Hydrogen measuring device

A device is described for measuring free hydrogen concentration in the coolant of a nuclear reactor, the device comprising: a gamma thermometer comprising an elongated rod that is housed in a sheath, the rod is provided with a series of enclosed annular spaces, disposed longitudinally along the gamma thermometer, that are formed between the rod and the sheath. Difference thermocouples are disposed along the rod having hot and cold junctions, the hot junctions of the thermocouples are positioned adjacent the annular spaces and the cold junctions are positioned between the annular spaces; whereby the annular spaces form annular heat insulating chambers for the hot junctions of the thermocouples, two adjacent annular spaces and the related two difference thermocouples form a hydrogen measuring zone. A vacuum is formed in one of the annular heat insulating chambers of measuring zone for insulating the hot junction and providing a space in which free hydrogen can collect. In the other annular heat insulating chamber of each hydrogen measuring zone a gas other than hydrogen is disposed therein to prevent hydrogen contamination of the annular space; and the free hydrogen partial pressure can be calculated from the comparison of the indicated power generation rates of the two difference thermocouples in each hydrogen measuring zone