Heavy water is used as coolant in primary heat transport systems of pressurised heavy water reactors. Hydrogen is added at regular intervals to the primary heat transport system to suppress the radiolysis of heavy water. However, the added hydrogen is not fully utilised in the system and finds its way to the heavy water storage tank after passing through the bleed condenser. Owing to the different temperatures and two phase regions present in these systems, hydrogen gets redistributed. It is important to know the concentration of dissolved hydrogen in these regions in order to ensure a dissolved hydrogen concentration as prescribed in the technical specification for the primary heat transport system. Different power stations report variations in the frequency and quantity of hydrogen added to achieve the concentration range prescribed. This paper makes an attempt to account for the inventory of hydrogen and model its transport in primary heat transport system. (c) 2011 Canadian Society for Chemical Engineering