A FIVE YEARS EXPERIENCE OF PULSE COLUMNS EXTRACTION CYCLES FOR THE REPROCES- SING OF FAST BREEDER REACTOR FUELS AT THE MARCOULE PILOT PLANT (SAP)

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1 FIVE YERS EXPERIENCE OF PULSE COLUMNS EXTRCTION CYCLES FOR THE REPROCES- SING OF FST BREEDER RECTOR FUELS T THE MRCOULE PILOT PLNT (SP) MM. CHRVILLT - FBRE - LE BOUHELLEC - HENRY The reprocessing of Phenix fast breeder reactor started at the MRCOULE PILOT PLNT in 1977 with the enriched UO2 first core (2.3 tons U) followed by several campaigns of UO2-P11O2 Phenix-core II (6.5 tons U-Pu). fter a short description of the Pilot Plant, characteristics of the pulse columns extraction flow-sheets are presented. Pulse columns are used for extraction and scrubbing of uranium and plutonium and for uranium backwashing whilst plutonium stripping and U-Pu partition are carried out in mixer settlers with HN and in-line electrolytic U IV generation. Performances of pulsed columns including recovery yields and decontamination factors are discussed : they show a good B y decontamination can be reached with two cycles and partition carried out at the second cycle. INTRODUCTION The Marcoule spent fuel reprocessing pilot plant was built in for extended testing of various fuel reprocessing methods under conditions approximating those of actual production facilities. The initial line, designed for reprocessing natural uranium base metallic fuels, was modified and upgraded on several occasions, notably for MTR fuel ( U - 1, Pu-1) reprocessing and neptunium recovery. In 1973 it was adapted to handle oxide fuels, especially those from fast breeder reactors. fter several reprocessing campaigns with oxide fuels from the Rapsodie an KNK reactors between 1974 and 1977, systematic reprocessing began for the Phenix fast breeder reactor core charge. total of 2.6 metric tons were reprocessed from the first Phenix enriched uranium core (26 % 5u after irradiation), followed by 6.6 metric tons from the second core load, containing approximately 25 %plutonium (core II). fter mechanical treatment and disolution, solvent extraction is carried out in 3 cycles for uranium and 2 cycles for plutonium, with partitioning in the second cycle. ll of the extraction steps use pulse columns except for the partition and for solvent treatments specific to each cycle, which are carried out in batteries of mixer-settlers (30 % TBP). This report covers five years of extraction cycle experience in reprocessing the second Phenix core charge. 1

2 EXTRCTION CYCLES : DESCRIPTION, THROUGHPUT CPCITY ND PROCESS FLOWSHEETS The following three extraction cycles are used continuously with no intercycle concentration (Figure 1). 1st CYCLE 2nd CYCLE 3rd CYCLE (uranium) FUNCTION Extraction & decontamination of solvent solutions (after NO reduction of PuVI formed) U + Pu coextraction Solvent treatment dditional U+Pu extraction & decontamination U/Pu partition and Pu scrubbing Diluent scrubbing of PuIII solutions Pu concentration to 250 g.l~ 1 Solvent treatment U extraction (after reduction of residual Pu with HN-N2H4) U backwashing Solvent scrubbing of backwashed U Uranium concentration to 400 g.l~1 Solvent treatment EQUIPMENT 1 extraction column 1 scrubbing column 2 backwashing columns 4 mixer-settler stages 1 extraction column 1 scrubbing column Battery of mixer-settlers 10 Pu backwashing stages 4 Pu scrubbing stages 1 diluent scrubbing column 1 evaporator 4 mixer-settler stages 1 extraction column 1 scrubbing column 1 backwashing column 1 solvent scrubbing column 1 evaporator 3 mixer-settler stages third plutonium purification cycle is implemented only in exceptional cases to process substandard plutonium batches. It comprises two pulse extraction & scrubbing columns and banks of mixer-settlers for plutonium backwashing (16 stages), solvent scrubbing of the backwashed plutonium (6 stages) and diluent scrubbing (4 stages). The extraction throughput capacity is 1500 g.h"^ of U+Pu under equilibrium conditions, and is reached gradually because of the weekend shutdowns. Maximum capacity operation is achieved for no more than 3 days a week, limiting the weekly throughput to 120 kg maximum. The process layout has remained virtually constant throughout the different campaigns. Figure 2 details the most recent campaign, on fuel with the highest burnup values. 2

