Dimensional Instability and Irradiation Growth of Zirconium Alloys
Unique aspects of material behaviour in a nuclear power plant include the component’s dimensional stability. In light water reactors, zirconium alloy structural components creep and change their dimensions through the approximately constant volume and stress-free process called irradiation growth. Radiation effects are not unexpected since during the lifetime of a typical component every atom is displaced from its normal lattice position at least 20 times.
Practical effects of dimensional instabilities are well known and it is a rare technical conference in the reactor performance field that does not include discussions on the topic. In addition to lengthening due to stress-free irradiation growth, many components are subjected to creep induced by stresses or by the restraint of irradiation growth. In adverse conditions, such dimensional changes have been observed to degrade the performance and safety margins of fuel components; e.g., produce lateral bowing of guide tubes or bulging of fuel channels that interfere with the movement of control rods. Differential growth due to neutron flux or hydrogen gradients can also lead to bowing issues.
Corrosion leading to hydrogen absorption in zirconium alloys contributes to component dimensional instability due to the fact that the volume of zirconium hydride is about 16% larger than zirconium and that hydrogen increases irradiation growth.
Such conditions illustrate the importance of understanding the empirical details and mechanisms of dimensional instability in the aggressive environment of the nuclear core for very practical reasons. Reliability of materials and structure performance can depend on such understanding.
In this STR review, aimed specifically at irradiation growth, addressed are conditions of direct interest to LWRs and CANDUs, including information that has mechanistic implications. Irradiation creep was covered earlier by ZIRAT14 Special Topic Report: In-reactor Creep of Zirconium Alloys, authored by Ron Adamson, Friedrich Garzarolli and Charles Patterson, 2009.
The STR addresses all data deemed relevant to understanding irradiation growth, including a broad review of growth mechanisms, and a summary of practical effects of growth on component performance.