NDTnet 1998 Aug, Vol.3 No.8

Abstract

Corrosion of zircaloy pressure tube by high temperature heavy water coolant produces hydrogen during reactor operation. A part of this hydrogen penetrates into the pressure tube. In case of contact between hot pressure tube and cold calandria tube, hydrogen absorved in the pressure tube migrates to the cold spot on the outer surface under thermal and stress gradients. The localised concentration of massive zirconium hydride is called "Blister" which can crack after attaining a critical size and may eventually lead to premature failure of pressure tube. Hence, it is important to develop a non-destructive testing technique to detect formation of a blister at an early stage.

Hydrogen in zircaloy increases ultrasonic longitudinal wave velocity and decreases shear wave velocity. But due to difficulty in accurately measuring the thickness of a radioactive pressure tube, it is not possible to rely on either of the velocities to predict hydrogen concentration or blister. However, the ratio of longitudinal to shear wave velocity is independent of thickness and increases with hydrogen concentration.

The velocity ratio of unhydrided zircaloy is 2.05 and 2.8 for a complete hydride blister. Velocity ratio is measured to predict hydrogen concentration and size of a blister using the calibration graph. Artificial blisters grown in laboratory were detected distinctly by velocity ratio measurements. The prediction by measurement made on reactor operated contact region of a number of pressure tubes were confirmed later by neutron radiography and metallography. It was possible to map the hydrogen distribution pattern at the contact location of pressure tube using this technique which is shown in figure 1.

Fig. 1. Ultrasonic velocity ratio plot showing contact patches of a reactor operated pressure tube.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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