Master Curve approach for the fracture toughness evaluation is 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 the Master Curve approach by means of Mini-CT and larger specimens for the determination of fracture toughness of typical Japanese RPV steels. A round robin program was organized with the participation of a university, industries and a government institute in Japan. The first-round test results (PVP2012-78661[1]) with limited number of reference temperature To data points indicated the possible loading rate dK/dt dependence in To. However, increasing number of data points in second-round test, which was conducted in specified loading rate of 0.5 MPa m0.5 /sec, ensures that there is no clue of such a remarkable dK/dt dependency. Concerning the effect of large plastic deformation on dK/dt, dK/dt calculated by several definitions were compared with each other. Maximum one order difference was found between dK/dts with and without consideration of plastic deformation. dK/dt - T0 relationships showed similar amount of scatter in T0 regardless of dK/dt definitions. Difference in dK/dt definition seems less effective on scatter of T0 in the present results.

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