Development of System Code Model for Reflood Phenomena on CRUD Cladding

Abstract

There are more than 400 nuclear power reactors in operation around the world. With the developing technology and the globalizing world, the electrical energy needed is increasing day by day. In this context, an unlimited number of nuclear power plants cannot be built everywhere. Instead, the power of the reactors has started to be increased with the improvement studies in the nuclear power reactors. Some problems have emerged with the increasing power of the reactors, which have been producing power to certain standards based on the design principle for many years. It has been observed that new material is formed in the nuclear reactor fuel rods during long-term and high-power operations. This material, called CRUD, has become an important safety topic in the nuclear industry. Recent research has highlighted the advantages and disadvantages of the CRUD material. However, as it is a new research area, it still contains many uncertainties about safety. While having low thermal properties is a disadvantage, the special surface feature 'chimney' raises the question of whether it is an advantage or a disadvantage. Most of the research done to date is on better understanding the CRUD morphology, and there is limited research on its effect on fuel rods. There is also a lack of experimental data. Computer thermal-hydraulic analysis codes, which have been used since the 70s and still continue to be used with their reliability today, are of great importance for accident scenarios. Thanks to these codes, many different scenarios have been tried and safety systems have been developed. However, most of these codes are designed for standard pressure water reactors and do not contain CRUD material information. This has also become a safety issue. There are deficiencies in how the CRUD material will affect the safety margin, how it can cause accidents, or what situations can be experienced after standard accidents. The effect on the fuel rod cladding, which is one of the most important safety barriers, should be well understood, especially since it occurs on the fuel rod. Studies have shown that CRUD material has a negative effect on the fuel rod cladding. However, it is assumed that it can also contribute to heat transfer with its different surface structure. In line with all this information, the development of simulation codes is important for nuclear power plant safety. In this study, the effects of MARS-KS code, whose reliability has been proven experimentally, and CRUD material are studied. The results of the MARS-KS thermal-hydraulic system code on the cladding of the fuel rod with the newly defined material were evaluated. The ability of the code for the CRUD material was evaluated, and also its effects on the post-accident reflooding phenomenon were examined. In the light of all these analyzes, it has been evaluated how the CRUD material affects the fuel rod cladding in terms of safety.