Radiochimica Acta 55, 2 9 - 3 5 (1991) © R. Oldenbourg Verlag, München 1991 - 0033-8230/91 $3.00+0.00

Development of an Electro-Oxidative Dissolution Technique for Fast Reactor Carbide Fuels By A. Palamalai, S. K. Rajan, A. Chinnusamy, M. Sampath, P. K. Varghese, Τ. N. Ravi, V. R. Raman and G. R. Balasubramanian Reprocessing Programme, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India

(Received May 22, 1990; revised September 27,1990)

Electro-Oxidation / Dissolution / Carbide fuels / Destruction / Soluble organics / Reprocessing

Abstract While dissolving Uranium Carbide and the mixed carbide fuel (30% UC-70% PuC) employed in the Fast Breeder Test Reactor (FBTR) at Kalpakkam in nitric acid medium, the soluble or-ganics formed in the dissolver solution interfere in the solvent extraction of Pu diverting it into the raffinate waste, stripping off Pu from the organic phase and in phase separation due to stable emulsion formation. The Electro-Oxidative Dissolution Technique (EODT) developed in our laboratory is found to destroy the oxalic acid completely and the other soluble organics to a level which corresponds to less than 0.2% of the carbon present in the carbide specimen. The effects of various par-ameters such as time of the electro-oxidation, concentration of nitric acid, temperature, concentration and nature of redox intermediates on the electro oxidative destruction of organics are presented. Ruthenium interference is studied. Operation of the equipment namely Electrolytic Dissolver-Cum-conditioner employed in the scaled-up version of the present EODT is de-scribed. Advantages of the present method over other techniques are highlighted.

1. Introduction

H e a d - e n d p r o c e s s e s

The head-end processes so far investigated for the carbide fuels are pyrohydrolysis [1], pyrolytic oxi-dation [1], direct dissolution [1] in nitric acid and photochemical process [2].

Pyrohydrolysis is a high temperature process wherein the mixed carbide is reacted with superheated steam at 650 to 750 °C. In addition, the hydrogen gas evolved as one of the products may form an explosive mixer with air. Also loss of Pu due to volatilisation and entrainment of the mixed oxide dust in the condensate water are the other problems. The advantage of this method is the complete dissolution of the oxide prod-ucts in nitric acid solution when it contains less than 20% of Pu0 2 .

Pyrolytic oxidation is also a high temperature op-eration requiring 400 — 500 °C to be carried out on the pyrophoric carbide. In the irradiated fuels, owing to the migration of Pu, phase segregation would occur leading to the formation of Pu02-rich oxide phase. In the two processes described so far separate equipment

for calcination and dissolution operations are required with different materials of construction and an ad-ditional step of transfering radioactive solid material from calciner to the dissolver is involved. Also a sepa-rate super dissolver is necessary for dissolving the Pu rich solid phase and the U and Pu alloyed with noble metal fission products.

Direct dissolution in H N 0 3 is preferred to the other two methods described above, because the fuel pins cut into pieces are directly immersed in H N 0 3 dissolvent without any prior high temperature treat-ment. Ferris and Bradley [3] have reported that Uranium Carbide reacts with 4 and 16 Μ H N 0 3 at 25 to 120°C yielding Uranyl nitrate and soluble organic acids corresponding to 21 to 44% of carbon in the UC specimen, besides oxides of nitrogen (N0 2 , NO) and C0 2 . No hydrogen (H2), carbon monooxide(CO), or gaseous hydrocarbon has been produced. The soluble organics include oxalic acid, mellitic acid and unidenti-fied organics probably benzene poly hydroxy car-boxylic acids. According to Flanary et al. [1], the solu-tion resulting from dissolution of mixed carbides (1.05 Μ U 0 2 ( N 0 3 ) 2 and 6.7 Μ H N 0 3 containing Pu(IV)) is refluxed for 27 h and periodically tested by batch extraction and stripping. A constant Pu loss of 0.35% during extraction is experienced whereas loss during stripping is erratic ranging from 0.02 to 1%. Extended refluxing in strong nitric acid slowly pro-motes degradation of the soluble organics. Even after refluxing with 0.2 Μ K M n 0 4 , 5% of the original carbon remains as soluble oganics. The precipitate M n 0 2 formed can adsorb Pu thereby leading to its loss and the filtration of this precipitate is an ad-ditional step.

