Laboratory and Pilot Plant Studies on Conversion of Uranyl Nitrate to

37 Laboratory and Pilot Plant Studies on Conversion of Uranyl Nitrate to Uranium Hexafluoride I.J.URZA and D. C. KILIAN

Downloaded by PURDUE UNIV on July 7, 2016 | http://pubs.acs.org Publication Date: April 16, 1980 | doi: 10.1021/bk-1980-0117.ch037

Exxon Nuclear Company, Inc., 2955 George Washington Way, Richland, WA 99352

Exxon Nuclear Company has conducted various development pro grams to support the design and licensing of a commercial nuclear fuel reprocessing plant. The uranium conversion portion of the reprocessing plant will use fluidized-bed processes for conver sion of uranyl nitrate hexahydrate (UNH) to uranium hexafluoride (UF ). This paper describes the laboratory and pilot plant studies conducted at Oak Ridge National Laboratory (1) for Exxon Nuclear Company on the conversion of UNH toUF ,and on the puri fication of UF . 6

6

6

Laboratory Studies Experimental laboratory studies on the removal of residual nitrate from UO and the fluorination and sorption of technetium on MgF were conducted to support the pilot plant work. These studies covered a wide range of conditions and were used primar ily to guide the pilot plant effort rather than to determine quantitative thermodynamic and kinetic data. 3

2

Removal of Residual Nitrate From UO : Uranium trioxide pro duced by thermal decomposition of uranyl nitrate solution in a fluidized-bed contains a small amount (usually about 0.4 to 1.0 wt%) of residual nitrate. If UO is to be converted toUF for feed to a gaseous d i f f u s i o n enrichment p l a n t , the n i t r a t e content of the UOg must be reduced to meet UF^ p u r i t y s p e c i f i c a t i o n s . F l u o r i n a t i o n of U0^ in the presence of n i t r a t e r e s u l t s in forma t i o n of n i t r o s y l and n i t r y l hexafluorouranates and h e p t a f l u o r o u ranates (NO UF where χ = 1 or 2 and y = 6 or 7) ( 2 ) . These com pounds form p o t e n t i a l l y troublesome s o l i d s . The removal o f r e s i d u a l n i t r a t e from U0^ was s t u d i e d as a f u n c t i o n of time and temperature under n i t r o g e n , a i r and hydro gen-nitrogen atmospheres. The procedure used f o r these t e s t s was to p l a c e a U0^ sample (10g) in a v e r t i c a l 2.54 cm diameter by 30.5 cm long s t a i n l e s s s t e e l r e a c t o r , heat it t o the d e s i r e d temperature, and then s t a r t the gas flow. When the t e s t was terminated, the sample was cooled and sampled f o r a n a l y s i s . 3

3

O.841 -Ο^Ρ 5^7^.00/0 2

©

1980

A&fflfîfyduèfi&f3i Society 7

1155

16* St. N. W.

Navratil and Schulz; Actinide Separations D. C. 20036 ACS Symposium Series;Washington, American Chemical Society: Washington, DC, 1980.

6

ACTINIDE SEPARATIONS

548

Data from t e s t s w i t h t y p i c a l UCL product produced in the p i l o t p l a n t f l u i d i z e d - b e d c a l c i n e r l e a to the f o l l o w i n g general conclusions : 1. Sparging of UO^ w i t h n i t r o g e n , a i r or hydrogen a t temperatures above 450°C is e f f e c t i v e in l o w e r i n g the n i t r a t e cont e n t of UO . 2. Treatment w i t h hydrogen g i v e s the h i g h e s t n i t r a t e r e moval r a t e s and lowest r e s i d u a l n i t r a t e but as expected, r e s u l t s in the h i g h e s t conversion of UO^ to U^0 and UO^. 3. Treatment w i t h n i t r o g e n at temperatures above 550°C a l s o causes s i g n i f i c a n t decomposition of UO- to the lower oxides. 4. An a i r sparge is n e a r l y as e f f e c t i v e as n i t r o g e n f o r n i t r a t e removal a t temperatures above 550°C and s i g n i f i c a n t l y suppresses the decomposition of UO^. Downloaded by PURDUE UNIV on July 7, 2016 | http://pubs.acs.org Publication Date: April 16, 1980 | doi: 10.1021/bk-1980-0117.ch037

