  This study explores a simplified one-dimensional subchannel of a
graphite-moderated nuclear reactor operating with a gaseous core in
steady-state conditions, reproducing a neutronic-thermal-fluid-dynamics coupled
problem with thermal feedback. The fuel gas, consisting of a homogeneous
mixture of uranium hexfluoride (\ce{UF_6}) and helium, is assumed to be ideal,
with simplifications made to its thermodynamic state. Due to the high thermal
expansion of the fuel, a possible interesting strong coupling is anticipated.
The discrete ordinates' method is used to compute the one group scalar flux,
the effective multiplication factor and the power released by the core. Six
groups of Delayed Neutron Precursors (DNPs) are used to take into account the
fuel motion drift. Compressible Euler equations are solved with a monolithic
approach and the two-physics problem is treated with Picard iterations. As
expected, the effective multiplication factor of a subchannel is shown to
increase with the inlet pressure. The critical pressure, representing the
threshold at which the system achieves criticality, changes as the fuel mixture
changes. High thermal feedback coefficients are observed due to the high
thermal expansion of the fuel. The amount of helium in the mixture greatly
affects the temperature at core outlet in critical configurations. This study
shows that a gaseous fuel reactor can be brought to criticality varying the
inlet pressure. The thermal feedback is strong and should be taken into account
in the design of the system.
