11 Subsection NG—Core Support Structures

This chapter provides commentary and some practical examples on the materials, design, fabrication, installation, and examination requirements for core support structures (CSS) in pressurized water reactors and boiling water reactors in Section III, Division 1, Subsection NG of the ASME Boiler and Pressure Vessel Code. It begins with background information on the development of Subsection NG and the definition of CSS, internal structures (IS), and threaded structural fasteners (TSF). The chapter then outlines the jurisdictional boundaries between CSS and reactor pressure vessels and the unique service loadings for reactor internals. The chapter details the material requirements in NG-2000; describes the examples of typical materials used in CSS, IS, and TSF; and discusses the salient subarticles of NG-3000 with regards to the design intent. Guidance is provided on specifying load combinations for the various service loadings and on development of the Owner’s Design Specification and Code Design Report. The chapter lists the requirements for fabrication and installation, including forming, fitting, and aligning; examining and repairing welds; and heat treatment and mechanical joining. The differences between NG-5000 and NB-5000 in the context of examination and repair during manufacture are also summarized. In addition, the chapter outlines requirements for hot functional testing for reactor terminals and the environmental effects that can lead to premature failures in CSS and IS. It also summarizes some notable Code Cases applicable to Subsection NG as well as some notable interpretations that apply to Subsection NG, most of which are related to materials, inspection procedures, and documentation. Finally, the chapter discusses the current and future issues and recommendations associated with Subsection NG in the design and manufacture of CSS and IS.