History. The UCOR process, developed by the Uranium Enrichment Corporation of South Africa, has been operated on a large pilot-plant scale at Valindaba, Union of South Africa. Partial information on the process, its separation factor and specific power demand, and its projected economics was given by Roux and Grant [R3]. The ingenious Helikon cascade technique developed for this process, in which a single axial-flow compressor handles several process streams simultaneously, was described by Grant et al. [G2] and analyzed theoretically by Haarhoff [HI]. Cost estimates, prepared in 1974 and converted to dollars with the purchasing power of that year, predicted that the capital cost of the 5000 MT/year plant would be $1,350 million, and that the cost of separative work from it, using electricity priced at 6 mills/kWh, would be $74/kg SWU. This cost was close to the price then charged by the U. S. Atomic Energy Commission.

The UCOR project is a major effort. In 1975, some 1200 persons were employed, and $150 million had already been spent on development. Extensive experiments had confirmed the separation performance and power consumption of individual stages. A “prototype module” with design separative capacity of 6000 kg SWU/year had been built and tested. The design of a full-scale prototype, expected to have a capacity of 50,000 kg SWU/year, was well advanced. On February 14, 1978, S. P. Botha, South African Minister for Mines and Industry, announced [B18] that South Africa would expand the pilot enrichment plant to meet domestic needs, but had abandoned plans to build a full-scale plant.

Description of process. Because many features of the process, including details of the separating element, have not been disclosed, this description is necessarily incomplete. The following partial description has been given by Roux and Grant [R3]:

The South African-ог UCOR-process is of an aerodynamic type. It has been possible to develop a separating element which in effect is a high performance stationary-walled centrifuge using UF6 in hydrogen as process fluid. All process pressures throughout the system will be comfortably above atmospheric and depending on the type of “centrifuge” used, the maximum process pressure will be in a range of up to 600 kPa (6 bar). The UF6 partial pressure will however be sufficiently low to eliminate the need for process heating during plant operation, and the maximum temperature at the compressor delivery will not exceed 75°C.

The process is characterised by a high separation factor over the element, namely from

1.25 to 1.030 depending on economic considerations. Furthermore it has a high degree of asymmetry with respect to the UF6 flow in the enriched and depleted streams, which emerge at different pressures. The feed to enriched stream pressure ratio is typically 1.5 whereas the feed to depleted stream pressure ratio is typically only 1.12.

To deal with the small UF6 cut, a new cascade technique was developed, the so-called “helikon” technique, based on the principle that an axial flow compressor can simultaneously transmit several streams of different isotopic composition without there being significant mixing between them. The UCOR process must therefore be regarded as a combination of the separation element and this technique, which makes it possible to achieve the desired enrichment with a relatively small number of large separation units by fully utilising the high separation factor available. . . .

The theoretical lower limit to the specific energy consumption of the separation element can be shown to be about 0.30 MWh/kg USW. The minimum figure we have been able to obtain with laboratory separating elements is about 1.80 MWh/kg USW, based on adiabatic compression and ignoring all system inefficiencies. Although we do not believe that the present energy consumption can, in the short term be drastically reduced, the discrepancy between the above figures illustrates that the UCOR process still has a large development potential.

In discussion following presentation of the above information, the actual power consumption of a complete UCOR plant, allowing for pressure drops, and other process inefficiencies, was given as 3.5 MWh/kg SWU, or 0.40 kW/(kg SWU/year). This is to be com­pared with 0.50 estimated by Geppert [Gl] for a complete nozzle plant and 0.266 for the improved U. S. gaseous diffusion plants.

An additional important bit of process information, from Grant et al. [G2], is: “For the UCOR process, the cut is typically 0.045 to 0.055.” Figure 14.29 is a flow sheet for one stage of the UCOR process on which the preceding information has been represented, with a particular cut of в = 0.050. This cut requires use of a 19-up, 1-down cascade. The only important process variable not stated in published information is the UF6 content of the mixture with hydrogen fed to the stage. As will be shown in the next section, a UF6 feed composition of 0.032 mole fraction is consistent with the reported process information.

Enriched stream to stage i-i-19, pressure * (УІ5

Depleted stream from stage i*l, pressure = p/l.12

Enriched stream (Л from stage і -19, pressure * p/l.5

I Depleted stream to stage і — I, pressure * p/l. 12

Cut: 0.045 < в < 0.055 Separation factor: 1.025 < a: < 1.030 Specific power: Q/A — і.80 MWh/kg SWU Temperatures < 75°C

Theoretical analysis of UCOR process. Because the UCOR process has been characterized [R2] as a “stationary-wall centrifuge,” its performance for 23SUF6/238UF6 separation can be repre­sented by Eq. (14.276). The speed parameter A2 is related to the given pressure ratio р/р =

1.5 by

as may be seen from Eqs. (14.288) and (14.289).

