Three VVER-1000 Tests Suitable for the Validation of Coupled Neutronic / Thermal-hydraulic Codes

In the first test, carried out in the NPP Balakovo-4 (Russia), one of the two working feed water pumps was turned off at nominal power. The control system automatically reduced the reactor power down to 45 % of the nominal level. The coolant heat-up decreased proportionally to the power reduction. In the secondary circuit, the pumping power of the second feed water pump was increased in order to compensate some part of the missing feed water flow. In the following, the power of the second pump was reduced again to match the decreasing thermal power of the primary circuit. Corresponding changes in the steam generator water levels were observed. - The initiating event for a transient observed in the Zaporoshye-6 VVER-1000 (Ukraine) was the degradation of the turbo-generator power from 1000 MW down to the house load level of 50 MW. The reaction of the core control system and the behaviour of the primary circuit were similar to that in the Balakovo case. Immediately after the drastic reduction of the load level, the turbine started increasing its rotation speed. In order to prevent turbine damage, the steam control valves immediately reduced the steam flow to the turbine. Thus the turbine speed was stabilized, but the main steam header pressure started increasing. The turbine bypass valves were opened reducing the pressure quickly to its previous level. - The third test was carried out in the NPP Kozloduy-6 (Bulgaria). At 90 % of nominal power, two neighbouring main coolant pumps were switched off. The power was reduced to 35 %. Immediately after the transient initiation, the pressure in the primary circuit began to drop. All four pressurizer heaters started working to stabilize pressure. - The data measured in the three tests are useful for the validation of coupled codes.