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Comparative economics of pulse columns and centrifugal contactors for solvent extraction in nuclear fuel reprocessing plants
Annals of Nuclear Energy. Vol. 3, pp. 73 to 83, Pergamon Press 1976. Printed in Northern Ireland
COMPARATIVE ECONOMICS OF PULSE COLUMNS AND CENTRIFUGAL CONTACTORS FOR SOLVENT EXTRACTION IN NUCLEAR FUEL REPROCESSING PLANTS J. M. COSTELLOand D. M. LEVINS Australian Atomic Energy Commission (Received 4 Auffust 1975; Revised 10 November 1975)
Abstract--Pulse columns and centrifugal contactors are compared as equipment alternatives for solvent extraction operations in nuclear fuel reprocessing plants. Preliminary design and cost data for the solvent extraction sections of plants of 3 tonnes U/day capacity are presented. In both designs, an attempt has been made to reduce capital costs by making maximum utilization of cell space. Cost estimates indicate that a plant incorporating pulse columns would be more economical. Uncertainties of maintenance requirements and of long-term reliability are additional problems to be faced in the operation of centrifugal contactors in fuel reprocessing plants. INTRODUCTION
Solvent extraction has become the established route for recovering uranium and plutonium from spent nuclear fuel. One of the first and most critical decisions to be taken in designing a reproce~ing plant is the choice of phase contactor for the solvent extraction operation. The choice lies between four types ofphasedispersalequipment: packedcolumns; pulse columns; mixer-settlers; and centrifugal contactors. Packed columns were used in the first reprocessing plants because of their simplicity and lack of moving parts and because their technology was already well established. However, with the development of pulse columns their comparative inefficiency became apparent--a packed column needs to be about 50 y, taller than a pulse column designed for the same separation (Long, 1967). Plant size and shielding requirements are thereby significantly increased. Packed columns are usually not considered for the primary solvent extraction operations in modern plants although they are used for solvent washing (Nuclear Fuel Services, 1962). Mixer-settlers have been used very successfully in many reprocessing plants. Their design is well established but they have the largest liquid holdup of any contactor. Equipment volume is large and criticality restrictions are usually more severe than for other solvent extraction equipment. The longer contacting time also results in greater solvent degradation by radiolysis. This is especially important whenever high burn-up fuels, such as those generated by advanced LWRs and LMFBRs, are processed after a short cooling period. For such
fuels, pulse columns and the newer centrifugal contactors appear to be more attractive alternatives. Bernard et aL (1971) compared the economics of pulse columns and centrifugal contactors for a single extraction cycle of a 5 tonnes U/day plant and found a slight advantage for the centrifugal contactor design in a conventional layout of plant equipment. This paper, which is part of a broader study of cost reduction in reprocessing plants (Costello and Levins, 1973) considers the total solvent extraction component of capital cost for a 3 tormes U/day plant featuring pulse columns and centrifugal contactors, with special emphasis on ways of obtaining maximum utilization of cell space. FLOWSHEET
A conceptual reprocessing plant treating advanced LWR fuel at a nominal capacity of 3 tonnes U/day and 28 kg Pu (total)/day was selected as the basis for equipment sizing and costing. The reference fuel assumed for the design consisted of UO~ pellets clad in Zircaloy, at an initial enrichment of up to 3.3 Yo 235U. After an exposure of 33,000 MWd/tU at an average power level of 34.8 MW/tU, the discharged fuel will contain up to 0.84~o z35U and 9.3 kg Pu (total)]tU at a fissile content (zagPu and ~41Pu) of about 70 %. Figure 1 shows the solvent extraction flowsheet which is based on that used at the Nuclear Fuel Services Plant in New York State (Nuclear Fuel Services, 1962). The solvent is 30yo tributyl phosphate (TBP) in kerosene diluent. There are four solvent extraction cycles; (a) a partition cycle where uranium and plutoPaper presented at the European Nuclear Conference, nium are extracted from the bulk of fission products and then separated using ferrous sulfamate, which Paris, April 1975. 73
6.3m 3
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Fresh Solvent 35"7m3
iO.Sm3 5"Sm3
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-Pu - Cycle Feed Pu 2.66g/£ NaNO z O.OSM
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1"95M HNO3
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Solvent aecycl_~e U 2Og/£ HNO30'25M ,(
14
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| 11.3 m~
4m3
Scrub I-aM HNO~
. 35"7m3
lu.so_,
I 37.5m 3
Strip O-OI M HNO3
~-
I'., STRIP J
0.TSm3
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2nd CYCLE J U EXTRACTION~ AND SCRUB
I 7.2m 3
Scrub 2M HNO3
Waste Raffinat¢ Fresh Solv¢nt P93M HNO~-I 35'7m3
I 7.1m 3
I
Feed Adjust 12M HNO 3
O.7Sm]
37.5 m~ -
Pu Product Pu 37.2g/C HNO31,4M
Used Solvent 35.7m 3
-
37.5m3
Strip_ O,OIM HNO 3
U Product U 8Og/¢ O'IM HNO3
Fig. 1. Solvent extraction flowsheet. Solvent is 30 % tributyl phosphate (TBP) in kerosene diluent. Basis--3 tonnes of uranium or one day's operation.
Was'~e Ra f finate U O'3 g/t Pu 163 gle HNO~I-85M Fp 780 Ci/1~
30 °Io TBP Solvent
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L~acher Product U 32Og 18 Pu 2.97g/~, HNO3 2M Fp i3OO Ci / g
S~ub 3 M HNO3
Reductant
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Feed Adjust 12M HNO3
z
P O
o=
Comparative economics of pulse columns and centrifugal coatactors reduces the plutonium to the less extractable trivalent state; (b) two consecutive cycles of uranium purification from residual contamination by plutonium and fission products; (c) one cycle of plutonium purification from uranium and fission products, Cross contamination of activity between the partition cycle and the two uranium cycles is reduced by segregation of the solvent into three separate solvent cycles each with its ownequipment for solvent treatment,
75
of the column. The pulsed columns are controlled by maintaining the interface of solvent and aqueous phase at a preset position by controlling the rate of withdrawal of aqueous phase from the base of the columns. The extraction columns are operated with organic solvent as the continuous phase; all other columns are operated with a continuous aqueous phase. The interface for the extraction column is in the bottom disengagement section (to reduce carryover of activity with the loaded solvent) and is located by sensing pots, filled with organic solvent connected to the interface control lines of the column. These sensing pots are located near the top of the column to balance the hydrostatic head in the column. The position of the interface in the bottom disengagement section is known from the relative level of solvent in the sensing pots, monitored by pneumercator probes. For the aqueous-continuous columns, the interface is located in the upper disengagement section by pneumercator probes. The aqueous streams from both aqueous and solvent continuous columns are withdrawn from the bottom disengaging section to a level control pot located near the top of the column. The position of the interface in the columns is stabilized by a varying air pressure applied to the control vessel, the control air pressure being determined by the interface sensing equipment. The aqueous phase passes through a decanter to remove traces of entrained solvent phase. The relative position of the solvent extraction columns and associated vessels carrying fl~,-active solution have, in the main, been selected to allow transfer of active liquor by gravity flow, and
PULSE COLUMN PLANT Continuous countercurrent solvent extraction is carried out in ten pulse columns whose dimensions are given in Table 1. Each column contains cartridges of perforated plates spaced 50 mm apart and having a free area of approximately 25 ~ . Based on reported values of HETS (height equivalent of theoretical stage) of 0.7-1.0m (de Witte, 1966; Joseph et aL, 1971), the effective column heights, which vary from 6.2 to 12.0 m, are believed to be adequate to meet flowsheet specifications. Column diameters have been calculated from the data of De Witte (1966) who found that flooding occurred at a combined feed rate (aqueous + organic) of 20--40 m3/h per m 2 of cross-sectionai area. The operating flow rates are assumed to be 70 ~o of the values at flooding. Two expanded disengagement sections, containing borated stainless steel plates for criticality control, are located above and below the perforated plate assembly. Each column is pulsed by application of a varying air pressure to a solvent line connected to the base
Table 1. Pulse coltmm dimensions Column No. 1 2 3 4 5 6 7 8 9 10
Function Partition cycle Extraction/scrub Partition Plutonium scrub Uranium strip 1st uranium cycle Extraction/scrub Uranium strip 2nd uranium cycle Extraction/scrub Uranium strip Plutonium cycle Extraction/scrub Plutonium strip
Diameter (m)
Effective length *(m)
Overall length (m)
0.34 0.30 0.15 0.39
10.4 10.4 6.2 10.4
13.0 13.0 8.2 13.0
0,46 0.37
10.4 10.4
13.0 13.0
0.46 0.37
10.4 10.4
13.0 13.0
0.19 0.13
12.0 9.5
14.7 10.7
* Excludes length of disengagement sections.
