8 for supplying a fluidizing gas 9 f an outlet 10 for unreacted reaction gas fluidizing gas as well as the gaseous or vapor form products of the reaction 11 which accumulate above the fluidized bed surface 19 g an outlet 12 for the product 13 h a heating device 14 i a power supply 15 for the heating device 14 wherein additional nozzles 18 are provided in the region of the reactor surfaces to be protected via which a gas composition 17 containing from 99 5 to 95 mol of hydrogen and from 0 5 to 5 mol of the gaseous silicon compound is introduced into the reactor insulation 16 is preferably arranged around the inner reactor tube 3 in order to reduce energy losses the heating device 14 is preferably a radiation source for thermal radiation which is arranged annularly outside the inner reactor tube 3 and without direct contact with it high transmission for thermal radiation is preferably intended to mean that the reactor tube has a transmission of preferably more than 80 for the thermal radiation emitted by the heater in one embodiment of the invention the inlet device 6 is designed annularly and the fluidized bed is divided into a heating zone and a reaction zone lying above the latter in this embodiment the heating device 14 is preferably designed so that it heats the silicon particles in the heating zone by means of thermal radiation to a temperature such that the reaction temperature is set up in the reaction zone in the context of the present invention u201c reactor surface u201d and u201c surface of the reactor u201d are preferably intended to mean the reactor wall 39 s surface facing the reaction space that surface of the inner reactor tube.