INTRODUCTIONWhen the first commercial nuclear power reactors began operation in the late 1950s and early 1960s, it was well known that the pressure-vessel steel would undergo radiation embrittlement,1 although the related safety implications were not clear. Today, federal regulations require the operators of nuclear power plants to conduct surveillance programs that monitor radiation embrittlement during the service life of a nuclear pressure vessel.Commercial nuclear power reactors in the U.S. are light-water-cooled, pressurized-water reactors (PWRs) and boiling-water reactors (BWRs). The reactor core is contained within pressure vessels made of heavy-section ferritic steel. Because PWRs operate at higher pressures than BWRs (about 16.5 MPa versus about 8.25 MPa), the PWR vessels are smaller and thicker. PWR pressure vessels are typically 3.4-4.3 m in diameter and 20-23 cm thick, while BWR pressure vessels are 5.2-6.4 m in diameter and 15-18 cm thick. Since there is less distance and water shielding between the core and the pressure vessel wall for PWRs, the neutron flux incident on the inner wall is higher. Typical neutron fluxes (fluxes and fluences refer to neutron energies greater than 1 MeV) range from 0.7 x