ORNL-4224 by Jon Morrow

ORNL-4224

Contract No. W-7405-eng-26 CHEMfCAL TECHNO1 G Y DlVlS ION Chemical Development Section B

~ ~ ~ OF FALLlNG ~ R DROPLETS ~ OFNMOLTEN A FLUORIDE ~ ~ SALT AS A MEANS OF RECOVERING URANIUM AND P L ~ T

NOVEMBER 1968

OAK RlDGE NATIONAL LABORATORY Oak Ridge, Tennessee operated by UNlON CARBIDE CQRPORATlON for the U. S. ATOMIC ENERGY COMMISSION

3 4456 0535828 L

... I I1

CONTENTS Page

......................................... 1 . (Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Fluoride Salts Used . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Apparatus and Procedure ........................ 2.3 Velocity and Time of Fall of Droplets . . . . . . . . . . . . . . . . 3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Sorption of UF and PwF on the Surfaces of Frozen Droplets . 6 6 3.2 Fluorination of Uranium-Containing Salts . . . . . . . . . . . . . 3.3 Fluusitxltion of Plutonium-Containing Salts . . . . . . . . . . . . 3.4 Fluorination sf Pro~actinium-ContainingSalts . . . . . . . . . . . 4 . Possible Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Abs~ract

. 5.

. ............

1 2 2

3 6 6 6 8

17 17

4.1 Removal of Uranium from Mslten-Salt Breeder Reactor Fuel

4.2 Processing of Law-Enrichment Reacfor Fuels

................................

.....................................

Ac know! edgrnents References

FLUORINATION OF FALLING DRQPLETS OF MOLTEN $ALT AS A MEANS OF RECOVERING ~ R A

DE ~

Experimental equipment WQS developed, anal smal I-scale fluorination experiments were mode, usir-6~~ several salt d ~ t i o m . data obtained from these experirnen caleolatc3d that, by using a ram

100+-di~m

w r b r fuel.

droplets !Urn

+-diarn

droplets of

can

be re-

in similar experiments using salt dropiets containing t ~ was ~ fluorinated, ~ ~ ~ ~ even w m temperatures a5 high Q

In the malten-salt fluoride volatility process a d i n moifen-salt reactor fuel processing,

fluorination.

u r ~ n i u mand/or plutonium are removed from molten fluoride salts

Sparging of a gaol of molten salt w i t h fluorine would appear to be the

simplest method, but i t reswjts i n undesirably high corrosion of the vessel and a low rate of plutonium removal.

The latter is caused, in part, by the thermodynamic

Six different c~mposltionswsw used Fn cxperirncn;~involving urai7;bm:

NaF-ZrF4-UF

NaF-ZrF -\IF (48-48-4 mole %) 4 4 NaF-ZrF4-LlF (45.5-45.5-9 mole %I 4 NaF-ZrF4-UFq (37.4-56.1-65 mole %)

3. 4.

(48.75-48.25-2.5

mole 96)

3 (31.7-31.7-31.7-5

NaF-hiF-ZrF4-UF4

LiF-BeeF2 (66-34 moiie %) containing 9.4

mole %> of uranium p r g of salt

The salt used i n the protactinium experiments consisted of NaF-ZrF, (5 Jt 23 1 mole %> containing about 0 . Z mg of Pa per g of salt. 2.2 Apparahvs and Procedure

~ and sizing ~ the fluoride v sa

The procedure consjsted i n ~ feeding the size

powder through the glass feeder (Fig. 21, y turning a nickel-wise

helix, info the preheater sectian of the feeder profecfed i t from 04: the

e l i v m flowing h w g h the powder

CC"

uorine and s e r v d

a5 Q

bisrklkea gas far the psehe

column,) T h e ยงma drops (58 to 2'88 Y i n iameter) sf molten salt, obtained

when the sized powders w e r e rnelte flvsrination section,

0 P in

the preheaser section, then f e l l t

Bn s m e of the experiments w i t h uranium, large

et^^^ were

produced direcf

After passing through

means of

drops (about

high-lam

the f u ~ r j ~ section, ~ t ~ ~then "'depleted" salt drop were

frozen and collected i n a stainlia~steel cup

located i n the bottom of the column, ln

POWDER FEEDER

SIX CLAMSHELL HEATERS \

FLUORINATION SECTION

-----4P

---

-----B=

COOLING WATER

-=CUebECrlON I

CUP

I I

ILOWER GLOVE BOX

Fig. 1. Equipment for Fluorinating Falling Droplets of PIutOFiium-Csntajning Salts. Apparatus for recovering uraniwn was essentially the same.

