ANALYSIS OF HETEROGENOUS BORON DILUTION TRANSIENTS DURING OUTAGES WITH APROS 3D NODAL CORE MODEL
24th Symposium of AER on VVER Reactor Physics and Reactor Safety (2014, Sochi, Russia)
Reactor dynamics and safety analysis
Abstract
ANALYSIS OF HETEROGENOUS BORON DILUTION TRANSIENTS DURING OUTAGES WITH APROS 3D NODAL CORE MODEL
Kuopanportti, J.
Fortum Power and Heat Ltd, Espoo, Finland
jaakko.kuopanportti@fortum.com
ABSTRACT
A diluted water plug can form inside the primary coolant circuit if the coolant flow has stopped at least temporarily. The source of the clean water can be external or the fresh water can built up internally during boiling/condensing heat transfer mode, which can occur if the primary coolant inventory has decreased enough during an accident.
If the flow restarts in the stagnant primary loop, the diluted water plug can enter the reactor core. The flow can restart if a primary coolant pump is activated or if the natural circulation starts. In this paper, it is assumed that the natural circulation moves the plug into the core.
During outages after the fresh fuel has been loaded and the temperature of the coolant is low, the dilution potential is the highest because the critical boron concentration is at the maximum. The lid of the pressure vessel can be open or closed depending on the stage of the outage.
The idea of the analyses is to examine the behaviour of the core as clean or diluted water plugs of different sizes enter the core during outages. Therefore, the actual build-up of the diluted plugs was not analysed but the plugs were simply assumed to appear in the lower plenum or the downcomer in the beginning of the transient.
The analysis were performed with the Apros 3D nodal core model, which contains an own flow channel and 10 axial nodes for each fuel assembly. The wide-range cross section data was calculated with CASMO-4E.