From 28 to 29 May 2004 a scheduled plant shutdown for repairing a primary leakage (gland of a control valve ) inside the containment was carried out. During preparation of the restart on 28. Mai (control room check at 19.00 hours) the operators failed to recognise that containment ventilation was in a condition not allowed for plant start-up (as required by Technical Specification). The check was done in the control room by checking the indicator lights: large purge (light was on), small purge (light was off), this was errorneous interpreted. In effect, only the “small purge” was allowed. The consequence of the difference lay in the open drywell isolation valves, which degraded the integrity of the room surrounding the rector vessel, which is needed for pressure suppression about the water pool. The pressure suppression is required at reactor coolant temperatures > 100°C in case of a loss of coolant accident. However, the drywell isolation valves, if they are open, receive a closure signal “drywell temperature > 55 °C” and also from other LOCA signals. The highest reactor coolant temperature during the event was 142°C. Temperatures above 100°C and open drywell lasted only 2 hours. Therefore, there was no identifiable risk increase. During the event, there was a radioactive release to the environment. The measurement shows 74 MBq Iodine-131 during the weak of 2004-05-25 to 2004-06-01, corresponding to about 2% of the short term authorized limit for Iodine. No persons were contaminated during the event.