The Nuclear Safety Analysis Center (NSAC) has performed a thermal-hydraulic analysis of the Crystal River Unit 3 nuclear plant incident that occurred on February 26, 1980. The incident was initiated at 2:23 p.m. by an instrument and control system electrical malfunction that resulted in loss of power on the nonnuclear instrumentation (NNI) “X” bus. This failure caused the loss of several control and indication parameters, including pressurizer and steam generator level, and all reactor coolant system (RCS) temperatures. The loss of control parameters fed erroneous signals to the integrated control system, which in turn initially increased reactor power level, terminated feedwater flow to the steam generators, and opened steam turbine throttle valves to maintain outlet steam conditions. In addition, the power-operated relief valve (PORV) opened prematurely and remained open as a result of faulty circuit design in the NNI. This transient culminated in a reactor trip, turbine trip, and an engineered safeguards actuation, discharging ≈40 000 gal of primary system coolant to the floor of the containment building. The thermal-hydraulic analysis of the above event was performed by NSAC, using the RETRAN computer code. The objectives were as follows:

  1. To confirm by calculational techniques that the NSAC understanding of the transient was consistent with the recorded data from the plant.
  2. To derive by calculations the temperature transient that had occurred in the RCS, since complete data were not available. (The loss of power on the NNI “X” bus had caused loss of the normal temperature recordings.)
  3. To establish the timing of events that occurred during the transient such as closing of the PORV block valve, which was not automatically recorded.
  4. To assess the capability of the RETRAN code to predict accurately intricate operating plant transients and feed needed improvements back to the code developers.
The analysis results reveal that the objectives were accomplished satisfactorily. This paper highlights the extent to which the last objective was accomplished, i.e., assessment of the RETRAN code in accurately analyzing plant transients.