[en] One of the options to mitigate the effects of irradiation on reactor pressure vessels (RPVs) is to thermally anneal them to restore the toughness properties that have been degraded by neutron irradiation. Even though a postirradiation anneal may be deemed successful, a critical aspect of continued RPV operation is the rate of embrittlement upon reirradiation. There are insufficient data available to allow for verification of available models of reirradiation embrittlement or for the development of a reliable predictive methodology. This is especially true in the case of fracture toughness data. Under the U.S.-Russia Joint Coordinating Committee for Civilian Nuclear Reactor Safety (JCCCNRS), Working Group 3 on Radiation Embrittlement, Structural Integrity, and Life Extension of Reactor Vessels and Supports agreed to conduct a comparative study of annealing and reirradiation effects on RPV steels. The Working Group agreed that each side would irradiate, anneal, reirradiate (if feasible), and test two materials of the other, so far, only Charpy impact and tensile speciments have been included. Oak Ridge National Laboratory (ORNL) conducted such a program (irradiation and annealing) with two weld metals representative of WWER-440 and WWER-1000 RPVs, while the Russian Research Center-Kurchatov Institute (RRC-KI) conducted a program (irradiation, annealing, reirradiation, and reannealing) with HSST plate 02 and HSSI Weld 73W. The results for each material from each laboratory are compared with those from the other laboratory. The ORNL experiments with the WWER welds included irradiation to about 1x10¹⁹ n/cm² (>1 MeV), while the RRC-KI experiments with the U.S. materials included irradiations from about 2 to 18x10¹⁹ n/cm² (>1 MeV). In both cases, irradiations were conducted at ∼290 C and annealing treatments were conducted at ∼454 C. The ORNL and RRC-KI experiments have shown generally good agreement for both the Russian and U.S. steels. While recoveries of the Charpy 41-J transition temperatures were substantial in all cases, significantly less recovery of the lateral expansion and shear fracture in some cases (no recovery in one case) deserves further attention. Although the RRC-KI results for the U.S. steels showed reirradiation embrittlement rates which are conservative relative to the lateral shift prediction based on Charpy impact energy, the desired material property is fracture toughness and uncertainties exist because of the paucity of fracture toughness data upon both annealing and reirradiation. (author)