76
J. M. CosT~LLOand D. M. LEVINS
eliminate the capital and maintenance costs of active pumps and shielded pump bulges. Air lifts have been employed in some areas to increase the head available for gravity flow.
Solvent wash There are three identical but separate solvent wash systems for the partition cycle and both uranium purification cycles. Waste solvent from the plutonium cycle is diverted to the partition cycle. In each wash system, the solvent is scrubbed countercurrently with sodium carbonate solution in a column packed with Raschig rings to remove the bulk of lightly complexed or entrained activity. It then flows by gravity to a long residence time, 'Holley-Mott' type contactor where it is washed in a single stage with recirculated sodium carbonate solution for an average residence time of about
45 minutes to remove strongly complexed fission products. Finally, the solvent is washed with dilute nitric acid to neutralize and remove traces of entrained sodium carbonate solution. The solvent is collected in stock tanks for recycle to the appropriate extraction cycle.
Equipment layout and maintenance Because the pulse columns are much taller than other equipment, they are located in a single cell (Fig. 2). This approach results in a great saving in shielded space compared to a more conventional layout (Costello and Levins, 1973). The only disadvantage is that highly active partition cycle columns are located adjacent to equipment from the less active U and Pu purification cycles. This would pose additional decontamination and shielding problems in the event of failure of the less active
15 I
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7
. 11 8
5
i
5
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2
4
106
4
3 1201
SECTION A-A
Ii •
SECTIONB-B
SECTIONC-C
o47 LEGEND Cmajor vessels} I- I0 See Table I 11 Aqueous decanter for column 1 12 Aqueous decanter for column 3 13 Aqueous decanter for column 4 14 Aqueous decanter for column 5
A At_
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i
SECTIONO-O
F--~
F--]~',~
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03 L _ _ j x~;~l 17Aqueous decanter for column 8 ~'- ,o3,~ ,B ~5 )1 -"Iv 18 Aqueous decanter for column 9 C
°°s~l C 19Aqueousdecanterforcolumn10
oJ PLAN-4m
L~B x 4m
Fig. 2. Equipment layout--pulse column well.
Comparative economics of pulse columns and c.¢ntfifugalcontactors equipment. Maintenance of equipment in the pulsed column cell should be unnecessary because the columns operate at normal temperatures and have no moving mechanical parts. A central access area is provided for installation of the columns; this would also permit repair by contact maintenance following a major plant failure. Other vessels associated with solvent extraction, such as those required for accountability and conditioning as well as feed and waste tanks, are located in the nearby active liquor cells (Fig. 3). Solvent wash equipment is housed along with offgas treatment vessels in another cell adjacent to both the pulse column well and the active liquor cells. CENTRIFUGAL
CONTACTOR
PLANT
Centrifugal solvent extraction contactors operate on similar principles to those of gravity mixersettlers, where mass transfer of solute between two immiscible liquid phases is effected by successive mixing and separation of the phases in separation chambers. The main drawback of the mixer-settler is the relatively large area and hence equipment size required for phase disengagement. In centrifugal contactors, centrifugal fields in excess of 1000g
~
PULSE COLUMN WELL ~..':t
•~;.~':'
.:~!!i
77
(g = 9.81 m/s z) achieve rapid phase separation in highly compact equipment. The advantages of the centrifugal contactor in the context of fuel reprocessing derive primarily from its relatively small size when compared with alternative contactor types having similar throughput. This results in the following design features: (a) a small shielded volume for contactor installation, and hence a reduction in the civil costs of the plant. Against this, however, must be set the increased cost of the contactor, which may be four times as expensive as alternative solvent extraction units; (b) a low holdup of process solutions, resulting in reduction of inventory charges for uranium and plutonium; (c) a short contact time, resulting in an increased solvent life in the highly active extractor of the partition cycle; and (d) ease of startup and rundown operations, and acceptance of a wide range of flowrates of fluids having a variety of physical properties. High speed centrifugal contactors with holdup times of the order of 5 s per stage and capacities up to 23 ma/h have been developed at Savannah River
SOLVENT WASH AND O F F (;AS TREATMENT CELL
-i~-';:
\
i.:p:
High Activity Liquor Cell Dimensions- 6.1rn x9.gmx 4.3m h i g h
Low Activity Liquor Cell Dimensions10.Smx9,9m x5.2m high
LEGEND 1 Partition cycle feed tank 2 Accountability and feed adjust tank 3 Rework waste evaporator feed tank 4. Partition cycle waste catch tank 5 High level waste accountability & neutralizer tank 6 High level waste evaporator feed tank 7 1st U cycle feed tank conditioner 8 2nd Ucycle feed tank conditioner
9 Pu cycle feed conditioner 10 Low level waste accountability and neutralizer tank 11 2nd U cycle waste catch tank 12 1st U cycle waste catch tank 13 Low level waste evaporator feed tank 14 Hot analytical drain catch tank 15 Pu cycle waste catch tank
Fig. 3. Equipment layout for active liquor cells--pulse column plant.
78
J. M. Cos~nno and D. M. LBvms
Plant, South Carolina (Long, 1967). These are single stage units comprising a mixing chamber, centrifugal separator, an air-controlled weir for interface control, collection chambers for separated phases, and interstage piping. Each contactor has its own drive motor, and a cascade of eight units necessary for countercurrent extraction could require a shielded space of about 2 × 6 m area and 1.5 m height for the active contactors alone. The motors would increase the overall height by about 1 m. This design of centrifugal contactor occupies shielded space similar to that of a mixer-settler of equivalent capacity. The Savannah River Centrifugal Contactor is presently unable to handle solutions rich in 289pu because of criticality restrictions. Modified designs have been proposed to overcome this limitation (Bernstein et aL, 1973a; 1973b). The " R o b a t e l " contactor
A major advance in compaction of the centrifugal contactor has been achieved in a design marketed by SGN-Robatel, where up to ten actual stages of solvent extraction with a liquor throughput (solvent + aqueous) of 5.5 m3/h are contained in a single unit of approximate dimensions 1 m (dia) ×
: PHASE
AQUEOUS PHASE INLET
Fig. 4. Robatel centrifugal contactor (model 670-1'4) (Drawing supplied by SGN-Robatel, Genoa, France).
