A monitoring system has been developed for detecting leaks in LMFBR steam generators by the detection of the hydrogen produced in the sodium-water reaction. The principal capabilities of this detection system are rapid response, high sensitivity, and high stability and reliability. The monitoring system is based on the detection of a change in hydrogen concentration in sodium by measurement of the change in the rate of hydrogen diffusion through a nickel membrane immersed in the sodium. A vacuum is drawn on the membrane by an ion pump, and the partial pressure of hydrogen on the vacuum side, a measure of the hydrogen flux and the hydrogen activity in the sodium, is determined by the measurement of the current to the ion pump. The response time of the monitor depends chiefly on the hydrogen-diffusion properties of the nickel membrane. Transient-diffusion calculations indicate that 10 sec after a sudden change in the hydrogen concentration in the sodium, the change in the hydrogen flux from the membrane would be 70% of the eventual total change in flux for a 10-mil-thick nickel membrane at 500°C. With a stable high-voltage power supply, the noise on the recorded ion pump signal was <0.2% of the signal output for a hydrogen concentration in sodium of 0.1 to 1.0 ppm. This level of sensitivity and stability are adequate to detect a water leak from the steam generator into the LMFBR secondary sodium of 10−4 lb/sec at a sodium flow of 107 Ib/h in <1 min, if the hydrogen concentration in the secondary sodium is ∼0.1 ppm.