A limiting transient under single-loop operation (SLO) conditions was analyzed to establish the analytical capability to support SLO for Washington Public Power Supply System's WNP-2.

In the WNP-2 RETRAN model, each of the two recirculation loops is simulated separately. Under the SLO condition, the unaffected loop provides ~50% core flow with a power of ~72%.

Two steady-state cases were run using actual plant data under single-loop conditions to show that the model initializes correctly. The plant data were collected from the cycle 2 SLO. An additional benchmark was performed using the single-recirculation-pump test data collected during the power ascension test in the initial startup phase of WNP-2. The calculated reverse flow of the idle loop compared well with the measured data from the single recirculation pump trip test.

After the benchmarking, a limiting transient, i.e., generator load rejection without bypass, was analyzed for cycle 8. The resulting power, flow, and pressure histories were compared with the fuel supplier's results. The Supply System results for the limiting transient showed reasonably good comparison with the fuel supplier's results, with the Supply System model yielding more conservative results. In addition, hot-channel analyses indicate that the SLO thermal limits are bounded by those from the two-loop operations.