ASME NTB-4:2021 pdf download

ASME NTB-4:2021 pdf download.Background Information for Addressing Adequacy or Optimization of ASME BPVC Section III, Division 5 Rules for Nonmetallic Core Components
2 CODE APPROACH
This section provides the basis for the code and a typical design sequence for a graphite core component. Because there is not a standardized graphite grade for nuclear applications, the code places the responsibility for determining the design properties of the graphite used on the core designer. The approved properties for the selected graphite grade are then determined through material testing and listed in the form of a materials data sheet [4]-[6], which is used to justify the design. Previous studies [7], [8] determined that variations of the Weibull distribution best describe the graphite reliability curve. HHA-II-3000 [9] describes how to statistically characterize graphite material based on specimen test results so that a material reliability curve can be derived. The approach is supported by many other studies [10]-[17]. To perform a stress-based analysis, the rules derive an equivalent stress state (from a multiaxial stress analysis) to determine the peak equivalent stress for a component for a given load condition. In general, parts are designed by comparing calculated stresses to strength limits based on specimen test results and adequate design margins. But in the case of graphite, fixed design margins do not ensure uniform reliability because of the variability in the material. The stochastic strength (large random fluctuations from the population mean) and the nonlinear stress-strain response (quasi-brittle) of graphite [18], as well as billet-to-billet variation [19], require that the material be statistically characterized. That characterization is then used to determine the design margin [20], [21]. The identified modes of failure for graphite are brittle fracture, fatigue, excessive deformation (including both elastic instability and irradiation-induced dimensional changes) and environmental effects such as irradiation and chemical attack.
The graphite code treats design, service, and test loadings consistently with the rest of the ASME BPVC. The design loadings, defined in HHA-3123.1 through HHA-3123.4 [22], include the distributions of pressure, temperature, fast neutron flux or damage dose rate, and various forces applicable to nonmetallic core components. According to HAB-2142 [23], the design specification defines the design limits (the enveloping case for the design) and service limits according to the designer’s classification with regard to functional performance (Level A); the ability to withstand damage requiring repair (Level B); and the extent to which large deformations in the areas of structural discontinuity (Level C) and gross deformations with consequential loss of dimensional stability and damage requiring repair (Level D) are permitted. To determine stress in the ASME BPVC, it is customary to distinguish between primary stress (a normal stress or shear stress developed by an imposed loading that is necessary to satisfy the laws of equilibrium of external and internal forces and moments), secondary stress (a normal stress or a shear stress developed by the constraint of adjacent material or by self-constraint of the structure), and peak stress (the increment of stress that is additive to the primary plus secondary stresses by reason of local discontinuities or local thermal stress). Because of the brittle nature of graphite, no distinction is made among the three types of stress; instead, a combined stress approach is used that combines them. The theory of failure is based on the maximum deformation energy theory, in which an equivalent stress is derived from an arbitrary stress state at a point. The POF is determined by comparing the peak equivalent stress (the highest equivalent stress computed from the total stress) with the results of a uniaxial strength test as specified in HHA-3213 and HHA-3214 [22]. Reducing the risk of failure requires incorporating adequate design margin.