The project investigated the fracture‑mechanical behaviour of reactor pressure vessel (RPV) steels and welds using sub‑sized specimens, a technique that allows the reuse of limited irradiated material. The study focused on three material families that are representative of German pressurised‑water reactors: the 22NiMoCr3‑7 alloy (P7 BM), the NiCrMo1‑UP weld metal (P370 WM) and the S3NiMo/OP41‑TT weld metal (P16 WM). Two unirradiated and two irradiated specimens of each material were examined. The irradiated welds were exposed to neutron fluences of 4.79 × 10¹⁹ n cm⁻² and 4.86 × 10¹⁹ n cm⁻², respectively, and the unirradiated welds contained high copper or nickel contents that are known to increase susceptibility to irradiation embrittlement.
Mini‑C(T) specimens of 0.16 T thickness were produced from two Charpy V‑notch specimens for each material. From each Charpy specimen up to sixteen mini‑C(T) samples could be fabricated, providing a larger data set than is normally available from irradiated material. Eight tests were performed on each material in the unirradiated state and ten on the unirradiated weld, while the irradiated specimens were tested in the same configuration. The results showed that the ASTM E1921 criteria for a valid reference temperature (T₀) could not be satisfied with only eight or ten mini‑C(T) specimens; at least sixteen specimens are required to meet the statistical requirements. When the T₀ values obtained from the mini‑C(T) tests were compared with those from standard‑size Compact Tension (C(T)) specimens, differences were observed. These differences can be attributed to the smaller specimen size and the altered stress field, as well as to the limited number of tests. Despite these discrepancies, the T₀ values for the base metal were in good agreement with the reference data, indicating that the mini‑C(T) approach can provide reliable information when sufficient specimens are used.
Fractographic analysis was carried out on the fracture surfaces using scanning electron microscopy and energy‑dispersive spectroscopy. The initiation sites were predominantly located near the mid‑plane of the specimens, a distribution that matches previous studies on RPV steels. The analysis confirmed that both weld materials exhibit a high propensity for irradiation embrittlement, consistent with their copper‑rich (P370 WM) and nickel‑rich (P16 WM) compositions.
The project was funded by the German Federal Ministry of Environment, Nature Conservation, Nuclear Safety and Consumer Protection. It was carried out in collaboration between the industrial partner Framatome GmbH and the research institutions Technische Universität Dresden and Helmholtz‑Zentrum Dresden‑Rossendorf (HZDR). Framatome conducted sub‑project 1501592A, focusing on specimen preparation and mechanical testing, while HZDR handled sub‑project 1501592B, which included detailed material characterisation and data analysis. The collaboration aimed to enhance the acceptance of fracture‑mechanics testing of irradiated and non‑irradiated RPV steels using mini‑C(T) specimens, thereby extending the useful life of European nuclear reactors beyond their originally planned operating periods.