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The Gaseous Diffusion Process

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The Gaseous Diffusion Process The Gaseous Diffusion Process The Portsmouth Gaseous Diffusion Plant, built in 1953-1954, produces enriched uranium hexafluoride (UF6) for commercial power reactors. In the past highly enriched uranium hexafluoride 235 was produced for the U.S. Navy. Enriched UF6 is material that contains the isotope U in assays (concentrations) greater than 0.711 weight percent. Commercial power plants currently use uranium enriched to the 3 to 5 percent range. The plant is capable of producing a range of U235 assays from 0.2% to over 97%. In late 1991, the equipment that produced high assay uranium was taken off line and placed in long term shutdown. High assay material that was declared excess by the US Navy and Department of Energy was fed into the cascade and blended down to the assays used by commercial power plants. This refeed project was completed in July 1998. It is possible that other high assay material will be declared excess in the future and would possibly be blended down at PORTS. It is also possible that high assay production could be resumed in the future. The plant's certification under the Nuclear Regulatory Commission (NRC) limits the maximum assay to 10%. Enrichment is accomplished using gaseous diffusion technology as the name implies. The gaseous diffusion cascade consists of many stages of equipment connected in series, i.e., in a cascade arrangement. Each stage consists of an electric motor, a compressor, and a converter which contains 235 a porous membrane through which the lighter U F6 diffuses at a slightly faster rate than the heavier 238 U F6. Separation takes place at a rate which is expressed as the square root of the ratio of the molecular weights of the materials being separated. For UF6, the degree of enrichment in each stage is about 1.004 (the square root of 352/349). Thus many stages of equipment are required. The cascade contains over 4140 stages of equipment which are connected in series. At the current level of enrichment, 2760 stages are required, including purge cascades. Stages are grouped into cells, a cell being the smallest group of equipment that can be removed from service for maintenance. Cells are further grouped into units and units into buildings. Each cell contains bypass piping and inlet and outlet block valves to allow a cell to be taken off stream for maintenance or repair. All of the equipment in a unit is the same size; there are five sizes of equipment which are designated (in order of decreasing size) as 33 (or 000), 31 (or 00), 29 (or 0), 27 and 25. The X-333 Building contains only one size of equipment (33 size) while the X-330 Building contains 31 and 29 size equipment and X-326 Building contains 27 and 25 size equipment. The largest equipment is at the bottom, i.e., the lowest enrichment, of the cascade and the smallest equipment is at the top. The largest equipment uses 3300 hp motors and the smallest uses 15 hp motors with several steps in between. The process piping which connects the various pieces of equipment ranges up to 42 inches in diameter. Virtually all connections in the cascade are welded to reduce the probability of leakage. Removing any major component besides the motor involves the use of cutting torches; the replacement equipment must be welded in place. f:\starship\docs\cascade.wpd 1 The process begins with the arrival of natural assay UF6 from a fluorination plant or slightly enriched UF6 (currently 1.9%) from USEC's Paducah (KY) Gaseous Diffusion Plant. In accord with treaties and agreements made at the national level, USEC may also receive low enriched (LEU) uranium from Russia. This material may be sold as is or it may be fed to the cascade. The UF6 is received in 10- or 14-ton cylinders which are heated in steam autoclaves in the X-342 or X-343 Feed Vaporization Facilities to vaporize the UF6. The UF6 is fed to the cascade as a gas. Compressors drive the process gas (UF6) through the porous membrane in each stage. The enriched portion which passed through the porous membrane travels up the cascade while the depleted stream is returned to the intake of the compressor two stages below for recycling. The process of compressing the gas creates large quantities of heat which must be removed. This is accomplished by means of a three stage cooling process. Each converter contains a cooler which is flooded with liquid Refrigerant-114 (R-114) which is vaporized due to the heat it absorbs from the process gas. The refrigerant then rises to a water-cooled condenser where it is condensed and runs back to the converter. All of the cooling systems are thermosyphon systems which contain no moving parts. The refrigerant is cooled in the condensers by recirculating cooling water which