<p>One of the major degradation mechanisms in a nuclear power plant structural or mechanical component is the neutron embrittlement of the irradiated steel component. High energy neutrons change the microstructure of the steel, so the steel loses its fracture toughness. This neutron embrittlement increases the risk of the brittle fracture. Meanwhile, the reactor pressure vessel support is exposed in low temperature with high neutron irradiation environment which is an unfavorable condition for the fracture failure. In this study, the failure assessment of a reactor pressure vessel support was conducted using the fitness-for-service failure assessment diagram of API 579-1/ASME FFS-1(2016, API) with quantifying the structural margin under the maximum irradiation and extreme load events. </p> <p>Two interrelated studies were conducted. For the first investigation, the current analytical methods were reviewed to estimate the embrittled properties, such as fracture toughness and the yield strength incorporates the low irradiation temperature. The analytical results indicated that the reactor pressure vessel support may experience substantial fracture toughness decrease during the operation near the lower bound of the fracture toughness. A three-dimensional (3D) solid element finite element model was built for the linear stress analysis. Postulated cracks were located in the maximum stress region to compute the stress intensity and the reference stress ratio. Based on the stress result and the estimated physical properties, the structural margin of the reactor pressure vessel support was analyzed in the failure assessment diagram with respect to the types of the cracks, level of the applied load and the level of the neutron influence. </p> <p>The second study explored the structural stress analysis approaches at hot-spot which was found to be key parameter in failure analysis. Depending on the methods to remove the non-linear peak stress and the stress singularities, the accuracy of the failure assessment result varies. As an alternative proposal to evaluate the structural stress in 3D finite element analysis (FEA), the 3D model was divided into two-dimensional (2D) plane models. Five structural stress determination approaches were applied in 2D FEA for a comparison study, the stress linearization, the single point away approach, the stress extrapolation and the stress equilibrium method and the nodal force method. Reconstructing the structural stress in 3D was carried by the 3x3 stress matrix and compared to the 3D FEA results. The difference in 2D FEA structural stress results were eliminated by the constructing the stress in 3D. </p> <p>This study provides the failure assessment analysis of irradiated steel with prediction of the failure modes and safety margin. Through the failure assessment diagram, we could understand the effects of different levels of irradiation and loadings. Also, this study provides an alternative structural stress determination method, dividing the 3D solid element model into two 2D models, using the finite element analysis. </p><br>

The embrittlement of reactor pressure vessel (RPV) steels due to neutron irradiation restricts the operating lifetime of nuclear reactors. The reference temperature 𝑇0, obtained from fracture mechanics testing using the Master Curve concept, is a good indicator of the irradiation resistance of a material. The measurement of the shift in 𝑇0 after neutron irradiation, which accompanies the embrittlement of the material, using the Master Curve concept, enables the assessment of the reactor materials. In the context of worldwide life time extensions of nuclear power plants, the limited availability of neutron irradiated materials (surveillance materials) is a challenge. Testing of miniaturized 0.16T C(T) specimens manufactured from already tested standard Charpy-sized specimens helps to solve the material shortage problem. In this work, four different reactor pressure vessel steels with different compositions were investigated in the unirradiated and in the neutron-irradiated condition. A total number of 189 mini-C(T) samples were fabricated and tested. An important component of this study is the transferability of fracture mechanics data from mini-C(T) to standard Charpy-sized specimen. Our results demonstrate good agreement of the reference temperatures from the mini-C(T) specimens with those from standard Charpy-sized specimens. RPV steels containing higher Cu and P contents exhibit a higher increase in 𝑇0 after irradiation. The fracture surfaces were investigated using SEM in order to record the location of the fracture initiators. The fracture modes were also determined. A large number of test results formed the basis for a censoring probability function, which was used to optimally select the testing temperature in Master Curve testing. The effect of the slow stable crack growth censoring criteria from ASTM E1921 on the determination of 𝑇0 was analysed and found to have a minor effect. Our results demonstrate the validity of mini-C(T) specimen testing and confirm the role of the impurity elements Cu and P in neutron embrittlement. We anticipate further research linking microstructure to the fracture properties of materials before and after neutron irradiation and the optimization of Master Curve testing using the results from our statistical analysis.