An investigation of steady-state radiation embrittlement of reactor vessels, which consists of surveillance capsule supplemental testing and mechanistic studies, is being conducted by the Westinghouse Electric Corporation for the Electric Power Research Institute. A summary of the increases in the 41-J (30-ft · lb) transition temperature exhibited by submerged arc weld material contained in eight supplemental surveillance capsules included in this investigation is presented. The data indicate that radiation damage for high-copper submerged arc welds does not saturate but does tend to show a very low rate of embrittlement at neutron fluences greater than ∼1 × 10¹⁹ n/cm². The data also indicate that radiation damage predictions based on U.S. Nuclear Regulatory Guide 1.99, Revision 1, are not realistic for fluences greater than ∼1 × 10¹⁹ n/cm², for most pressure vessel steels.