The spherical harmonics method has long been used to obtain approximate Solutions to the Boltzmann equation of neutron transport theory. In spite of this wide use, the boundary conditions to be applied at a vacuum boundary have never been uniquely specified. Presented here is a derivation of a unique set of consistant boundary conditions for both the flux and iraportanoe function in the Pn approximation. This derivation is based on only the use of a spherical harmonics expansion and the principle of importance weighting (or the variational method). The boundary conditions so obtained are shown to give better vahies for the extrapolated end-point than eitlier of the more commonly used boundary conditions. The boundary conditions are tabulated numerically for direct use in most spherical harmonics calculations.
展开▼
