ITER MELCOR model for the assessment of the Cryostat design

Abstract

ITER cryostat is a cylindrical chamber that will surround the tokamak machine and maintain the vacuum and super-cool operating conditions. The scope of this report is to present the development of an ITER MELCOR model to simulate the behavior of the machine in case of cryostat loss of vacuum accident (Cr LOVA), in-cryostat ingress of coolant event (Cr ICE) and helium ingress in galleries (HIG). The ultimate objective is to obtain the evolution of pressures, temperatures and heat transfer coefficients to feed the mechanical assessment of the cryostat’s design. The first step to build the model is to define a nodalization made out of heat structures, control volumes and flow lines, capable of representing the thermal-hydraulic behavior of the tokamak. The radiation heat transfer system is set externally using control functions. In parallel, a deep analysis of ITER components is carried out to obtain a complete and traceable geometrical and materials database which is subsequently used to feed the MELCOR input parameters. Special attention is given to the cryostat structures where more precision in the results is required. Different approaches are followed to model every system according to an optimization of the documentation phase and the reproduction of the thermal-hydraulic behavior. The initial conditions corresponding to the cooling power values and the temperatures at normal operation are determined with a steady state simulation. An accidental scenario consisting of an in-cryostat leakage of 500 kg of helium coinciding with a loss of off-site power (LOOP) is chosen to prove the performance of the model built. The Cr ICE He II accidental sequence is defined in the input deck before running the calculations. Finally, the evolution of pressures, temperatures and heat transfer coefficients in the different cryostat’s structures and volumes is obtained. They will be used later on as inputs for the mechanical assessment of the cryostat’s design with finite elements methods.