Impact of Control Rod Position and Homogenization on Sodium Void Effect in CFV-type SFR

Paper i proceeding, 2016

In complex innovative fast reactor concepts, fairly detailed core modeling is essential for reliable safety analysis during severe accident scenarios. The CFV core with its axially heterogeneous design, has a negative sodium void reactivity effect, a favorable feature wich increases the inherent system safety in case of sodium boiling. In this work, we studied the impact that the control rod homogenization model used, and the control rod position, have on the sodium void-reactivity effect and the control rod worth, in the case of a voided CFV core. Three different control rod homogenization models were studied, the traditional 2D equivalence procedure, and two models based on a 3D equivalence procedure, taking into account the axial heterogeneity of the CFV core. It was found that the impact of control rod homogenization has a negligible effect on the sodium void reactivity effect. However, between different control rod positions, a difference of up to 1$ in the sodium void reactivity effect was found, hence the control rod position has to be carefully considered when calculating the sodium void reactivity effect. For the control rod worth in a voided CFV core, the traditional 2D procedure, could lead to discrepancies of up to 11% for control rod positions at the top of the core. These discrepancies could be much reduced by control rod homogenization with the 3D equivalence procedure. For the total control rod worth, all models and procedures produced results within the desired error margin of 5%.