ASME NTB-4-2021 pdf download
ASME NTB-4-2021 pdf download.Background Information for Addressing Adequacy or optimization of ASME BPVC Section lll，Division 5 Rules for Nonmetallic Core Components.
3 GRAPHITE STRENGTH
The ASMF. graphite design code is essentially a design methodology that compares the stress on a graphite component with the material’s strength distribution. The major Ilictors affecting graphite strength are presenied in this seclion, along with the relevant sections of the ASMF. graphite code.
The lnd anisotropy of single-crystal graphite (in-plane strong covalent tionds. between-planes weak van der Waals bonds I contributes to the inherent anisotropy of the polycrystalline material, even though the behavior is more isotropic than single-crystal graphite. Moreover, the manufacture of polycrystalline graphite gives rise to the distribution of the size, location, and orienlation of porosity in a graphite 1361. The variability in the porosity in proportion to the length scale of the filler particles of a specific size (and thus density) influences the strength of the material, with higher-density graphite exhibiting higher strength. The structure, and hence the strength, of a graphite is a strong function of the exact manufacturing route employed. Several manufacturing features can hc identified which impact the structure and properties of the material:
• Filler particle type and size
• Forming method t extrusion, molding. isostatic pressing)
• Process variables (filler particle type. impregnation type. processing temperatures)
Depending upon the extent of the materials anisotropy. the strength may be quoted as pertaining to the with- grain WG) or against grain (AG) orientation. Generally, the WG strength is greater than AG strength. Because of the many potential variations in production of different grades of graphite. each grade will have different properties. It is important, therefore, that the graphite grade be carefully specified. The ASME graphite code (371 requires that the graphite be compliant to either ASTM D72l9 1381 or ASTM 1)7301 1391, II irradiation-induced dimensional change is not a significant design consideration ((he amount of dimensional change is not considered), an isotropy ratio (aAo’an) of greater than 1.15, defined in terms of the coefficient of thermal expansion (CTE). ASTM D730l, may be used.
Because of the inherent variability of the graphite flaw strucure within the microstructurc. the strength is usually described statistically. The ASME code requires that the designer determine and record the parameters of the Weibull distribution that best describe the strength (41.
Several environmental factors also influence the strength of graphite. For example, the strength increases with increasing temperature. This increase is —50% over the temperature range from 20°C to greater than 1500°C for most graphite grades 1361. Typically, the data from which the Wcibull distribution of strength is derived are obtained by ambient-temperature tensile testing. No credit is taken for this increase in the graphite strength caused by temperature, which provides added conservatism. However, if the designer chooses to consider the strength increase induced by higher temperature. elevated-temperature strength testing must be conducted over the anticipated operating temperature conditions of the core. Additionally, other required material property values must he tested at elevated temperatures. as required by the material data sheets 141-
Moreover, the strength is markedly influenced by neutron irradiation. At low and intermediate doses, the strength will increase with increasing neutron dose, the initial increase being very rapid. At high doses, the strength will decrease and eventually tat much higher doses) reach zem The ASME graphite code posits a cohesive life limit 1401 (Le. the dose at which the WG dimensional change is +l(W.). Beyond this limit, the graphite cannot be considered to contribute to a component’s structural integrity. The exact behavior of strength with neutron dose for any selected graphite will need to be determined experimentally. Doing so is the responsibility of the designer.
‘I’he ASME graphite code additionally provides lower dose limits 1411. For example. if the dose at any point in a component is greater than 0.25 displacement per atom (dpa). all effects of irradiation (including strength i shall be considered. Thus, a low-dose component can, for the purpose of structural assessment.
