Analysis of an actual reactor trip operating event due to a high variation of neutron flux occurring in the Vandellòs-II nuclear power plant

The Asociación Nuclear Ascó-Vandellòs-II (ANAV) is a utility that runs three operating reactors. All of them are Westinghouse-design 3-loop Pressurized Water Reactor (PWR), with an approximate electrical power of 1000 MW. In the past ANAV prepared an Integral Plant Model for each plant using Relap5. The model is used to support plant operation and control from the point of view of safety and operation. The Technical University of Catalonia (UPC) has been working with ANAV since 1991 in order to establish, qualify and use these Best Estimate (BE) models. Among the different uses of such models, simulation of actual transients has special significance, since it is usually extremely helpful in order to understand their dynamic behaviour. This paper shows an example of how the analysis helped to clarify some questions about the sequence of events and the influence of manual actions. As some parameter time trends are not measurable by plant instrumentation, the model helped to provide a complete explanation of a transient for which the initiating event was a loss of off-site power.