The unit was in stable operation with a load of 290 MWe on December 15, 1995. Because some tubes of no. 3 high pressure heater leaked seriously, at 10:18 a.m. control room operator isolated the leaking no. 3 high pressure heater. The Main feedwater automatic regulation could not meet the requirement of water level control of SG and then manual control failed, which led to turbine trip and reactor scram because of "SG water level high-high" at 10:26:50 a.m.