Crack Arrest Behavior of Pressure Vessel Plates and Weldments as Influenced by Radiation and Copper Content

An experimental investigation was conducted to determine the effects of fast-neutron irradiation on the crack-arrest behavior and other mechanical properties of four reactor-pressure-vessel materials, including both plates and weldments. Two of the materials were typical of early melting and welding practices and had copper contents of at least 0.20% by weight. The other two materials were typical of current melting and welding practices and had copper contents of no greater than 0.07% by weight. The principal objectives of the study were to: (1) determine the pre- and post-irradiation static crack-arrest toughness (K_Ia), and (2) evaluate and improve procedures for estimating radiation effects on K_Ia.

As was expected, irradiation of the higher copper materials to a neutron fluence of about 1.5 × 10¹⁹ neutrons (n)/cm² (E ≥ 1 MeV) at 288°C produced relatively large increases in yield strength, Charpy V-notch transition temperature, and Jïia transition temperature, and sizable decreases in Charpy upper-shelf energy and 7ic values. For the lower copper materials, the effects of radiation on the preceding properties were more modest and no lowering of the Charpy upper-shelf energy was observed. Of various Charpy transition parameters examined, the radiation-induced shift in fracture-appearance transition temperature (FATT) provided the closest correlation with the observed shift in the K_Ia, transition curve— within 10°C for the high copper materials. Various empirical expressions to estimate radiation effects likewise were compared with observed shifts in the K_Ia transition curve. The Nuclear Regulatory Commission Regulatory Guide 1.99 equation tended to overestimate the effect of radiation on the K_Ia transition, particularly at the higher copper levels, where the overestimate ranged from 55 to 80°C. The Metals Property Council (MPC) trend curve, on the other hand, while slightly nonconservative for low-copper materials, provided a reasonable, but conservative, estimate of the effect of radiation on the K_Ia-transition-curve shift for the higher-copper materials—within 20°C of that observed.

Based on these results, it appears that Regulatory Guide 1.99 is overly conservative in estimating radiation effects on the crack-arrest behavior of higher copper plates and weldments. Estimates of the K_Ia-transition-curve shift based on the Charpy FATT shift or on the MPC trend curve were found to be 45 to 70°C less conservative than were Regulatory Guide 1.99 estimates. Use of either of the more realistic estimation procedures would improve the ability to predict the arrest depth of a rapidly propagating crack in an irradiated pressure vessel.