is pumped under pressure to large cooling towers where the heat is removed by air flowing upward through water falling through the cooling towers. R-114 is also known as CFC-114 and Freon-114, Freon being a registered DuPont trademark for its original line of refrigerant products. The production of R-114, a Class I ozoned-depleting substance, has been banned since January 1, 1996, and is now available only as recycled material. The uranium enrichment plants have conducted extensive research into replacement coolants and have identified several potential replacements, three of which are under active consideration - perfluorobutane (C4F10), perfluorocyclobutane (C4F8), and perfluorotetrahydrofuran, also called perfluoro-THF (C4F8O). All three substances have received USEPA approval under the Significant New Alternative Policy (SNAP). One of these substances may be introduced as supplies of R-114 are depleted. Current plans call for deploying the replacement coolant at the Paducah, KY plant and shipping R-114 from Paducah to PORTS. However, the replacement coolant may also be deployed at PORTS. Small amounts of the replacement coolant will be introduced into the cascade at PORTS as impurities in the partially enriched UF6 received from Paducah for further enrichment. The plant's process coolant systems contain approximately 6.5 million pounds of R-114. Some of this refrigerant leaks to atmosphere and some leaks into the process gas where it is an impurity. Due to its low molecular weight, it moves to the top of the cascade where it accumulates in a "bubble" behind air, nitrogen, and other light gases. The accumulation of excessive amounts of refrigerant in the bubble degrades cascade performance. In order to more efficiently purge the refrigerant from the cascade, a side stream device called the Freon® Degrader is located within the Top Purge Cascade. Gas from the refrigerant bubble is shunted to an electrically heated furnace into which fluorine gas is introduced. The resulting reaction breaks the R-114 (MW = 171) into several lighter molecular weight substances, mostly carbon tetrafluoride (MW = 88) in order to more easily purge it from the cascade. The degradation products are returned to the top purge cascade. The Freon® Degrader may or may not be used depending on operating conditions. 2 After processing, the product (enriched uranium) and tails (depleted uranium) are withdrawn from the cascade into 10-ton and 14-ton cylinders respectively and allowed to solidify. Product withdrawal occurs in the X-333 Low Assay Withdrawal Area (LAW) or the X-326 Extended Range Product Withdrawal Area (ERP). Tails withdrawal occurs in the X-330 Tails Withdrawal Area (Tails). The product cylinders are then taken to the X-344 Toll Enrichment Facility where they are again heated and sampled to verify the enrichment and determine customer charges. The product is then transferred into 2.5 ton customer cylinders. The customer cylinders are placed into protective overpacks and shipped to either the customer or a fuel fabrication plant. Several ancillary processes are associated with the gaseous diffusion cascade; without them, the cascade would not be able to operate. These are the cold recovery systems, the building wet air evacuation systems, the seal exhaust systems, and the purge cascades. Each of these systems has a vent to atmosphere and each has a continuous stack sampler which operates continuously any time the source is in operation. These systems will be discussed in the following sections. Table 1 lists the radiological vents from the gaseous diffusion cascade. Table 1. Gaseous Diffusion Cascade Vents OEPA Source Name PORTS Source Stack ID P458 X-326 Top Purge Vent (X-326-P-2799) P459 X-326 Side Purge Vent (X-326-P-2798) P460 X-330 Cold Recovery/Building Wet Air Evacuation (X-330-P-272) P461 X-333 Cold Recovery Vent (X-333-P-852) P462 X-333 Building Wet Air Evacuation Vent (X-333-P-856) P424 X-333 Seal Exhaust System Area 1 (X-333-A-851) P425 X-330 Seal Exhaust System Area 2 (X-330-A-262) P426 X-330 Seal Exhaust System Area 3 (X-330-A-279) P427 X-326 Seal Exhaust System Area 4 (X-326-A-512) P465 X-326 Seal Exhaust System Area 5 (formerly P428) (X-326-A-528) P429 X-326 Seal Exhaust System Area 6 (X-326-A-540) 3 Top and Side Purge Cascades The Top and Side Purge Cascades are an integral part of the gaseous diffusion cascade; it would not operate properly without them. The UF6 process gas contains small amounts of other gases which were either deliberately or inadvertently introduced. Wet atmospheric air continuously leaks into the cascade past the compressor seals and through cracks and pinholes that sometimes develop. UF6 reacts with the moisture in the air to form gaseous hydrogen fluoride (HF) and uranyl fluoride (UO2F2) which is solid at operating temperatures.
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