Pebble bed, helium cooled, molten salts breeder blanket concept for a tokamak reactor

Abstract

An important issue met during the design of a tokamak reactor is the one due to the pulsed way of operation of the machine, which induces fatigue phenomena in most basic components. In the pesent study the objective has been to analyse the behaviour of a pebble bed, helium cooled blanket, in order to be able to compare its performances with those of the other blankets concepts. The idea is to use the latent heat of fusion of some litium alloys (FLIBE, LiF+BeF 2) to reduce the outlet temperature oscillation of helium coolant and the thermal stress in the module.In fact, during the burn time, the power produced by the breeder can bring the FLIBE alloy to melting conditions, which should keep also during the reactor's dwell time, making use of the capacity of thermal storage of the FLIBE litium alloy. The breeder is contained in spherical SiC shells to allow the batch tritium recovery operations. Further, the lay-out of the module results simplier, since no helium pipes are needed. Spheres, 6 cm diameter, are randomly packed in a blanket module and cooled by 10 bars pressurized helium. Ansaldo D.N.T. has brought forward some analyses to prove the feasibility of this blanket design and in particular: (i) compatibility analysis of structural and breeder materials; (ii) verification of spheres behaviour under operating conditions; (iii) thermohydraulic study to evaluate the blanket behaviour in the event of accident for coolant loss or critical pressure drop. For the second and third item an original Fast Running Code was developed to simulate some different working conditions of the reactor. The first results of this study show that the objective of minimizing outlet temperature oscillations of helium has been reached.