Analysis of the correlation for energy deposition in PWR coolant by coupled neutron-photon transport calculations

This paper presents a thorough investigation of the correlation governing the energy deposition in PWR coolant, resulting from coupled neutron-photon transport. The study examines the effects of varying pitch size, fuel enrichment, and the presence of neutron absorbers in the fuel pellet on coolant heating. First, the correlation is checked against Monte Carlo simulations of a UO₂ fresh-fuel pin cell with different pitch sizes, and significant discrepancies are observed. These discrepancies are addressed by applying a correction factor based on the proportional relation between the reaction rate (involving both photons and neutrons) and the moderator mass. The correction successfully resolves the discrepancies, yielding relative gaps of less than 1% between the correlation and the simulations. The validity of this correction is demonstrated for both square and hexagonal cell configurations. The subsequent section investigates the effects of varying fuel enrichment and the presence of xenon and gadolinium absorbers in UO₂ pins. A gadolinium-poisoned fuel element is simulated in a 3×3 arrangement, with the poisoned pin positioned at the center, to simulate the proper spectrum around the gadolinium pin. Furthermore, the correlation is validated in fuel assembly design from the VERA benchmark problems 2B, 2O, and 2P. In problem 2B, the presence of water-filled guide tube cells leads to an increase in the mass of the moderator, which is taken into account by applying a correction factor in the correlation. In problems 2O and 2P, the combined influence of guide tubes and gadolinium pins on the energy deposition in the coolant is examined. Across all the case studies, the differences between the simulation data and the predictions from the correlation are minimal. Throughout the investigation, the study characterizes and analyzes the impacts of changes in design parameters on the energy spectrum, few-group homogenized cross sections, and reaction rates on coolant heating. This research provides a deeper understanding of the physical processes involved in radiation energy deposition in the coolant and offers valuable insights for future analyses utilizing the proposed correlation.