Gap structures in a shielding plug

Purpose: To reduce the size of the shielding plug and the nuclear reactor container, as well as to improve the gap structure so as to facilitate the manufacture of the shielding plug. Constitution: If a gap portion is of a stepped structure, the diameter of the shielding plug is increased to enlarge the size of the reactor container and increase the construction cost, which is undesired from an economical point of view. On the other hand, if the gap is made linear in the axial direction, nuclear fission products in cover gases reach the upper portion of the shielding plug to require thick flanges, which is undesirable as well. In view of the above, resins (ex. silicones) containing great radiation shielding power material are filled in the gap portion. In this way, since the leakage of radioactivity from the inside of the reactor container can be inhibited, there is no requirement for the stepped structure or increase in the thickness of the rotational plug flange. Thus, the size of the shielding plug and that of the reactor container can be reduced and production thereof is facilitated. Lead, zirconium, etc. of high density is used as the material to be charged in the gap. (Kamimura, M.)