PURPOSE:To execute uniformly cooling of the peripheral direction of a closed vessel by providing radially a vertical fin on a cooling gas passage of the outside periphery of the closed vessel, and also placing an annular ring provided with a passage hole, in th4 vicinity of an outlet and an inlet of a cooling material. CONSTITUTION:In a passage of a cooling gas formed by a cylindrical body 3 of an atomic reactor cold trap, and a closed vessel 4, a vertical fin 11 is provided radially on the outside periphery of the closed vessel 4, also an annular ring 12 is placed in the vicinity of a cooling material inlet 9 and a cooling material outlet 10, and the annular ring is provided with a passage hole 13 with respect to the whole periphery. According to this constitution, when a cooling gas is led in from the cooling material inlet 9, the cooling gas flowing to the vertical fin area generates no dispersion in the periphery of the closed vessel 4, and the periphery direction becomes a uniform cooling state.
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