O combustível nuclear queimado (CNQ) é retirado do reator nuclear após exaurir sua eficiência de geração de energia. Após ser retirado do reator, esse combustível é temporariamente armazenado em piscinas com água na própria instalação do reator. Durante esse tempo, o calor gerado e os elementos radioativos presentes, de meia-vida média e curta, caem até níveis que permitem retirar o combustível queimado da piscina e enviá-lo para depósitos temporários de via seca. Nessa fase, o material precisa ser armazenado segura e eficazmente de forma que possa ser recuperado em futuro próximo, ou disposto como rejeito radioativo. A quantidade de combustível queimado cresce anualmente e, nos próximos anos, vai aumentar mais ainda por causa da construção de novas instalações de geração de energia de origem nuclear. Nos dias de hoje, o número de instalações novas voltou a atingir os níveis da década de 1970, porque é maior que a quantidade de ações de descomissionamento de instalações antigas. Antes que seja tomada qualquer decisão, seja a de recuperar o combustível remanescente ou considerar o CNQ como rejeito radioativo, o mesmo precisa ser isolado em um dos diferentes tipos de armazenagem existentes no mundo. No presente estudo mostra-se que a armazenagem do CNQ, via seca, em cascos é a opção mais vantajosa. Propõe-se um modelo de casco autóctone para combustível de reatores de potência e de uma instalação de armazenagem para abrigar esses cascos. É um estudo multidisciplinar no qual foi desenvolvida a parte conceitual de engenharia e que poderá ser usada para que o CNQ nacional, retirado dos reatores brasileiros de potência, seja armazenado com segurança por um longo período até que as autoridades brasileiras decidam o local para deposição final. / Spent nuclear fuel (SNF) is removed from the nuclear reactor after the depletion on efficiency in generating energy. After the withdrawal from the reactor core, the SNF is temporarily stored in pools at the same site of the reactor. At this time, the generated heat and the short and medium lived radioactive elements decay to levels that allow removing SNF from the pool and sending it to temporary dry storage. In that phase, the fuel needs to be safely and efficiently stored, and then, it can be retrieved in a future, or can be disposed as radioactive waste. The amount of spent fuel increases annually and, in the next years, will still increase more, because of the construction of new nuclear plants. Today, the number of new facilities back up to levels of the 1970s, since it is greater than the amount of decommissioning in old installations. As no final decision on the back-end of the nuclear fuel cycle is foreseen in the near future in Brazil, either to recover the SNF or to consider it as radioactive waste, this material has to be isolated in some type of storage model existing around the world. In the present study it is shown that dry SNF storage is the best option. A national cask model for SNF as well these casks storage installation are proposed. It is a multidisciplinary study in which the engineering conceptual task was developed and may be applied to national SNF removed from the Brazilian power reactors, to be safely stored for a long time until the Brazilian authorities will decide about the site for final disposal.

O combustível nuclear queimado (CNQ) é retirado do reator nuclear após exaurir sua eficiência de geração de energia. Após ser retirado do reator, esse combustível é temporariamente armazenado em piscinas com água na própria instalação do reator. Durante esse tempo, o calor gerado e os elementos radioativos presentes, de meia-vida média e curta, caem até níveis que permitem retirar o combustível queimado da piscina e enviá-lo para depósitos temporários de via seca. Nessa fase, o material precisa ser armazenado segura e eficazmente de forma que possa ser recuperado em futuro próximo, ou disposto como rejeito radioativo. A quantidade de combustível queimado cresce anualmente e, nos próximos anos, vai aumentar mais ainda por causa da construção de novas instalações de geração de energia de origem nuclear. Nos dias de hoje, o número de instalações novas voltou a atingir os níveis da década de 1970, porque é maior que a quantidade de ações de descomissionamento de instalações antigas. Antes que seja tomada qualquer decisão, seja a de recuperar o combustível remanescente ou considerar o CNQ como rejeito radioativo, o mesmo precisa ser isolado em um dos diferentes tipos de armazenagem existentes no mundo. No presente estudo mostra-se que a armazenagem do CNQ, via seca, em cascos é a opção mais vantajosa. Propõe-se um modelo de casco autóctone para combustível de reatores de potência e de uma instalação de armazenagem para abrigar esses cascos. É um estudo multidisciplinar no qual foi desenvolvida a parte conceitual