When a nuclear reactor is used to supply steam for a desalination plant, the method of coupling has a significant technical and economic impact. The exact method of coupling depends on the type of reactor and the type of desalination plant. As a part of Nuclear Desalination Demonstration Project (NDDP), BARC has successfully commissioned a 4500 m³/day multi-stage flash desalination plant coupled to Madras Atomic Power Station at Kalpakkam. A desalination plant coupled to nuclear reactor of pressurised heavy water reactor type is a good example of dual-purpose nuclear desalination plant. This paper presents the thermal coupling system analysis of this plant along with technical and safety aspects.