The Photo chemical method for the destruction of the organic acids recently developed in Germany by Bokelund et al. [2] requires longer time of the order of 34 to 45 h for completion of the reaction. Incorpora-tion of fragile glass or quartz window in the dissolver equipment in order to irradiate the solution is hazard-ous. These facts make this head-end approach rather difficult for plant application.

The present Electro-Oxidative Dissolution Tech-nique(EODT) independently developed in our labora-tory is similar to the Catalysed Electro-Chemical Plu-tonium Oxide Dissolution (CEPOD) process reported recently by Bray et al. [4], But the EODT has been

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30 A. Palamalai et al.

Table 1. Characteristics of the UC pellets

Isotopic composition

U u235

u2 3 8

c12

c13

Carbon analysis By combustion method By titrimetry

U2C3 phase

By X-ray diffraction method Oxygen

0.7% 99.3%

98.9% 1.1%

4 ±0.08% 4 + 0.1 %

2 to 5% 10, 850 ppm

applied to other nuclear fuel materials as well, such as UC, 30% UC-70% PuC pellets and an unirradiated fuel pin containing the abovesaid pellets, in addition to Pu0 2 [5].

2. Experimental

2.1. M a t e r i a l s and r eagen t s

Uranium Carbide pellets, mixed carbide fuel pellets of FBTR and the fuel pin containing the abovesaid pel-lets obtained from Radiometallurgy Division, Bhabha Atomic Research Centre, Bombay are used in the ex-periments. The characteristics of UC pellets are given in Table 1. Nitric acid, Silver nitrate and cerous nitrate used are Guaranteed Reagent grade of Sarabhai M. Chemicals. All other reagents used are GR/AR grade.

Ruthenium nitrate is prepared from a solution of Ruthenium chloride, acidified with 0.5 ml of 1:1 nitric acid, by precipitating the chloride as silver chloride with 1% excess of stoichiometric amount of silver nitrate solution. The precipitate is filtered and the filtrate, Ruthenium nitrate, is employed in the exper-iment.

2.2. L a b o r a t o r y scale e lec t ro ly t i c cell f o r u r a n i u m ca rb ide

Dissolution of Uranium carbide and electro-oxidative destruction of the soluble organics formed are carried out in the simple 2-compartment electrolytic cell shown in Fig. 1. A porcelain tube serving as dia-phragm separates the two compartments. A titanium coil of 10 mm dia made from titanium wire of 1 mm dia is used as the cathode. A cylindrical platinum gauze of 5 cm height and 6 cm dia is used as anode. Stirring of the solution is effected by a magnetic stirrer. The temperature is maintained by circulating water through the jacket of the cell from a thermostat. The voltage required to sustain one Amp current in the cell

Pt. gauze anode

Pt. foil cathode

Ceramic diaphragm

Water jacket

Nitric acid

• Water

UC pellets dissolving in HN03

Fig. 1. Laboratory scale two-Comparmental cell for dissolution of UC pellets.

Stirring bar

Glass beads

Condenser

Platinum gauze anode Platinum foil cathode

• N2 out let

• N2 in let

^ — ' — Lid with •r j/c joint

Stirring bar

Fig. 2. Laboratory scale Electrolytic dissolver for plutonium containing fuel materials.

is applied which ranges from 3.5 to 5 V. When a cur-rent of 2 Amp is passed, the voltage ranges from 5.5 to 6.5 V. For this purpose, a D.C. power supply of 30 V and 3 Amp capacity is used.

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Development of an Electro-Oxidative Dissolution Technique for Fast Reactor Carbide Fuels 31

Off gas { Dissolvent ft feed Γ X — - Chopped

ι η pin feed

X7 Π • Hot (anodic) limb

Heating coil

LJ Air sparging •

ΤΎΊ

Table 2. Percentages of organics destroyed in the case of the three experiments

Electrolytic limb

Fig. 3. Electrolytic Dissolver-cum-Conditioner.

