Analysis and Calculation of an Accident with Delayed Scram at NPP Greifswald Using the Coupled Code DYN3D/ATHLET

Analysis and calculation of an abnormal event which occurred in 1989 at the NPP Greifswald (VVER-440/213) are reported about. This event was a scram failure at about 50% nominal power with two of six working main coolant pumps (MCP). The analysis revealed remarkable differences between the active and passive loops due to non-homogeneous coolant mixing in the downcomer and lower plenum in the initial state before the accident. An existing input data deck for the coupled code DYN3D/ATHLET was modified to carry out calculations of this accident. On account of the absence of a mixing model considering loops with forward and loops with backward flow at the same time, the six reactor loops were modeled by a 3-fold (passive), a 2-fold (active) and a single (with the MCP to be switched-off at the beginning of the transient) loop. The comparison of the stationary calculation with the reported data shows a good agreement taking into account the mentioned deviations between the loops. Most of the calculated thermohydraulic parameters during the accident are very close to the measured data. Deviations in the loop temperatures can be explained by the differences in the measured stationary data and by the fact of non-homogeneous coolant mixing in the downcomer and the lower plenum. A small, but continuous power increase was observed during the whole time in the calculation while the measured power was remaining nearly constant. The power behaviour was brought into agreement with the measured data by a correction of the moderator temperature coefficient within the accuracy recommendations of the Atomic Energy Research (AER).
The calculation of this accident is an important contribution to the validation of the coupled code DYN3D/ATHLET. It demonstrated problems of the analysis of real abnormal events on NPP`s.