Mixing and stirring device for high level liquid waste storage vessel

A liquid stored in a hollow cylinder of a high level waste storage vessel is mixed and stirred by charging/discharging pressurized gases to/from the top of the cylinder to jet the stored liquid into/out of a small space between the cylinder and the bottom plate of the vessel. A back flow check mechanism is disposed at a position in the storage liquid which allows only inflowing of the liquid stored at the upper portion to the inside of the cylinder irrespective of charge or discharge of the pressurized gases to/from above the upper portion of the cylinder. When the pressurized gases are charged, the stored liquid is jetted into/out of the bottom to conduct mixing and stirring with the back flow check mechanism being closed, and when the pressurized gases are discharged, a liquid stored at the upper outer side of the cylinder is inflown with the mechanism being opened, and the liquid stored at the upper portion of the storage vessel is introduced into the cylinder and caused to be jetted out and in. Then, the composition and the temperature of the whole storage vessel can be made uniform. (I.S.)