A U-tube heat exchanger for the heat transfer from a primary gas circuit to a secondary gas circuit in a high temperature reactor. At the high temperatures of approximately 950.degree. C., the additionally permissible stresses on the used materials are low. The cold gas collector is therefore flexibly attached at the housing. The arrangement permits complete and also remote-controlled testing from the secondary gas-side of all parts of the primary gas circuit which are stressed by pressure. The flexible elements are neither stressed by the weight of the heat exchanger nor endangered by high temperatures.