Patent Number: 
Section: claims

1. A seal-welding method for a closed vessel containing radioactive substance, comprising:filling water into a substantially tubular vessel body closed at the bottom and having a top opening; placing radioactive substance in the vessel body and immersing the substance in the water; setting a lid in the top opening of the vessel body to close the top opening; evacuating the vessel body through a discharge hole formed in the lid and discharging steam generated in the vessel body to the outside, while charging air into the vessel body through the discharge hole; and welding a peripheral edge portion of the lid to the vessel body, thereby sealing the top opening of the vessel body, while discharging the steam to the outside through the discharge hole.  2. A seal-welding method for a closed vessel according to claim 1, wherein the lid has 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 space portion located on the bottom side of the vessel body with respect to the welding portion, and a shield gas is filled into or run through the space portion to prevent the steam from getting into the welding portion, as the lid is welded. 3. A seal-welding method for a closed vessel according to claim 2, wherein the shield gas is an inert gas. 4. A seal-welding method for a closed vessel containing radioactive substance, comprising:filling water into a substantially tubular vessel body closed at the bottom and having a top opening; placing radioactive substance in the vessel body and immersing the substance in the water; setting a shielding plate in the upper end portion of the vessel body to close the top opening, and sealing a gap between the inner peripheral surface of the vessel body and the shielding plate by means of a seal member; setting a lid in the top opening of the vessel body to be lapped on the shielding plate, thereby closing the top opening; evacuating the vessel body through a discharge hole formed in the lid and the shielding plate and discharging steam generated in the vessel body to the outside, while charging air into the vessel body through the discharge hole; and welding the peripheral edge portion of the lid to the vessel body, thereby sealing the top opening of the vessel body, while discharging the steam to the outside through the discharge hole.  5. A seal-welding method for a closed vessel according to claim 3, wherein the lid has 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 space portion located on the bottom side of the vessel body with respect to the welding portion, and a shield gas is filled into or run through the space portion to prevent the steam from getting into the welding portion, as the lid means is welded. 6. A seal-welding method for a closed vessel according to claim 5, wherein the shield gas is an inert gas. 7. An exhaust method for a closed vessel containing radioactive substance, comprising:filling water into a substantially tubular vessel body closed at the bottom and having a top opening; placing radioactive substance in the vessel body and immersing the substance in the water; setting a lid in the top opening of the vessel body to close the top opening; evacuating the vessel body through a discharge hole formed in the lid and discharging steam generated in the vessel body to the outside, while charging air into the vessel body through the discharge hole; welding a peripheral edge portion of the lid to the vessel body, thereby sealing the top opening of the vessel body, while discharging the steam to the outside through the discharge hole; passing a charging pipe through the discharge hole, the charging pipe having a charging port opening into the vessel body and a suction port opening to the outside of the vessel body; disposing an exhaust pipe in the charging pipe to form a double-pipe structure, the exhaust pipe having an exhaust port opening into the vessel body and an extending portion extending to the outside of the vessel body; and connecting a suction device to the extending portion of the exhaust pipe; evacuating the vessel body through the exhaust pipe; and charging the open air into the vessel body through the charging pipe.  8. An exhaust method according to claim 7, wherein the charging port of the charging pipe and the exhaust port of the exhaust pipe are trumpet-shaped and substantially coaxial with each other. 9. An exhaust method according to claim 7, further comprising:disposing a flow regulating portion in the charging pipe near the suction port; and regulating a quantity of air charged into the vessel body.