Estudo por simulação de Monte Carlo do retroespalhamento em água para a dose de nêutrons em uma embarcação nuclear submersa

Abstract

This work presents a computational simulation study intended to analyze the modifications induced in the internal radiation field within a circular hull section of a hypothetical nuclear vessel, resulting from the presence of an external water layer. The study uses numerical modelling based on the Monte Carlo Method (MCM), implemented through the Geant4 particle transport code, with the objective of quantifying possible changes in the dosimetry inside the geometry of interest when scenarios with water or air in the external environment of the structure are compared. Initially, a computational methodology is developed for geometry generation, definition of the neutron source, and recording of the energy deposited in a tissue-equivalent sphere standardized by the International Commission on Radiation Measurements and Units (ICRU), positioned in the region of interest. Subsequently, a validation simulation with monoenergetic neutron fields is performed in order to verify the model's ability to reproduce available reference results. The validation simulations demonstrate good agreement with the reference data, reproducing the expected behaviour of the absorbed dose throughout the energy spectrum. After validation of the sensing element and the physics library, the main simulation is conducted, in which scenarios with monoenergetic neutron fields in different ranges of the energy spectrum are analyzed to evaluate the influence of the presence of water outside the geometry. The results indicate that, for very low energies, the differences between the scenarios with water and with air remain within the statistical uncertainties of the study, whereas for intermediate energies of the spectrum a significant increase in absorbed dose is observed in the case in which the structure is immersed in water. This effect is mainly associated with neutron moderation promoted by the hydrogen present in water, which promotes the occurrence of nuclear interactions and the production of secondary radiation capable of contributing to energy deposition in the sensitive volume. In the final stage of the work, a simulation is performed using a neutron spectrum representative of the containment region of a land-based Pressurized Water Reactor (PWR), allowing evaluation of the behaviour of the dosimetry inside the geometry of interest under a condition closer to a real operational scenario. The statistical analysis of the deposited energy distributions demonstrates that the presence of water outside the structure results in an approximate average increase of 10.2% in the absorbed dose in the tissue-equivalent sphere when compared with the scenario with air. It is concluded that the external environment may influence the radiation field inside confined structures, mainly due to the combined effects of neutron moderation, production of secondary photons, and partial reflection of the particle flux, demonstrating that the developed methodology constitutes an appropriate tool for dosimetry studies and radiation field analysis in shipboard nuclear systems.