1.4 m. Twelve stages have been considered as a possible maximum for the design (Bernard et al., 1971). A stage of extraction consists of an annular gap between a fixed central drum and a rotating bowl (Fig. 4). The annulus is divided into a mixing chamber and a disengagement chamber by radial blades attached to the rotating bowl. A disc attached to the stationary drum is located between a pair of radial blades forming the mixing compartment. In operation, the bowl is rotated about a vertical axis. The relative velocities between moving and stationary surfaces give rise to an intense turbulent mixing of the two phases. The moving blades and fixed disc further act as a centrifugal two-stage pump and transfer the mixed phase from the mixing compartment into the adjoining settling chamber, where phase disengagement occurs under centrifugal force. Solvent is transferred from a seJtler to the successive mixer over a circular weir of precise radius. The aqueous phase is transferred through a series of axial and radial channels drilled in the periphery of the cylinder, channelling the liquor into the mixer of the preceding stage. The geometry of the weirs and ducts is determined by hydrostatic balance for the practical range of phase densities to be handled by the contractor. Ducts may be provided in the central cylindrical drum to introduce auxiliary process feeds into intermediate stages of the contactor. In the nuclear application of the contactor shown in Fig. 4, the bowl is attached to the lower end of a pendular shaft, which passes through a seal in the roof of the cell in which the contactor is located. The bowl is contained in an external cylindrical casing to which all pipe connections are made. The mechanical bearing support for the pendular shaft and the electrical driving unit are supported by a removable, steel shielding plate recessed into the shielding of the cell roof. Table 2 gives the dimensions of the centrifugal contactors which meet the plant design capacity of 3 tonnes U/day. The contactors were selected by comparison of flowsheet duties with data supplied by SGN-Robatel (1972) on the number of stages and capacity of production models. Fourteen contactors are needed with separate units for extraction, scrubbing and stripping. A criticality assessment indicates that all proposed contactors are safe under normal operating conditions. The critical concentration for Robatel centrifugal contactors exceeds 20g/l of fissile plutonium (Bernard et al., 1971). The smallest contactors processing plutonium are geometrically
Comparative economics of pulse columns and centrifugal contaetors safe. Preliminary estimates indicate that, under worst conditions of maloperation all except two of the contactors would be safe based on limitations on volume, mass, or concentration. Detailed calculations which are beyond the scope of this study would be required for these two contactors taking account of the complex annular shapes of the stages which are favourable to geometric safety. If necessary the shape of the external vessel could be modified and the central cavity filled with a neutronabsorbing material (SGN-Robatel, 1972). It has been assumed that the nuclear safety of all of the contactors could be achieved by a combination of fixed neutron absorber, geometrical restriction, and by the monitoring of flowrates and concentrations of inactive feeds.
79
desirable to remove solids from the leacher product. While these solids could be removed by filtration, a centrifugal separator is considered the logical choice here because the problems in its design, operation and maintenance are similar to those for the contactors it serves and periodic removal and replacement of highly active filters is not required.
Solvent wash
The solvent wash systems are similar to those used in the pulse column plant except that it has proved more convenient to substitute mixer-settlers for packed columns. Waste solvent is scrubbed countercurrently with sodium carbonate solution in a four-stage mixer-settler, followed by a long residence wash with sodium carbonate in a 'HollyMott' type contactor, and final washing with dilute Solids clarification nitric acid in a two-stage mixer-settler. Sizing The solution from the fuel leachers will contain calculations for the mixer-settlers were based on some finely divided solid particulates in suspension mixing residence times of 60 s and throughputs of which are slow to settle under gravity. The solids 5.4 mZ]h/m2 of settling area. range from particles of Zircaloy cladding and ceramic fuel spacers fragmented by the shear, to insoluble carbonaceous material resulting from the Equipment layout and maintenance organic binder used in fuel manufacture, elemental The centrifuges are located in the roof of the fission products and possibly undissolved plutonium Active Liquor Cells which house the ancillary vessels oxide. associated with solvent extraction. Figures 5 and 6 Pulsed column solvent extraction equipment show the layout of the centrifuges. In addition to accepts these solids to some degree without opera- the vessels used in the pulse column plant, two tional difficulty; however, solids may settle out and drainage tanks are required to accept the contents block internal liquor routes under the high settling of the contactors whenever the centrifuge drives are forces of a centrifugal contactor, and it is considered stopped. Table 2. Centrifugal contactor dimensions* Contactor No. 1E 1S 2 3 4 5E 5S 6 7E 7S 8 9E 9S 10
Function
Partition cycle Extraction Scrub Partition Plutonium scrub Uranium strip 1st uranium cycle Extraction Scrub Uranium strip 2nd uranium cycle Extraction Scrub Uranium strip Plutonium cycle Extraction Scrub Plutonium strip
SGN-gobatel Model No.
Number of stages
Overall diameter (m)
Overall height (m)
Holdup per stage (1)
530-N 530-N 530-N 420-N 670-N
10 10 I0 10 10
0.94 0.94 0.94 0.78 1.10
1.35 1.35 1.35 1.20 1.50
5.5 5.5 5.5 2.3 10.7
670-N 530-N 670-N
10 10 10
1.10 0.94 1.10
1.50 1.35 1.50
10.7 5.5 10.7
670-N 530-N 670-N
10 10 10
1.10 0.94 1.10
1.50 1.35 1.50
10.7 5.5 10.7
420-N 240-N 240-N
10 8 8
0.78 0.45 0.45
1.20 0.90 0.90
2.3 0.48 0.48
* Excludes thickness of shielding. External dimensions in this table represent the spatial requirements for installation.
80
J. M. COSTELLOand D. M. LEVlNS
None of the centrifuges have bearing surfaces inside the active cell, and routine maintenance is confined to the external shaft support bearings and motor drive. In the event of contactor failure, the entire unit may be removed from its casing into a flask mounted externally on the cell roof. This operation is preceded by emptying the bowl and performing a preliminary decontamination to reduce gross levels of contamination and radiation. The mechanical driving system and bearing support are removed; remote couplings are operated to withdraw pipe connections from the path of the bowl. A shielded flask carrying a winch is located by a travelling crane over the plug housing the contactor, and the unit (comprising the shielding plug, bowl, shaft and fixed drum) is withdrawn into the flask, which is closed by a horizontal door and transported to maintenance or disposal. A standby closure is used for the aperture in the cell roof; the disposal flask is subsequently used to install a new contaetor and shielding plug. PLANT DESIGN
It is not possible to examine the comparative economics of pulse columns and centrifugal contactors without considering how the choice affects other areas in the reprocessing chain and the overall plant design. For example, because pulse columns are long but slender, the process building for this type of contactor will tend to be taller than a plant based on centrifugal contactors. This, in turn, affects the arrangements of other process cells.
The work reported here is part of a broader study in which overall plant designs were developed for both solvent extraction alternatives (Costello and Levins, 1973). Figures 7 and 8 show plan views at ground level for the two designs. To reduce civil costs, cells have been arranged so that they share common shielding walls. In the centrifugal eontactor plant, the Active Liquor Cells have been aligned with the ShearLeach and Waste Evaporator Cells so that the centrifuges can be serviced by the main building crane.
