AbstractThe coolant flow distribution among parallel passages in a nuclear reactor (or boiler or heat exchanger) can be very sensitive to variations in heat input, channel dimensions, etc. In a previous paper this flow sensitivity was defined in terms of certain partial derivatives, which were related by analytical expressions to fluid properties and operating characteristics. Flow sensitivity contributes largely to potential malfunction, reduced efficiency, or failures. The use of valves and orifices was quantitatively evaluated for supercritical water in the earlier paper. The scope of this paper is to consider the utility of mixing headers. These mixing chambers are located along the flow passage as a common receiver for parallel flow from many channels. The headers, in turn, supply subsequent lengths of heated passages in parallel. Analytical expressions are derived for the effect of headers on flow, outlet‐fluid enthalpy, and channel‐wall temperatures. The limiting cases of minimum and complete mixing in the headers are considered and numerical results for water at supercritical pressures are given to show the marked increase in stability obtained by use of intermediate mixing head