g

F l u o r i n a t i o n and S o r p t i o n of Technetium on Magnesium F l u o r i d e : Trace q u a n t i t i e s of technetium w i l l accompany the uranium through the Purex s o l v e n t e x t r a c t i o n p r o c e s s , the convers i o n of UNH to U0 and the f l u o r i n a t i o n of UO^. The technetium in the UO w i l l be f l u o r i n a t e d (probably t o TcF^) and w i l l v o l a t i l i z e w i t h the gaseous UF^ product. The technetium must be removed to meet gaseous d i f f u s i o n p l a n t feed s p e c i f i c a t i o n s . G o l l i h e r e t a l . (3) developed the use of MgF^ f o r removal of technetium from UF^. The process was demonstrated on both a l a b o r a t o r y and p r o d u c t i o n s c a l e a t the Paducah Gaseous D i f f u s i o n P l a n t where UF^ is produced by the h y d r o f l u o r i n a t i o n p r o c e s s , 3

4HF U0

2

F - UF

4

ÛF . 6

The o b j e c t i v e s of the l a b o r a t o r y s t u d i e s were: (1) to study the behavior of v o l a t i l e technetium compounds produced by d i r e c t f l u o r i n a t i o n of UO^ w i t h elemental f l u o r i n e ; (2) to study the technetium s o r p t i o n c h a r a c t e r i s t i c s o f MgF ; and (3) t o determine the f e a s i b i l i t y of repeated r e g e n e r a t i o n of MgF^ by d e s o r p t i o n of technetium w i t h f l u o r i n e . A batch of UO^-TcÔ c a l c i n e was prepared in the l a b o r a t o r y by adding ammonium pertechnetate (NH^TcO^) t o UNH which was then t h e r m a l l y d e n i t r a t e d a t 350°C. Alumina was subsequently added to form a g r a n u l a r product. The technetium l o s s e s due to v o l a t i l i z a t i o n d u r i n g the c a l c i n a t i o n process were n e g l i g i b l e . The analyzed technetium conc e n t r a t i o n was 1060 ppm (based on U) and the c a l c u l a t e d concent r a t i o n based on the ammonium pertechnetate i n p u t was 1070 ppm. This c o n c e n t r a t i o n is more than 10 times t h a t expected in the f u e l r e p r o c e s s i n g p l a n t U0-. The Tc used in t h i s work was analyzed d i r e c t l y by beta s c i n t i l l a t i o n counting. Analyses of samples were c a r r i e d out using a r a d i o c h e m i c a l s e p a r a t i o n procedure, w i t h perrhenate as c a r r i e r , and beta p r o p o r t i o n a l counting. Samples, which were always run in d u p l i c a t e , g e n e r a l l y showed a spread of 99% of the technetium from the UF^ was achieved. Repeated regeneration of MgF by d e s o r p t i o n of technetium w i t h f l u o r i n e was t e s t e d a t 350 and 500°C. The same MgF^ bed was used during these t e s t s . At the completion of each f l u o r i n a t i o n run, the MgF^ was desorbed. The off-gas was passed through a c o l d t r a p to c o l l e c t the desorbed technetium. Less than 10% of the technetium was desorbed a t 350°C., but e s s e n t i a l l y all of the technetium was desorbed a t 500°C. 9

Navratil and Schulz; Actinide Separations ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


