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On this paper it is presented the main physics features of the thorium as a nuclear material and a security analysis due to insertion of ThO2 in a PWR nuclear reactor fuel by the determination and assessment of the sensitivity coeficients through the Standardized Computer Analyses for Licensing Evaluation - SCALE 6.1 nuclear code system. This work was motivated by the world tendencies to reseach and development of nuclear fuels based on thorium, because the limitations of uranium resources and the importance of nuclear energy to the humanity.

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In this work, heat conduction in a typical spherical fuel element (pebble) of a pebble-bed high temperature reactor was studied. The fuel element is composed by a particulate region with spherical inclusions, the UO2 fuel particles (TRISO), dispersed in a graphite matrix. The two energy equation model was applied to the particulate region, generating two macroscopic temperatures, respectively, of the fuel and of the matrix. Analytical solutions are obtained for steady-state heat conduction. Transient analysis was carried out by using the generalized integral transform technique (GITT), which requires low computational efforts and allows a fast evaluation of the two macroscopic transient temperatures of the particulate region. The solution by GITT leads to a system of ordinary differential equations with the unknown transformed potentials, which is solved numerically to obtain the hybrid solution of the original partial differential equation. Numerical results for several testing cases are presented.

Neste trabalho, o núcleo de um reator modular de pequeno porte (SMR) hipotético do tipo PWR, com um enriquecimento do combustível de 20% e potência de 50 MWe (150 MWt), foi simulado na condição de criticalidade. A 300 cm do vaso de pressão do SMR foi medida a dose efetiva de 1,33E+9 mSv, que mostrou a necessidade de calcular e instalar uma blindagem multilaminada no contexto do plano de proteção radiológica da instalação nuclear para fins de licenciamento e cumprimento das diretrizes pertinentes. Os dados disponíveis foram utilizados no dimensionamento do projeto básico do serviço de proteção radiológica da instalação, além da possibilidade de serem aplicados na elaboração do respectivo PSAR (Preliminary Safety Analysis Report).