On 21 November 1996, it was discovered that a 15 cm diameter unfiltered containment ventingline was actually open to the outside air, while its 4 block valves all registered "closed". It appeared that the valves had all been mispositioned, apparently as a result of maintenance work 9 months earlier.
The deviation was corrected within 2 hours. The 4 valve ball spindles were 90 degrees rotated with respect to the motor drives and correctly attached to the position indicators.
Based on the Users Manual Table I: Criteria for Degradation of Defence in Depth without an initiator, with frequency medium and safety function inadequate the event is classified as level 2.
Event date: Thu, 21-11-1996