Nuclear combined heat and power - analyses of hot water pipeline breaks in a service tunnel with Apros simulation software

This paper presents a computer model and simulation results for a long-distance heat transport system. The system can be used e.g. to transport heat from a nuclear power plant with combined heat and power (CHP) production. CHP production is considered for new build NPP projects in Finland. Emphasis is on the environmental conditions during a hot water pipeline break in a service tunnel. The modelled pipeline system is designed to transport 1000 MW of heat over a distance of 77 km for district heating purposes. The hot water pipeline is assumed to be 1200 mm diameter with a water temperature of 120 deg. C. Cooled water returns with a temperature of 55 - 60 deg. C in a similar 1200 mm diameter pipe. Both pipelines are installed to a service tunnel which is excavated into bedrock and divided into 2 kilometres long compartments. Both the 77 km long pipeline and the tunnel are modelled with Apros simulation software. A leak is modelled from the pipeline to the tunnel and the results are analyzed. This paper includes three different leak sizes (1 %, 10 % and 100 % of the pipeline's cross-sectional area). The leaks are calculated with water temperatures of 95 deg. C and 120 deg. C in the pipeline. Apros calculates dynamically the phenomena inside the pipeline with two-phase 6-equation calculation model. The tunnel conditions are calculated with a lumped parameter model. The size of the leak has a substantial effect on the leak's consequences in the tunnel. Also the water temperature in the pipeline influences the results strongly. If the water temperature is over 100 deg. C, a considerable amount of the water boils as it leaks to the tunnel. The boiling of water makes the conditions in the tunnel much more severe than they would otherwise be. If there is a substantial flow out of the tunnel, the air in the tunnel can be replaced by hot steam. Obviously, this can mean hazardous conditions in the tunnel. (authors)