One does not normally associate small items such as garter springs with large facilities such as CANDU reactors. In fact, they play a critical role.
In a CANDU reactor, on average, every atom in the core is knocked off its lattice position by a neutron once per year. In almost all cases the displaced atom returns immediately to its initial location. Over many years of operation, however, the odd time this does not recur, resulting in swelling, lengthening or sagging of the material with a change in mechanical properties.
This is particularly the case for the long (~6 m) thin-walled, horizontal, zirconium pressure tubes that contain the relatively-heavy fuel bundles and coolant in a CANDU reactor. The pressure tubes, which operate at a relatively-high temperature of ~350 C, are separated from the much cooler calandria tubes by a relatively small gap, typically 7 to 9 mm. Contact between the two surfaces, which can lead to a cool spot on the pressure tube and subsequent hydride blister formation and corrosion concerns, must be avoided. Brittle hydride formations led to a pressure tube failure in Pickering unit 2 on August 1 1983, with a subsequent expensive cleanup and retubing of Pickering units 1 to 4.
The gap between a pressure tubes and its calandria tube was maintained by coiled spacers (aka garter springs) at a number of locations (initially 2, later 4) along the pressure tube to avoid the potential contact. During the earliest experience with these springs, it was found that they did not necessarily remain where initially placed due to small vibrations of the pressure tubes. Thus a technique called SLAR (Spacer Locating And Repositioning) was developed to reposition the springs with a device placed in a defueled pressure tube that produced a magnetic pulse to move the springs to the desired locations.
Experiments with different spring designs and materials as well as different tension in the springs eventually solved the spring-shifting problem and the relatively small and inexpensive garter springs continue to play an important role in the world-wide fleet of CANDU reactors.
Visitors to the Canadian Nuclear Heritage Museum (CNHM) can see a number of different versions of garter springs. Their usage is perhaps best illustrated in the base of lamp donated by Fenella Lane. The lamp had been given to her late husband, Al Lane, as a retirement gift. The lamp base, built around a CANFLEX fuel bundle, shows the bundle positioned within a pressure tube and then within the calandria tube. A garter spring which separates the two tube completes the display.
Visits to the CNHM can always be arranged by contacting info@nuclearheritage.com
