AbstractA theoretical model, supported by extensive experimental verification, has been used to examine the transient thermal behaviour of water cooled components, installed in the vessel of the world’s largest fusion tokamak (JET), under normal operational conditions and abnormal scenarios. Relevant freezing and boiling times for a full range of adverse scenarios were determined by the model, while protective actions were prescribed to safeguard the equipment against freezing or boiling risks. A completely automatic safety protection system, based on the results of the model, has been designed and implemented. The system reacted several times during real off-normal events protecting the relevant JET components.