Fusion reactor design parameters relevant to the passive removal of decay heat

Abstract

The bulk quantity of decay heat produced by the activated structural materials of fusion reactors is a matter of concern. In adiabatic conditions it has the potential to cause at least a partial meltdown of the reactor. Although a fusion reactor, even in conditions of total loss of coolant accident (LOCA), is not adiabatic, it will be an extremely well-insulated thermal system. Extensive studies have been performed in the frame of the ITER/CDA, DEMO, SEAFP and ITER/EDA programmes with the scope of predicting the post-accident temperature transient for the “envelope” conditions of instantaneous, total and permanent loss of every form of active cooling of the reactor. These studies included numerical calculation of the neutron fluxes and decay heat histories and consequently the simulation of the “envelope” post-accident temperature transient of the whole tokamak for as long as 3 months after the accident. A variety of design configurations, including various structural materials, breeding and shielding blankets, etc., have been simulated. The results from these studies indicated that in most cases, even under “envelope” LOCA conditions, the structural integrity of the containment would not be compromised. It also became apparent that, even in cases where the resulting temperatures were too high, slight design modifications in the regions outside the vacuum vessel were sufficient to significantly lower the post-accident temperature peaks. All these studies and analysis being done a posteriori to the design, any modifications, even if not relevant to the first wall, blanket or vacuum vessel, are not easily accepted. The present paper responds to the need for integrating the considerations on decay heat generation and passive dissipation into the design activity by identifying the factors that affect the post-accident transient temperatures and providing some general design guidelines.