(An) information-theoretic approach to the reconstruction of pointwise flux distribution in nodal calculations

Abstract

The modern nodal method in nuclear reactor analysis calculates global corewise flux distributions very efficiently. However, it does not provide detailed local flux distribution for a heterogeneous assembly. A special technique is thus needed to reconstruct local information where and when desired. This thesis develops a new reconstruction method that is based on the maximum entropy principle in information theory. The probability distribution that maximizes the entropy provides the most unbiased objective probability distribution within the known partial information. The flux distribution on the boundary of a fuel assembly, which is the boundary condition for the neutron diffusion equation, is transformed into the probability distribution in the entropy expression. The most objective boundary flux distribution is deduced using the results of the nodal calculation by the maximum entropy emthod. This boundary flux distribution is then used as the boundary condition in a procedure of the imbedded heterogeneous assembly calculation to provide detailed flux distribution. The results of the new method applied to several PWR benchmark problems show that the reconstruction errors are comparable with those of the form function methods in inner region of the assembly while they are relatively large near the boundary of the assembly. The incorporation of the surface-averaged neutron currents in the constraint information (that is not done in the present study) should provide better results. Improvements in the finite difference solution method for the neutron diffusion equation used in the study would also lead to great reduction in the computing time.