Growth, Fracture, and Nondestructive Evaluation of Hydride Blisters in Zr-2.5 Nb Pressure Tubes

Rupture of a cold-worked Zircaloy-2 pressure tube in Pickering Unit 2 in 1983 occurred when a crack developed from an array of hydride blisters. These had formed on the outside surface of the pressure tube where it contacted the surrounding calandria tube. Fol-lowing this event, a comprehensive research program was initiated by Ontario Hydro. Its objective was to determine the consequences of pressure tube-calandria tube contact for channels in other reactors with Zr-2.5 Nb pressure tubes. The purpose of this paper is to present some of the results that have been obtained in the areas of blister growth, fracture, and nondestructive evaluation and to show how these results may be applied to Canada Deuterium Uranium (CANDU) reactor inspection.

Blisters were grown both on small specimens and on pressurized tubes. Blisters were ob-served to grow by gradual precipitation of hydride platelets throughout a volume that also increased with time. Blister growth rates increased rapidly with increasing average temperature. The hydride distribution in the blisters has been observed to be highly variable.

Four-point bending fracture tests were carried out on the small specimen blisters. These fracture tests indicate that blister size is a key parameter in determining the stress required to crack the blister with larger blisters cracking at lower applied stresses. This trend is shown to be consistent with previous finite-element modelling, which incorporated hydride plasticity.

Two of the pressurized tubes were inspected with a specially designed ultrasonic head, and responses were correlated with metallographic information. The ultrasonic inspection system was capable of detecting cracks as small as 0.15 mm deep in small blisters. This gives some expectation that inspection will be able to identify blistered tubes.