In order to predict fracture behavior in the ductile-brittle transition temperature (DBTT) region of a structure with low constraint, a plastic constraint correction coefficient χ was proposed. It was defined as a ratio of KJ of a fracture toughness specimen to KJ of a reactor pressure vessel with a specific flaw shape with the same Weibull stress level, and it was given as a function of a shape factor of the Weibull distribution for the Weibull stress, and Ry, yield strength/ultimate strength. The Weibull stress was calculated based on the original Beremin model even in the DBTT region because no ductile crack initiation was confirmed by the RPV model under the four representative pressurized thermal shock (PTS) transient. The coefficient χ was proposed for a simplified facture evaluation to predict an accurate fracture behavior under pressurized thermal shock. In order to introduce χ to the codes, a simplified structural FE model, a flat plate model, was applied. After confirmation of conservativeness of the flat plate model and unirradiated material on χ, the coefficient χ was prepared for a draft standard of the Japan Welding Engineering Society, which was to be extended to other codes and standards, such as ASME Sec. XI, JSME. A main structure of the draft standard was also shown.

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