7c 1.3 1.6
Per cycle for each gas. * Rare earths. Zr decontamination is questionable. Same material as for previous4-cycle and treated the same, except for an additional 1250' C. exposure. a
the oxygen-uranium ratio of the product was between 2.00 and 2.05. An optimum reducing temperature of 625' C. (3) has been suggested for crystallite cracking, but it has been implied that low oxidation temperatures of about 400' C. are more suitable for preparing activated urania ( 7 ) . That urania powder is activated by additional oxidation and reduction treatments ( 6 ) as previously reported (7) is confirmed (Table 11). The second treatment yielded powder which gave pellets having a density superior to that for pellets made from powders which were prepared by conventional methods (2, 7).
The process successfully pulverizes melted, refractory fuel shapes also A, melted uranium dioxide.
46
B, melted fissia
Fission P r o d u c t S t u d i e s Following heat-up to operating temperature in a stream of argon, irradiated uranium dioxide powder was exposed to flowing oxygen at 1 atm. of pressure. After purging with argon, the sample was treated with hydrogen. T o collect radioactive particles, two glass fiber filters were used in the furnace and a Chemical Warfare Service-type paper filter operated at room temperature, outside the furnace. The former filters are about 90% efficient for 1-micron particles, but the latter is 99% efficient on particles between 0.2 and 1 micron. Radiochemical analyses (Table 111) indicated significant removals of some fission products, which tended to increase with higher operating temperatures. Though the material used in this series of experiments had been out of pile too long to follow iodine, a similar study on short cooled uranium dioxide showed a decontamination factor of about 8 after six IO-minute oxidationreduction cycles at 800" C. Though uranium dioxide dust was
INDUSTRIALA N D ENGINEERINGCHEMISTRY
anticipated, ruthenium was the only radioactive material detected on the filters. Most of the activity was retained on the glass fiber filters, with an unidentifiably small amount collected on the fine filter. The response of the G-M counter was unusual since a large burst of activity was noted about 1 minute (residence time of gas flowing through the system) after either oxygen or hydrogen was admitted to the sample. Approximately 15 seconds later (counter flushing time), the counting rate had returned essentially to background level. This pattern continued throughout the experiment with each peak-counting rate being less than that of the previous cycle, with the same gas. In one test, the gas counter was valved off during peak indication, and the half life of the contents was greater than 25 days. In another experiment, there was no reduction in counting rate when a cold trap (at -100" C.) was interposed in front of the gas counter. I t was inferred that the radioactive material in the gas counter was krypton -85.
literature Cited (1) Bard, R. J., others, U. S. Atomic Energy Comm. Rept. LA-1952 (Octo-
ber 1953).
(2) Belle, J., Lustman, B., Ibid.,WAPD-
(SeDtember 19571.
184 _._
( 3 ) DdHdllander, W. R.,Ibid., HW-46685 (Nov. 8.1956).
(4$-S;niz&, D. 'I., Kendall, E. G., others, Ibid.,NAA-SR-3269 (1959). (5) Strausberg, S., Luebben, T. E., Ibid., NAA-SR-3910, Pt. I (August 1959). ( 6 ) Strausberg, S.,others, Ibid.,NAA-SR3911; Pt. I1 (1960). (7) Watson, L. C., .4tomic Energy of Canada, Ltd., Chalk River, Ont. Project., CRL-45 (November 1957). RECEIVED for review March 5, 1959 ACCEPTED August 31, 1959