Table 3 compares the process cell requirements for solvent extraction operations in the two plants. The compactness of the centrifugal contactor leads to a 25 ~o saving in cell volume. COSTING
Building costs for the solvent extraction cells were assessed by civil engineering consultants. Where cells shared common shielding walls, one half of the construction costs have been assigned to each adjoining cell. Equipment costs for both plant concepts were obtained by detailed assessment of materials plus labour charges. This approach was supplemented and verified by direct quotations from Australian fabricators on a number of representative vessels. Robatel, Genas, France supplied quotations for its centrifugal contactors. Equipment layouts were used to calculate piping costs from a compilation of pipe lengths, number of welds, bends, wall 11.3m
,r
i
"a"
I
I 9-2 rn' c,
, ~
,,
L
0.0m
'
L
;: ;!.:;T!H!ghActivity Liquo¢ Cell Low A ,,vl,y L,,uor Co,
Section "D- 13" Legend 1-10 See toble 2 11 Solids removal centrifuge
Fig. 5. Plan showing centrifugal contactor layout.
L
Comparative economics of pulse columns and centrifugal contactors
"" .~', ~;qh Ac tivity Liquor Cell ~
..E13 . 7 -
"I" 1E Steel Shielding _
81
Low A}:tivity Liquor i , • CellI PART SECTION A-A ~ ~
~ T
m °
+k.~
Jk.
~-'-I
m'=
PART SECTION"B-B'
PANT SECTION "C-C'
Fig. 6. Elevation view of centrifugal contactor~.
DISCUSSION penetrations and testing man-hours. Instrumentation and control costs were estimated by extracting Bernard et aL (1971) compared the economics of requirements from the engineering flow diagrams mixer-settlers, pulse columns and centrifugal conand applying a unit cost for each control function. tactors for the first extraction cycle of a 5 tonnes U/day reprocessing plant. Qualitatively their ECONOMIC COMPARISON breakdown is similar to that reported here; the A summary of solvent extraction cost estimates centrifugal contactor plant has highest costs for updated to mid-1974 Australian dollars is shown in process equipment but the lowest costs for building, Table 4. Building costs for the centrifugal contactor piping and instrumentation. However, their analysis plant are 25 ~o less than those for the pulse column shows a slight cost advantage (4 ~o) for the centriplant, in direct proportion to the savings in shielded fugal contactor plant. volume. Process piping is one-third less because of The principal discrepancy between this work and the compactness of the centrifugal contactor layout. that of Bernard et aL is the difference in chemical The additional cost of pulse column instrumentation equipment costs. The French estimate shows only results in a 20% increase in instrumentation and a relatively small saving (20~o) in equipment costs electrical costs for that design. for the pulse column plant. Another possible However, chemical process equipment costs in explanation for the discrepancy in overall costs is the pulse column plant are less than half those in the the rather unconventional layout of pulse columns centrifugal contactor plant and this more than in this study, which leads to a significant reduction offsets the cost disadvantages mentioned above. in building costs. Centrifugal contactor costs are typically about three The French study was based on a design capacity to four times those for pulse columns. Total solvent of 5 tormes U/day compared to 3 tonnes U/day in extraction costs are estimated to be 30yo more this work. Centrifugal contactors become more expensive when centrifugal contactors are used. economically attractive at high plant throughputs When these estimates are translated into total because criticality and scale-up problems arise as capital investment for a complete reprocessing pulse column diameter is increased. For very large facility, the choice of pulse columns for solvent pulse column plants, parallel duplication of extracextraction results in an 8 ~o capital saving (Costello tors will probably be necessary. and Levins, 1973). Other considerations that are difficult to cost
82
J . M . COSTELLOand D. M. LEVINS
To Fuel Storage Pond
1 2 3 4 5 6
Sheor-leoch cell Cladding disposal cell Evaporator cell Maintenance pond High Activity liquorcell Low Activity liquor cell
7 8 9 10 11 12
Acid recovery cell Pulse column well Solvent wash 0ff-gas treatment Plutonium ceil Uranium ceil
13 14 15 16 17
Plutonium pockaging Laboratories, offices, etc. Shear maintenance pond Pump aisles Analytical cells
Fig. 7. Ground level plan view of pulse column plant.
To Fu, Stora!
Legend See Figure 7
Fig. 8. Ground level plan view of centrifugal contactor plant.
Comparative economics of pulse columns and centrifugal contactors
83
Table 3. Process cell requirements Pulse column plant
High activity liquor cell Low activity liquor cell Pulse column well Solvent treatment cell
Centrifugal contactor plant
Area (In2)
Volume (mz)
Area (m2)
Volume (m3)
60 108 16 30 214 m S
258 557 335 309 1459 m 2
70 114 -45 229 m 2
297 588 -235 1120 m a
Table 4. Solvent extraction costs (mid 1974 Australian dollars*) Pulse column plant Buildings Chemical process equipment Process piping Instrumentation and electrical
$
644,000 1,295,000 824,000 502,000 $A3,265,000
Centrifugal contactor plant $
491,000 2,844,000 546,000 414,000 $A4,295,000
* In mid 1974 SA = 1.49 SU.S.
must also be weighed in making a decision on solvent extraction alternatives. For example, centrifugal contactors have a low residence time, so solvent damage from high performance fuels will be minimal. Recent research, however, suggests that the solvent damage problem is less than was previously thought (van Geel et aL, 1971). Centrifugal contactors require more maintenance than pulse columns, leading to a poorer utilization factor for a plant based on this type of extractor. Moreover, pulse columns have performed reliably in many plants whereas the long-term reliability of centrifugal contactors has yet to be fully demonstrated. CONCLUSIONS Centrifugal contactors have often been proposed as replacements for pulse columns in nuclear reprocessing plants based on advanced technology. However, estimates of capital cost show that these contactors cannot be economically justified for plants processing L W R fuels at plant capacities of 3 tonnes U]day or less. Maintenance requirements and long-term reliability are further uncertainties
associated with the operation of centrifugal contactors in a highly active environment. REFERENCES
Bernard, C., Michel, P. and Taruero, M. (1971) Proe. Int. Solv. Extr. Conf. (ISEC 71) 2(8]3), 1282. Bernstein, G. J., Grosvenor, D. E., Lent, J. F. and Levitz, N. M. (1973a) Argonne National Lab. Report, ANL-7968. Bernstein, G. J., Grosvenor, D. E., Lent, J. F. and Levitz, N. M. (1973b) Argonne National Lab. Report, ANL-7969. Costello, J. M. and Levins, D. M. (1973) Australian Atomic Energy Commission Report, AAEC[E275. De Witte, R. (1966) Atompraxis 12, 42. Van Geel, J., Joseph, C., Detilleux, E., Heinz, W., Centeno, J. and Gustafsson, B. (1971) Proc. lnt. Solv. Extr. Conf. (ISEC 71) 1(3C), 577. Joseph, C. J., van Geel, J., Detilleux, E. and Centeno, J. L. (1971) Proc. Int. Solv. Extr. Conf. (ISEC 71) 1(3C), 593, Long, J. T. (1967) Engineering for Nuclear Fuel Reprocessing. Gordon & Breach, N.Y. Nuclear Fuel Services (1962) USAEC Docket 50201. SGN-Robatel (1972) Multi-stage centrifugal contactor for liquid-liquid extraction (brochure). Saint-Gobain Techniques Nouvelles, Gcnas, France.
