Design Concept and Heat Transfer Analysis for a Double Pipe Channel for SCWR Type Reactors

Current CANDU-type nuclear reactors use a once-through fuel channel with an annulus gas insulating it from the moderator. The current reference design for a CANDU-type SCWR is to eliminate the annulus gap and use a ceramic insert to insulate the coolant from the moderator. An alternative approach being considered is to use the annulus gap as a coolant pre-heater in a double-pipe configuration (so-called re-entrant channel). The alternative design consists of two tubes, the inner tube (flow channel) and the outer tube (pressure tube). The fuel bundles, similar to those of the current CANDU reactors, are placed in the inner tube. The inner and outer tubes form an annulus through which flows the primary coolant. The coolant will flow through the annulus receiving heat from the inner tube from one end of the channel to another. At the far end, the flow will reverse direction and enter the inner tube, and hence the fuel bundle. At the inlet, the temperature is 350°C for a high-pressure coolant (pressure of 25 MPa), which is just below the pseudocritical point. At the outlet, the temperature is about 625°C at the same pressure (the pressure drop is small and can be neglected). Channel power and flow rate are variable initial conditions. The objective of this work is to model the heat transfer in the proposed fuel channel design. The channel has been divided into several nodes for the inner tube and the annulus gap. The power in each node in the pressure tube is considered to be uniform. The numerical model calculates the temperature profiles, the heat transfer coefficients and the overall heat transfer across the double-pipe fuel channel for a given set of flow, pressure and temperature boundary conditions and the power initial conditions. With the results from the numerical model, the design of the re-entrant (double-pipe) fuel channel can be optimized to improve its efficiency.