ORNI.. - DWG 68-.7627

POWDER 2 ES E RV Q 1 R *

TEFLON SLIP -181NT

NICKEL ROD

NlCKEC WIRE HELIX

SCREW FEEDER FOR POWDERED

//-WITH

5 WAGE 1-0K FIT TI NG

TOP F L A N G E

i i

TEFLON FERRULES

FLUORIDE SALT (TURNED B Y HAND)

6 almost a l l of the experiments w i t h uranium-containing salt, chilled liquid fluorocarbon t o prevent

this cup contained

UF i n the gas from sorbing on the frozen 6

salt. After collection, the frozen droplets were sieved; then, the different size fractions were sampled for chemical analysis.

2.3 Velociiy and Time of Fall o f Droplets

The velocity of the droplets must be known i n order to calculate! the time of contact i n the fluorination tower. The velocity of 137+-dian

dropleb falling

through fluorine at 600째C was calculated as a SCPRICP~O~ of time, using drag coefficienis. Figure 3 i s a plat of these calculated velocities. At droplet reaches i t s terminal *relocity of about

6M"C and 1 a h , the falling

60 cni/ses.

after falling only 8.7 cm.

Thus, the important velocity for the droplets (except. for the 3000-c~ droplets, which do not attuin terminal velocity i n a short fall) i s the terminal velsctty. Terminal velocities for particles of other sizes were easily cnlccllated because, far smal I droplets, the terminal velocity i s nearly propartianal to the square most of the fluorination experiments were done af

600

the diameter. Since

* 5WC, the rate of full was not

corrected for temperature. I n these experiments, the residence time of a small drop i n the fluorination tower can be expressed as::

t (sec) = C

where

-3 2

_ I

1 . 2 4 6 ~10 d

i s the particle diameter i n microns, and

i s the length of the fluorinutian

section i n inches.

3. RESULTS 3.1 Sorption af UF and PuF on the Surfaces o f Frozen Droplets 6 6 Since, i n our system, the frozen salt was collected i n the bottom of the fallingdrop tower, i t

WQS

necessary to examine the possibility that ciranlurn and plutanivrn

hexafluorides were being sorbed o n this salt and biasing the experimental results. I n

8 R NL DWG. 68- 882

Fig. 3. Calcvlatd Rate af Fall of ?37+-diarn OO@ and 1 atm.

Scald.Droplets i n Fluorine

8 most o f the uranium experiments, the droplets were caught i n a p o l o f chilled liquid fluorocarbon, which prevented direct contact between the frozen salt and the UF

significant increase i n uranium removal was obtained i n these riins, particularly when the particle diameter was large (Fig.

4); the maximum removal without use of the

fluorocarbon was about 99.5%, whereus the maximum removul

with i t s use was greater

than 99.9%. In the plutonium work, this procedure was not used because the potential kazard

of contacting fluorine w i t h

fluorocarbon was considered f a be too great.

If i t had

been used, some sorption on the frozen salt undoubtedly would have bocn prevented.

by analogy w i t h the uranium results (Fig. 4), only 1 t o 2% of the

However,

volatilized plutonium would have been expected t o sorb on the frozen droplets. l e s s than

58% o f the plutonium was volatilized

in any experiment (Sect.

sorption would change the amownt of plutonium found i n a drop

Since

3.3), surface

by less than 15%. This

i s less than the experimental variation.

The data shown i n Fig. 4 suggest that., i n a large-scale falling-drop FIuorinatiorr tower, some method of preventing contact of the gas and the depleted salt would be desirable.

Probably the best solution would be to admit fluorine a t the 'sattom of the

column at a flow rate sufficient to prevent significant amounts of UF and PvF from

contacting the solidified salt.

3.2 Fluorination of Uranium-Containing Salts Table 1 shows

few examples of the data obtained i n these experiments. I n

general, the recoveries increased w i t h decreasing particle diameter and w i t h increasing temperature, other conditiuns being the same.

The uranium recovery data obtained

with the six salt compositions were correlated by using the foallowing equation:

where

D = droplet diameter, P,

T = fluorination temperature,

OK,

COLLECTED UNDER L i Q U i n FLUOROCARBON,c

0 COLLECTkD IN DRY CLIP

1-.

