The US Nuclear Regulatory Commission (NRC) has confirmed that as incident at unit 1 of the Three Mile Island plant last October, in which a previously-plugged steam generator tube split causing damage to adjacent tubes, is “not a generic problem” for plants with recirculating steam generators.

The split, which the NRC reported on January 8 this year, occurred after water was trapped inside a plugged tube. As the plant heated up water expanded, causing the plugged tube to swell and lock within its supporting structure of the steam generator. Once locked, the tube was more vulnerable to flow-induced vibrations, causing it to split and strike other tubes. The industry will now carry out a five-month study to identify potential mechanisms that could cause plugged tubes to damage adjacent tubes.