The Fitness-for-Service assessment of Zr 2.5Nb pressure tubes in CANDU reactors requires the evaluation of fatigue. As-designed, the pressure tubes do not contain notches, but in operation various degradation mechanisms, such as fretting and crevice corrosion, can generate flaws with small root radii. Since these flaws are exposed to the coolant, an assessment of environmental effects on fatigue life is needed, and fatigue tests are being conducted to obtain the fatigue curve.
The pressure tubes are located in areas of high neutron flux, and thus the material is subject to irradiation effects. Since tests of irradiated material in reactor coolant environment are very difficult to conduct, a mechanistic understanding of fatigue at notches in Zr 2.5Nb material is sought, to allow the best possible use of fatigue tests of irradiated material in air and unirradiated material in water.
As part of the effort to develop a mechanistic understanding, continuum-level simulations of the mechanical behaviour of unirradiated and irradiated Zr 2.5Nb material were performed using finite element analyses. Both monotonic and cyclic runs were conducted to investigate the difference between first-cycle and shakedown behaviour near the notch tip. To increase confidence in the result, finite element notch strain predictions were benchmarked with a Neuber notch strain model. The paper discusses observations and possible consequences of these simulations.