2.3. L a b o r a t o r y scale e l e c t r o l y t i c d i s -so lver f o r mixed c a r b i d e

The schematic of the laboratory scale (100 ml) electro-lytic dissolver employed for mixed carbide pellets in-side a glove-box is shown in Fig. 2. The solution is mixed by a magnetic stirrer and is heated using infra-red lamp. A cylindrical platinum gauze anode of 3 cm height and 3 cm in diameter and a platinum foil cath-ode of 1 χ 0.5 cm are used. The cathode is enclosed in a glass tube which ends in a 3 mm dia hole through which ions move during the electrolysis. The voltage required to sustain the passage of one Amp current through the cell is applied which ranges from 3 to 4 V. The condenser containing the glass beads is used to reflux down the vapours evolved. Anode gas is sepa-rated from the cathode gas to avoid the possible ex-plosion hazard due to hydrogen. The gas stream from both compartments are separately passed through gas wash bottles containing 2 Μ NaOH solution for ab-sorption of oxides of nitrogen and filtered through 2 absolute filters in series before they are let into con-trolled ventilation.

Ruthenium interference is studied in a separate dissolver of this type.

2 .4 . E l e c t r o l y t i c d i s s o l v e r - c u m -c o n d i t i o n e r

The scaled-up version of the present EODT (100 g scale) employs the thermosyphon dissolver which is suitably modified to accommodate the electrodes sep-arated by a porcelain diaphragm, along with an ad-ditional limb for the cathode compartment.

These modifications facilitate the anodic processes during dissolution to proceed, unaffected by those occuring at the cathode. This modified thermosyphon dissolver is given the name Electrolytic Dissolver-cum-Conditioner and is shown in Fig. 3. This equipment has been fabricated with borosilicate glass and por-celain tube diaphragm for studies with UC pellets.

Time/ 6 Μ HNOj- 6 Μ HN0 3 - 6 Μ H N 0 3

(h) 0.05 Μ Ag 0.05 Μ Ce

Percentage Percentage Percentage destroyed destroyed destroyed

1/2 60.9 62.1 55.2 1 75.1 70.0 61.9 2 86.4 75.2 65.6 4 98.9 83.1 77.9 6 99.8 90.2 84.1 8 99.8 95.5 88.8 10 99.8 96.3 91.1

The UC pellets enclosed in a clad tube of AISI 316SS, the mock-up fuel pin of FBTR are chopped into half-an-inch pieces under water. The chopped pieces are taken in a stainless steel (304L) basket and the basket is lowered into the hot limb. The dissolvent 6 Μ HN03-0.05 Μ AgN0 3 is filled in the anode com-partment. 6 Μ nitric acid is filled in the cathode com-partment. Suction is applied using ejectors, causing a vacuum of 20 cm of water in the cathode compartment and 10 cm of water in the anode compartment, in order to reduce transport of catholyte into the anode compartment. Nitrogen is passed through both anode and cathode compartments to circulate the liquid. A cylindrical platinum gauze of 10 cm height and 8 cm dia is used as the anode and a titanium coil of 2 cm dia, 20 cm height and made of 1 mm dia wire is used as the cathode. Two Amp current is passed, the voltage being 5.5 V.

A 20 g — scale Electrolytic Dissolver-cum-Condi-tioner has been fabricated with borosilicate glass and porcelain tube diaphragm, for the FBTR fuel.

3. Results and discussion

3.1. S t u d i e s w i t h u r a n i u m c a r b i d e pe l l e t s in the l a b o r a t o r y cell

The UC pellets dissolve rapidly (within 0.5 h time) with frothing in 4 to 6 Μ H N 0 3 on heating to 65 °C forming a dark brown solution containing uranyl ni-trate and the abovesaid soluble organic acids. The brown coloured organics and oxalic acid resulting from the dissolution of UC pellets in H N 0 3 in the cell are observed to be completely destroyed in 2 h at 65 °C under electro-oxidation. Destruction of brown coloured organics is observed visually. The oxalic acid is determined by Potentiometrie titration against standardized K M n 0 4 solution employing Pt-calomel electrodes. On further electrolysis, other organic acids are also destroyed. Estimation of soluble organic acids in the nitric acid solution is carried out as per the wet chemcial method of analysis for carbon reported elsewhere [6].

