Heat　pipe　radiator,　featured　with　remarkable　advantages　in　heat　transfer　efficiency　and　inherent　safety　with　small　specific　mass　and　little　weight,　are　widely　adopted　to　the　heat-rejection　system　for　space　nuclear　power　reactors.　In　this　paper,　physical　and　numerical　models　are　developed　to　obtain　the　startup　and　transient　behaviors　of　radiator　unit　with　a　potassium　(K)　heat　pipe　covered　by　fin　under　space　environment.　The　heat　transfer　limit　theory　is　adopted　as　criteria　for　heat　pipe　operation　success.　Numerical　results　indicated　that　according　to　the　internal　vapor　flow　regimes,　the　K　heat　pipe　startup　could　be　divided　into　three　distinct　stages.　The　K　heat　pipe　started　up　from　frozen　state　successful　and　rapidly　until　the　expected　operation　state　is　reached.　Among　the　different　heat　transfer　limits,　only　the　sonic　limit　due　to　choked　flow　restricts　the　K　heat　pipe　during　the　second　stage.　Overall,　for　the　heat-rejection　system　of　space　nuclear　power　reactor,　the　heat　pipe　radiator　unit　can　effectively　radiated　waste　heat　to　the　space　environment　in　5　min　and　responses　fast　under　transient　conditions.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.