All three reactors, Rapsodie, Phenix and Superphenix, encountered significant safety problems during start-up, operation and dismantling periods; including sodium leaks, reactivity incidents, explosions and material failures.

Rapsodie — sodium leaks and a lethal explosion

After a rather smooth operational period from Rapsodie’s start-up at the beginning of 1967, at the end of 1978 a small primary sodium leak was detected, which led to the decision to reduce the operational capacity from 40 MWt to approximately 22 MWt. In January 1982, another small sodium leak was detected in the nitrogen system surrounding the primary vessel. Localization of the leak was believed to be too costly and too uncertain. The reactor was therefore shut down on 13 October 1982.

The secondary sodium was drained in April 1983 and is still stored on the Cadarache site. The primary sodium was drained by April 1984. It took two years to retrieve the 468 highly irradiated reflector assemblies from around the core (222 made of nickel, 246 made of steel) from the vessel, wash them to eliminate traces of sodium, and install them in a storage container. The 37 tons of primary sodium were treated in a specially designed facility (DESORA) that turned it into 180 cubic meters of concentrated sodium hydroxide.

On 31 March 1994, an explosion occurred during the cleaning of the residual primary sodium contained in a tank located in a hall outside the containment building.37 An experienced, highly specialized 59 year old CEA engineer was killed instantly and four people were injured. Approximately 100 kg of residual sodium had remained at the bottom of a tank at the end of the treatment campaign. An analysis of the accident concluded later:

The process selected to perform this clean up operation consisted in progressively introducing in the tank a heavy alcohol called ethylcarbitol, while monitoring the reaction through temperature, pressure, hydrogen and oxygen measurements. The major cause of the accident was due to the formation of an heterogeneous physical — chemical environment, complex and multiphasic made of three basic components: alcohol, alcoholate and sodium. This environment turned out to be particularly favourable to the development of thermal decomposition reaction and/or catalytic exothermal reactions. Large quantities of gases (including hydrogen and light hydrocarbon compounds) were thus produced. Shortly after the last alcohol injection on 31 March, the phenomenon ran out of control, leading to a sudden rupture of the overpressurised tank, then to the explosion of the gases mixture blown out in the hall.38

Since this accident, the use of ethylcarbitol or other heavy alcohol has been forbidden in the treatment of sodium. But the circumstances of the accident are subject to an ongoing legal dispute. In 2001 an expert court-commissioned analysis accused the CEA, the IPSN (Institute for Nuclear Protection and Safety, predecessor of IRSN) and the safety authorities of "faults by imprudence, negligence and violation of safety obligations."39 As of December 2009, there still is no published information indicating that there has been a final judgment.