550


The f o l l o w i n g c o n c l u s i o n s were drawn from r e s u l t s o f the technetium f l u o r i n a t i o n - s o r p t i o n s t u d i e s : 1. Greater than 99% o f the technetium and uranium was v o l a t i l i z e d from the f l u o r i n a t o r in all f l u o r i n a t i o n t e s t s . 2. Excess f l u o r i n e in the f l u o r i n a t o r o f f - g a s does not ap pear t o improve the c o l l e c t i o n e f f i c i e n c y o f MgF^ f o r technetium compounds. 3. Greater than 99% o f v o l a t i l e technetium compounds pro duced by d i r e c t f l u o r i n a t i o n o f UO- can be removed from UF^ by p a s s i n g the o f f - g a s through a bed o f HgF^. 4. Desorption t e s t s confirmed t h a t technetium removal from MgF^ w i t h f l u o r i n e is p o s s i b l e but optimum temperature, time and e f f e c t s o f repeated s o r p t i o n - d e s o r p t i o n c y c l e s were n o t ade quately defined. 5. Pretreatment c o n d i t i o n i n g o f MgF^ s i g n i f i c a n t l y a f f e c t s technetium s o r p t i o n c a p a c i t y . Uranium C a l c i n a t i o n P i l o t P l a n t Studies The o b j e c t i v e s o f the uranium c a l c i n a t i o n development pro gram were t o : 1. Test the performance o f a r e c t a n g u l a r uranium c a l c i n a tion vessel. 2. Produce f l u i d i z e d - b e d UO product s u i t a b l e f o r f l u o r i n a tion. 3. V e r i f y o p e r a t i n g and design parameters needed f o r s t a r t of d e t a i l e d design. 4. Determine t h e behavior o f technetium and ruthenium during c a l c i n a t i o n and the removal r a t e o f r e s i d u a l n i t r a t e by heat treatment of U0^. J

Process D e s c r i p t i o n : Uranium t r i o x i d e is produced by t h e r mal decomposition o f u r a n y l n i t r a t e hexahydrate (UNH) by the f o l l o w i n g endothermic r e a c t i o n : heat U0 (N0 ) .6H 0 2

3

2

2

- U 0 + NO + Ν 0 + 0 + 6H 0 3

£

£

2

ΔΗ = 150 kcal/g-mole @ 300°C The r e a c t i o n is accomplished by s p r a y i n g UNH i n t o a bed o f f l u i d i z e d U 0 p a r t i c l e s a t a temperature o f about 300°C. The UNH is deposited on the U0^ p a r t i c l e s and decomposed. F l u i d i z e d - b e d r e a c t o r s are w e l l s u i t e d f o r t h i s process because h i g h r a t e s o f heat and mass t r a n s f e r are r e q u i r e d between the s o l i d and f l u i d . Under normal o p e r a t i n g c o n d i t i o n s , UO- produced in a f l u i d i z e d bed is g r a n u l a r , f r e e f l o w i n g m a t e r i a l w i t h a p a r t i c l e s i z e d i s t r i b u t i o n s u i t a b l e f o r subsequent f l u i d i z e d - b e d operations ( 4 ) . The V0„ is a c h e m i c a l l y s t a b l e , m i l d l y hygroscopic s o l i d w i t h a c r y s t a l d e n s i t y of 7.3 g/cm3. 3



37.

URZA

AND

KILIAN

Conversion of UNH

to

UF

6

551

The use of f l u i d i z e d - b e d s f o r producing UCL has been studied e x t e n s i v e l y by various i n v e s t i g a t o r s (4-9) at p i l o t p l a n t and production scale. The e f f e c t s of operating v a r i a b l e s such as temperature, feed composition, feed r a t e , and s u p e r f i c i a l v e l o c i t y , have been i n v e s t i g a t e d e x t e n s i v e l y . A s i m p l i f i e d flowsheet of the uranium c a l c i n a t i o n p i l o t p l a n t is shown in Figure 1. Uranyl n i t r a t e hexahydrate was prepared in two steam heated feed tanks. The UNH, which has a f r e e z i n g p o i n t of about 65°C and a b o i l i n g p o i n t of 120°C., was pumped i n t o the c a l c i n e r through an a i r atomized feed nozzle. The UNH was c a l c i n e d to UO in the f l u i d i z e d - b e d c a l c i n e r using preheated a i r as the f l u i d i z i n g gas. The UCL product overflowed continuously by g r a v i t y from the c a l c i n e r to the product v e s s e l where the weight was monitored with a load c e l l . Off-gas from the c a l c i n a t i o n process was passed through sintered-metal f i l t e r s l o c a t e d in the upper s e c t i o n of the c a l c i n a t i o n v e s s e l . The process off-gas was then cooled in the o f f gas condenser where most of the N0 released in the d e n i t r a t i o n process was recovered as n i t r i c a c i a and c o l l e c t e d in the conden sate tank. Uncondensed vapors were passed s u c c e s s i v e l y through the de-entrainer, the off-gas preheater, the HEPA f i l t e r , and then vented to the off-gas system. 9