In the UCOR process, unlike the separation nozzle process, the depleted stream is recompressed through a smaller pressure ratio (1.12) than the enriched stream (1.5). Hence, to evaluate the energy used in compression it is necessary to know the hydrogen cut 0H, the fraction of hydrogen fed that leaves in the enriched stream, and the composition of the enriched stream represented by the mole fraction fx of UF6 in it. A development analogous to the one that led to Eq. (14.271) for the UF6 cut results in Eq. (14.296) for the hydrogen cut:

because the molecular weight of hydrogen is mH =2. Because the fraction of flow area used by the enriched stream, c2/a2, is given by (14.272), the hydrogen cut 0H is related to the UF6 cut в by

л [6 + (1 — 6) exp (—A2)] ‘^176 — exp (—A2/176) вн= —

The mole fraction UF6 in the enriched stream 1, Fig. 14.29, is

_ Bf

Bf+e„( 1 — f) and the moles of UF6 (M) plus hydrogen (MH) in the enriched stream per mole of UF6 fed (M

+ N) is

Temperatures at points 1, 3, 6, 7, and 8 are assumed to equal T, the temperature of feed to the separating element. Then, the power input from compression is

K = (M + MH+N + NH) (T5 — T) (14.306)

because the heat capacity yR/{y — 1) = Cp is a linear function of mole fraction, Eq. (14.281). The energy input in joules per kilogram of UF6 fed, K/Z, is

the adiabatic, reversible energy input in joules per kilogram separative work is

К 2yR(TS — T)

A " 238(7 — 1)/ 6(1 -0X«-l)a

From an assumed feed temperature T = 313 K, a UF6 cut в = 0.05, and the stated expansion pressure ratios of 1.12 and 1.5 for the heavy and light fractions, a value for the mole fraction of UF6 in feed of / = 0.03225 was found by trial to lead to the value of 1.80 MWh/kg SWU given by Roux and Grant [R2] for the energy per kilogram uranium separative work.

Table 14.19 summarizes the steps in the calculation of compositions, properties, and flow rates of the numbered streams in Fig. 14.29, and from them, the energy per kilogram of uranium fed, the separation factor, and the separative work.

The following should be noted:

1. The high hydrogen cut, 0.73, coupled with the low UF6 cut, 0.05, causes the mole fraction UF6 in the enriched stream, 0.0029, to be much lower than in the feed, 0.032, and the mole fraction UF6 in the depleted stream, 0.105, to be much higher.

2. For every mole of UF6 fed, 21.9 mol of enriched stream and 9.1 mol of depleted stream are processed.

3. The maximum calculated temperature, 340.35 K, provides margin below the 75°C (348 K) maximum temperature cited by Roux and Grant, to allow for process inefficiencies.

4. The heavy fraction containing 0.105 mole fraction UF6 would start to condense at a pressure of 3.8 bar at 313 K. Hence the pressure of the heavy stream must be below this value and the feed pressure, p, must be below (1.12X3.8) = 4.3 bar. This pressure is much higher than the subatmospheric pressures reported for the nozzle process and would result in much lower volumetric flow rates in a UCOR plant than in a nozzle plant of the same separative capacity.

Table 14.19 Steps in calculating separation performance of UCOR process Variable Symbol Equation Value Mole fraction UF6 in feed f Assumed 0.03225 Temperatures to stage T і and Tз Assumed 313K Molecular weight feed m (14.283) 13.3036 R/Cp of feed (Г-П/7 (14.281 & 2) 0.259363 Speed parameter A2 (14.295) 11.31261 UF6 cut в Given 0.05 Hydrogen cut Єн (14.298) 0.728919 Mole fraction UF6 in enriched stream /і (14.299) 0.0022807 R/Cp of enriched stream (Ті — D/Ті (14.281 & 2) 0.286761 Mole fraction UF6 in depleted stream /з (14.301) 0.104573 R/Cp of depleted stream (Тз — D/Тз (14.281 & 2) 0.210766 Moles enriched stream^ Ш + МН)/(М + Ю (14.300) 21.9232 Moles depleted stream ^ (N + МН)/(М + Ю (14.302) 9.0845 Compression ratio, heads compressor Рі/Рі Given 1.5/1.12 Temperature from heads compressor т2 (14.303) 340.3507 К Temperature to feed compressor т* (14.304) 330.4904 К Compression ratio, feed compressor Ps/Pa Given 1.12 Temperature from feed compressor Ts (14.305) 340.3488 К Energy, MWh/kg U fed К/3.6 X 10* z (14.307) 3.1728E-5 Separation factor a (14.276) 1.027239 kg U separative work/kg U fed д/z (14.308) 1.7622E-5 MWh/kg SWU *73.6 X 109 A (14.309) 1.8005 ^Per mole UF6 fed.

