The through-wall attenuation of neutron fluence of reactor pressure vessel (RPV) steels is often expressed using an exponential decay function based on some estimate of displacements per atom (dpa). In order to verify this function, an irradiation project was performed in which 18 layers of Charpy specimens and one central temperatue control layer were stacked in a block to simulate a 190 mm thick RPV wall. Three western-type RPV steels (medium and low copper plates and a high copper Linde 80 flux weld) were irradiated in this project. Mechanical property tests of these materials have been performed under a consortium of EPRI, CRIEPI, NRI-Rez and ATI Consulting to fully characterize the mechanical properties in terms of Charpy transition temperature and upper-shelf energy, as well as reference fracture toughness using the Master Curve. Some results have been reported at previous PVP conferences. In this paper, we report the results of microstructural characterization using three-dimensional atom probe tomography (APT) of the medium copper plate and the high copper weld metal. The microstructures obtained by APT reasonably explain the changes in mechanical properties of these materials, and the difference in the response of these materials to irradiation was also identified. The mixed effect of fluence/flux/spectrum is discussed from the microstructural point of view.

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