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32 A. Palamalai et al.

Fig. 4. Effect of nature of redox intermediate on the destruction of organics resulting from Carbide dissolution in nitric acid.

Introduction of kinetically fast redox intermedi-ates such as Ce(IV)/Ce(III) or Ag(II)/Ag(I) is found by us to enhance the rate of destruction of the organics, similar to that observed by Ryan and Bray [7] for the rate of dissolution of Pu0 2 . In order to unambigu-ously establish the role of redox couple in the destruc-tion of soluble organics, three experimental dissolu-tion runs have been carried out at 85°C using the solutions: 6 Μ HN03-0.05 Μ AgN03 , 6 Μ HN0 3 -0.05 Μ Ce(N03)3 and pure 6 Μ HN0 3 . The results of these runs are summarized in Table 2.

The percentages of organics destroyed have been plotted as a function of time and the curves are shown in Fig. 4. It is observed from Fig. 4 that at any time after 0.5 h, from the starting of the electrolysis, the extent of organics destroyed is higher in case of electro-lyte containing the redox intermediate Ce(IV)/Ce(III) than in case of pure nitric acid and it is maximum in case of the electrolyte containing Ag(II)/Ag(I) redox couple. This may be explained as follows: Being highly substituted in 5 or 6 positions with — COOH and — OH groups, the soluble aromatic acid molecules may become overcrowded and strained and hence the molecule may be unstable towards oxidation to C 0 2 and H 2 0 . In case of pure nitric acid electrolyte, the nitric acid itself (being an oxidizing acid) and the oxidizing agents such as atomic oxygen, hydroxyl rad-ical and peroxide species, generated at the anode, react with the aromatic acids (such as mellitic acid etc.) oxidizing them to C 0 2 and H 2 0 . The generation of the oxidizing agents at the anode are represented by the following equations.

Table 3. Electro oxidative destruction of soluble organics/result-ing from UC dissolution in the electrolytic dissolver-cum-

conditioner"

Sample no. Time/h Soluble organics/ mgC/ml

1 2 0.02 2 4 0.02 3 5 0.72 4 6 0.51 5 9 0.07 6 11 0.013 7 12 0.01 8 14 0.011 9 15 0.013

10 16 0.013

a Dissolvent: 6 Μ H N 0 3 and 0.05 Μ AgNOj.

H 2 0 Ο + 2H+ + 2e~

H 2 0 "OH + H + + e~

2 Ό Η H 2 0 2 .

This mechanism does not warrant the discharge of aromatic acids at the anode as the primary oxidation step. This is in accordance with the more positive oxidation potentials of the aromatic acids than the discharge potentials of aqueous solutions.

In case of H N 0 3 electrolyte containing the inter-mediate redox couple of Ce(IV)/Ce(III) or Ag(II)/ Ag(I), the Ce(IV) or Ag(II) regenerated at the anode, in addition to the abovesaid oxidizing agents, oxidize the organic acids and hence the increased extent of destruction of the organics, in presence of the redox intermediate. The redox couple of Ag(II)/Ag(I) has appreciably higher standard electrode potential (E° = 1.98 V) than Ce(IV)/Ce(III) (£° = 1.61 V) and hence the extent of organics destroyed is higher (99.8%, this includes the initial oxidation of carbide carbon to C 0 2 also) in presence of Ag intermediate than in presence of Ce(96.3%), after 10 h of electrolysis.

From Fig. 4, it is also inferred that for the destruc-tion of 84% of the organics, pure 6 Μ H N 0 3 has taken 6 h, 6 Μ HN03-0.05 Μ Ce has taken 4 h, 9 min and 6 Μ HN03-0.05 Μ Ag has taken only 1 h, 39 min. Presence of kinetically fast redox intermediate, cerium or silver couple enhances the rate of destruction of the organics. The Ag(II)/Ag(I) couple has very high electron exchange rate (K= 27401 M o l s _ 1 ) a s com-pared to Ce(IV)/Ce(III) ( t f=0.3 1 Mol"1 s" 1 ) under almost comparable conditions and hence presence of silver intermediate gives faster rate of destruction of the organics than the presence of cerium intermediate. Thus the Ag(II)/Ag(I) redox couple has both favour-able thermodynamics and kinetics and hence it is ob-served to offer the best result of almost complete (99.8%) destruction of the organic acids in relatively shorter time of 6 h.

