Lessons learnt from the resin release into the primary circuit of the Fessenheim NPP unit 1 in January 2004. Impact on the nuclear safety
EUROSAFE Forum, Berlin, November 8-9, 2004,
Rapport DSR 39
On January the 24th, at the Fessenheim NPP Unit 1, a human error was committed during a boron demineralizer line-up, caused by lack of preparation. Consequently, a quantity of resin estimated at about 300 liters was released from this demineralizer, through its safety valve, into the head-tank of the Chemical and Volume Control (CVC) System and after that, into the primary circuit. The incident had a real impact on the unit: the CVC filters were clogged, the seal injection flow of the primary circuit main pumps was lost, the primary circuit main pump 2 tripped four days after the incident, as the rate of the recirculated seal leak flow (downstream the seal 1) increased up to the automatic trip set point, the shaft of the running primary circuit feed pump was found seized into the rear hydrostatic bearing following the pump stop (after ten days of successful operation), the thimble plugs were jammed into their guide tubes, the small diameter pipes were plugged. The unit shutdown for over five months was necessary to clean the primary circuit components, repair or replace the affected equipment items and carry out inspections and tests. The reinforced unit in-service monitoring program, set up during the unit start-up, confirms that, up to now, the unit operation has not been adversely affected by the residual amounts of resin which subsist in certain areas of the primary circuit. Nevertheless, it remains to verify that, in the long term, these deposits will have no negative chemical effect in the potential confined areas, such as the thermal barriers of the primary circuit main pumps. Finally, the occurrence of this incident underlines, once more, the importance of normal operating activity preparing and checking. It also reveals the implementation of an “unforgiving” design change allowing the installation of a boron demineralizer safety valve having its outlet connected to the primary circuit.