Nuclear power plant life extension and management aspects; neutron irradiation embrittlement and stress corrosion cracking - two possible degradation mechanisms and methods for their mitigation

The response of a mock-up low alloy ferritic reactor pressure vessel (RPV) steel and associated weldments to neutron irradiation has been studied using a combination of hardness, tensile, fracture mechanical and toughness tests in combination with annealing treatments. Thermal analysis using isochronal and isothermal techniques has indicated that annealing at a minimum of 440oC for 168h is needed to mitigate neutron embrittlement received at 290oC. Rates of re-embrittlement after annealing and reirradiating are no faster than initial rates, even up to neutron fluences as high as 5x1019cm-2 (energy E>1 MeV). All mechanical properties measured benefited from annealing. Thus, annealing is indicated as one measure for maintaining mechanical properties in irradiated low alloy steels and welds and should be considered in plant life management strategies. The influence of simulated reactor coolant water chemistry on the stress corrosion cracking propensity of ferritic low alloy steel specimens in autoclave loop experiments has also been studied. The double cantilever bend specimens were fatigue pre-cracked and wedge-loaded to different degrees to induce nominal stress intensity factors between 15-95 MPa.m1/2. Other specimens were subjected to stress using a tensile loading device integral with the test autoclave. The importance of close control of the dissolved oxygen content and the conductivity of the water has become evident under these experimental conditions. The RPV material and degree and mode of loading are also important parameters in SCC studies; stress intensity factors above 30 MPa.m1/2 have been associated with SCC in these studies. (author) 2 figs., 13 refs