Surveillance testing is the basis for all reactor pressure vessel (RPV) integrity and lifetime assessments to evaluate the validity of the RPV embrittlement trend curve used for a specific reactor. Most of the standard surveillance programmes, do not contain sufficient number of surveillance specimens for beyond the original design life of 40 years. Thus, testing methods based on small / sub-sized specimens are considered. Experimental programmes on un-irradiated RPV steels have proven that miniature compact tension (∼4*10 *10 mm3) specimens, mini-CT, can successfully be used for fracture toughness characterization in the transition regime to determine reference temperature (T0) based on the master curve method. However, application of miniature testing techniques on irradiated materials in hot cells comes along with several technical challenges such as remote controlled machining, application of the extensometers, manipulator sensitivity, accurate sample geometry measurement etc. In addition, the optimal test temperature range for obtaining valid KIc values for the mini-CT specimen is narrow and consequently sufficient number of specimens should be tested to guarantee a reliable valid T0-value.

Within the FRACTESUS project, European and International efforts are joined to establish the foundation of small specimen fracture toughness validation and demonstration to achieve change in code and standards allowing to address the various national regulatory authority concerns. As part of the third work package (WP3), high-fluence fracture toughness data, on well characterized RPV materials, will be produced to serve as input for the guidelines of future testing.

To assure the robustness of the testing procedures to the difference in testing machines or operators, a round robin (RR) exercise on un-irradiated materials is organised among the partners of WP3. This work presents the preliminary results of the mini-CT fracture toughness data on un-irradiated RPV SA508 Cl. 3 material which was independently tested by four institutes: CIEMAT, KIT, SCK CEN and NRG. All four institutes determined T0 according to the ASTM E1921 standard and the results are compared and discussed.

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