Post Accident Afterheat Generation & Dissipation in Fusion Reactors

In a fusion reactor, the neutron flux will cause activation of the plasma chamber. The volumetric decay heat associated with this activation is removed, during normal operation, by forced flow cooling circuits. Its effects under post accidental conditions are a matter of concern since they can cause temperatures higher than allowed leading to the degradation of the properties and even structural failure of all or some of the reactor components. Here, an overview of the post accidental temperature transients, performed under the European Fusion Program for a variety of tokamak devices, is presented. The modelling activities, including the neutronic, activation and thermal part, are described. The latest results concerning the SEAFP reactor study are given. Generally, the most dangerous temperature peak happens long after the accident (typically 1 – 2 months) thus allowing for a considerable margin for intervention. Appropriate design of the region outwards from the vacuum vessel can provide the necessary thermal links so as not to compromise the structural stability of the containment even in the envelope conditions of complete and permanent loss of every form of active cooling.