scope:

ASPECTS OF CONSTRUCTION COVERED BY THESE RULES

Subsection NG establishes rules for materials, design, fabrication, examination, and preparation of reports required in the manufacture and installation of core support structures.^1,2

DEFINITION OF STRUCTURES AND APPLICATION OF THESE RULES TO THEM

Core Support Structures

Core support structures shall be constructed to the rules of this Subsection. Core support structures are those structures or parts of structures which are designed to provide direct support or restraint of the core (fuel and blanket assemblies) within the reactor pressure vessel. Structures which support or restrain the core only after the postulated failure of core support structures are considered to be internal structures (NG-1122).

Internal Structures

(a) Internal structures are all structures within the reactor pressure vessel other than core support structures, fuel³ and blanket assemblies, control assemblies, and instrumentation.

(b) The rules of this Subsection apply to internal structures as defined in (a) above, only when so stipulated by the Certificate Holder manufacturing core supports, hereafter referred to in this Subsection as Certificate Holder.

(c) The Certificate Holder shall certify² that the construction of all internal structures is such as not to affect adversely the integrity of the core support structure.

Temporary Attachments

A temporary attachment is an element in contact with or connected to the core support structure, which is removed prior to operation. Temporary attachments include items such as alignment lug tie straps and braces.

BOUNDARIES OF JURISDICTION APPLICABLE TO THIS SUBSECTION

Boundary Between Core Support Structure and Reactor Pressure Vessel

The jurisdictional boundary between a core support structure and the reactor pressure vessel shall be the surface of the core support structure. The first connecting weld of a core support structure to the reactor pressure vessel shall be considered part of the reactor pressure vessel unless the weld is more than 2t from the pressure retaining portion of the reactor pressure vessel, where t is the nominal thickness of the pressure retaining material. Beyond the first connecting weld to the reactor pressure vessel, or beyond 2t from the pressure retaining portion of the reactor pressure vessel, the first weld shall be considered part of the core support structure, unless otherwise specified in the Design Specification. Mechanical fasteners used to connect a core support structure to the reactor pressure vessel shall meet the requirements of this Subsection. Figure NG-1131-1 is provided as an aid in defining the boundary and construction requirements of this Subsection.

Boundary Between Core Support Structure and Internal Structure

(a) Internal structures may bear on or may be welded, cast, or fastened to core support structures.

(b) The jurisdictional boundary between a core support structure and an internal structure is the surface of the core support structure. The means by which the internal structure is connected to the core support structure shall fall within the jurisdiction of this Subsection.

(c) One or more portions of a casting may be classified as core support structures and different portions of the same casting may be classified as internal structures. The portions of the casting so classified shall be defined by the Design Specification or on the drawing. The entire casting (core support and internal structural portions) shall meet the material property requirements of NG-2000 with the additional nondestructive examinations of the internal structure portion sufficient to meet the requirement of NG-1122(c).

Boundary Between Core Support Structure and Temporary Attachment

The jurisdictional boundary between a core support structure and a temporary attachment shall be the surface of the core support structure.

¹ (a) The rules of Subsection NG are not directed to sealing against coolant leakage. Further, gross hydrostatic loading may not be typical of the loads experienced by core support structures. Thus, hydrostatic testing of the structures is not required.

(b) The most severe loads on which design must be predicated usually result from abnormal, rather than normal, load conditions. Core support structures need not remain leak tight to perform their function. However, if leak tightness is required for a structure, the rules for pressure boundary construction may be appropriate.

² In Subsection NG it is recognized that the design functions are frequently handled separately from the fabrication functions of the Certificate Holder manufacturing core supports. The separation of these functions is necessary because the design of core support structures cannot be performed completely independent of the nuclear or hydraulic design of the coolant system. Furthermore, portions of a set of structures may be fabricated by a specialist and assembled at the site without any one fabricator having a controlling position for fabrication of the complete structure. Therefore, provisions are made herein for separate organizations to perform the design and fabricating functions of the Certificate Holder. This Section (NCA-3510), however, requires that one organization have overall responsibility for compliance with the requirements of this Subsection NG. The use of the term Certificate Holder therefore must be understood to mean the design or fabricating organization assuming the overall responsibility for compliance with this Section. The term Certificate Holder, as used in this Subsection, applies to the Certificate Holder manufacturing core supports who shall be responsible for certifying that all core support structures and internal structures, as defined in NG-1120, comply with the requirements of this Subsection.

3 Note that some fuel and blanket positions in the core matrix may be filled with structures related to reflector or shielding functions. These structures also fall into the fuel and blanket category described above.