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The brittle–ductile transition temperature (BDTT) and fracture toughness are useful indicators of the brittleness of reactor pressure vessel materials. However, current methods to analyze these parameters are expensive and time-consuming; moreover, the relevant mechanism is not very clear. Thus, in this study, a body-centered cubic crystal plasticity finite element model (CPFEM) based on the densities of dislocation and irradiation defects, which are full- and partial-absorption dislocation loops, was numerically implemented on A508-3 steel. Irradiation embrittlement was investigated in the CPFEM framework. The cleavage failure model for a representative volume element was proposed to calculate the BDTT, the results of which agreed well with experimental results in the literature for both unirradiated and irradiated samples. The risk for brittle failure is higher for irradiated samples at constant temperature. Furthermore, the proposed model combined with the Beremin cleavage fracture local approach, could estimate the fracture toughness of A508-3 steel at different temperatures before and after irradiation. The results obtained were consistent with reported experimental data, indicating that irradiation exacerbates the embrittlement. This work is beneficial to the calculation and evaluation of irradiation effect of RPV steel, providing reference for the research of reactor life extension. [ABSTRACT FROM AUTHOR]

The fitness for service code of the Japan Society of Mechanical Engineers (JSME) provides a fracture evaluation methodology based on linear elastic fracture mechanics (LEFM) for boiling water reactor (BWR) internal components irradiated to a neutron fluence of 3.0 × 1024 n/m2 or more. On the other hand, it is known that austenitic stainless steel maintains its ductility and toughness even after neutron irradiation up to the fluence expected for BWR internal components. Accordingly, it is preferable to apply the ductile fracture evaluation methodology for BWR internal components. The present study examined the applicability of damage mechanics analysis based on the Gurson‒Tvergaard‒Needleman (GTN) model for irradiated stainless steel. The changes of mechanical properties of the material due to neutron irradiation were simulated by cold working. As a result, in terms of material strength properties the applicability of the GTN model was confirmed for irradiated stainless steel using the simulated material for neutron fluence up to about 1.5 × 1025 n/m2. • Damage mechanics analysis based on the Gurson‒Tvergaard‒Needleman (GTN) model for irradiated stainless steel was investigated. • Changes of mechanical properties of the material due to neutron irradiation were simulated by cold working. • Fracture tests and analyses were performed with plate specimens. • Applicability of GTN model was examined by comparing the maximum loads and fracture behaviors between the tests and analyses. • Applicability of the GTN model was confirmed for irradiated stainless steel using the simulated material for neutron fluence up to about 1.5 × 1025 n/m2. [ABSTRACT FROM AUTHOR]

• The changes in fracture behavior of stainless steel with in neutron dose can be predicted from the presented method. • The multiaxial fracture strain model is determined by simulating tensile and fracture toughness test. • The neutron irradiation embrittlement constant is determined by simulating tensile test. • The neutron irradiation embrittlement constant is multiplied with the multiaxial fracture strain model to simulate fracture toughness tests, enabling the prediction of degradation in fracture toughness due to neutron irradiation. This paper proposes a numerical method for predicting the reduction in the fracture toughness with neutron dose using the concept of an embrittlement constant. Tensile and fracture toughness test data for type 304 stainless steel in the literature [ 41 , 42 ] are used to validate the proposed method. Fracture toughness tests are simulated using a stress-modified fracture strain damage model implemented in an ABAQUS user-subroutine. The true stress-strain curve and one point of the fracture strain locus are obtained from the tensile test simulation. The remaining parameters of the fracture strain locus are determined by simulating a fracture toughness test. The fracture strain locus is modified to reflect the irradiation effect on the fracture toughness using the proposed irradiation embrittlement constant C I. The function of the constant with the neutron dose is calculated from the tensile tests and simulations. Subsequently, the modified damage model is used to predict the reduction in the fracture toughness. The predicted simulation results are compared with experimental data, showing good agreement. [Display omitted] [ABSTRACT FROM AUTHOR]

Life extension of light water reactors requires robust models to predict the embrittlement of reactor pressure vessel (RPV) steels under long-term neutron irradiation. Embrittlement is due to nanoscale precipitation hardening by phases containing Cu, Mn, Ni, and Si, resulting in increases of the ductile to brittle transition temperature. Embrittlement data for low flux, high fluence extended life conditions, up to 80 years or more, are largely not available. Thus, higher flux, accelerated test reactor irradiations have been used to help develop models to predict low flux embrittlement at high fluence. Here flux, or dose rate, is expressed in units of displacements per atom per second (dpa/s). The dose rate can significantly influence precipitate evolution in a way that depends on fluence (dpa), temperature, and alloy composition, as well as flux itself. Here, we explore precipitation in surveillance steels with varying compositions at high fluxes (10−5–10−6 dpa/s) produced using 70 MeV Fe2+ ion irradiations. Atom probe tomography was used to characterize the dose dependence of precipitation at very high ion irradiation dose rates, and to evaluate corresponding similarities or differences compared to neutron irradiated steels at a much lower dose rate of ≈ 5 × 10−9 dpa/s. The data show that, on average, the ion irradiation precipitate volume fractions increase by factors of ∼ 1.5 to 2 between 0.3 dpa and 1.2 dpa. Limited data show that the major effect of neutron versus ion irradiation is the former has lower precipitate number densities and larger sizes. Otherwise, the precipitates are remarkably similar in composition and volume fraction. [ABSTRACT FROM AUTHOR]