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Development of an Electro-Oxidative Dissolution Technique for Fast Reactor Carbide Fuels 33

100

0.05 0.1 0.15 Molarity of A g N 0 3 / Μ

Fig. 5. Effect of Concentration of redox intermediate, 6 Μ HNOj, 85°C, 2 A, 5 V, 2 h.

0 2 4 6 8 10 12 14 16 Molarity of H N 0 3 / Μ

Fig. 6. Effect of initial concentration of nitric acid, 0.05 Μ AgN0 3 , 85 °C, 2 A, 5 V, 2 h.

3.2. Ef fec t of d i f f e r e n t p a r a m e t e r s on the d e s t r u c t i o n of o rgan ics by the E O D T

The various parameters studied are concentration of intermediate, nature of the redox couple, the initial concentration of nitric acid and temperature of the solution.

Concentration of redox intermediate

In order to determine the optimum concentration of the recommended Ag redox intermediate, a series of experiments have been carried out varying the concen-tration of silver nitrate from 0.005 to 0.2 M. The per-centage of organics destroyed are plotted against the concentration of silver nitrate in Fig. 5. From Fig. 5 it is obvious that optimum concentration of silver nitrate is 0.05 to 01 Μ as the slope is very high below 0.05 Μ and it is very small above 0.1 M. Use of silver nitrate concentration beyond 0.1 Μ will unnecessarily increase the cost as well as the salt content in the raffinate waste.

Concentration of nitric acid

The effect of initial concentration of nitric acid on the destruction of the organics has been studied in another series of experiments and the results are plotted in Fig. 6. It is seen from the graph that increase in nitric acid concentration upto 8 Μ increases the extent of destruction of the organics, and beyond 8 Μ upto 16 Μ H N 0 3 it is almost constant. Thus the optimum initial concentration of nitric acid is 8 Μ for Electro-oxidative destruction of organics in presence of silver.

Table 4. Effect of temperature of electrolysis on the destruction of organics by EODT

8 Μ H N 0 3 , 0.1 Μ AgN0 3 , 3 A, 4 V, 2 h

Temperature/°C Organics remaining undestroyed/wt%

45 15.6 55 13 65 14 75 6.1 85 3.6 92 1.3

Temperature of electrolysis

The temperature of electrolysis has been varied from 45 to 92 °C and corresponding organics remaining undestroyed are shown in Table 4. It is observed that generally increase in temperature of electrolysis in-creases the extent of organics destroyed. Deviations in two experiments may be due to experimental error.

3.3. Studies with mixed ca rb ide pel le ts of F B T R in the l a b o r a t o r y cell

In 6 Μ HN03-0.1 Μ Ce(IV) under electro-oxidation at 65 °C complete dissolution of FBTR fuel pellets has taken more than 36 h. The FBTR fuel pin cut into 0.5 inch pieces, dissolves in 13 Μ HN03-0.1 Μ Ce(IV) at boiling conditions, under electro-oxidation in about 24 h and with Ag redox couple under similar con-ditions it takes 14 h time. About 0.1% of the fuel pellets taken has been left behind as residue which dissolves in the subsequent experiment. In contrast to this, Bokelund et al. [9] and Flanary et al. [1] have observed that the mixed carbide pellets of the compo-sition 80% UC-20% PuC dissolve completely in 8 to 13 Μ H N 0 3 at 50 °C to boiling point in 3 to 6 h time.

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34 A. Palamalai et al.

0.8

0.7

0.6

σ> _c

0.4 Ε ω <n •a 0.3 <o σ> O

0.2

0.1

0 0 1 2 3 4 5 6 7 8 9 10 12 14 16

Time / h

Fig. 7. Concentration of organics remaining, as a function of time during the Electro-Oxidative destruction of the organics,

in the Electrolytic Dissolver-cum-Conditioner at 45 °C.

