As of 1988, Phenix had a cumulative average load factor of 60.5 percent. Operation was not without problems, however. The first fuel pin leak occurred in June 1975, secondary sodium leaks occurred in September 1974, March and July 1975 (approximately 20 liters each for the first two and 1 liter for the last). "Leakage generally led to the slow spontaneous combustion of this sodium in the insulation, without triggering fires external to the insulation."40 Repair operations proved ineffective and valves in the three secondary systems were eventually replaced by diaphragms.

On 11 July 1976, a sodium leak occurred at the intermediate heat exchanger (between the primary and secondary sodium loops) that led to what was later labeled as the "first real sodium fire in the Phenix plant." The fire was extinguished manually. On 5 October 1976, another sodium fire broke out at an intermediate heat exchanger and was again manually brought under control. Figure 2.7 provides an illustration of the impact of a sodium fire at an unidentified date. A further sodium leak was identified in August 1977. Further secondary sodium leaks were identified in the 1980s, including incidents in March and November 1984, and in September 1988.

In July 1978, two control rods showed a level of swelling that prevented normal extraction from their guide tubes. However, since the blocking was positioned above the insertion level during normal operation, the phenomenon was considered not to constitute an immediate safety issue.

In the first years no events directly impacted the steam generators. Steam generator failures, which can lead to violent sodium-water reactions are the most feared incidents in fast-neutron reactors. But various incidents took place in the steam generator environment, including four water leaks in the economizer-evaporator inlet of the steam generators between November 1975 and September 1976. The first cladding failure was detected in May 1979. It led to the "greatest release of fission gas (xenon-135) ever seen in the Phenix plant".

Between April 1982 and March 1983, sodium-water reactions in the reheater stages affected all three steam generators in at least four incidents. In the first event, on April 1982, approximately 30 liters of water leaked into the sodium and created a combustion flame that burned a hole in two tubes and damaged the reheater module’s shell. The other three events apparently involved quantities of water limited to a few liters. These four sodium-water incidents resulted in a total of six months of outage and nine months of operation limited to two-thirds capacity.

The most costly and potentially most significant incidents were rapid reactivity transients in the core on three occasions in 1989 (6 and 24 August, 14 September) and on 9 September 1990. In spite of a research program costing hundreds of millions of francs, 200 person-years of work, and the elaboration of some 500 documents, the cause of the phenomenon was never conclusively identified.

The events were particularly worrying since following reactivity and power drops of 28 percent to 45 percent within 50 milliseconds, power actually increased above the original state of the reactor. The fear was that such an event could trigger a power excursion. The cause could possibly have been an argon gas bubble going through the core, but this hypothesis was never confirmed. Subsequent investigations revealed that similar events had taken place in April 1976 and June 1978 and that the explanation at the time (control rod slippage) was wrong.