COMPARATIVE ECONOMICS OF PULSE COLUMNS AND CENTRIFUGAL CONTACTORS FOR SOLVENT EXTRACTION IN NUCLEAR FUEL REPROCESSING PLANTS J. M. COSTELLOand D. M. LEVINS Australian Atomic Energy Commission (Received 4 Auffust 1975; Revised 10 November 1975)
Abstract--Pulse columns and centrifugal contactors are compared as equipment alternatives for solvent extraction operations in nuclear fuel reprocessing plants. Preliminary design and cost data for the solvent extraction sections of plants of 3 tonnes U/day capacity are presented. In both designs, an attempt has been made to reduce capital costs by making maximum utilization of cell space. Cost estimates indicate that a plant incorporating pulse columns would be more economical. Uncertainties of maintenance requirements and of long-term reliability are additional problems to be faced in the operation of centrifugal contactors in fuel reprocessing plants. INTRODUCTION
Solvent extraction has become the established route for recovering uranium and plutonium from spent nuclear fuel. One of the first and most critical decisions to be taken in designing a reproce~ing plant is the choice of phase contactor for the solvent extraction operation. The choice lies between four types ofphasedispersalequipment: packedcolumns; pulse columns; mixer-settlers; and centrifugal contactors. Packed columns were used in the first reprocessing plants because of their simplicity and lack of moving parts and because their technology was already well established. However, with the development of pulse columns their comparative inefficiency became apparent--a packed column needs to be about 50 y, taller than a pulse column designed for the same separation (Long, 1967). Plant size and shielding requirements are thereby significantly increased. Packed columns are usually not considered for the primary solvent extraction operations in modern plants although they are used for solvent washing (Nuclear Fuel Services, 1962). Mixer-settlers have been used very successfully in many reprocessing plants. Their design is well established but they have the largest liquid holdup of any contactor. Equipment volume is large and criticality restrictions are usually more severe than for other solvent extraction equipment. The longer contacting time also results in greater solvent degradation by radiolysis. This is especially important whenever high burn-up fuels, such as those generated by advanced LWRs and LMFBRs, are processed after a short cooling period. For such
fuels, pulse columns and the newer centrifugal contactors appear to be more attractive alternatives. Bernard et aL (1971) compared the economics of pulse columns and centrifugal contactors for a single extraction cycle of a 5 tonnes U/day plant and found a slight advantage for the centrifugal contactor design in a conventional layout of plant equipment. This paper, which is part of a broader study of cost reduction in reprocessing plants (Costello and Levins, 1973) considers the total solvent extraction component of capital cost for a 3 tormes U/day plant featuring pulse columns and centrifugal contactors, with special emphasis on ways of obtaining maximum utilization of cell space. FLOWSHEET
A conceptual reprocessing plant treating advanced LWR fuel at a nominal capacity of 3 tonnes U/day and 28 kg Pu (total)/day was selected as the basis for equipment sizing and costing. The reference fuel assumed for the design consisted of UO~ pellets clad in Zircaloy, at an initial enrichment of up to 3.3 Yo 235U. After an exposure of 33,000 MWd/tU at an average power level of 34.8 MW/tU, the discharged fuel will contain up to 0.84~o z35U and 9.3 kg Pu (total)]tU at a fissile content (zagPu and ~41Pu) of about 70 %. Figure 1 shows the solvent extraction flowsheet which is based on that used at the Nuclear Fuel Services Plant in New York State (Nuclear Fuel Services, 1962). The solvent is 30yo tributyl phosphate (TBP) in kerosene diluent. There are four solvent extraction cycles; (a) a partition cycle where uranium and plutoPaper presented at the European Nuclear Conference, nium are extracted from the bulk of fission products and then separated using ferrous sulfamate, which Paris, April 1975. 73
6.3m 3
5"0.'
7.4m 3
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=
Fresh Solvent 35"7m3
iO.Sm3 5"Sm3
~11-,
Fred Adjust HNO~ 6M NaNO20"IM
l
-Pu - Cycle Feed Pu 2.66g/£ NaNO z O.OSM
Fresh Solvent
1"95M HNO3
I 7"2m 3
ScrUb 2M HNO3
o. sovo Waste.2.3, Raffinate
43.3m~
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Solvent aecycl_~e U 2Og/£ HNO30'25M ,(
14
O.IM HNO3
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4"
Roffinate 3"6M HNO3
| 11.3 m~
4m3
Scrub I-aM HNO~
. 35"7m3
lu.so_,
I 37.5m 3
Strip O-OI M HNO3
~-
I'., STRIP J
0.TSm3
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2nd CYCLE J U EXTRACTION~ AND SCRUB
I 7.2m 3
Scrub 2M HNO3
Waste Raffinat¢ Fresh Solv¢nt P93M HNO~-I 35'7m3
I 7.1m 3
I
Feed Adjust 12M HNO 3
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37.5 m~ -
Pu Product Pu 37.2g/C HNO31,4M
Used Solvent 35.7m 3
-
37.5m3
Strip_ O,OIM HNO 3
U Product U 8Og/¢ O'IM HNO3
Fig. 1. Solvent extraction flowsheet. Solvent is 30 % tributyl phosphate (TBP) in kerosene diluent. Basis--3 tonnes of uranium or one day's operation.
Was'~e Ra f finate U O'3 g/t Pu 163 gle HNO~I-85M Fp 780 Ci/1~
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z
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Comparative economics of pulse columns and centrifugal coatactors reduces the plutonium to the less extractable trivalent state; (b) two consecutive cycles of uranium purification from residual contamination by plutonium and fission products; (c) one cycle of plutonium purification from uranium and fission products, Cross contamination of activity between the partition cycle and the two uranium cycles is reduced by segregation of the solvent into three separate solvent cycles each with its ownequipment for solvent treatment,
75
of the column. The pulsed columns are controlled by maintaining the interface of solvent and aqueous phase at a preset position by controlling the rate of withdrawal of aqueous phase from the base of the columns. The extraction columns are operated with organic solvent as the continuous phase; all other columns are operated with a continuous aqueous phase. The interface for the extraction column is in the bottom disengagement section (to reduce carryover of activity with the loaded solvent) and is located by sensing pots, filled with organic solvent connected to the interface control lines of the column. These sensing pots are located near the top of the column to balance the hydrostatic head in the column. The position of the interface in the bottom disengagement section is known from the relative level of solvent in the sensing pots, monitored by pneumercator probes. For the aqueous-continuous columns, the interface is located in the upper disengagement section by pneumercator probes. The aqueous streams from both aqueous and solvent continuous columns are withdrawn from the bottom disengaging section to a level control pot located near the top of the column. The position of the interface in the columns is stabilized by a varying air pressure applied to the control vessel, the control air pressure being determined by the interface sensing equipment. The aqueous phase passes through a decanter to remove traces of entrained solvent phase. The relative position of the solvent extraction columns and associated vessels carrying fl~,-active solution have, in the main, been selected to allow transfer of active liquor by gravity flow, and
PULSE COLUMN PLANT Continuous countercurrent solvent extraction is carried out in ten pulse columns whose dimensions are given in Table 1. Each column contains cartridges of perforated plates spaced 50 mm apart and having a free area of approximately 25 ~ . Based on reported values of HETS (height equivalent of theoretical stage) of 0.7-1.0m (de Witte, 1966; Joseph et aL, 1971), the effective column heights, which vary from 6.2 to 12.0 m, are believed to be adequate to meet flowsheet specifications. Column diameters have been calculated from the data of De Witte (1966) who found that flooding occurred at a combined feed rate (aqueous + organic) of 20--40 m3/h per m 2 of cross-sectionai area. The operating flow rates are assumed to be 70 ~o of the values at flooding. Two expanded disengagement sections, containing borated stainless steel plates for criticality control, are located above and below the perforated plate assembly. Each column is pulsed by application of a varying air pressure to a solvent line connected to the base
Table 1. Pulse coltmm dimensions Column No. 1 2 3 4 5 6 7 8 9 10
Function Partition cycle Extraction/scrub Partition Plutonium scrub Uranium strip 1st uranium cycle Extraction/scrub Uranium strip 2nd uranium cycle Extraction/scrub Uranium strip Plutonium cycle Extraction/scrub Plutonium strip
Diameter (m)
Effective length *(m)
Overall length (m)
0.34 0.30 0.15 0.39
10.4 10.4 6.2 10.4
13.0 13.0 8.2 13.0
0,46 0.37
10.4 10.4
13.0 13.0
0.46 0.37
10.4 10.4
13.0 13.0
0.19 0.13
12.0 9.5
14.7 10.7
* Excludes length of disengagement sections.
