From the developing strategy of nuclear industry throughout the world, small modular nuclear reactor is of great advantage. Besides of its basic function of generating electricity, it has many other industrial functions, such as hydrogen production, industrial heat supply and sea water desalination, which can take good use of ideal electricity and heat source. So the research on reactor core analysis of small reactor is of great importance.In this book, the independent designed small integral reactor is studied. In this reactor hexagonal fuel assembly and plate type control rod is used. So the reactor has compact core, large heat transfer area and high power density. In this book, the coupling between Neutron Kinetics and Thermal Hydraulics is completed and was taken to do the thermal hydraulic analysis of reactor core. The coupling between NK/TH evaluate the thermal feedback effects which is used to simulate the accident scenarios. This techniques involves core spatial power distribution and feedback effect between the two phenomenon. We researched some of the accidents by considering the NK/TH coupling technique and have been included in this book.