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• A model of the ITER chiller water system (CHWS-1B) has developed using RELAP5 mod3.3. • Several scenarios of decay heat exchanger leak/break in the drain tank room have investigated. • Event evolution has analysed, required measures to isolate the affected zone have defined. • Accident management strategy is included in the analysis effort as well as a definition of the instrumentation and control fulfilling a relevant role in the event detection and mitigation. 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. [ABSTRACT FROM AUTHOR]

• A model of the ITER Chiller Water system (CHWS-1B) has developed using RELAP5 mod3.3. • Several CHWS-1B pipe break scenarios in the Tokamak Complex Drain Tank Room has investigated. • Event evolution have analysed and required measures to isolate the affected CHWS-1B zone have defined. • Accident management strategy is included in the analysis effort. • Definition of the instrumentation and control fulfils a relevant role in the event detection and mitigation. The design of the Chilled Water System (CHWS-1) of ITER (Nuclear Facility INB-174) considers two independent and segregated subsystems, CHWS-1 A & B, which provide cooling water to safety-relevant components in Tokamak and Hot Cell Complexes. CHWS-1 is a Safety Important System needed for the proper functioning of Protection/Safety Important Activities & Components and therefore Single Failure Criterion is included in its design, while physical separation between its trains is maintained as much as possible. However, both CHWS-1 trains are located in the Tokamak Complex Drain Tank Room (DTR) and thus, measures are taken in order to protect them from common causes of failure (i.e. Fire or High Energy Line Break). A failure of a given train in the DTR is investigated in order to analyse the consequences and take required additional measures so as to guarantee full safety cooling to CHWS-1 components outside DTR. A RELAP5 model of CHWS-1B is developed and a pipe break inside the DTR modelled, to perform thermal hydraulic transient analyses of this event. From the results, the transient evolution of the main thermal hydraulic parameters is determined and the required times of intervention evaluated in order to restore the cooling function of the affected CHWS-1 train for its remaining components. Thus, an accident management strategy is included in the analysis effort as well as a definition of CHWS-1 instrumentation and control fulfilling a relevant role in the event detection and mitigation. This paper summarizes the results of this activity. [ABSTRACT FROM AUTHOR]