Thermal hydraulic transient analysis of ITER chilled water system. Decay heat exchanger leak

Chilled Water System (CHWS-1) is an ITER safety cooling system comprised of two independent and segregated trains (CHWS-1A and CHWS-1B), which provide cooling water to components located in Tokamak and Hot Cell Complexes. VV-PHTS is the only component that, during water baking mode, operates at high pressure than CHWS-1. In case of leaks, it represents a risk in terms of over pressurization and contamination of the CHWS-1 (due to tritiated water and activated corrosion products (ACPs)). Therefore, this paper postulates CHWS-1 leak events at one interface: Vacuum Vessel Primary Heat Transfer System. (VV-PHTS). Three scenarios have been analysed: an incident event (heat exchanger pipe leakage) and two accident events (heat exchanger tube break and multiple tubes break). A simplified RELAP5–3D model of CHWS-1B was developed to perform thermal hydraulic transient analysis as well as to define and size system operation and control. A postulated accident management strategy is included while also defining CHWS-1 instrumentation and control (I&C), which play a relevant role in event detection and mitigation procedures. The simulation results are studied to analyze and verify the system behavior as well as the required actions to reach and maintain a safe state after the given events. With respect to the events studied, this paper reports and confirms the system design to properly and safely bring CHWS-1B to a shutdown, as per corresponding ITER project requirements. This operating scenario has been studied for academic purposes; it is not related to the actual CHWS-1B design. During VV-PHTS baking mode, decay HX is not foreseen to be operative. In order to avoid affecting the redundancy of the system, this study supports this decision.