A study is conducted on a newly designed radiometric monitoring system for detecting the water level in a high-pressure steam chamber that can be used to monitor the high-pressure boiler operation state in a power plant. The principle of this system is based on recording the rate of gamma radiation (137Cs) that goes through the boundary between the steam and the water and then calibrating the rate into the actual level with computer processing. A practical mathematical model and a special buildup factor family for this calibration are provided. A particular treatment of errors for this application is given. This system, which has been used in a power plant for more than 30 months and has worked smoothly, has a relative accuracy of 1.5% and has long-term reliability.