Fracture Toughness Vis-a-Vis the Master Curve for Some Advanced Reactor Pressure Vessel and Structural Steels

R.K. Nanstad (Sp), M.A. Sokolov, Oak Ridge National Laboratory, Oak Ridge, Tennessee (USA) 
The evolutionary concepts under consideration for advanced nuclear reactors naturally compel the need for advanced structural materials. In the Generation IV reactor program, for example, there are four high temperature reactor systems. The preliminary designs for the RPVs in these systems envision operating temperatures from 280 to 850 degrees C, operating pressures from atmospheric to 25 MPa, and radiation doses from 0.01 to 40 dpa. Moreover, they will operate with internal environments of helium, helium/supercritical CO2, supercritical water, lead, or lead/bismuth coolants. The RPVs that will operate at very high pressures are also conceived to have very large vessel sizes that will require scale-up of ring forging and joining technologies and ensuring thick-section properties. Very large forgings have been fabricated in the past, but not as large and thick as that projected for some of these systems. In these cases, high-strength alloys can reduce sections sizes with concomitant advantages to through-thickness properties, welded construction, transportation, etc. In this study, the fracture toughness of four different structural materials of varying structural experience will be discussed, two ferritic-martensitic alloys used for high temperature structures, an oxide-dispersion-strengthened (ODS) alloy with very good high temperature strength, but a very new alloy with essentially no database, and a high strength, low alloy structural steel with improved toughness often used for naval structures. It is shown in this paper that, in general, the master curve appears to adequately describe fracture toughness vs temperature behavior for ferritic-martensitic steels, although some steels tend to exhibit relatively high scatter outside of tolerance bounds, and that fracture toughness data in the ductile-brittle transition region are sparse, especially in thick sections. Available data on fracture toughness in the irradiated condition for all materials are also discussed.

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