The substitution of a large-scale clean energy for burning coal is needed in China because of serious pollution in cities and CO₂ emission. Nuclear energy is an option of the clean energy, it can not only be used for generating electricity but also for urban district heating in stead of coal. A non-pressurized heating reactor has been designed based on experience of pool type research reactors of China. It can supply heat for urban district heating system and has a high level of safety and reliability. This is a realizable and economical solution for clean energy supply. Since the district heating systems are plentifully developed in cities of China, the heating sources which can be used in the systems will be able to improve air-environment and decrease greenhouse gas emission in a large-scale.