One of the most promising concepts for replacing the conventional power plants in the clean energy transition is the conversion of Coal-Fired Power Plants (CPP) to Nuclear Power Plants (NPP) or Hybrid Power Plants (HPP). This approach has the advantage that it not only replaces CPPs with clean energy sources but also could take their role in the energy system compared to the option of replacing with Renewable Power Plants. The conversion of CPPs into NPPs (C2N) could be done in different ways: repowering, retrofitting, revamping or replacement. The focus of this work is to explore and evaluate different variants for combining pressurized water small modular reactor (SMR) with a subcritical steam turbine of CPP, resulting in C2N or different options of CPP conversion into HPPs. The difficulties arise from the fact that the turbine is designed with high live steam parameters and the SMR produces slightly superheated steam with low temperature and pressure. Also, the temperature and pressure of the feed water for the steam generator in SMR and in a CPP are very different which in the best case scenario leads to additional losses. Despite the obstacles, successful combinations are achieved, but which of them would be the most economically feasible depends on the specific conditions of the CPP and the regulations in the country, in which it is located.

One of the most promising approaches for replacing conventional power plants during the transition to clean energy is the conversion of existing coalfired power plants (CPPs) into nuclear power plants. This strategy offers numerous ecological and economic advantages. However, integrating a nuclear reactor with a steam turbine originally designed for a coal plant is far from trivial and involves significant technical challenges. The purpose of this work is to analyze and evaluate various options for coupling a HighTemperature GasCooled Small Modular Reactor (HTGR SMR) with a potentially suitable subcritical steam turbine from an existing CPP, thereby creating several repowering configurations. The main difficulties stem from the fact that the turbine was designed to operate with live steam at lower flow rates, temperatures, and pressures than those typically provided by an HTGR SMR. In addition, the feedwater temperature and pressure requirements for the HTGR SMR steam generator differ substantially from those in a CPP, leading at best to additional efficiency losses. Moreover, the overall thermal cycle layouts of the two systems are fundamentally different. Despite these challenges, technically feasible combinations can be achieved. However, determining which option is the most economically viable depends on numerous additional factors, including the specific characteristics of the individual CPP and the regulatory framework of the country in which it operates.

From the different technologies proposed and used in the energy transition, the combining of a steam turbine from a Thermal power plant with a nuclear reactor is one of the most promising but also one of the most technically challenging. This approach offers numerous ecological and economic advantages, including reduced or zero CO ₂ emissions, the usage of an already well-known site with its connection to the grid and water source, and the possible usage of auxiliary equipment and facilities. However, integrating a nuclear reactor with a steam turbine originally designed for a coal-fired power plant (CPP) is far from trivial and involves significant technical challenges. The main purpose of this work is to analyse and evaluate various options for coupling with minimum changes a Small Modular High-Temperature Gas-Cooled Reactor (SMHTGR) with a potentially suitable subcritical steam turbine from an existing CPP, thereby creating several repowering configurations. The second goal is to optimize these configurations as much as possible. The main difficulties stem from the fact that the turbine was designed to operate with live steam at lower flow rates, temperatures, and pressures than those typically provided by an SMHTGR. In addition, the feedwater temperature and pressure requirements for the SMHTGR steam generator differ substantially from those in a CPP, leading at best to additional efficiency losses. Moreover, the overall thermal cycle layouts of the two systems are fundamentally different. Despite these challenges, reliably working combinations can be achieved, but at the cost of some losses in efficiency and, therefore, produced power. The most economically viable variant of a modified power plant is case-specific and depends on numerous additional factors, including the specific characteristics of the individual CPP and its site and the legislation and economics of the country in which it operates.