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Fuel Assembly Instrument Tube Tie-Rods

[+] Author Affiliations
John D. Wood, Larry Montgomery

Westinghouse Electric Company, LLC, Monroeville, PA

Paper No. ICONE16-48913, pp. 333-339; 7 pages
doi:10.1115/ICONE16-48913
From:
  • 16th International Conference on Nuclear Engineering
  • Volume 1: Plant Operations, Maintenance, Installations and Life Cycle; Component Reliability and Materials Issues; Advanced Applications of Nuclear Technology; Codes, Standards, Licensing and Regulatory Issues
  • Orlando, Florida, USA, May 11–15, 2008
  • Conference Sponsors: Nuclear Engineering Division
  • ISBN: 0-7918-4814-0 | eISBN: 0-7918-3820-X
  • Copyright © 2008 by Westinghouse Electric Company

abstract

In March 2001, a 17×17 fuel assembly top nozzle separated from a fuel assembly while being handled in the spent fuel pool and suspended from the spent fuel handling tool. The failure mechanism was later determined to be Inter-granular Stress Corrosion Cracking (IGSCC) of the stainless steel sleeves that provide the connection between the top nozzle and the thimble tubes to form the fuel assembly skeleton. The IGSCC was attributed to spent fuel pool chemistry. Further investigation found that the failure occurred at the bulge joints which connect the 304 sleeves to the zirconium thimble tubes. The corrosion occurred at the peak of the bulge in a high stress region. The susceptible fuel assemblies are the ones having their sleeves fabricated out of “304” stainless steel. The sleeves were changed to “304L” stainless steel to avoid this risk with subsequent fuel. There are more than 18,000 IGSCC susceptible fuel assemblies in spent fuel storage pits at approximately 25 different plants. In order to meet handling needs for IGSCC affected fuel, Westinghouse developed a solution known as the Instrument Tube Tie-Rod (ITTR) as a solution for long term handling and storage. The ITTR is a metal tube that extends from the top nozzle adapter plate through the instrument tube to the bottom nozzle. It then acts as a means to reinforce the connection between the top nozzle and the balance of the fuel assembly. The ITTR is only applicable to 15×15 and 17×17 fuel types because the center location of the instrument tube. The ITTR has been designed to carry the weight of the fuel assembly during lifting/handling so that in the event that a top nozzle separates from the fuel assembly, the load is transferred directly from the top nozzle to the bottom nozzle. This design allows the fuel assembly to be handled with the standard spent fuel handling tool and procedures. Utilizing the instrument tube for the ITTR installation also allows fuel assembly core components, such as RCCAs, Thimble Plugs and Burnable Poison Assemblies to be stored in or removed from the fuel since the thimble tubes remain open. Westinghouse’s ITTR solution not only returns and ensures the structural integrity of an IGSCC affected fuel assembly (FA) while stored in either the spent fuel pool or in a dry cask but also allows any insert design to be stored in the fuel assembly. The following paper will highlight the considerations that Westinghouse undertook while in development of the ITTR project. These include the fundamental design considerations for the Instrument Tube Tie-Rods, analysis and installation tooling system used to install these at a reactor site.

Copyright © 2008 by Westinghouse Electric Company

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