D e s c r i p t i o n of Equipment: The dimensions of the rectangular c a l c i n a t i o n v e s s e l (15.2 cm χ 12.7 cm) were s e l e c t e d to t e s t geo metric parameters t h a t are important to the design of a p l a n t scale s l a b u n i t , thereby reducing u n c e r t a i n t i e s involved in scaleup. The f l u i d i z e d - b e d s e c t i o n of the c a l c i n e r contained the a i r d i s t r i b u t o r p l a t e , the bed drainage o u t l e t , the feed nozzle, the product overflow l i n e , and the bed charging l i n e . An expanded f i l t e r s e c t i o n containing porous s t a i n l e s s s t e e l f i l t e r s and the off-gas vent l i n e was l o c a t e d d i r e c t l y above the f l u i d i z e d - b e d section. The UNH feed tanks had a volume of 227 l i t e r s each; the v e s s e l s were jacketed f o r 15 p s i g steam and contained steam c o i l s f o r 125 p s i g steam. A p o s i t i v e displacement metering pump was used to feed the UNH. The off-gas condenser was a shell-and-tube (water-cooled) heat exchanger; a 260 l i t e r c a p a c i t y s t a i n l e s s s t e e l tank was used to c o l l e c t condensate; and a s t a i n l e s s s t e e l wire-mesh de-entrainer, a shell-and-tube (steam-heated) preheater and a HEPA f i l t e r were used in the off-gas system. The p r i n c i p a l process instrumentation included: (1) thermo couples to monitor the temperature at various l o c a t i o n s ; (2) a flow c o n t r o l l e r to measure the a i r flow rate to the c a l c i n e r ; and (3) d i f f e r e n t i a l - p r e s s u r e t r a n s m i t t e r s to monitor the pressure drop across the d i s t r i b u t o r p l a t e , the f l u i d i z e d - b e d and the sintered-metal f i l t e r s .



552


R e s u l t s of C a l c i n a t i o n Runs: F i f t e e n c a l c i n a t i o n runs were made using UNH feed concentrations ranging from 512 to 1172 g U/L and feed rates ranging from 150 to 419 mB/min. A t o t a l of 2540 kg of UO^ was produced during these runs. During the first two runs, d i l u t e UNH feed (500 gU/L) and a low feed rate (150 mL/min) was used to t e s t the o p e r a t i o n of the system. In the subsequent runs, the UNH feed rate was progress i v e l y i n c r e a s e d to 230, 315 and 419 mL/min and the UNH concent r a t i o n was i n c r e a s e d to 900 and 1200 g U/L. During the first 8 runs, excessive p a r t i c l e growth and a p r o g r e s s i v e i n c r e a s e in pressure drop across the o f f - g a s f i l t e r s caused o p e r a t i o n a l problems. These problems were r e s o l v e d by e n l a r g i n g and heating the f i l t e r housing, i n c r e a s i n g the f i l t e r area and modifying the f i l t e r blowback system. The c a l c i n a t i o n system operated s a t i s f a c t o r i l y during the l a s t 7 runs; the run l e n g t h was l i m i t e d by the UNH feed supply r a t h e r than process instabilities. Caking of UO^ around the feed nozzle was evident in most runs but d i d not cause o p e r a t i o n a l problems. During the l a s t 7 runs (9 t h r u 15) 1900 kg of UO^ was produced in 76 hours of o p e r a t i o n . The o p e r a t i n g c o n d i t i o n s and r e s u l t s of these runs are summarized in Table 1. The average p a r t i c l e diameter of the U0 bed m a t e r i a l remained in the d e s i r e d range (about O.25 mm) f o r good f l u i d i z a t i o n of the^ bed. The maximum U0^ p r o d u c t i o n rate t e s t e d was 1,770 kg/hr-m (based on the f l u i d i z e d - b e d cross s e c t i o n ) during run 14. The UO product was t y p i c a l of UO^ produced in f l u i d i z e d beds a t other f a c i l i t i e s . The UO- was granular, f r e e flowing m a t e r i a l with a bulk d e n s i t y of 3.7 to 4.1 g/dm3and a tap d e n s i t y of 3.9 to 4.2 g/cm3. The average n i t r a t e content ranged from O.37 to O.92 wt% and the water content ranged from O.02 to O.25 wt%. The UÔg content was l e s s than O.2 wt%. q

