FUSION-REACTOR DESIGN PARAMETERS RELEVANT TO THE PASSIVE REMOVAL OF DECAY HEAT

The bulk quantity of decay heat produced by the activated structural m aterials of fusion reactors is a matter of concern. Tn adiabatic condi tions 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 i n the frame of the ITER/CDA, DEMO, SEAFP and ITER/EDA programmes with the scope of predicting the post-accident temperature transient for th e ''envelope'' conditions of instantaneous, total and permanent loss o f every form of active cooling of the reactor. These studies included numerical calculation of the neutron fluxes and decay heat histories a nd consequently the simulation of the ''envelope'' post-accident tempe rature transient of the whole tokamak for as long as 3 months after th e accident. A variety of design configurations, including various stru ctural materials, breeding and shielding blankets, etc., have been sim ulated. The results from these studies indicated that in most cases, e ven under ''envelope'' LOCA conditions, the structural integrity of th e containment would not be compromised. It also became apparent that, even in cases where the resulting temperatures were too high, slight d esign modifications in the regions outside the vacuum vessel were suff icient 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 vac uum 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 factor s that affect the post-accident transient temperatures and providing s ome general design guidelines.