3 Notable points include the following : specific pulse column flowrates ranging from 0.7 to 2.2 l.h,cm" c in continuous aqueous phase except for the 1st cycle extraction and scrubbing and the diluent scrubbers, - the absence of any intercycle concentration steps, - the use of specific solvents for each cycle. FUEL REPROCESSING CMPIGNS The second Phenix reactor core charge is a U0o-Pu02 mixed oxide fuel containing natural uranium. Each fuel pin includes a fertile portion (the integral axial blanket) that is dissolved together with the fissionable core ; as a result, the Pu/U+Pu ratio is between 18 an 20 %. The specified burnups correspond to the fissionable portion : the^burnup values increased form to MWd.mt-1 with cooling times ranging from 14 months to 3.5 years. number of experimental sub-assemblies were reprocessed, including one irradiated to MWd.mt-1 and two others containij^rplutonium already submitted to two irradiation-reprocessing cycles. Table I summarizes the data for the campaigns completed between 1979 and PULSE COLUMN CHRCTERISTICS Major pulse column specifications are shown in table II. Continuous Organic Phase Pulse Columns These columns have a separate lower settler : an airlift raises the dispersion into this unit from which the solvent is recycled to the bottom of the column. The recycled solvent flowrate is held constant as a setpoint for regulating the emulsion airlift. These columns are equipped with plates supporting 3.5 mm diameter nozzles on a 50 mm pitch with 18 % transparency. The 1st cycle extraction and scrubbing columns have been in use since Continuous queous Phase Pulse Columns These columns are equipped with a pressure pot letdown system : the pressure is slaved to the interphase level in the upper settler, allowing the letdown flowrate to be regulated bu controlled siphoning. These columns are fitted with conventional perforated plates (3 mm holes on a 50 mm pitch with % free-area. 3

4 The first cycle columns were commissioned in 1974, the 2nd cycle columns in 1976 and the third uranium cycle units replaced the centrifugal extractors in 1979 when an additional 2nd cycle extraction column was also installed. RESULTS Hydraulic Operation Throughput Variations in Continuous queous Phase Pulse Columns Despite relatively long experience in operating pulse columns since 1964, no change in their throughput capacity was noted during the pilot campaigns, lasting from 1 to 4 months. The phenomenon was only detected during the extended campaigns of 1976 (1.6 metric tons of KNK fuel) and the Phenix uranium core campaign or (2.6 metric tons), and subsequently reproduced in non-radioactive facilities : in order to maintain a constant flowrate it was necessary to modify the column pulse conditions on several occasions by reducing the amplitude to prevent fouling. Several conclusions were drawn from the experience of multiple campaigns : - the phenomenon stabilizes before the amplitudes are reduced below a value incompatible with proper pulse column operation, and the column efficiency remains satisfactory, - initial conditions can be restored, although only temporarily, simply by flushing the columns with a 1-2 N NaOH solution, - the capacity variations are not identical in all columns, but are observed after 2 or 3 months of operation, especially in the first and second cycles. Studies on non-radioactive solutions showed that the phenomenon involves gradual clogging of the plates by the organic phase which reverses the wetting properties of the perforated plates used in columns operated in continuous aqueous phase. The final treatments were carried out after flushing the columns with a soda solution only between campaigns. The throughput capacity decreases stabilized at values that did not require revision of the extraction flow-sheets as the column efficiency was unaffected by the decreased amplitudes. 4

5 Column Shutdown and Restart Conditions During operation, for brief shutdowns such as over weekends the columns are depleted of fissionable material to a few milligrams of plutonium per liter, and left filled with their continuous phase. For longer shutdowns, the columns are filled with 3-4N nitric acid. Restarts are conducted step bu step after first obtaining stable hydraulic operation by simultating the feed solution with an equivalent flowrate of acid. Plutonium and Uranium Recovery Refined material losses into the backwashed solvents and evaporation distillates are totally negligible under equilibrium conditions, and correspond to recovery yields exceeding % in every case. In actual operation, because of the weekend shutdowns, the overall losses are somewhat higher, but remain fully acceptable. Table III shows the overall losses (in grams) recorded in the first six months of 1983 during which kg of uranium and kg of plutonium were reprocessed. The principal source of plutonium losses is the 2nd cycle refined solutions, but the total amount of the extraction losses is less than tose incurred during high-level laboratory analyses. $ y Emitter Decontamination Feed Solution Composition s a result of the wide diversity of irradiation conditions and especially the highly variable cooling times for the spent fuels reprocessed, the composition of the 1st cycle feed solutions has varied in considerable proportions. The $ activity limits (as measured by gamma spectrometry and thus excluding pure 3 emitteurs such as 90$r) have ranged from 30 to 250 Ci.l"1 as shown in Table IV. Decontamination Factors Table V showsan example of determinations corresponding to a general sampling carried out after 50 to 70 operating hours. This duration certainly does not correspond to the achievement of a stable physicochemical equilibrium, hence the relative dispersion of the values indicated in the table. From the whole results, the following remarks can be made : - irrespective of the 3 activity of the 1st cycle feed solutions, the activity of the co-backwashed U+Pu solutions remains virtually 5

