Patent Number: 059404639
Section: claims

1. An in-vessel structure for a fast reactor comprising a reactor core disposed in a reactor vessel;  an upper core structure disposed above said core;  an upper plenum occupied in the upper portion of said reactor vessel above said core, a free liquid surface of a coolant being in said upper plenum;  a cold leg piping inserted into said reactor vessel from the top thereof and led to said core; and  a hot leg piping extended from said upper plenum to the outside of said reactor vessel through the top thereof;  the coolant being introduced through said cold leg piping to said core in which it is heated, flowing out from said core into said upper plenum and being discharging through said hot leg piping to the outside of said reactor vessel;  characterized in that a plurality of annular fins are fixed horizontally in an axially spaced manner to both the portions of an outer circumferential surface of said upper core structure and the opposite portions of an inner circumferential surface of said reactor vessel, said portions being under said free liquid surface during a rated operation of said reactor. 2. An in-vessel structure for a fast reactor according to claim 1, wherein said annular fins fixed to both said upper core structure and said reactor vessel are provided in a plurality of axially spaced stages respectively, the width of each of said fins being 5 to 15% of a distance between the inner circumferential surface of said reactor vessel and the outer circumferential surface of said upper core structure.