Boron dilution transient calculation with the hextran code using CFDPLIM

A main analysis tool in Finland for reactor dynamical RIA calculations has been the three-dimensional HEXTRAN code which also includes full circuit models. Reliable calculation of propagating boron fronts is very difficult with standard numerical algorithms because numerical diffusion tends to smoothen the front. In this way, the reactivity effect of the boron dilution can be significantly lowered and conservatism of the analyses cannot be guaranteed. In normal flow conditions this problem has been avoided in HEXTRAN analyses by simulating the dilution front directly to the core inlet. In natural circulation conditions there occurs significant numerical diffusion even during the propagation of boron front inside the core. The new hydraulics solution method PLIM (Piecewise Linear Interpolation Method) is capable to avoid the excessive errors, the numerical diffusion and also the numerical dispersion.Therefore a new hydraulics solver CFDPLIM using PLIM has been applied to HEXTRAN. Examples are given of comparative analyses made with HEXTRAN in both flow conditions. The results show excellent performance of the solver also in varying flow conditions when the rigid discretization is dictated by neutronics