Removal of R e s i d u a l N i t r a t e : Product from runs 4 and 15 was heat t r e a t e d at a bed temperature of 600°C and a s u p e r f i c i a l f l u i d i z i n g a i r v e l o c i t y of 30.5 cm/sec to study the removal of residual nitrate. The n i t r a t e content was reduced to l e s s than O.05 wt% in 3 hours. Most of the n i t r a t e removal occurred during the first hour of heat treatment. Less than 1% of the UO^ decomposed to UÔg and the p a r t i c l e s i z e was not a f f e c t e d significantly. Behavior of Technetium and Ruthenium: A p i l o t plant test was conducted to study the behavior of technetium and ruthenium in the uranium c a l c i n a t i o n process. The UO- product from these t e s t s was used in^subsequent f l u o r i n a t i o n s t u d i e s . The UNH feed was spiked with Tc (as ammonium pertechnetate) and nonradioact i v e ruthenium (as ruthenium n i t r a t e ) , and then d e n i t r a t e d under normal run c o n d i t i o n s . As expected, most of the technetium was found in the UO- product as technetium oxide. Over h a l f of the ruthenium (RuO^) was v o l a t i l i z e d and found in the condensate from the o f f - g a s .


37.

URZA

A N D KILIAN

Conversion of UNH to UF

553

6

TABLE 1 SUMMARY OF DATA FOR CALCINATION RUNS 9 THROUGH 15

UNH Feed Cone. g U/£

Feed Rate mA/min

9

1125

315

300

3.0

10

1145

315

300

8.4

11

1160

315

300

9.3


Run Number

Run D u r a t i o n hrs

Bed Temp. C°

12

1100

315

300

12.4

13

1130

290

300

16.0

14

1145

315

300

15.9

15

1130

419

300

11.4

Avg. U 0 Prod. Rate kg/hr-m 3

Run Number

2

U0 Produced kg 3

Avg. U 0 P a r t i c l e S i z e (mm) End Beginning 3

9

1,320

77

O.25

O.25

10

1,200

195

O.23

O.25

11

1,270

229

O.30

O.25

12

1,120

269

O.23

O.23

13

1,090

337

O.28

O.30

14

1,320

406

O.25

O.28

15

1,770

390

O.28

O.25



FILTER B L O W B A C K AIR

DE-ENTRAINER


UNH FEED TANK

CONDENSATE

ATOMIZING AIR

FLUIDIZED BED CALCINER

UNH PUMP

FLUIDIZING AIR

U0 PRODUCT 3

PREHEATER

Figure 1. Simplifiedflowsheetof the U calcination pilot plant

FILTER BLOW B A C K AIR

uo FEEDER 3

OFF G A S COOLER

O-

FLUIDIZED BED FLUORINATOR

AIR WATER COOLANT

Na F TRAP

i

SODA LIME TRAP

ACTIVATED ALUMINA TRAP

PREHEATER

Figure 2.

Simplifiedflowsheetof thefluorinationpilot plant


37.

URZA AND

Conversion of UNH

KILIAN

Uranium F l u o r i n a t i o n

to

UF

6

555

P i l o t P l a n t Studies

Uranium t r i o x i d e can be converted to UF^ by a one-step, d i r e c t f l u o r i n a t i o n process, or by the h y d r o f l u o r i n a t i o n process which i n v o l v e s three steps: (1) r e d u c t i o n of UO to UO with hydrogen; (2) conversion of U0 to UF^ with HF; an

Laboratory and Pilot Plant Studies on Conversion of Uranyl Nitrate to

Recommend Documents