Implications of advanced computational methods for reactivity initiated accidents in nuclear reactors.

Abstract

Este exemplar foi revisado e corrigido em relação à versão original, sob responsabilidade única do autor e com a anuência de seu orientador. ACKNOWLEDGEMENTSI would like to express special thanks to my wife, Sandra, and my daughter, Giovanna, for always believing and supporting me during the moments when "having a job and pursuing a PhD" was a challenge by itself. I never would have made it here without you both.My deepest gratitude to my advisor, Dr. Mujid Kazimi, who took me as his student during my graduate studies at MIT and who agreed to remain as my advisor on this endeavor. His immense knowledge and expertise in the nuclear field were a key motivation.I am deeply grateful to my friend and co-advisor, Dr. José Jaime da Cruz, for the long discussions and comments on the revisions of this thesis. His leadership, support and hard work have set an example since I was an undergraduate student at USP. I wish to thank Dr. Koroush Shirvan for his insightful and detailed discussions on the codes used in this work, and for sharing his knowledge and views on a number of issues related to the thesis´ topic. The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge.Stephen Hawking ABSTRACTAdvanced computational tools are applied to simulate a nuclear power plant (NPP) control rod assembly ejection (CRE) accident. The impact of these reactivity-initiated accidents (RIAs) on core reactivity behavior, 3D power distribution and stochastic reactivity estimation are evaluated. The three tools used are: the thermal-hydraulic (TH) RELAP5 (R5) code, the neutronic (NK) PARCS (P3D) code, and the coupled version P3D/R5, with specially developed linkage using the environment code MATLAB. This study considers three different-size cores: NPP1 (2772 MWt); NPP2 (530 MWt); and NPP3 (1061 MWt). The three cores have the same general design and control rod assembly (CRA) positions, and the ejected CRA has similar worth and at the same rod ejection pace. The CRE is assessed under both hot zero power (HZP) and hot full power (HFP) conditions.The analyses indicate that RIA modeling and simulation should be carried out through a systematic coding and configuration approaches, otherwise the results will not capture the true transient behavior of the core under analysis. The simulation of one code depends on the appropriate configuration of parameters generated by the other code and on the correct determination of the TH/NK mapping weight factors for the various mesh regions in each of the models. From the design point of view, the standalone codes predict milder magnitude of power and reactivity increase compared to the coupled P3D/R5 simulation. The magnitudes of reduced peak power and reactivity become larger as the core size shrinks. The HFP simulation shows that the three NPPs have the same transient peak value, but the post-transient steady power is lower for a smaller core. The HZP analysis indicates that the transient peak is lower for the smaller core, but the post-transient power occurs at the same level.The t...