Small reactors have become a new hotspot of international nuclear energy research.The nuclear heating reactor(NHR)technology developed by Tsinghua University is an important multipurpose small reactor solution with features such as high integration,modular design and full power natural circulation.A new small reactor based on the existing NHR-200 reactor was developed by the Institute of Nuclear and New Energy Technology of Tsinghua University.A full-scale natural circulation test loop with the same operating parameters as the actual reactor was built in order to experimentally validate the natural circulation ability of the reactor primary loop and heat-transfer ability of fuel assemblies and heat exchangers.Corresponding results are given in detail,including parameter validation of the reactor primary loop,flow rules of the natural circulation and heat-transfer coefficients of heaters and heat exchangers,which can be directly used in the actual reactor as a reference for optimization design.Finally,a characteristic parameter k is proposed to represent the natural circulation ability of a system.By using the new data arrangement method in the form of parameter k,comprehensive experimental results of the natural circulation can be represented by a simple integrated expression.The work in this paper is of importance in broadening application fields and pushing forward commercialization of the NHR type reactors. Small reactors have become a new hotspot of international nuclear energy research. Nuclear heating reactor (NHR) technology developed by Tsinghua University is an important multipurpose small reactor solution with features such as high integration, modular design and full power natural circulation.A new small reactor based on the existing NHR-200 reactor was developed by the Institute of Nuclear and New Energy Technology of Tsinghua University. A full-scale natural circulation test loop with the same operating parameters as the actual reactor was built in order to experimentally validate the natural circulation of the reactor primary loop and heat-transfer ability of fuel assemblies and heat exchangers. Coresponding results are given in detail, including parameter validation of the reactor primary loop, flow rules of the natural circulation and heat-transfer coefficients of heaters and heat exchangers, which can be directly used in the actual reactor as a reference for optimization design.Finally, a characteristic parameter k is proposed to represent the natural circulation of a system. By using the new data arrangement method in the form of parameter k, comprehensive experimental results of the natural circulation can be represented by a simple integrated expression. The work in this paper is of importance in broadening applications fields and pushing forward commercialization of the NHR type reactors.