The reason for this difference may probably be due to the higher percentage of PuC in the FBTR fuel pellet (70% vs. 20%) and higher degree of sintering. Unlike the UC pellet which dissolves rapidly (0.5 h) in 6 Μ H N 0 3 at 65 °C leading to a porous pellet floating subsequently, the FBTR fuel pellet is observed to dis-solve layer by layer at the surface and the pellet sinks at the bottom of the cell until last.

3.3.1. Studies on the interference of Ruthenium

The dissolvent, 4 Μ HN03-0.1 Μ Ceric Nitrate has worked well for unirradiatd Fast Reactor Fuel whereas it failed for the irradiated fuel. The reason for this failure has been attributed to the presence of Ruthenium, a fission product in the irradiated fuel. Ruthenium is oxidized to the volatile R u 0 4 by the ceric ion. The R u 0 4 is decomposed into Ru0 2 and oxygen in the cooler part of the condenser. This Ru0 2 is brought back to the dissolver solution by the con-densate. The RuOz is oxidized back to R u 0 4 by ceric ion which once again is decomposed in the cooler portion of the condenser to R u 0 2 and oxygen. Thus Ruthenium present, consumes the effective dissolution reagent Ce(IV) in the above described cyclic re-actions.

10 grams of UC pellets along with 1% of ru-thenium with respect to the U taken, are added into the electrolyte of 6 Μ HN03-0.05 Μ AgN0 3 in a separate cell of this type having the condenser. Once the entire uranium carbide is dissolved electrolysis is started passing 2 amperes current at a voltage of 4.5 V at 85°C. At the end of 6 h of electrolysis 99.8% of the

organics has been found to be destroyed confirming that interference of Ruthenium is avoided in the Electro-Oxidative Dissolution. The passage of elec-tricity offsets the catalytic reduction of Ag(II) by Ru-thenium. From this it is inferred that the EODT can be applied to irradiated fuels also.

3.4. D i s so lu t ion s tudies of U C and 70% PuC-30% U C in the E lec t ro ly t i c D i s s o l v e r - c u m - C o n d i t i o n e r

Within a few minutes of passing the current through the cell, the dark brown colour of Ag(II) is observed in the anode compartment before adding UC pellets. Then, 5 grams of UC pellets are added into the anode compartment and the heater is switched on to raise the temperature of the anolyte to 45°C. The soluble organics present in the solution in the anode compart-ment are determined by withdrawing samples at reg-ular intervals of time and analysing for the carbon content. The results are plotted as function of time in Fig. 7. A peak value of the concentration of organics is observed in Fig. 7 at 5 h due to the competing reac-tions of dissolution of UC with the corresponding formation of soluble organics and electro-oxidative destruction of the organics. Complete destruction of organics has taken 11 h, probably due to the low tem-perature, 45 °C. In another experiment, 100 grams of UC pellets are dissolved in 800 ml of 6 Μ HN0 3 -0.05 Μ AgN0 3 kept in the anode compartment at 85 °C. In the cathode compartment 950 ml of 6 Μ nitric acid has been introduced. The entire UC pellets have been found dissolved in 1 hour. Then the elec-trolysis is started passing 3 amps current at 4.5 V and at 85 °C. Samples have been drawn just prior to the starting of the electrolysis and at the end of 6 hours of electrolysis. The soluble organics destroyed just before start of electrolysis are 53% and after 6 h of electroly-sis 99.8%.

When 20 g of FBTR fuel pin chopped into half-an-inch pieces, taken in the basket in the 20 g-scale electrolytic Dissolver-Cum-Conditioner (200 ml ano-lyte and 240 ml catholyte) is dissolved in 200 ml of 13 Μ HN03-0.05 Μ AgN0 3 under, near boiling con-ditions passing 3 A current at 6 V, it has taken 15 h for the almost complete dissolution of the fuel leaving behind about 0.1% of the fuel material, taken. There is scope to reduce the time required for the dissolution of the fuel as there is room to enhance the surface area of the anode by 2 —3 times.

