Prediction of fast fracture in Zr-2.5%Nb pressure tubes using elastic-plastic fracture mechanics

The CANDU nuclear reactor system uses cold-worked Zr-2.5% Nb pressure tubes as primary coolant containment. At present, critical crack length data are obtained by burst-testing full size tube sections. Burst-testing programs are time consuming, costly and experimentally awkward, and the development of a technique for predicting critical crack length from a small-specimen test would allow the collection of many more data, and improve the statistical basis of failure predictions. It would also provide a quick and easy method of assessing unusual material or environmental conditions, which may arise in the future, and provide a framework for predicting critical conditions in new tube sizes, which might become relevant through changes in reactor design. Because of the ductile condition of the pressure tube alloy under most circumstances, linear elastic fracture mechanics does not provide a suitable fracture criterion. Furthermore, Zr-2.5% Nb can accommodate considerable stable crack extension under rising load, hence, a fracture criterion based on initiation of crack extension is extremely conservative. Therefore, an R-curve approach has been selected, for relating small specimen behaviour to the instability of pressure tube cracks. The results are presented and discussed. (author)