Nodal diffusion methods and lattice physics data in LWR analyses: Understanding numerous subtle details

In this paper, we present numerous important refinements of nodal diffusion methods that are required to obtain accurate commercial LWR power distributions using single-assembly lattice data. Many of these refinements have never been published (because of the proprietary nature of their development), and consequently, many subtleties of nodal methods are not well known in the nuclear reactor analysis community. The importance and impact of numerous refinements of basic nodal methods are documented here using the Hot Zero Power (HZP) BEAVRS PWR benchmark. This paper documents only those nodal refinements needed to accurately predict assembly-averaged power distributions without thermal-hydraulic feedback (e.g. isothermal conditions) and fuel depletion. Additional refinements needed for thermal feedback, fuel depletion, and pin power reconstruction are deferred for subsequent publications.