3-Dimensional Analysis of a Baron Dilution Accident by Using the Code DYN3D/M2

An incorrect startup of a pump in an isolated loop containing a plug of diluted absorber can initiate a reactivity accident in a VVER-440. It is assumed that the main gate valve is opened after starting the main coolant pump. When the plug enters the core, it gives rise to a positive reactivity. An asymmetric distribution of diluted water in the core is expected which requires a 3-dimensional analysis for a more correct description of this accident.
The code DYN3D/M2 developed for investigations of reactivity initiated accidents in thermal reactors with hexagonal fuel elements is used for analysis. A model describing the mixing of water from the different loops in the lower plenum of a VVER-440 reactor gives the boron concentration at the inlet of each fuel element. The two limiting cases, ideal mixing and no mixing, are investigated for comparisons.