[en] The SODEMME code provides a Solution to the One Dimensional Energy, Mass and Momentum Equations for a system of ''volumes'' and ''segments'' that comprise a circulating system containing a gaseous coolant. Input consists of a description of the system's geometry, the user's choice of some control variables, specification of the initial and time-dependent boundary conditions and setting various options for items such as pumping head, heat transfer and frictional effects. The program automatically finds the steady state solution corresponding to the initial conditions and then proceeds to compute the transient solution corresponding to the input boundary conditions. The present code is designed to operate with helium as the coolant but can very easily be altered to function with any gas by changing a few program statements. It is somewhat unique in that it is particularly suitable for solving natural circulation problems. The program has been written in modular form so that applications to diverse systems can be accommodated in the future by rewriting the subroutine that computes the heat transfer in particular components of the system being studied. The version described in this report is applicable to the 3000 MWth HTGR design