76
J. M. CosT~LLOand D. M. LEVINS
eliminate the capital and maintenance costs of active pumps and shielded pump bulges. Air lifts have been employed in some areas to increase the head available for gravity flow.
Solvent wash There are three identical but separate solvent wash systems for the partition cycle and both uranium purification cycles. Waste solvent from the plutonium cycle is diverted to the partition cycle. In each wash system, the solvent is scrubbed countercurrently with sodium carbonate solution in a column packed with Raschig rings to remove the bulk of lightly complexed or entrained activity. It then flows by gravity to a long residence time, 'Holley-Mott' type contactor where it is washed in a single stage with recirculated sodium carbonate solution for an average residence time of about
45 minutes to remove strongly complexed fission products. Finally, the solvent is washed with dilute nitric acid to neutralize and remove traces of entrained sodium carbonate solution. The solvent is collected in stock tanks for recycle to the appropriate extraction cycle.
Equipment layout and maintenance Because the pulse columns are much taller than other equipment, they are located in a single cell (Fig. 2). This approach results in a great saving in shielded space compared to a more conventional layout (Costello and Levins, 1973). The only disadvantage is that highly active partition cycle columns are located adjacent to equipment from the less active U and Pu purification cycles. This would pose additional decontamination and shielding problems in the event of failure of the less active
15 I
1 10
7
7
. 11 8
5
i
5
[
2
4
106
4
3 1201
SECTION A-A
Ii •
SECTIONB-B
SECTIONC-C
o47 LEGEND Cmajor vessels} I- I0 See Table I 11 Aqueous decanter for column 1 12 Aqueous decanter for column 3 13 Aqueous decanter for column 4 14 Aqueous decanter for column 5
A At_
0
i
SECTIONO-O
F--~
F--]~',~
~ Aqueousd~onter for colum~ 7
03 L _ _ j x~;~l 17Aqueous decanter for column 8 ~'- ,o3,~ ,B ~5 )1 -"Iv 18 Aqueous decanter for column 9 C
°°s~l C 19Aqueousdecanterforcolumn10
oJ PLAN-4m
L~B x 4m
Fig. 2. Equipment layout--pulse column well.
Comparative economics of pulse columns and c.¢ntfifugalcontactors equipment. Maintenance of equipment in the pulsed column cell should be unnecessary because the columns operate at normal temperatures and have no moving mechanical parts. A central access area is provided for installation of the columns; this would also permit repair by contact maintenance following a major plant failure. Other vessels associated with solvent extraction, such as those required for accountability and conditioning as well as feed and waste tanks, are located in the nearby active liquor cells (Fig. 3). Solvent wash equipment is housed along with offgas treatment vessels in another cell adjacent to both the pulse column well and the active liquor cells. CENTRIFUGAL
CONTACTOR
PLANT
Centrifugal solvent extraction contactors operate on similar principles to those of gravity mixersettlers, where mass transfer of solute between two immiscible liquid phases is effected by successive mixing and separation of the phases in separation chambers. The main drawback of the mixer-settler is the relatively large area and hence equipment size required for phase disengagement. In centrifugal contactors, centrifugal fields in excess of 1000g
~
PULSE COLUMN WELL ~..':t
•~;.~':'
.:~!!i
77
(g = 9.81 m/s z) achieve rapid phase separation in highly compact equipment. The advantages of the centrifugal contactor in the context of fuel reprocessing derive primarily from its relatively small size when compared with alternative contactor types having similar throughput. This results in the following design features: (a) a small shielded volume for contactor installation, and hence a reduction in the civil costs of the plant. Against this, however, must be set the increased cost of the contactor, which may be four times as expensive as alternative solvent extraction units; (b) a low holdup of process solutions, resulting in reduction of inventory charges for uranium and plutonium; (c) a short contact time, resulting in an increased solvent life in the highly active extractor of the partition cycle; and (d) ease of startup and rundown operations, and acceptance of a wide range of flowrates of fluids having a variety of physical properties. High speed centrifugal contactors with holdup times of the order of 5 s per stage and capacities up to 23 ma/h have been developed at Savannah River
SOLVENT WASH AND O F F (;AS TREATMENT CELL
-i~-';:
\
i.:p:
High Activity Liquor Cell Dimensions- 6.1rn x9.gmx 4.3m h i g h
Low Activity Liquor Cell Dimensions10.Smx9,9m x5.2m high
LEGEND 1 Partition cycle feed tank 2 Accountability and feed adjust tank 3 Rework waste evaporator feed tank 4. Partition cycle waste catch tank 5 High level waste accountability & neutralizer tank 6 High level waste evaporator feed tank 7 1st U cycle feed tank conditioner 8 2nd Ucycle feed tank conditioner
9 Pu cycle feed conditioner 10 Low level waste accountability and neutralizer tank 11 2nd U cycle waste catch tank 12 1st U cycle waste catch tank 13 Low level waste evaporator feed tank 14 Hot analytical drain catch tank 15 Pu cycle waste catch tank
Fig. 3. Equipment layout for active liquor cells--pulse column plant.
78
J. M. Cos~nno and D. M. LBvms
Plant, South Carolina (Long, 1967). These are single stage units comprising a mixing chamber, centrifugal separator, an air-controlled weir for interface control, collection chambers for separated phases, and interstage piping. Each contactor has its own drive motor, and a cascade of eight units necessary for countercurrent extraction could require a shielded space of about 2 × 6 m area and 1.5 m height for the active contactors alone. The motors would increase the overall height by about 1 m. This design of centrifugal contactor occupies shielded space similar to that of a mixer-settler of equivalent capacity. The Savannah River Centrifugal Contactor is presently unable to handle solutions rich in 289pu because of criticality restrictions. Modified designs have been proposed to overcome this limitation (Bernstein et aL, 1973a; 1973b). The " R o b a t e l " contactor
A major advance in compaction of the centrifugal contactor has been achieved in a design marketed by SGN-Robatel, where up to ten actual stages of solvent extraction with a liquor throughput (solvent + aqueous) of 5.5 m3/h are contained in a single unit of approximate dimensions 1 m (dia) ×
: PHASE
AQUEOUS PHASE INLET
Fig. 4. Robatel centrifugal contactor (model 670-1'4) (Drawing supplied by SGN-Robatel, Genoa, France).