NUMBERS BESIDE POINTS ARE NUMBER OF DATA POINTS C Q l N C l D i N G

DROPLET DIAMETER ( p )

Fig. 4. Removal of Uranium QS a Function of Droplet Diameter Using Different Methods for Gdlecting the Frozen Droplets.

Initial soh: NaF-ZrF4 (50-50 mole 94) containing U f

170,1m0

556

88- 105 105- 125 125-1 4P 149477

1,708 2,70c 9,?00 2 1,9CO

39.8 98.4 94.3 87.1

54,400

559

63-88 58- 105 105- 125 125- 149

202 2 12 419

93.6 99.6

m;mo

5 isc)

-3200

5 T ,a0

538

125- 149 149- 177 l/7-2 10

1,724

95.2 96.8

74,600

5.6

73 12h '260

152

99.9 99.8

99.5 ?9.?L 99.46 99.9 3

=I.

fluorination t i m e , sec,

53,W = i n i t i a l uranium concentration i n the salt, w t fraction, C U = Gina! k and

uranium concentration i n the sah, w t fraction, and

are empirical constants.

Values for lag k and

are given i n Tab%e2, and plots of Eq. (I), using data for Four

of the salt compositions, are shown as Fig. otted vs the iiquiders temperature

5. In Fig. 6, the empirical

af the salt.

constants are

explanation for the lineor

r d a t i ~ t ~ ~between h i p ~ iog k a d ~ h eliquidus tempera-ature and between

and the

lliquidws temperature i s known.

All the results indicate !hat,

in o

4-ft-long fluorination calumn at 650째C, m o r e

than 99.5% of the uranium can be removed from droplets that are less than 200

d F a met er .

3..3 F Buarination sf P B u tan iurn -Can f ain i ng Sa Ib

12 Table 2. Correlation Constants for Equation (1) for Six Salts Containing Urani urn Tetrafluoride

NaF

Salt Composition (mole %)

Li6

ZrF4

UF4

Weight

5cF2

Fraction Uranium In Salt

Liquidus

Temp. of Salt

(“C9

h J k

x 10

31.7

0,132

47sQ

15.78

1.45

48.75

2.5

0.0542

51Q

21.58

2.84

48.0

0.0843

5 15

2 1.58

2.84

45.5

0.1733

550

28.79

4.35

37.4

56.1

6-5

0.1 la

630

40.25

7.16

0.13 33.29

O.OO942

4-80

15“60

1.42

31.7

66.58

By cooling curve.

13 ORNL DWG. 64-7513

Liquidws temp., 41aO°C NaF-ZrF4-UF4 (48.75-48.75-2.5 mole %I Liquidws temp., 510°C

\ m

NaF-ZrF4-UF4

(48-48-4 mole %) Liquidus temp., 515’ NaF-ZrF -UF4 (45.5-45.4-9 mole 9 Liquidus temp., 55OC

4.06

1. io

1.14 I/T x

I 1.48

4.22

(OK-’)

Fig. 5. Plots of Equation ( l ) , Using Data Obtained with Four Salts.

O R N i DWG 68-887

Fig. 6. Plots of Liquidus T e m p e r a t ~ ~ rof e the Salt vs Covstanfs ~PQTIS Equation ( 1 ) for Salt Solutions Containity Uranium.

15 Table 3. Results of Fluorination of Plutonium i n Falling Molten-Salt Droplets (50-50 mole %) containing PuF 3 Length of fluorinator: 52 in.

Initial salt:

Average Temperature

("(3

NaF-ZrF4

Range of Drop Diameters

(

Initial Plutoni urn Concentration

Final

Piutoni urn Concentration

(W/d

(Wh)

Plutoni um Removed From Salt

F Iuorination

(%)

Time bc)

