NEF Safety Analysis Report-1.1.3 Process Descriptions
1.1.3 Process Descriptions
This section provides a description of the various processes analyzed as part of the Integrated Safety Analysis. A brief overview of the entire enrichment process is provided followed by an overview of each major process system. Additional details are provided in Chapter 3, IntegratedSafety Analysis Summary.
18.104.22.168 Process Overview
The enrichment process at the NEF is basically the same process described in the SAR for the Claiborne Enrichment Center (LES, 1991). The Nuclear Regulatory Commission (NRC) staff documented its review of the Claiborne Enrichment Center license application and concluded that LES’s application provided an adequate basis for safety review of facility operations and that construction and operation of the Claiborne Enrichment Center would not pose an undue risk to public health and safety (NRC, 1993). The design of the NEF incorporates the latest safety improvements and design enhancements from the Urenco enrichment facilities currently operating in Europe.
The primary function of the facility is to enrich natural uranium hexafluoride (UF6) by separating a feed stream containing the naturally occurring proportions of uranium isotopes into a product stream enriched in 235U and a tails stream depleted in the 235U isotope. The feed material for the enrichment process is uranium hexafluoride (UF6) with a natural composition of isotopes 234U, 235U, and 238U. The enrichment process is a mechanical separation of isotopes using a fast rotating cylinder (centrifuge) based on a difference in centrifugal forces due to differences in molecular weight of the uranic isotopes. No chemical changes or nuclear reactions take place. The feed, product, and tails streams are all in the form of UF6.
22.214.171.124 Process System Descriptions
An overview of the four enrichment process systems and the two enrichment support systems is discussed below. Numerous substances associated with the enrichment process could pose hazards if they were released into the environment. Chapter 6, Chemical Process Safety, contains a discussion of the criteria and identification of the chemicals of concern at the NEF and concludes that uranium hexafluoride (UF6) is the only chemical of concern that will be used at the facility. Chapter 6, Chemical Process Safety, also identifies the locations where UF6 is stored or used in the facility and includes a detailed discussion and description of the hazardous characteristics of UF6 as well as a detailed listing of other chemicals that are in use at the facility. Additional details on each of the enrichment process systems are provided in Chapter 3, Integrated Safety Analysis Summary. The enrichment process is comprised of the following major systems:
UF6 Feed System
he first step in the process is the receipt of the feed cylinders and preparation to feed the UF6 through the enrichment process. Natural UF6 feed is received at the NEF in 48Y or 48X cylinders from a conversion plant.
Pressure in the feed cylinders is below atmospheric (vacuum) and the UF6 is in solid form. The function of the UF6 Feed System is to provide a continuous supply of gaseous UF6 from the feed cylinders to the cascades. There are six Solid Feed Stations per Cascade Hall; three stations in operation and three on standby. The maximum feed flow rate is 187 kg/hr (412 lb/hr) UF6 based on a maximum capacity of 545,000 SWU per year per Cascade Hall.
The function of the Cascade System is to receive gaseous UF6 from the UF6 Feed System and enrich the 235U isotope in the UF6 to a maximum of 5 w/o. Multiple gas centrifuges make up arrays called cascades. The cascades separate gaseous UF6 feed with a natural uranium isotopic concentration into two process flow streams – product and tails. The product stream is the enriched UF6 stream, from 2 – 5 w/o 235U, with an average of 4.5 w/o 235U. The tails stream is UF6 that has been depleted of 235U isotope to 0.20 – 0.34 w/o 235U, with an average of 0.32 w/o 235U.
Product Take-off System
The function of the Product Take-off System is to provide continuous withdrawal of the enriched gaseous UF6 product from the cascades and to purge and dispose of light gas impurities from the enrichment process.
The product streams leaving the eight cascades are brought together into one common manifold from the Cascade Hall. The product stream is transported via a train of vacuum pumps to Product LTTS in the UF6 Handling Area. There are five Product LTTS per Cascade Hall; two stations in operation and three stations on standby.
The Product Take-off System also contains a system to purge light gases (typically air and hydrogen fluoride) from the enrichment process. This system consists of UF6 Cold Traps which capture UF6 while leaving the light gas in a gaseous state. The cold trap is followed by product vent Vacuum Pump/Trap Sets, each consisting of a carbon trap, an alumina trap, and a vacuum pump. The carbon trap removes small traces of UF6 and the alumina trap removes any hydrogen fluoride (HF) from the product gas.
Tails Take-off System
The primary function of the Tails Take-off System is to provide continuous withdrawal of the gaseous UF6 tails from the cascades. A secondary function of this system is to provide a means for removal of UF6 from the centrifuge cascades under abnormal conditions.
The tails stream exits each Cascade Hall via a primary header, goes through a pumping train, and then to Tails LTTS in the UF6 Handling Area. There are ten Tails LTTS per Cascade Hall. Under normal operation, seven of the stations are in operation receiving tails and three are on standby. In addition to the four primary systems listed above, there are two major support systems:
Product Blending System The primary function of the Product Blending System is to provide a means to fill 30B cylinders with UF6 at a specific enrichment of 235U to meet customer requirements. This is accomplished by blending (mixing) UF6 at two different enrichment levels to one specific enrichment level.
The system can also be used to transfer product from a 30B or 48Y cylinder to another 30B cylinder without blending. This system consists of Blending Donor Stations (which are similar to the Solid Feed Stations) and Blending Receiver Stations (which are similar to the Product LTTS) described under the primary systems.
Product Liquid Sampling System
The function of the Product Liquid Sampling System is to obtain an assay sample from filled product 30B cylinders. The sample is used to validate the exact enrichment level of UF6 in the filled product cylinders before the cylinders are sent to the fuel processor. This is the only system in the NEF that changes solid UF6 to liquid UF6.