A flexible mathematical model has been developed to simulate the transient thermal response of a number of nuclear fusion components, including cryogenic devices which operate inside the JET Tokamak. The present work reports on the simulation of an accident scenario, as well as on further studies on hypothetical off-normal scenarios concerning an out of vessel cryopump. These studies resulted in a complete safety protection system for the LHCD cryopump which has been implemented into the JET operating routines.