a method and apparatus for in - line tritium - in - water monitoring in which a sample of water is vaporized at a temperature sufficient to transform it to dry water vapor substantially free of liquid water droplets . the water vapor is transferred to a tritium detection device having a chamber heated to a temperature at which there is substantially no condensation of the water vapor , the detection device comprising volume detection device adapted to detect β - decay of tritium atoms in the water vapor . the volume detection device may comprise ionization chamber detector , gas scintillation counting detector and gas electron multiplier detector . the method and apparatus are preferably used to detect leaks of tritiated water from the primary to secondary side of the heat transport systems associated with certain types of nuclear reactors , and when used on an in - line , continuous basis , provide a time response as low as 10 seconds with the detection limits depending on the type of volume detection equipment used . this method also lends itself to in - line monitoring of tritium in water discharged to lakes , rivers and sewers , and may also be amenable to environmental monitoring of tritium in waterways .