Impacts to Project from {sup 99}Tc Contamination on the K-25 / K-27 Decontamination and Decommissioning Project
- Bechtel Jacobs Company LLC, PO Box 4699, Oak Ridge, TN 37831 (United States)
Like so many good ideas that evolved as the U.S. was learning about nuclear materials, the introduction of recovered uranium from the fuel reprocessing efforts is causing additional headaches in the decontamination and decommissioning (D and D) of the gaseous diffusion plants at the East Tennessee Technology Park in Oak Ridge, Tennessee. Technetium-99 ({sup 99}Tc) is a low-energy beta emitter that is a product of the fission reaction. Its introduction into the in an operating gaseous diffusion plant is of little or no operational or dose concern but creates many issues when planning and conducting the D and D of those facilities. These issues include control of contamination and beta exposure to workers and significant waste management issues when trying to ship and dispose of the waste materials. Technetium-99 is very long lived and mobile in the environment and is therefore has very restrictive concentration limits in wastes going to disposal facilities. The gaseous diffusion process was developed for isotopic enrichment of uranium. Gaseous uranium hexafluoride (UF{sub 6}) was passed through a series of stages to affect the separation of the fissionable U-235 from U-238. Because of the low concentration of U-235 in naturally occurring uranium ({approx}0.711%), the number of separation stages required to enrich the uranium to a level usable for reactor fuels or nuclear weapons was quite large. For instance, the K-25 Building, where uranium was enriched, is a U-shaped structure approximately one mile in length. Similar sizes of stages were grouped in cells, and up to 14 cells were contained in a single building unit. Multiple building units were arranged in series to process the uranium, with the capability of isolation and bypass of each cell. This arrangement of multiple stages was referred to as the cascades. Since UF{sub 6} is a heavy molecule, lighter gases that were contained in the process gas stream moved at a higher rate than UF{sub 6}. These gases tended to quickly flow to the latter stages of the cascade (also known as the 'top of the cascade'). The removal of certain undesired lighter gases was done in the purge cascades. Technetium forms volatile fluoride compounds under the conditions used to chemically convert uranium oxide (UO{sub 2}) in reactor tails to UF{sub 6}. Thus, Tc vaporized and flowed with the process gas in the cascade. However, because Tc is much lighter than U, it rapidly migrated to the top of the cascade and into the purge cascades. Most of the Tc is thought to be located in the purge cascade equipment. The purge cascade equipment is generally located in Area 1 of K-25 and Areas 7 and 8 in K-27. Fourteen building units are now known or believed to be contaminated with {sup 99}Tc. During the demolition activities, the equipment used is expected to become contaminated. To prevent cross-contamination of other wastes with {sup 99}Tc, separate equipment will be used for the southern end of the K-25 east wing. This same demolition equipment will be employed in K-27 to help minimize equipment decontamination efforts at the end of the project. However, because of this sequential schedule, any delays for demolition of the K-25 Building will adversely impact the schedule for K-27. The {sup 99}Tc contamination of the K-25 and K-27 D and D Project is now better known, but considerable planning will still be needed to control migration of this material to meet radiological controls and waste management limits. Since {sup 99}Tc has been an emerging issue over the last decade, the measurement, controls, and ultimately the impacts (e.g., cost and schedule) to the project have caused a significant increase in the planning effort for the D and D of these facilities and are expected to increase the ultimate cost of remediation. The cost increase will not be known until the planning effort is restarted (probably in FY 08-09). The contamination controls to be put in place to minimize the spread of this contaminant are not expected to be fully demonstrated until the demolition of the affected areas later in the project.
- Research Organization:
- American Nuclear Society, 555 North Kensington Avenue, La Grange Park, Illinois 60526 (United States)
- OSTI ID:
- 21144228
- Resource Relation:
- Conference: DD and R 2007: ANS Topical Meeting on Decommissioning, Decontamination, and Reutilization 2007, Chattanooga, TN (United States), 16-19 Sep 2007; Other Information: Country of input: France; Related Information: In: Proceedings of the 2007 ANS Topical Meeting on Decommissioning, Decontamination, and Reutilization - DD and R 2007, 336 pages.
- Country of Publication:
- United States
- Language:
- English
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