1.4 m. Twelve stages have been considered as a possible maximum for the design (Bernard et al., 1971). A stage of extraction consists of an annular gap between a fixed central drum and a rotating bowl (Fig. 4). The annulus is divided into a mixing chamber and a disengagement chamber by radial blades attached to the rotating bowl. A disc attached to the stationary drum is located between a pair of radial blades forming the mixing compartment. In operation, the bowl is rotated about a vertical axis. The relative velocities between moving and stationary surfaces give rise to an intense turbulent mixing of the two phases. The moving blades and fixed disc further act as a centrifugal two-stage pump and transfer the mixed phase from the mixing compartment into the adjoining settling chamber, where phase disengagement occurs under centrifugal force. Solvent is transferred from a seJtler to the successive mixer over a circular weir of precise radius. The aqueous phase is transferred through a series of axial and radial channels drilled in the periphery of the cylinder, channelling the liquor into the mixer of the preceding stage. The geometry of the weirs and ducts is determined by hydrostatic balance for the practical range of phase densities to be handled by the contractor. Ducts may be provided in the central cylindrical drum to introduce auxiliary process feeds into intermediate stages of the contactor. In the nuclear application of the contactor shown in Fig. 4, the bowl is attached to the lower end of a pendular shaft, which passes through a seal in the roof of the cell in which the contactor is located. The bowl is contained in an external cylindrical casing to which all pipe connections are made. The mechanical bearing support for the pendular shaft and the electrical driving unit are supported by a removable, steel shielding plate recessed into the shielding of the cell roof. Table 2 gives the dimensions of the centrifugal contactors which meet the plant design capacity of 3 tonnes U/day. The contactors were selected by comparison of flowsheet duties with data supplied by SGN-Robatel (1972) on the number of stages and capacity of production models. Fourteen contactors are needed with separate units for extraction, scrubbing and stripping. A criticality assessment indicates that all proposed contactors are safe under normal operating conditions. The critical concentration for Robatel centrifugal contactors exceeds 20g/l of fissile plutonium (Bernard et al., 1971). The smallest contactors processing plutonium are geometrically
Comparative economics of pulse columns and centrifugal contaetors safe. Preliminary estimates indicate that, under worst conditions of maloperation all except two of the contactors would be safe based on limitations on volume, mass, or concentration. Detailed calculations which are beyond the scope of this study would be required for these two contactors taking account of the complex annular shapes of the stages which are favourable to geometric safety. If necessary the shape of the external vessel could be modified and the central cavity filled with a neutronabsorbing material (SGN-Robatel, 1972). It has been assumed that the nuclear safety of all of the contactors could be achieved by a combination of fixed neutron absorber, geometrical restriction, and by the monitoring of flowrates and concentrations of inactive feeds.
79
desirable to remove solids from the leacher product. While these solids could be removed by filtration, a centrifugal separator is considered the logical choice here because the problems in its design, operation and maintenance are similar to those for the contactors it serves and periodic removal and replacement of highly active filters is not required.
Solvent wash
The solvent wash systems are similar to those used in the pulse column plant except that it has proved more convenient to substitute mixer-settlers for packed columns. Waste solvent is scrubbed countercurrently with sodium carbonate solution in a four-stage mixer-settler, followed by a long residence wash with sodium carbonate in a 'HollyMott' type contactor, and final washing with dilute Solids clarification nitric acid in a two-stage mixer-settler. Sizing The solution from the fuel leachers will contain calculations for the mixer-settlers were based on some finely divided solid particulates in suspension mixing residence times of 60 s and throughputs of which are slow to settle under gravity. The solids 5.4 mZ]h/m2 of settling area. range from particles of Zircaloy cladding and ceramic fuel spacers fragmented by the shear, to insoluble carbonaceous material resulting from the Equipment layout and maintenance organic binder used in fuel manufacture, elemental The centrifuges are located in the roof of the fission products and possibly undissolved plutonium Active Liquor Cells which house the ancillary vessels oxide. associated with solvent extraction. Figures 5 and 6 Pulsed column solvent extraction equipment show the layout of the centrifuges. In addition to accepts these solids to some degree without opera- the vessels used in the pulse column plant, two tional difficulty; however, solids may settle out and drainage tanks are required to accept the contents block internal liquor routes under the high settling of the contactors whenever the centrifuge drives are forces of a centrifugal contactor, and it is considered stopped. Table 2. Centrifugal contactor dimensions* Contactor No. 1E 1S 2 3 4 5E 5S 6 7E 7S 8 9E 9S 10
Function
Partition cycle Extraction Scrub Partition Plutonium scrub Uranium strip 1st uranium cycle Extraction Scrub Uranium strip 2nd uranium cycle Extraction Scrub Uranium strip Plutonium cycle Extraction Scrub Plutonium strip
SGN-gobatel Model No.
Number of stages
Overall diameter (m)
Overall height (m)
Holdup per stage (1)
530-N 530-N 530-N 420-N 670-N
10 10 I0 10 10
0.94 0.94 0.94 0.78 1.10
1.35 1.35 1.35 1.20 1.50
5.5 5.5 5.5 2.3 10.7
670-N 530-N 670-N
10 10 10
1.10 0.94 1.10
1.50 1.35 1.50
10.7 5.5 10.7
670-N 530-N 670-N
10 10 10
1.10 0.94 1.10
1.50 1.35 1.50
10.7 5.5 10.7
420-N 240-N 240-N
10 8 8
0.78 0.45 0.45
1.20 0.90 0.90
2.3 0.48 0.48
* Excludes thickness of shielding. External dimensions in this table represent the spatial requirements for installation.
80
J. M. COSTELLOand D. M. LEVlNS
None of the centrifuges have bearing surfaces inside the active cell, and routine maintenance is confined to the external shaft support bearings and motor drive. In the event of contactor failure, the entire unit may be removed from its casing into a flask mounted externally on the cell roof. This operation is preceded by emptying the bowl and performing a preliminary decontamination to reduce gross levels of contamination and radiation. The mechanical driving system and bearing support are removed; remote couplings are operated to withdraw pipe connections from the path of the bowl. A shielded flask carrying a winch is located by a travelling crane over the plug housing the contactor, and the unit (comprising the shielding plug, bowl, shaft and fixed drum) is withdrawn into the flask, which is closed by a horizontal door and transported to maintenance or disposal. A standby closure is used for the aperture in the cell roof; the disposal flask is subsequently used to install a new contaetor and shielding plug. PLANT DESIGN
It is not possible to examine the comparative economics of pulse columns and centrifugal contactors without considering how the choice affects other areas in the reprocessing chain and the overall plant design. For example, because pulse columns are long but slender, the process building for this type of contactor will tend to be taller than a plant based on centrifugal contactors. This, in turn, affects the arrangements of other process cells.
The work reported here is part of a broader study in which overall plant designs were developed for both solvent extraction alternatives (Costello and Levins, 1973). Figures 7 and 8 show plan views at ground level for the two designs. To reduce civil costs, cells have been arranged so that they share common shielding walls. In the centrifugal eontactor plant, the Active Liquor Cells have been aligned with the ShearLeach and Waste Evaporator Cells so that the centrifuges can be serviced by the main building crane.