de engenharia e que poderá ser usada para que o CNQ nacional, retirado dos reatores brasileiros de potência, seja armazenado com segurança por um longo período até que as autoridades brasileiras decidam o local para deposição final. / Spent nuclear fuel (SNF) is removed from the nuclear reactor after the depletion on efficiency in generating energy. After the withdrawal from the reactor core, the SNF is temporarily stored in pools at the same site of the reactor. At this time, the generated heat and the short and medium lived radioactive elements decay to levels that allow removing SNF from the pool and sending it to temporary dry storage. In that phase, the fuel needs to be safely and efficiently stored, and then, it can be retrieved in a future, or can be disposed as radioactive waste. The amount of spent fuel increases annually and, in the next years, will still increase more, because of the construction of new nuclear plants. Today, the number of new facilities back up to levels of the 1970s, since it is greater than the amount of decommissioning in old installations. As no final decision on the back-end of the nuclear fuel cycle is foreseen in the near future in Brazil, either to recover the SNF or to consider it as radioactive waste, this material has to be isolated in some type of storage model existing around the world. In the present study it is shown that dry SNF storage is the best option. A national cask model for SNF as well these casks storage installation are proposed. It is a multidisciplinary study in which the engineering conceptual task was developed and may be applied to national SNF removed from the Brazilian power reactors, to be safely stored for a long time until the Brazilian authorities will decide about the site for final disposal.

Chloride-induced stress corrosion cracking (CISCC) has been identified as the main degradation mechanism for spent nuclear fuel dry storage canisters. This type of induced cracking is complex and depends on several factors, such as material composition, exposure temperature, relative humidity, applied tensile stress, and atmospheric salt concentration. An accelerated experiment was designed to simulate marine environments in a controlled fogging chamber to examine 304 and 304L stainless steel U-bend and welded U-bend samples. The samples were exposed to chloride rich and humid fogging in a corrosion chamber at 35℃ continuously for 4 weeks, 8 weeks, and 12 weeks. The same experiment was repeated at 50℃ for 4 weeks, 8 weeks, and 14 weeks to study the sensitivity of CISCC to temperature changes. A qualitative evaluation of optical micrographs from a 3D Surface Profiler was performed for 16 corroded samples and compared with 2 reference samples. Cracking was observed on 12 out of 16 samples exposed to 35℃ and 50℃ for durations ranging from 8 to 14 weeks. Likely cracking observations were noted on 4 out of 16 samples. A quantitative statistical analysis was also performed using surface profile depth (valley) data from corroded and reference samples. The quantitative analysis examined the effect of temperature, welding, exposure duration, and material composition. The quantitative results were compared with the qualitative results and literature published in CISCC. / Master of Science / Most nuclear power plants are currently using dry storage canisters (DSCs) which are made of a concrete vault and a stainless steel canister that houses the spent nuclear fuel (SNF) assemblies. Multiple conditions must be present simultaneously for chloride-induced stress corrosion cracking (CISCC) to develop, such as the presence of a susceptible alloy, high relative humidity, high temperature, high atmospheric salt concentrations, and applied tensile stresses. DSCs are typically made from 300-series austenitic stainless steels which are susceptible to this type of corrosion during long-term storage near marine environments. Therefore, understanding of the factors leading to CISCC is critically important for proper management and mitigation and to estimate the service life of DSCs for the safe long-term storage of SNF. An accelerated experiment was designed to examine the effects of temperature, exposure duration, and welding on pitting and cracking for 304 and 304L U-bend samples. The experimental results concluded that stainless-steel grades 304 and 304L are susceptible to CISCC when exposed for 8 weeks or longer to fogging at temperatures between 35℃ and 50℃, 95% relative humidity, and 5% salt concentration. This study also concluded that increasing exposure duration from 8 to 12 weeks or the temperature from 35℃ to 50℃ had no significant effect on the acceleration of CISCC. Also, unwelded samples were deemed more susceptible to CISCC than welded samples and the susceptibility of 304 and 304L grades were relatively similar.