COMPARISON OF BR3 SURVEILLANCE AND VESSEL PLATES TO THE SURROGATE PLATES REPRESENTATIVE OF THE YANKEE ROWE PWR VESSEL

摘要

The sister pressure vessels at the BR3 and Yankee Rowe PWR plants were operated at lower-than-usual temperature (≈260°C) and their plates were austenitized at higher-than-usual temperature (≈ 970°C)- a heat treatment leading to a coarser microstructure than is typical for the fine grain plates considered in development of USNRC Regulatory Guide 1.99. The surveillance programs provided by Westinghouse for the two plants were limited to the same A302-B plate representative of the Rowe vessel upper shell plate; this material displayed outlier behavior characterized by a 41J. Charpy-V Notch shift significantly larger than predicted by Regulatory Guide 1.99. Because lower irradiation temperature and nickel alloying are generally considered detrimental to irradiation sensitivity, there was a major concern that the nickel-modified lower Rowe plate and the nickel-modified BR3 plate may become too embrittled to satisfy the toughness requirements embodied in the PTS screening criterion. This paper compares three complementary studies undertaken to clarify these uncertainties: 1) The accelerated irradiation and test program launched in 1990 by Yankee Atomic Electric Company using typical vessel plate materials containing 0.24% copper at two nickel levels: YA1, 0.63% (A533-B) and YA9, 0.19% (A302-B). These were heat-treated to produce the coarse and fine grain microstructures representative of the Yankee/BR3 and the Regulatory Guide plates, respectively; 2) The BR3 surveillance and vessel testing program: this vessel was wet-annealed in 1984, relicensed for operation till the plant shutdown in 1987, and was trepanned in early 1995; 3) The accelerated irradiations in the Belgian test reactor BR2 of the Yankee coarse grain plates YA1 and YA9 together with BR3 vessel specimens extracted at nozzle elevation, a location with negligible radiation exposure. It is contended that the PTS screening criterion was never attained by the BR3 and Rowe plates, and that the BR3 vessel anneal was neither necessary nor efficient. Furthermore, the practice to index fracture toughness to an absorbed energy level of 41 Joules in the CVN impact test is challenged in light of micromechanical modeling considerations and of supportive three-point slow bend precracked Charpy tests of the BR3 and YA1 plates. Finally, the sensitivity of embrittlement, annealing and post-anneal reembrittlement to irradiation temperature, nickel and other alloying or impurity elements is discussed, with a view to justify reducing the conservatism of regulatory prescriptions for these materials.