Fatigue Life Prediction for Steels in Pulsating Irradiated Systems

Abstract

EXECUTIVE SUMMARYContinuous energy pulses at a repetition rate of 10 Hz and the use of ferritic-martensitic (FM) steel for construction are assumed for a hypothetical high-temperature neutron source under study. Initial analyses based upon published literature data and established fracture mechanics models indicate that the service life of such a system would be limited by fatigue, and that viability would require designers to limit the magnitude of the alternating stress, keeping it well below 10 MPa in order to achieve a service life of 1 year with an assumed availability of 85 percent. These standard analyses use analytical expressions from published sources such as Roark's Handbook. The applied static stress is either assumed, or taken from parallel predictions made with finite element model (FEM) codes. The alternating stress is predicted with formulae that account for (1) thermal stress pulses associated with isochoric heating of the structural material; and (2) pressure pulses from any working fluid in contact with the vessel wall. With published mechanical property data for the irradiated structural material, including the yield strength ( YS ) and fracture toughness (K C ), the critical flaw size for initiation of a fatigue crack (a c ) can then be estimated. The calculated critical flaw size (a c ), and the largest expected manufacturing defect in the surface (a 0 ) are required to calculate the number fatigue cycles to failure (N f ) as a function of applied stress (). The fatigue limit ( CR ) can be estimated once the required number of fatigue cycles are specified, which in this case are in excess of 200 million (268,056,000) cycles. Given the fatigue limit and the yield stress, the Soderberg equation is used to establish the most conservative bounds for the mean and alternating stresses, levels that cannot be exceeded for safe operation. Less conservatively, and given the fatigue limit and ultimate tensile strength of the material, the Goodman equation can be used to establish these bounds. Even more optimistic estimates can be made with the Gerber equation. Of course, more complicated alternating stress waveforms can be accounted for using the damage accumulation model, with cycle counting based upon the rainfall criterion. Model parameters were based experimental data for FM steels from a number of published sources, and recently published data on the mechanical testing of EUROFER97 and EUROFER97 HT after irradiation to 71 dpa in the BOR-60 reactor in Russia. These data indicate that radiation-induced damage, strengthening and hardening, and ductile brittle transition temperature (DBTT) increases approach saturation levels after a damage level of 10 to 20 displacements per atom (dpa) is reached.