Experimental study on frozen startup and heat transfer characteristics of a cesium heat pipe under horizontal state

•A cesium heat pipe with good heat transfer performance is designed and fabricated.•A series of frozen startup experiments of the cesium heat pipe under horizontal state are systematically carried out for the purpose of supplementing the quantitative data of the cesium heat pipe.•The frozen startup process of the cesium heat pipe is described in detail.•The effects of heating powers and heating modes on the frozen startup process and heat transfer performance are systematically analyzed.