The molten fuel is first discharged into a drain tank as the molten salt reactor is shut down. Some heat transfer tubes need to be placed in the drain tank accounting for the generation of decay heat by the fission products. The tube of special structure has no need to pass through the entire drain tank. It's a two-layer assembly where a center tube is inserted in a thimble. A single prototype tube without scaling is studied in this paper. The results show that the absorption ratio by the center tube is approximately unchanged. There is a transition zone in the nucleate boiling region before the linear change of superheat and heat flux. The ONB data was used for the validation of the models and an improved correlation was developed. (C) 2019 Elsevier Ltd. All rights reserved.