Axial discontinuity factors for the nodal diffusion analysis of high conversion BWR cores

High conversion Light Water Reactor concepts typically rely on heterogeneous core configuration, where fissile zones are interspersed with fertile blanket zones in order to achieve high conversion ratio. Such a heterogeneous structure renders the modeling of these cores impossible using conventional reactor analysis methods. It was recently suggested to overcome such difficulties, in particular, for the case of axially heterogeneous reduced moderation BWRs, by introducing additional set of discontinuity factors in the axial direction at the interfaces between fissile and fertile fuel assembly zones. However, none of the existing nodal diffusion core simulators have the capability of accounting for discontinuity of homogeneous nodal fluxes in the axial direction since the fuel composition of conventional LWRs is much more axially uniform. In this work, we modified nodal diffusion code DYN3D by introducing such capability. The new version of the code was tested on a series of reduced moderation BWR cases with Th-U233 and U-Pu-TRU fuel. The library of few-group homogenized cross sections and the data required for calculation of discontinuity factors were generated using Monte Carlo transport code Serpent. The results obtained with the modified version of DYN3D were compared with the reference Monte Carlo solutions and were found to be in a good agreement. The current analysis demonstrated that high conversion LWRs can in principle be modeled using existing nodal diffusion core simulators.