Considerations of the effects of azimuthal fuel motion in a fast breeder reactor accident

A sizeable reactivity feedback can result from material movement in a large liquid metal fast breeder reactor (LMFBR). Previous investigations considered mainly fuel slumping under gravity and outward radial motion. Very little work has been done on azimuthal motion. Furthermore, studies of the effects of material motion either use point kinetics or very expensive space-time differencing solutions. This work was undertaken to develop an intermediate approach between point kinetics and the full space-time finite difference solutions. The approach was then applied to sample problems with azimuthal and radial material motion under possible accident conditions. Specifically, the objectives of the work were to: (a) develop a technique for treating space-time neutron kinetics during postulated accident conditions which include material motion; (b) apply the technique to sample problems. The technique was developed based on the use of the finite element method (FEM) for the spatial differencing of the multigroup, time dependent diffusion equation. The FEM was chosen for three reasons. First, the FEM gives good accuracy with relatively fewer unknowns than the finite difference method. Second, the FEM is very flexible in setting up a mesh for the geometry of concern. Last, the FEM could handle the spatial differencing of a mesh which became distorted as the material in the reactor moved. This material motion was handled by specifying the FEM mesh nodes as a function of time and periodically updating the spatial matrices. Finally, the method used to solve the time dependence was Gear's variable order predictor corrector scheme