On 22/06/2006 Unit 4 was in operational state E1 (232 oC). At 06:30 while monitoring computer displays after shift turnover the reactor operator discovered that the temperature of CRD housing 6/7 exceeded the limit specified in TS 184.108.40.206.2 (max. 100 oC-5 min). Subsequently, the operator lowered the CR drives from lower end switch into lower end stop by manual scram, thereby deenergizing them. 4 out of 6 RCPs which had been operating until that time, were shut down (RCPs 2,4,5,6). After interpretation of the archive data personnel determined that the parameter excess has occurred earlier and has remained so for about 8 hours. Local inspections revealed that the inrease in temperature was caused by leakage of the closed valves 40TX25S215, S216 through which reactor coolant flowed from the upper block central venting line toward the makeup water system. Personnel stopped the leakage by re-tightening the valves.