Reference temperature evaluation method by Master Curve approach for the fracture toughness evaluation had been standardized recently in Japan, and expected to be a powerful tool to ensure the reliability of long term used RPV steels. In order to get sufficient number of data for the Master curve approach coexistent with the present surveillance program for RPVs, the utilization of miniature specimens, which can be taken from broken halves of surveillance Charpy specimens, is important. CRIEPI had developed the test technique for the miniature C(T) specimens (Mini-CT), whose dimensions are 4 × 10 × 10 mm, and verified the basic applicability of Master Curve approach by means of Mini-CT for the determination of fracture toughness of typical Japanese RPV steels. A round robin program is organized with the participation of Japanese academia, industries and a government institute. The program aims to verify the reliability and robustness of experimental data of Mini-CT, and to pick out further investigation items to be solved before the actual application of the technique. As the first step of this program, four institutes separately carried out a series of Master Curve evaluation in conformity to ASTM E1921-10e1 by means of Mini-CT specimens, whose material (Japanese RPV material, SFVQ1A), machining process and pre-cracking process are in common in all the specimens. Valid reference temperature T0 could be successfully obtained in all of the institutes by means of Mini-CT specimen. However, the value, T0, have large difference with maximum of 34 °C among the institutes. The difference shows strong correlation with the difference in loading rate, which is selected by each organization to be meet the testing standard ASTM E1921-10e1.

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