5. The calculated separation factor of 1.0272 is in the range 1.025 to 1.03 cited by Roux and Grant and is higher than optimum in the nozzle process.

6. The value of A2 = 11.31 calculated for wheel flow is sufficiently high that even if the effective gas speed were well below that corresponding to the stated expansion ratio of 1.5, the separation factor would not be much below the calculated 1.027 value.

7. The specific power of 1.80 MWh/kg SWU, with no allowance for process inefficiencies, is equivalent to 0.205 kW/(kg SWU/year). This may be compared with 0.168 for gaseous diffusion (Table 14.9), and 0.31 for the nozzle process (Fig. 14.23). The higher value for the nozzle process may be due to its expanding the heavy stream through the full pressure ratio.

UCOR process equipment. The low cut, в = 0.045 to 0.055, selected for the UCOR process requires use of more stages than the gaseous diffusion or nozzle process, despite the higher UCOR separation factor. To reduce the number of independent items of process equipment, the UCOR process uses the ingenious Hilikon technique to consolidate as many as 20 stages in a single independently operable unit. Figures 14.30, 14.31, and 14.32, adapted from UCOR publications [G2, HI], provide a partial description of the Helikon principle and the process equipment used in it.

Each Helikon module uses two axial-flow compressors, one for the enriched streams (point 1, Fig. 14.29) and a second for the feed streams (point 4). The nature of flow through this type of compressor is such that there is rather little mixing of material fed into the barrel at one angular position with material of another composition fed at another angular position. Such streams of different composition flow through the compressor in helical paths and leave the compressor still relatively unmixed.

Figure 14.30 shows how the inlet end of the compressor would be divided into sectors to handle the streams fed to three stages with 23SU fractions increasing in the order zx < z2 < z3. Each feed stream is divided into two halves which are introduced symmetrically about plane AA through the axis into sectors formed by radial partitions. In this way, composition differences between adjacent streams are minimized. The partitions stop at the inlet rotor blades and begin again after the outlet blades. To deal with possible helical displacement during compression, the

Figure 14.30 Introduction of three streams of different 23SU content Zi <z2 <z3 into axial-flow compressor.

Figure 14.31 Schematic representation of flow through stage і of p-up, one-down Helikon module. (Reproduced with permission of the copyright holder, American Institute of Chemical Engineers, and Dr. W. L. Grant.)

outlet partitions may be displaced through an appropriate angle. In the UCOR plant with a cut of 55, 38 (2 X 19) sectors would be used.

The flow path through one sector of a Helikon module, containing all equipment of stage і except the light-stream compressor, is shown in Fig. 14.31. Depleted stream from stage і + 1 and enriched stream from stage і — p, both at intermediate pressure, are mixed and fed into one sector at the compressor inlet. At the compressor discharge the compressed feed is

Figure 14.32 Flow between modules of three-up, one-down Helikon cascade. (Reproduced with permission of the copyright holder, American Institute of Chemical Engineers, and Dr. W. L. Grant.)

collected in the appropriate sector, passed first through a stage cooler, and then through the separating element where it is divided into the low-pressure enriched stream and the intermediate-pressure depleted stream.

The enriched stream from each sector is transported to the enriched stream compressor for stage і + p in the module handling the next higher enrichment. The depleted stream is rotated by deflecting plates into the feed stream of stage / — 1 of the same module, or if from the least enriched stage, is sent to the highest stage of the module handling the next lower enrichment.

To illustrate the Helikon principle, flow between two adjacent modules of a three-up, one-down Helikon cascade is shown schematically in Fig. 14.32. The upper half shows the flow of depleted streams from one stage to the next lower stage; the lower half shows the flow of enriched streams from a sector of one module to the corresponding sector of the module of next higher enrichment. Because the figures are symmetric about the plane AA, the other half of the flow paths are not shown.

To permit construction of a complete plant with one size, or at most a few sizes, of compressor, while providing the variation in stage throughput desirable in an ideal cascade, it is proposed that the number of sectors in a module be varied to provide a smaller number of large sectors near the feed point and a larger number of small sectors toward the product and tails ends of the cascade.

Experiments reported by Grant et al. [G2] have shown that mixing of streams of different composition in an axial flow compressor can be kept acceptably low.

The number of stages needed for a given overall enrichment is inversely proportional to 6(a — 1). Because of its low cut the UCOR process needs more stages than the separation nozzle or gaseous diffusion process, despite its higher separation factor. This potential disad­vantage is dealt with by the Helikon technique, which combines a number of stages into a single module. Table 14.20 compares the gaseous diffusion process design of Table 14.9, the improved separation nozzle process of Table 14.18, and the UCOR process of Table 14.19 with respect to cut, separation factor, number of stages in an ideal cascade producing product containing 3 percent 235U and tails containing 0.25 percent 23SU, and the number of modules for such a UCOR plant cited by Grant et al. [G2].