Proposal of an alternative procedure using miniature C(T) specimen for USE-based screening in surveillance program

To extend the reactor operation period beyond 40 years, additional surveillance test requirements must be satisfied by using a limited amount of surveillance specimens. One of the solutions to satisfy the surveillance test requirements with less specimen consumption is the use of a miniature C(T) (Mini-C(T)) specimen with a thickness of 4 mm. 8 of Mini-C(T) specimen can be prepared from one broken Charpy specimen. Upper shelf fracture toughness evaluation in the surveillance program instructed by JEAC4206-2016 requires the screening to verify that the upper shelf energy USE exceeds 68 J by performing Charpy impact tests normally consuming three Charpy specimens. If USE does not exceed 68 J, the detailed evaluation based on J-R curve is required. If USE can be estimated from the J-R curve obtained by performing a fracture toughness test on Mini-C(T) specimen, the USE-based screening may be alternatively examined with the consumption of only a half of broken Charpy specimen. The purpose of this study was to develop an equation to estimate the USE from the J-R curve. 210 data of Japanese base metals and 110 data of weld metals, including both irradiated and unirradiated materials, were analyzed. The two equations to conservatively estimate USE from a J-R curve for base metals and weld metals were proposed respectively. If the USE value estimated from the proposed equation is higher than 68 J, the actual USE value of the material exceeds 68 J with 95% confidence. For the verification of the proposed equations, the USE values were estimated from the J-R curves obtained for the four unirradiated PRV steel base materials. As a result, all the estimated USE values were lower than the actual USE values. Therefore, it has been demonstrated that the USE-based screening may be conservatively conducted by the proposed equations. (author)