Evaluation of PBMR control rod worth using full three-dimensional deterministic transport methods

It is a well known fact that the neutron diffusion theory fails in the vicinity of strongly absorbing media, such as the control rods. This failure is much more pronounced in the PBMR because the location of control rods in the side reflector adds a directional dependence to the flux, and this complicates the problem further. Reactor control and safety can only be ensured if control rod worths are accurately predicted. In this work a thorough evaluation of different control rod models is performed by constructing an approximate two-dimensional (2D) model adopting the so-called grey curtain and this is compared to full 3D deterministic transport model. Both these models are compared to an explicit MCNP model. It is shown in this study that it is possible to have an accurate 2D model of control rods, utilizing appropriate equivalent cross sections and applying them to a control rod grey curtain. Further, this paper also shows that it is possible to obtain reasonably accurate control rod worths using a 3D deterministic transport method with the same fidelity of a Monte Carlo reference calculation, provided that the correct cross sections are used. These results confirm that the deterministic transport models can be successfully used for PBMR transient safety analysis.