6 for most of the values recor- constant at between 0.2 an 1 mci.l" 1 ded (U+Pu = 60 g.h), - similarly, the activity of the uranyl nitrate produced by the 2 nd cycle (U = 30 g.1-1) generally is less than 10 uci.1-1, i.e. near the finished product specification of 100 uci.kg-1 of uranium. This clearly shows the limited decontamination role of the 3rd cycle in ensuring the required 3Y activity value, - with regard to plutonium, the dispersion of the a activity after partitioning is greater, generally ranging from 0.01 to 0.2 mci.1-1 for a mean plutonium concentration of 13 g.1-1. This activity is essentially attributable to Zr-Nb and Ru-Rh pairs, and varies independently of the 2nd cycle feed solution activity. These variations may be due to interphase fouling or precipitates related to solvent decomposition products in the electrolytic partition units. This contamination is also due to weekly work schedule of the reprocessing facility : at equilibrium during the middle of the week, specification values are routinely obtained. Nevertheless, the residual contamination is still compatible with the decontamination factors obtained during oxalic conversion : all of the oxide batches produced by the plant have met the specified 1 uci.g-1 Pu limit required to permit reutilization of the plutonium for fuel fabrication. Uranium-Plutonium Separation The fundamental operation of uranium-plutonium separation is carried out in 10 mixer-settler stages (5 to 14), six of which are equipped with electrolysers to reduce a fraction of the uranium to valence 4. Stages 1 through 4 are used for plutonium scrubbing by fresh solvent to extract trace amounts of uranium IV or VI entrained with the plutonium. While the UO2 fuels were reprocessed using only electrolytic reduction, the high plutonium content of the Phenix fuels justified combining electrolytic reduction with reduction by hydroxylamine nitrate. The uranium IV makes it possible to ensure partial reduction of the plutonium in the organic phase, and thus complete the partition. 6

7 Uranium Removal from the Plutonium Despite the relative dispersion of the results due to the importance of the transient conditions arising from the semi-continuous operating schedule, the mean residual plutonium contamination in the uranium at the mixer-settler outlet is 0.25 ppm corresponding to a mean decontamination factor of 10^, while values of 0.05 ppm are reached under equilibrium conditions. The process without electrolysers is not efficient enough for the 3rd cycle to eliminate the residual plutonium, for which the contamination in the purified uranium must not exceed 0.04 ppm (i.e disintegrations per minute (dpm) per gram of uranium). This specification is almost met after partition, and is ensured in the 3rd cycle by adding a reducing agent (HN 0.05 M + N2H M) to the feed solution before acid adjustment ; this procedure permits an additional plutonium decontamination by a factor of 10 to 20. Plutonium Removal from the Uranium The uranium decontamination factor obtained during oxalic acid conversion of the plutonium is sufficient to ensure that the MRCOULE plant (UP1) entry specification of ppm of uranium in the plutonium batches can be easily reached. The residual contamination level routinely observed at the outlet of the partition unit is about 300 ppm, which is Pu O2 specification, while the batches transferred to the plant have had contamination levels of between and ppm (due once again to the discontinuous pilot operating conditions). Neptunium Behavior (Table VI) In the high level solutions, the neptunium concentration of the first cycle feed solutions was calculated, unlike the medium level solutions which were analyzed. The dissolution solutions, which should contain Np VI after prolonged boiling, are treated with NO before feeding in order to reduce the PuVI. The neptunium should therefore be present at several valences. In any event, 90 % of the theoretical quantity (17-25 mg.1-1 for a 150 g.1-1 U+Pu feed solution) is extracted with the uranium and plutonium during the first cycle, while about 1,5 mg.1-1 remain with the fission product solution. During the second cycle the acidity is raised from 0.6N to 3.5N, and extraction occurs about 5 hours after the end of the 1st cycle. The neptunium extraction results are highly scattered, ranging from 35 to 80 % ; this would seem to indicate considerable variation in the valence distribution. Thus, out of a total of about 9 mg.1-1 in the 2d cycle feed solution, from 1 to 3.2 mg.1-1 of Np pass into the refined solution. The presence of two reducing agents with 7

