Abstract

Most standard surveillance programs for pressurized water reactors (PWR), do not contain enough surveillance specimens for their long-term operation (LTO) beyond the original design life of 40 years. The use of testing methods based on small/sub-sized specimen such as mini-CT (0.16 CT, 4 mm thickness) is considered a promising approach to overcome this limitation. The feasibility of fracture toughness testing using sub-sized CT specimen has been generally demonstrated in Japanese-US lead Round Robin programs for un-irradiated materials. 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 etc. In addition, the dimensional tolerances of the standard CT specimen defined in the ASTM E1921 standard are defined as a function of specimen width, resulting in severer absolute tolerance for the miniature specimen. This paper presents an in-cell fabrication method of a modified mini-CT specimen and a testing facility suitable for hot cell use. This work is performed at NRG within the framework of the STRUMAT project, funded by the Netherlands Ministry of Economic Affairs. A slightly modified miniature CT specimen is chosen, which allows “outboard” clip gage mounting for direct load-line displacement (LLD) measurement. In addition, a wide notch configuration with a height of 0.6 mm, 0.1 mm wider than specified in the ASTM E1921 standard, is used. This modified specimen geometry facilitates the in-cell fabrication of the mini-CT specimen and eases the remote handling and testing procedure. Three-dimensional finite element analysis validation was carried out to determine the effect of these modifications on the stress state of the specimen. It is demonstrated that the modifications to the mini-CT specimen geometry will not lead to significant deviations in the stress patterns and J-integral values compared to the standard geometry.

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