Among　numerous　space　nuclear　power　sources,　the　high　temperature　gas-cooled　reactor　with　closed　Brayton　cycle　has　the　advantages　of　both　high　power　and　high　energy　conversion　efficiency.　An　open-grid　megawatt　gas-cooled　space　nuclear　reactor　(OMEGA)　was　investigated　in　this　paper.　It　mainly　consists　of　three　parts:　a　heat　source　provided　by　the　reactor　core,　an　energy　conversion　system　converting　heat　into　electricity,　a　heat　rejection　system　as　heat　sink.　Generally　there　are　four　structural　layouts　of　gas-cooled　space　reactor　(GCSR),　including　porous　prism　core　mainly　used　for　nuclear　thermal　propulsion,　pin-block　type　reactor,　pellet　bed　reactor　and　open-grid　type　reactor.　Not　alike　the　other　three,　there　is　no　solid　matrix　in　open-grid　type　reactor.　Consequently,　it　is　hard　for　the　core　removing　fission　and　decay　power　through　heat　conduction　under　the　transient　condition　of　core　coolant　flow　reduction　or　even　totally　loss.　Due　to　the　absence　of　relevant　research,　passive　decay　heat　removal　capability　during　shutdown　transient　condition　was　investigated　in　this　paper.　Especially　radiation　heat　transfer　among　fuel　rod　claddings　was　calculated.　Furthermore,　a　system　transient　analysis　code　of　start-up　and　shutdown　(TASS)　was　developed　by　staggered　grid　technique　and　solved　by　GEAR　algorithm,　including　core,　turbine-generator-compressor,　regenerator,　condenser,　heat　pipe　radiator　and　other　modules.　TASS　was　verified　by　comparing　the　design　value　with　the　program　calculation　value,　and　the　transient　conditions　of　system　start-up　and　shutdown　were　simulated　by　TASS.　The　results　show　that　the　core　flow　and　power　match　well　by　inserting　positive　reactivity　in　two　stages　and　elevating　the　rotating　speed　of　turbomachinery　in　a　ladder-type.　The　start-up　process　of　the　system　can　be　accomplished　in　3.5　h,　and　the　steady-state　operation　of　reactor　power　3　406　kW　and　flow　rate　14.2　kg/s　can　be　achieved.　At　the　beginning　of　start-up,　an　additional　power　supply　of　5　kW　is　required　for　the　turbomachinery.　During　scheduled　shutdown,　average　temperature　of　fuel　element　experiences　a　trend　of　decline,　rise　and　fall.　During　emergency　shutdown,　pellet　temperature　continues　to　drop　and　cladding　temperature　is　slightly　lower　than　pellet　temperature.　No　matter　it＇s　a　scheduled　shutdown　or　emergency　shutdown,　due　to　the　existence　of　radiation　heat　transfer　among　fuel　rod　claddings,　maximum　temperature　of　cladding　and　pellet　is　lower　than　the　safety　limits　of　their　materials,　which　reflects　the　passive　safety　of　core　design.　And　as　a　result　of　the　Biot　number　of　fuel　element　is　lower　than　0.1,　cladding　temperature　is　close　to　the　pellet　temperature,　which　requires　special　attention.　©　2022,　Editorial　Board　of　Atomic　Energy　Science　and　Technology.　All　right　reserved.