Abstract: The scheme of the diffusion--type hydrogen meter system used for detectingthe small leaks in the steam generators of LMFBR is described and analysed. Thetheoretical models of sodium--water reaction and diffusion of hydrogen throughnickel membrane are established. The performance of the system including sensiti-vity, response time, mechanic property and stability is discussed and evaluated onthe basis of experimental data. The result indicates the system has good propertiesin mechanics and hydrogen permeability and can meet the requirement of LMFBR.