8 different modes of action relative to neptunium complicates the interpretation of the partition results ; but due to the high /0 ratio required by the use of HN, almost all of the Np remains with the plutonium III. The uranium contains only trace amounts of Np at the beginning of the third cycle, which are further decreased by the presence of HN in the feed solutions, preventing any reoxidation. Solvent Treatments The first cycle solvent was most frequently analyzed. fter U+Pu coextraction, the mean composition of the discharged solvent was as follows : U = 5 mg.t 1 Pu = 0.3 mg.1-1 DBP = mg.t 1 yt5zr = 0.3 mci.l' 1 95 Nb = 0.1 mci Ru-Rh = 6 mci.1-1 The remaining fission products account for less than 5 % of the 6 activity measured by gamma spectrometry. The treatment is carried out in four mixer-settler stages in which the contact time per stage is about 7.5 minutes (the aqueous phases are recycled to obtain an /0 ratio near 1). Stages 1 and 4 are fed with 0.5M sodium carbonate, and stages 2 and 3 with 1N NaOH. The mean decontamination factors obtained are the following : DBP = 3 to 5 * Nb = 5 to Zr = 6 to Ru-Rh = 3 to 5 Comparable results were given by a brief solvent treatment test using hydrazine carbonate to decrease sodium in the effluents. The 2nd cycle solvent treatment was modified on several occasions by introducing an acid stage and by doubling the number of stages, primarily to improve the DBP decontamination : the dibutylphosphate concentration ranges from 30 to 50 mg.1'1. The results obtained were even less satisfactory than for the first cycle, although the equipment used was identical. 8

9 CONCLUSIONS The process layout is relatively flexible and the specific flowrates remain moderate, leaving appreciable range for modifications. Uranium 3 y decontamination is achieved in three cycles, the last of which is used more for plutonium removal than for 3 Y decontamination and could thus be eliminated if the 2nd cycle decontamination were improved (e.g. by a backflow process). Plutonium decontamination is achieved in two cycles, allowing for the decontamination factor obtained by oxalic acid conversion. Regardless of the 3 y activity of the initial feed solutions, the activity in the backwashed 1st cycle solutions remains stable at a value near 1 mci.1-1 and no variation has been observed in solvent performance. The partition during the 2nd extraction cycle provides two cycles in which the nearly saturated solvent ensures better 3 Y decontamination than a third Pu cycle while avoiding the need for an intercycle concentration step that is always detrimental to the plutonium quality. Moreover, the use of hydroxylamine nitrate avoids any problems of uranium contaminated by neptunium, virtually all of which remains with the plutonium. new first cycle facility, now under construction for the fast oxide reprocessing (TOR) project, will allow partition either at the first or the second cycle. It will be equipped with continuous aqueous phase pulse columns using disk & ring packing that is less sensitive to changes in the wetting properties of stainless steel, except for the fission product solvent extraction and scrubbing operations, which will be carried out in continuous organic phase columns. 9

10 T3LE I : Phgnix fuel reprocessing at the Marcoule Pilot Plant YER CMPIGN U + Pu kg MXIMUM IRRDITION RTE COOLING TIME (core) MWd.t- 1 months Pu PURIFIED kg PHENIX U a P9-core 1 P1-core 2 80 P2-core 2 P3-core 2 81 P3-core 2 P4-core 2 82 P4-core 2 P5-core a a a P5-core 2 -core TOTL Core 1 Pu U + Pu = 18 % Core 2 : Pu U + PU = 25 % 10

11 CYCLE COLUMN N - FUNCTION SHFT DIMETER (mm) COLUMN HEIGHT COLUMN SHFT VOLUME (1) EXTRCTION SCRUB U.PU STRIPPING (I) U.PU STRIPPING (11) , B 2d EXTRCTION (I) EXTRCTION (11) SCRUB PU DILUNT WSHING U STRIPPING 62, , ,8 2, , ,6 3d U EXTRCTION SCRUB U STRIPPING U DILUNT WSHING 66,9 66,9 68,8 53 4,5 4 4, ,5 13,8 16,4 4,0 3d Pu EXTRCTION SCRUB 68,8 68, ,4 17,9 TBLE II - PULSE COLUMNS : MIN CHRCTERISTICS ND FLOWRTES. UPPER SETTLER VOLUME (1) 4,6 4,6 3,3 3 6, ,4 6,4 4, , , TT0M SETTLER VOLUME (1) SEPRTE SEPRTE 6, ,8 SEPRTE 5.2 6,2 6,2 6.2 SEPRTE 8.6 8,6 FREE RE CONTINUOUS PHSE QUEOUS FLOW (LIT 1 ) ORGNIC FLOW SPECIFIC THROUGHPUT (LIT 1 ) (l.h^.cin 2 ] ,4-1,7 0, , ,5-2,0 0,7-1, ,1 1,5-1, ,3 1,8-2,2 0,7-0, ,5 1,2-1,4 - -

