Apparent Embrittlement Saturation and Radiation Mechanisms of Reactor Pressure Vessel Steels

The irradiation and annealing results of three different reactor pressure vessel steels are reported. Steel A, a basic material according to ASTM A 533-B, was produced by Lukens as HSST Plate 03; Steel B was a modified A 533 B having 0.15 percent vanadium; and Steel C contained 3.2 percent nickel. The steels were irradiated at 150, 300, and 400°C with neutron fluxes of 6 × 10¹¹ and 3 × 10¹³ neutrons (n)/cm²s [E > 1 MeV] to different neutron fluences in the research reactors Forschungs-Reactor Jülich (FRJ)-1 and FRJ-2 in Jülich, Germany.

In addition to the trend curve for the increase in transition temperature with irradiation, the trend curves are presented for the relative increase of Vickers hardness, the relative decrease of the upper-shelf energy, and the relative increase of yield strength.

An apparent saturation-in-irradiation effect was found within certain neutron fluence ranges. It was observed for irradiations with a high neutron flux of 3 × 10¹³n/ (cm²s), with a low neutron flux of 6 × 10¹¹ n/(cm²s), and also with 150°C irradiations as well as higher temperature irradiations.

The annealing behavior normally was measured in terms of the Vickers hardness increase produced by irradiation relative to the initial hardness as a function of the annealing temperature. During the annealing, various recovery processes occur in different temperature ranges. These are characterized by various activation energies. The individual processes were determined by the different time dependencies at various temperatures.

Two causes for the apparent saturation were discovered from the behavior of the annealing curves: first, the production of defects of higher activation energy without altering the effective defect density; and second, the tendency of one process to remain constant in spite of increasing neutron fluence.