Table 3 compares the process cell requirements for solvent extraction operations in the two plants. The compactness of the centrifugal contactor leads to a 25 ~o saving in cell volume. COSTING
Building costs for the solvent extraction cells were assessed by civil engineering consultants. Where cells shared common shielding walls, one half of the construction costs have been assigned to each adjoining cell. Equipment costs for both plant concepts were obtained by detailed assessment of materials plus labour charges. This approach was supplemented and verified by direct quotations from Australian fabricators on a number of representative vessels. Robatel, Genas, France supplied quotations for its centrifugal contactors. Equipment layouts were used to calculate piping costs from a compilation of pipe lengths, number of welds, bends, wall 11.3m
,r
i
"a"
I
I 9-2 rn' c,
, ~
,,
L
0.0m
'
L
;: ;!.:;T!H!ghActivity Liquo¢ Cell Low A ,,vl,y L,,uor Co,
Section "D- 13" Legend 1-10 See toble 2 11 Solids removal centrifuge
Fig. 5. Plan showing centrifugal contactor layout.
L
Comparative economics of pulse columns and centrifugal contactors
"" .~', ~;qh Ac tivity Liquor Cell ~
..E13 . 7 -
"I" 1E Steel Shielding _
81
Low A}:tivity Liquor i , • CellI PART SECTION A-A ~ ~
~ T
m °
+k.~
Jk.
~-'-I
m'=
PART SECTION"B-B'
PANT SECTION "C-C'
Fig. 6. Elevation view of centrifugal contactor~.
DISCUSSION penetrations and testing man-hours. Instrumentation and control costs were estimated by extracting Bernard et aL (1971) compared the economics of requirements from the engineering flow diagrams mixer-settlers, pulse columns and centrifugal conand applying a unit cost for each control function. tactors for the first extraction cycle of a 5 tonnes U/day reprocessing plant. Qualitatively their ECONOMIC COMPARISON breakdown is similar to that reported here; the A summary of solvent extraction cost estimates centrifugal contactor plant has highest costs for updated to mid-1974 Australian dollars is shown in process equipment but the lowest costs for building, Table 4. Building costs for the centrifugal contactor piping and instrumentation. However, their analysis plant are 25 ~o less than those for the pulse column shows a slight cost advantage (4 ~o) for the centriplant, in direct proportion to the savings in shielded fugal contactor plant. volume. Process piping is one-third less because of The principal discrepancy between this work and the compactness of the centrifugal contactor layout. that of Bernard et aL is the difference in chemical The additional cost of pulse column instrumentation equipment costs. The French estimate shows only results in a 20% increase in instrumentation and a relatively small saving (20~o) in equipment costs electrical costs for that design. for the pulse column plant. Another possible However, chemical process equipment costs in explanation for the discrepancy in overall costs is the pulse column plant are less than half those in the the rather unconventional layout of pulse columns centrifugal contactor plant and this more than in this study, which leads to a significant reduction offsets the cost disadvantages mentioned above. in building costs. Centrifugal contactor costs are typically about three The French study was based on a design capacity to four times those for pulse columns. Total solvent of 5 tormes U/day compared to 3 tonnes U/day in extraction costs are estimated to be 30yo more this work. Centrifugal contactors become more expensive when centrifugal contactors are used. economically attractive at high plant throughputs When these estimates are translated into total because criticality and scale-up problems arise as capital investment for a complete reprocessing pulse column diameter is increased. For very large facility, the choice of pulse columns for solvent pulse column plants, parallel duplication of extracextraction results in an 8 ~o capital saving (Costello tors will probably be necessary. and Levins, 1973). Other considerations that are difficult to cost
82
J . M . COSTELLOand D. M. LEVINS
To Fuel Storage Pond
1 2 3 4 5 6
Sheor-leoch cell Cladding disposal cell Evaporator cell Maintenance pond High Activity liquorcell Low Activity liquor cell
7 8 9 10 11 12
Acid recovery cell Pulse column well Solvent wash 0ff-gas treatment Plutonium ceil Uranium ceil
13 14 15 16 17
Plutonium pockaging Laboratories, offices, etc. Shear maintenance pond Pump aisles Analytical cells
Fig. 7. Ground level plan view of pulse column plant.
To Fu, Stora!
Legend See Figure 7
Fig. 8. Ground level plan view of centrifugal contactor plant.
Comparative economics of pulse columns and centrifugal contactors
83
Table 3. Process cell requirements Pulse column plant
High activity liquor cell Low activity liquor cell Pulse column well Solvent treatment cell
Centrifugal contactor plant
Area (In2)
Volume (mz)
Area (m2)
Volume (m3)
60 108 16 30 214 m S
258 557 335 309 1459 m 2
70 114 -45 229 m 2
297 588 -235 1120 m a
Table 4. Solvent extraction costs (mid 1974 Australian dollars*) Pulse column plant Buildings Chemical process equipment Process piping Instrumentation and electrical
$
644,000 1,295,000 824,000 502,000 $A3,265,000
Centrifugal contactor plant $
491,000 2,844,000 546,000 414,000 $A4,295,000
* In mid 1974 SA = 1.49 SU.S.
must also be weighed in making a decision on solvent extraction alternatives. For example, centrifugal contactors have a low residence time, so solvent damage from high performance fuels will be minimal. Recent research, however, suggests that the solvent damage problem is less than was previously thought (van Geel et aL, 1971). Centrifugal contactors require more maintenance than pulse columns, leading to a poorer utilization factor for a plant based on this type of extractor. Moreover, pulse columns have performed reliably in many plants whereas the long-term reliability of centrifugal contactors has yet to be fully demonstrated. CONCLUSIONS Centrifugal contactors have often been proposed as replacements for pulse columns in nuclear reprocessing plants based on advanced technology. However, estimates of capital cost show that these contactors cannot be economically justified for plants processing L W R fuels at plant capacities of 3 tonnes U]day or less. Maintenance requirements and long-term reliability are further uncertainties
associated with the operation of centrifugal contactors in a highly active environment. REFERENCES
Bernard, C., Michel, P. and Taruero, M. (1971) Proe. Int. Solv. Extr. Conf. (ISEC 71) 2(8]3), 1282. Bernstein, G. J., Grosvenor, D. E., Lent, J. F. and Levitz, N. M. (1973a) Argonne National Lab. Report, ANL-7968. Bernstein, G. J., Grosvenor, D. E., Lent, J. F. and Levitz, N. M. (1973b) Argonne National Lab. Report, ANL-7969. Costello, J. M. and Levins, D. M. (1973) Australian Atomic Energy Commission Report, AAEC[E275. De Witte, R. (1966) Atompraxis 12, 42. Van Geel, J., Joseph, C., Detilleux, E., Heinz, W., Centeno, J. and Gustafsson, B. (1971) Proc. lnt. Solv. Extr. Conf. (ISEC 71) 1(3C), 577. Joseph, C. J., van Geel, J., Detilleux, E. and Centeno, J. L. (1971) Proc. Int. Solv. Extr. Conf. (ISEC 71) 1(3C), 593, Long, J. T. (1967) Engineering for Nuclear Fuel Reprocessing. Gordon & Breach, N.Y. Nuclear Fuel Services (1962) USAEC Docket 50201. SGN-Robatel (1972) Multi-stage centrifugal contactor for liquid-liquid extraction (brochure). Saint-Gobain Techniques Nouvelles, Gcnas, France.