12 TBLE III RECOVERY YIELDS DURING CMPIGN (P5) 1 st CYCLE 2d CYCLE 3d CYCLE U TOTL Raffinates U (g) Pu (g) 72,2 79,7 0,5 152,4 Solvent U Pu 65,6 4,5 9 79,1 Evaporation U ,3 1,3 distillates Pu Other U losses Pu 30,9 14,2 0 45,1 Total U ,3 1882,3 Pu 168,7 118,4 17,5 304,6 Yields U % 99,96 99,96 99,94 99,88 Pu % 99,95 99,97-99,90 VLUES RELTIVES TO 1 644,5 kg URNIUM ND 307,6 kg PLUTONIUM REPROCESSED. 12

13 13 TBLE IV - FEED CHRCTERISTICS DISSO- DTE LUTION N F E " E D 1st CYCLE U Pu 9 5Zr 9 5Nb 106 Ru-Rh Sb 1 3 4CS 1 3 7Cs 144 Ce-Pu *T g.h mci. 1-1 (3) ,1 28, ,5 25, , ,5 27, ,8 30, , ,2 28, ,4 29, ^120 ^30 < ,7 29,6 < 50 < ,1 31, ,6 30, ,2 27, ,2 27, ,8 27, ,5 28, ,5 29, , ,7 22,

14 TBLE V - DECONTMINTION FCTORS (average results during high activity feed extraction) (activities in ) mci.l"1 Feed 1 st cycle Sortie 1d cycle FD 2d cycle Sortie U 2d cycle FD U 2d cycle Sortie Pu 2d cycle FD Pu 3d cycle Sortie U 3d cycle FD U total FD Pu FD U total u 118, <5.10' 3 40 Pu 27, ~ 6 14 < 10" 7 9 5Zr , <10 " 4 > ' , Nb 800 0, ' " * 106Ru.Rh ,66 1, ,4.10" , " 4 6 8, Sb Cs Cs Ce.Pr &T0TL ,85 8,6.10 4?, , " 4 6 3, ,

15 TBLE VI - NEPTUNIUM DISTRIBUTION FEED U+Pu : 150 g.1-1 HN0 3 : 3,8 N Np s mg.1-1 (0 mg.h-1) 10 l.h-1 I 1st CYCLE 1 U+Pu : 63 g.1-1 HNO3 : 0,6 N Np s 8,8 mg.1-1 (210 mg.h-1) J * RFFINTE Np 3 1,5 mg.1-1 ( mg.h-1) 15 l. h - 1,8 l. h - 1 2nd CYCLE RFFINTE Np : 1 a 3,2 mg.1-1 (43 a 135 mg.h-1) 42 l. h - 1 PRTITION 1 u U = 30,5 Np : = 7, mg.1-1 (0,3 mg.h-1) Pu Pu : 14 g.1-1 Np : 3,6 a 8,3 mg.1-1 (72 a 166 mg.h-1) 20 l. h l.h-1 15

16 FIGURE 1 - PILOT PLNT - SIMPLIFIED FLOW DIGRM 16

17 1 M.IHCMTftTICM UVMg fotvmrr /Kioi'tcmo* HCIThCTION K WCVTMCTION u U.Pw m»t.l».h TT Mi 0,1 II *it 1.). 11 II * *) 0,01 II MNB,. j,> M MHO,, 0,» N M. 0,7» M UNO,. O.0IM M.» i.n-«10 I.H' l«i.h-' MM..-" IT l.h-1 3» l.h-«j l.h-1»» l>,%h l.h-1 llwlnltinwi till IH j (l 0 1 *"'^ V ' I I I lrh»»fug» Si 3»-5M.l"M»j J U I U OWCTWTHI FIGURE 2 - PILOT PLNT GENERL FLOW SHEET -T,- J ~ F ± * 1 1 I BIIIIWT NTrRM IN 0.1 I.!.'' 17

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