ASME NTB-4:2021 pdf free download – Background Information for Addressing Adequacy or optimization of ASME BPVC Section lll，Division 5 Rules forNonmetallic Core Components
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 ofpressure, temperature, fast neutron flux or damage dose rate, and various forces applicable to nonmetalliccore components. According to HAB-2142 [23], the design specification defines the design limits (theenveloping case for the design) and service limits according to the designer’s classification with regard tofunctional performance(Level A); the ability to withstand damage requiring repair(Level B); and the extentto which large deformations in the areas of structural discontinuity (Level C) and gross deformations withconsequential 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 normalstress or shear stress developed by an imposed loading that is necessary to satisfy the laws of equilibriumof external and internal forces and moments), secondary stress (a normal stress or a shear stress developedby the constraint of adjacent material or by self-constraint of the structure), and peak stress (the incrementof stress that is additive to the primary plus secondary stresses by reason of local discontinuities or localthermal stress).Because of the brittle nature of graphite, no distinction is made among the three types ofstress; 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 isderived from an arbitrary stress state at a point. The POF is determined by comparing the peak equivalentstress (the highest equivalent stress computed from the total stress) with the results of a uniaxial strengthtest as specified in HHA-3213 and HHA-3214 [22]. Reducing the risk of failure requires incorporatingadequate design margin. As mentioned, fixed design margins do not ensure uniform reliability because ofthe strength variability in graphite. Instead an allowable POF is used.
The specified design allowable POF was derived from a review of design margins from several codemethodologies that included the proposed ASME CE draft methodology [24], the Japan Atomic EnergyAgency(JAEA) methodology [25], [26], the Kerntechnischer Ausschuss (KTA) 3232 methodology [27and the United Kingdom(UK) Advanced Gas-cooled Reactor methodology [28] as reported by Mitchell[20].
The JAEA methodology (and previous ASME BPVC Section IlI Division 2 Subsection CE draft method)used a predicted failure approach, applying the failure probability function with defined limits (or ratios),as shown in the Hopper diagrams in Figure la, the JAEA approach, and the Subsection CEapproach(Figure2).
The JAEA methodology defines primary , secondary, and peak stresses using the minimum ultimate strengthratio while assuming a linear cumulative damage law for fatigue evaluation with usage factors limited to1/3,2/3, or 1 for the different operating conditions [25]. The specified minimum ultimate strength(SA, witha survival probability of 99% and confidence level of 95%) is applied to obtain the acceptable design stresslimit, as shown in Figure 1a (reprinted from [25]).For example, the induced stress in the core supportstructure should be less than 0.25 of Sl, as shown in Figure 1a; but it is also shown that, by applying thisratio to a corresponding graphite with a tensile strength distribution, it is estimated that a fracture or failureprobability of 10-10 is attained, as illustrated in Figure 1b(reprinted from[26]).