Assessment of the Influence of a Mixing Model on a Boron Dilution Transient in the VVER-440 Core by Help of the Code DYN3D/M2

A reactivity initiated transient caused by entering a water plug with diluted boron concentration into the core during incorrect loop startup in a VVER-440/W-213 reactor is analyzed with the help of the code DYN3D/M2. The water with diluted boron concentration from the one loop is mixed with the water of the other loops before the core inlet. The results for the transient are influenced by three different mixing models that were investigated:

• ideal mixing: the boron concentration at all inlets of fuel assemblies is uniformly distributed,
• no mixing: the boron perturbation occurs only in the fuel elements of the sector belonging to the considered loop,
• mixing model: estimation of boron dilution at the core inlet by superposition of reference distributions, defined experimentally or pre-calculated by a simplified analytical model.

The results of DYN3D/M2 for these three cases presented in the paper show significant differences of the safety parameters, i. e. nuclear power, maximum fuel temperature or critical power ratio.
The asymmetric boron perturbation and the short length of the plug requires a more-dimensional core model for the treatment of this transient. The code DYN3D/M2 is capable of describing the space-dependent effects of the diluted boron in the core for all three cases. The power excursion is connected with strong deformations of the neutron flux distribution and power shape as a result of the space-dependent boron perturbation. Therefore the feedback effects are also influenced by the space effects.
The different boron mixing models lead to different boron distributions in the core, which influence the power excursion and the results for the safety parameters. By using the mixing model, more severe consequences of the transient are obtained than by assuming ideal mixing. Assuming no mixing, the perturbation results in a severe accident with partial core damage. The results indicate the importance of the applied mixing model for the inv ...