The “garter spring” with a minor diameter of 8 mm was used at NPD2 in the space between the pressure tubes (PT) and calandria tubes (CT) that surrounded the PT. The single “garter spring” was of “tight” design and gripped -as its name implies -the outside of the PT at the center point so the springs would generally stay in place despite eventually getting brittle from irradiation. It maintained the gap between the two tubes and prevented sagging of the fuel channel onto the CT.
Some later CANDU reactor designs, with longer pressure tubes, used multiple, loose “garter springs” which, when there was no fuel bundles in channels (e.g. during commissioning) to secure them in place, often vibrated away from their positions resulted in damage to some PTs that sagged and contacted the much colder CTs. Subsequently there was a return to using “tight” “garter springs” in all those reactors.
