Boundary Condition for the Adjoint Flux Corresponding to the Flux Discontinuity of the Nodal Diffusion Equation in the Perturbation Theory

Abstract

In order to take into account the heterogeneity of the fuel assemblies of light water reactors by the nodal diffusion method, a method is used where the discontinuity boundary condition is applied for fluxes at the node or assembly interfaces, although the currents are continuous. Then a problem arises what kind of boundary condition should be used for the adjoint flux in solving the adjoint equation to perform subsequent perturbation calculations.

As an appropriate boundary condition, it is shown mathematically, that the current of the adjoint flux must be discontinuous, although the adjoint flux itself must be continuous.

KEYWORDS:

• discontinuity factor
• discontinuous boundary condition
• perturbation theory
• fuel assembly homogenization
• adjoint equation
• adjoint function
• nodal method
• criticality calculation