Due to problems in the Process Salt water heat exchangers, the performance of control room air-conditioning chillers (which is cooled by the process salt water) was affected, resulting in rise of the control room temperature (up to 87.5 deg. F). this rise in control room temperature adversely affected the satisfactory operation of the two regulating computers. At 0153 hrs controlling computer B started internal initialization frequently, while controlling computer A stalled at 0200 hrs. without any alarm. Controlling computer B took control, but its control valve was observed 80 % closed with steam pressure at 580 psig and rising. To control the situation, the steam pressure set point was brought down to 550 from 560 psig. Meanwhile the reactor control display went off due to stalling of computer A and continuous re-initiating of computer B. However, the non-availability of display is significant only as an indication of the initialization attempt. it did not have much importance as far as availability of information to operate the reactor was concerned. Indication of control rod drive actuation was also observed. With the increasing steam pressure, turbine-generator load was observed around 80 MWe.
Due to malfunction of both computers, the operators decided to shutdown the plant, therefore, turbine-generator load was reduced to 20 MWe, and station load was transferred to starting transformer, immediately after this operation, therefore, turbine-generator load was reduced to 20 MWe, and station load was transferred to starting transformer. Immediately after this operation, the reactor tripped at 0204 hrs. No computer printout of the transient from 0201 hrs to 0203 is available as a result of malfunction of both the computers. The available printout suggests that the reactor tripped on high outlet header temperature. Justification: The event frequency falls in the category "expected" as per definition of annexure III-A.3.1 of the INES User's Manual. The safety system was available in full, therefore, using table-II of the manual the event is rated at scale "0" (below scale).