[en] Microstructural characterization was performed on long-term (∼100,000-h) thermally aged and neutron-irradiated surveillance materials obtained from the Babcock ampersand Wilcox Nuclear Technologies and a high phosphorus weld from a Russian reactor. Although mechanical testing indicated that thermal aging did not cause any significant changes in the Charpy impact properties, it is important to determine if there are any changes in the composition of the matrix and if any ultrafine precipitates had formed due to the thermal component of service environment only. The characterization of the Russian weld was performed to determine if the behavior of a steel with a phosphorus level in excess of that typically found in Western steels changes, and to ascertain whether the results for formation of copper-enriched regions are specific to the narrow composition band of the Western steels or a more general phenomenon. The ORNL APFIM is well suited to the microstructural characterization of neutron-irradiated RPV materials because of its near-atomic spatial resolution and ability to chemically analyze all elements. In addition to detecting, chemically identifying, and determining the size, morphology, and approximate number density of ultrafine features, the atom probe is able to quantify the amount of each element remaining in solution in the matrix and the amount of solute segregated to grain or lath boundaries