Patent Number: 
Section: claims

1. A closed vessel for a radioactive substance, comprising: a substantially tubular vessel body closed at the bottom, having a top opening, and configured to contain radioactive substance in a shielded state; and  a lid set in the top opening of the vessel body and welded to the inner peripheral surface of the vessel body,  the lid having an outer peripheral portion adjacently opposed to the inner peripheral surface of the vessel body, the outer peripheral portion including a welding portion welded to the inner peripheral surface of the vessel body and a groove formed on the outer peripheral portion throughout the circumference and defining a space portion which is located toward the bottom side of the vessel body with respect to the welding portion and which faces the inner peripheral surface of the vessel body, the space portion being configured to be filled with a shield gas or to allow the flow of the shield gas therein so as to shield the welding portion from the interior of the vessel body, as the welding portion is welded. 2. A closed vessel for a radioactive substance according to  claim 1 , wherein the lid has a discharge hole through which air is simultaneously charged into and discharged from the vessel body as the welding portion is welded. claim 1 3. A closed vessel for a radioactive substance, comprising: a substantially tubular vessel body closed as the bottom, having a top opening, and configured to contain radioactive substance in a shielded state;  a shielding plate set in the top opening of the vessel body and closing the top opening;  a seal member for sealing a gap between the inner peripheral surface of the vessel body and the shielding plate; and  a lid set in the top opening of the vessel body so as to be lapped on the shielding plate and having a peripheral edge portion welded to the inner peripheral surface of the vessel body,  the lid having an outer peripheral portion adjacently opposed to the inner peripheral surface of the vessel body, the outer peripheral portion including a welding portion welded to the inner peripheral surface of the vessel body and a groove formed on the outer peripheral portion throughout the circumference and defining a space portion which is located toward the bottom side of the vessel body with respect to the welding portion and which faces the inner peripheral surface of the vessel body, the space portion being configured to be filled with a shield gas or to allow the flow of the shield gas therein so as to shield the welding portion from the interior of the vessel body, as the welding portion is welded. 4. A closed vessel for a radioactive substance according to  claim 3 , wherein the lid and the shielding plate have a discharge hole through which air is simultaneously charged into and discharged from the vessel body as the welding portion is welded. claim 3 5. A closed vessel for a radioactive substance according to  claim 3 , which further comprises a support portion located on the inner peripheral surface of the vessel body near the top opening and a frame-shaped support plate placed on the support portion, and wherein the shielding plate is placed on the support plate, and the seal member has an O-ring provided between the shielding plate and the support plate. claim 3