528

88- 105 105-1 25 125- 149

0.026

0.020 0.0223 0.0220

22.9 14.3 15.3

4.5 3.2 2.2

570

88- 105 '105-125 125- 149 149- 177

0.026

0.0898 0.0171 0.0259 0.0205

62.1 34.4 19.7 21.0

4.5 3.2 2.2 1.6

624

63-88 88- 105 185-125

0.026

8.0156 0.0055 0.0088

40.0 79 .Q 66.0

7.3 4.5 3.2

506

63-88 88-1 05 105-8 25 125- 149 149- 177

2,58

1.232 1.45 1.58 1.09 1.34

52.2 43.8 38.7 57.8 48.2

7.3 4.5 3.2 2.2 1.6

543

53-63 63-88 88- 105 105- 125

2.58

0.79 1.19 1.56 2.04

69.5 53.8 39.3 2 1 ,o

12.4 7.3 4.5 3-2

609

53-63 63-88 105- 125 125-149

2.58

0.45 0.50 0.93 1 .Q6

82.7 80.4 64.0 58.8

12.4 7.3 3.2 2.2

640

53-63 63-88 88- 105 105-125

2.58

0.33 0.32

87.4 87.6 85 .O 81.3

12.4 7.3 4.5 3.2

0.39 0.48

O R N L DWG 68-888

IO0

LENGTH I

OF FLUQRINATBON

COLUMN, 5 2 INCHES

Fig. 7. Plutonium Removal os a Function of Drop Diameter at Coostant Temperattire. Ler?gth of fluorinator, 52 in.

17 The plwfoni'urn removal i n a tower having a length othm than 52 in. can

estimated

from the following equation:

F"/OQ

- (1

- F/IOQ) I.>"52

where

F ::: plutonium removed i n 52-in.-lang fluorinator (drop diameter and temperature constant), %#

pfutoniwm removed i n fluwinator of length

e',

and

L ' = fluorinator length, in. 3.4 Fluorination of Psotactinium-~:onb.ainiingSalts

(SO-SO mole %) cantaining protactinium.

231

Pa. Gamma counting was used for anaIysis of the

N o ciafectable amount sf ~

~was ~volatilized ~ t in i either ~ j

experiIlnenf., However, the f uorine used thraughwt t h i s entire investigation contained 9 about 'BOvol 5% ~ ~ including~ about 1%~ oxygen. ~The studies r of Stein~ at ~ ationat l a b a r a f w y indicate

that, when the oxygen concentratfan i n

fluorine i s this high, nonvolatile Pa OF i s formed i n preference to the more2 8 valcmtiBe PaF 5'

4. POSSIBLE APPLBCATIB 4.7 Removal of Uranium fram Molten-Salt Breeder Reactor Fuel One possible application of the falling-drop fluorination scheme i s for processing Iten-Salt

Breeder Reactor ~

~ fue S This ~ fuelR contains ~ about 1,6 wt % uranium

and must be processed at the rate of about 16 ft /day ( P = 2.0 g/'cm 1- The total fuel 3 5 salt volume w i l l be about 688 ft and will contain about 6.2 x 1 g of uranium. The annual increase i n uranium due to breeding will be about 7%. For procedures yielding a

99% removal of uranium i n the reprocessing step (assuming that the other 1% i s lost),

we calculate that the annual loss of uranium would be about 2.6% of the total charge, which i s more than one-third of the uranium that i s produced b y the reactor. Thus,

18 processsing scl~ernsthat do not permit recovery of at least '39.9% of the wranniurn would

be considered as economically infeasible. Using a falling-dray fluoriilator having a fluorination section that i s

5 f+ long (to keep the

unit re!atFve!y compact), w e sa!--

culated that the maximum diameters aF drop5 from which 99.95% of h e uranium cacllcd

be removed at temperatures of 6N9, 650, a i d 700째C would be 90, 112, and 137 P I res pec ti ve 1y

Drops; of these sizes can he generated i ~ ~ i go\ gtwo-fluid s p a y nozzle. With 10 such a nozzle, the !,knit Operations Ssctisrr of the Chsnical Y e c t i r d o ~ yDivision has succeeded Fn producing droplets, most of which ore 25 t o 100

i n diameter.

Significantly, several thousad volumes sf driving gas per voiunie of molten salt arc required t o obtain droplets of

this sine. Nitrogen has been ~ s 0 p o s das the driving

gas since fiuarine i s too corrosive to the spay r ~ o f r ! ehowever, f such inert gas i n ihe flcrorinution tower

quo

p.'mw!d diiute the cfflilea: fjuorine tw ~iiuchto

allow i t s recycle. In Ahlten-Sak Breeder R w e 6 c ~fuel p

sing, Clisposwa of the

unreactad fiucxiaae piobably would not be prc*h:lrsitive ~ i a e e ,far 563% uiilirwtian,

would a r m u n t t o only about 7588 Ib,/yecr.

this

Another approach might consist i n forming

droplets (tising a two-fluid spray norrCe), freezing them i n on i n n r ~ufmosphare, a d

feedirg the frozen droplets into the top of

fluwination tower.

