  There is a strong motivation to develop high-power output nuclear fission
reactors (around 1 MWe) for space applications, such as high-payload missions
and long-duration missions beyond Mars, where the reduced solar flux makes
using alternative energy sources challenging. Many published gas-cooled reactor
designs for space applications deliver outputs in the 100 kWe regime, with
typical power densities of around 4 MW/m$^3$. Here we present a gas-cooled
reactor design - referred to as JANUS - employing He-Xe coolant and operating
on a direct Brayton cycle, that can achieve a high power output (around 0.8
MWe) and higher power density (24 MW/m$^3$) than previously published designs.
The core employs a coated particle fuel form with uranium nitride kernels in a
graphite matrix and with a high packing fraction (45%).
  Starting from the CEA's published OPUS space reactor design, a variety of
approaches were considered for achieving high power densities in a gas-cooled
reactor, including changing the core aspect ratio. Our JANUS design uses a
decreased coolant channel diameter and fuel element pitch, whilst increasing
the number of fuel elements and employing radial enrichment zoning. The design
achieves a significant increase in power density whilst obeying core-life
limits of 2000 Effective Full Power Days and a peak fuel operating temperature
of 1900 K. The core has been modelled neutronically with the ERANOS and SERPENT
codes, with a simple yet robust thermal dimensioning tool developed by us to
study peak fuel temperatures, to ensure the fuel operates within its design
limitations and to aid optimisation.
