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ASME NTB-4-2021 pdf download.Background Information for Addressing Adequacy of Optimization of ASME BPVC Section III, Division 5 Rules for Nonmetallic Core Components.

Before its appearance in the 2011 edition of ASME BPVC, there was no internationally recognized graphite core design code. Although the need for such a code was recognized by stakeholders such as the US Nuclear Regulatory Commission (NRC) and several reactor designer/constructers. it was not until 2002 that ASME formed a project team to initiate a graphite design code. Graphite is used extensively for reactor internal components in high-temperature gas reactor concepts, as it is needed to establish core geometries allowing coolant flow, reactivity control, and shutdown element insertion; serve as a moderator while supporting the nuclear heat generation process: and provide a passive heat removal flow path in certain licensing basis events [1]. Those functions are key to the operation and safety of the reactor system. One critical difference between graphite core components and the pressure vessel is that the graphite core assembly is a structure comprising many hundreds of components. The designers take measures to ensure that the failure of a single component does not compromise the function of the assembly. The prior approach to graphite design was deterministic, similar to the approach applied for metallic components today. Graphite, with no strength in the plastic regime, was treated as a linear Hookean material. Any component that suffered cracking was considered a bbfailed component” and was removed. Additionally, only nonirradiated graphite use was addressed. This approach has been found to be inadequate for design and regulatory licensing. Graphite, because of its nature, is inherently cracked; and the absence of cracking cannot be ensured nor used as an indicator of absolute reliability, as it can be lbr metals.

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