The conservation equations of mass and momentum are derived for the average flow of gases in coupled, parallel channels, or rod bundles. In the case of gas-cooled rod bundles the pitch of the rods is relatively large so the flows in the channels are strongly coupled. From this observation a perturbation parameter, ε, is derived and the descriptive equations are scaled using this parameter, which represents the ratio of the axial flow area to the transverse flow area, and which is of the order of 10−3 in current gas-cooled fast breeder reactor designs. By expanding the velocities into perturbation series in ε the equations for two channels are solved as an initial value problem, and the results compared to a finite difference solution of the same problem. Then, the N-channel, problem is solved to the lowest order as a two-point boundary value problem with the pressures specified at the inlet and the outlet. It is concluded from the study that asymptotic methods are effective in solving the flow problems of rod bundles; however, further work is required to evaluate the possible computational advantages of the methods.

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