This method

WQO.J!~

require c soeitcmble nieam for removing decay heat from the frozen cLop!sts ts prevent pre~xmturemelting.

4.2 Processing of Law-Envichmer~tR x c t n r Fuels A t the prssent timc, i t appears unlikely t h a ~mo8ten-x'; toiatility met170ds

w i l l be

used for processing reactor foci; that contoin pB;etor.i~lnt- If such piacesing

i s contcmplutd, i t

should be noted that the facllling-diop methad i s trrc only up,prarrcII

that has shown promise for the rapid rcmoval sf p l v t a n i v r i ~w i t h Cow reaction-vcsse! corrosian rates. For examplc,

QUT

expsrbmentul dato indicate that fium.inw:ion in a n

11-ft-high tower w i l l result i n the reivrijval of 99% af the plutonium from

IOQ-J~-~;CIR~

drops at 64OI"C. This corresponds to u fluorination t i m e of ubowt 1 1 sec. In the prozeaing sf salt c o n t a i n i ~ gplutoniwrn, the fluorine would probably have to be rccycled

19 i n o r d e r to maintain a

high fluorine:PuF

(to prevent decomposition of the

PuF6).

m o l e ratio in the e x i t gas from the column Recycling the fluorine would, i n turn, r e q u i r e

that the d r o p l e t s be formed in a separate tower, where the driving gas for the s p a y nozzle would n o t dilute the f l u o r i n e i n the fluorination tower. The drops would then

be frozen and fed

the top of the fluorination fower.

1. G. I. Cathers, Nwc

5. A.

sci.

E%. 2, 768-77 (1957).

Florin e t ai., J. InoRg, v

ucl. Chern. 2, 368 (1~56).

20 7. J. H. Perry (ed.), Chemical Engineering Hadbook, pp. 7078-20, McGraw-Mil!, I

N e w York, 1950.

8. S. 1.

ANP Fluoride

!.*%

D. Powers, and N. D. Greene, A Physical Mixfures, -___ OR L-2 150 (Aug. 23, 1956).

Ccphen, W.

Stein, Inorg. Chem. 3,995 (1964).

_ I -

B P o p e ~ S u m n a r yfar

21 ORNL-4224 UC-10 - Chemical S e p a r a t i o n s P r o c e s s e s f o r Plutonium and Uranium INTERNAL DISTRIBUTION 1. Biology L i b r a r y C e n t r a l Research L i b r a r y 5-6. ORNL - Y - 1 2 T e c h n i c a l L i b r a r y Document Reference See t i o n 7-26. Laboratory Necords Department 27. Laboratory Records, ORNL R.C. 28. L. L. Bennett 79. M. R . Bennett 30. R. E. Blanco 31. C . A. Brandon 32. K . B. Brown 33. W . L. Carter 34. W. B. Carr 35. G. I . C a t h e r s 3 6 . F. L . C u l l e r 37. D. E. Ferguson 38. L. M. F e r r i s 39. C. Goldberg 40. J. K. J o n e s 41. P. R . Kasten 42. C . E. Larson 43. K. H. Lin 44. A. P. Litman

2-4.

45. R. Lowrie 46. H. G . MacPherson 47. J . C . Mailen 48. L. E. McNeese 49. J . R . McWrierter 50. R . P. M i l f o r d 51. E. L . Nicholson 52. J , T . Roberts 53. M. W. Rosenthal 54. M. J . Skinner 55. D. A. Sundberg 56. R . E. Thoma 57. 5. R. T a l l a c k s o n

58. A .

M. Weinberg

60. E . 61. J . 62 J 63. J . 64. J 65. E. 66. R .

L. Youngblood P. Young B. Emmett ( c o n s u l t a n t ) J, K a t z ( c o n s u l t a n t ) T Margrave ( c o n s u l t a n t ) A. Mason ( c o n s u l t a n t ) B. Richards ( c o n s u l t a n t )

59. M. E. Whatley

EXTERNAL DISTRIBUTION

67. D. E. Bloomfield, B a t t e l l e fiJorthwest, Richland, Washington 68. J . A. Swartout, Union Carbide C o r p o r a t i o n , New York

69. Laboratory and U n i v e r s i t y D i v i s i o n , AEC, OR0 70-214. Given d i s t r i b u t i o n as shown i n TID-4500 under Chemical

S e p a r a t i o n s P r o c e s s e s f o r P l u t o n i m and Uranium c a t e g o r y (25 c o p i e s - CFSTI)