Graphite is an important material being used to for core components in the next generation of high temperature gas cooled nuclear reactors. The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) Section III Subpart HHA Article 3000 outlines two semi-probabilistic design by analysis assessments to evaluate graphite core components against design reliability targets. This paper applies the margin metric to the assessments to quantify how conservative they are and how various aspects of the assessments contribute to the overall margin. The assessments are referred to as the simplified and full assessments. The simplified assessment using the 2-parameter Weibull distribution to describe the graphite grade?s tensile strength distribution to establish stress limits. The full assessment uses the 3-parameter Weibull distribution and a modified Weibull Weakest Link Theory approach to calculate a component design probability of failure (POF). Component design reliability targets are established based on a components? structural reliability class (SRC) to determine whether components meet the criteria. Conservatism in component design criteria is often evaluated by the safety factor metric. The safety factor metric is the ratio of the ultimate load capacity to the design allowable load. The ultimate load capacity is defined as the median experimental tensile load, by grade. Nuclear applications often shift the safety factor by reducing it by one, to find the margin. This paper evaluates margin in the simplified and full graphite component design by analysis assessments. The effects of least squares (LS) estimation vs. maximum likelihood estimation (MLE), lower bounds (LB) vs. point estimates, and three target probability of failures (POFs): 10^(-4), 0.01, and 0.5, were evaluated in the simplified assessment. The full assessment margin effects evaluated were: LB vs. point estimates, the three target POFs, grouping criteria, and threshold reduction criteria in the material reliability curve. Four grades of graphite, varying in grain size, were used in this study: 2114, IG-110, PCEA, and NBG-18. The experimental data used were from the Department of Energy?s Gas Cooled Reactor?s baseline (unirradiated) material property program. The analysis provided in this paper provides necessary baseline safety margin calculations for the ASME BPVC graphite core component design by analysis methodologies. Findings suggest that the design by analysis assessments are robust to variations in graphite material properties. Assessment design margins are higher for grades with lower characteristic strengths and higher tensile strength variability, such as PCEA-36, at the extreme of the graphite grades evaluated. PCEA-36 has the highest design margin among all grades evaluated, maximally at 4.2 in the simplified assessment and at 3.7 in the full assessment ?gold standard? condition. 2114 is the opposite extreme of the tensile strength and variability as PCEA-36. It has the lowest design margin at 0.36 in the simplified assessment and at 0.33 in the full assessment ?gold standard? condition. Graphite core component design by analysis margins are consistent with acceptable margins for other nuclear component materials.
