Incorporation of a rectangular void into a three-dimensional neutron diffusion nodal model
The problem of a parallelipiped void occupying a nodal position in the diffusion nodal method is investigated. To solve for the neutron partial currents that couple the void to its surrounding nodes, the concept of overall transfer matrix is introduced. This transfer matrix, when multiplied by the column vector containing the Legendre coefficients of the spatial flux in the neighboring node, produces the outbound neutron partial currents. This method does not require the solution of the neutron transport or diffusion equation inside the void but rather uses the faces of the void as P1 distributed neutron sources and finds the subsequent neutron streaming through the void. The benchmark problem consists of balancing the number of neutrons per second that enters the void with the number per second that exits from it and the results have been found satisfactory to better than 0.1%.