Phénix ran without problems through the 1970s and 1980s, but in the early 1990s it began to demonstrate a number of unexplained behaviours, including large power transients. This had serious safety implications, and the reactor was repeatedly shut down, spending most of the period from 1991 to 1994 being studied while offline. The long offline period required it to be recertified, so the plant also underwent a significant refurbishment between 1994 and 2002. It was finally recertified in June 2003, but only at a reduced power of 130 MWe.