Thermal hydraulic studies on cold start-up in a multichannel natural circulation system

Two-phase natural circulation has been adopted as the heat removal mechanism in the primary loop of Advanced Heavy Water Reactor (AHWR). For safe and reliable operation, it is required that the system should be operated in stable regions where no thermo-hydraulic oscillations are present. For the experimental study of thermo-hydraulic instabilities, a facility called Parallel Channel Natural Circulation Test Facility (PCNCTF) was erected and commissioned at the Indian Institute of Technology Bombay (IITB). The loop in this facility is geometrically similar to the primary loop of AHWR. The present paper focuses on the thermal hydraulics of cold start-up process in a Natural Circulation Systems (NCS) in general and AHWR in particular. Experiments were conducted in the three channel loop of PCNCTF as per the prescribed philosophy to be adopted in AHWR. The response of the system was then simulated satisfactorily using RELAP/MOD3.2 after some tuning. Subsequently, parametric studies were carried out with the developed numerical model to accelerate the cold start-up process by increasing power levels. Finally, a rational start-up methodology is developed which could further accelerate the cold start-up.