In the anolyte almost all the plutonium is expected to be present in the hexavalent state. Then catholyte which will be 13 Μ H N 0 3 will be drained off and the anolyte will be transferred in the cathode compart-ment making up the volume with 13 Μ HN0 3 . Fresh lot of chopped fuel pins will be charged in the basket which will be lowered in the hot limb and fresh dissol-vent will be fed into the anode compartment. Heating and electrolysis will be started. While dissolution of the

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Development of an Electro-Oxidative Dissolution Technique for Fast Reactor Carbide Fuels 35

fuel and destruction of the resultant soluble organics occur in the anode compartment, Pu(VI) will be re-duced to Pu(IV) in the cathode compartment, partly by reduction at the cathode and the rest by the N 0 2 gas formed at the cathode from the reduction of nitric acid. The above described proposal is a possibility which has to be confirmed experimentally. All the above said opt Jons are simple and can be carried out inside a hot cell using Master Slave Manipulator (MSM). Further works are in progress to establish the feasibility of the abovesaid operations with MSM.

agement and keen interest in this work. We are thank-ful to Shri. J. B. Gnanamurthy, Dr. Ο. M. Sreedharan and U. Kamachi Mudali of Material Development Lab. and Shri. R. B. Yadav of Radiochemistry Lab. for their valuable suggestions and discussions. We thank S/Shri. S. V. Mohan, R. Srinivasan, T. S. Thankachan, and A. K. Natarajan for their assistance in carrying out some of the experiments.

3.5. A d v a n t a g e s of the E O D High temperature operation such as p>. pyrolytic oxidation is avoided. Use of N a N 0 2 or N 0 2

gas for adjusting the valency of Pu in a separate step can be avoided. Like any other electro-chemical pro-cess, the EODT also is easily amenable for remote operation and control.

Conclusions Unirradiated uranium monocarbide pellets dissolve rapidly in nitric acid (0.5 h time) and the resulting soluble organic acids are destroyed almost completely (99.8%) in the EODT, in 6 h time. The mixed carbide fuel pellets of FBTR takes 15 h for dissolution. Further works are in progress to reduce this time of dissolution by adjusting the conditions of electro-oxi-dation. 100 g batches of UC pellets are dissolved (with the attendant destruction of the organics) in the Elec-trolytic Dissolver-Cum-Conditioner, the scaled-up version of the laboratory cell. The results similar to the laboratory scale studies have been obtained.

Acknowledgement The authors are thankful to the Director, Indira Gandhi Centre for Atomic Research for his encour-

References

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2. Bokelund, H., Caceci, M., Muller, W.: A photo chemical Head-end Step in Carbide Fuel Reprocessing, Radiochim. Acta 33, 115(1983).

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4. Bray, L. Α., Ryan, J. L., Wheel Wright, E. J.: Development of the CEPOD Process for Dissolving Plutonium Oxide and Leaching Plutonium from scrap or wastes. USA Report, PNL-5657 (1985).

5. Palamalai, Α., Balasubramanian, G. R.: Electro-Oxidative Dissolution of Refractory compounds of Actinides. Trans SAEST 19, 89 (1984).

6. Chandramouli, V., Yadav, R. B., Vasudevarao, P. R.: A wet chemical method for the estimation of carbon in Uranium Carbides. Talanta 34, No. 9, 807 (1987).

7. Bray, L. Α., Ryan, J. I.: Catalysed Electrolytic Dissolution of Plutonium Dioxide. In: Actinide Recovery from Waste Low Grade Sources (J. D. Navratil and W. W. Schulz, eds.) Harwood Publishers, New York 1982.

8. Harmon, H. D.: Dissolution of Pu0 2 with Cerium(IV) and Fluoride Promoters, USERDA Report, DP-137 (1975).

9. Bokelund, H., Caceci, M., Ougier, M.: Dissolution of Mixed carbide Fuels in Nitric Acid. Radiochim. Acta 30,49 (1982).

10. Horner, D. E., Grouse, D. J., Mailen, J. C.: USERDA Report, ORNL/TM-4716 (1977).

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