Patent Number: 
Section: claims

1. A jet pump disposed in a reactor pressure vessel of a boiling water reactor, the jet pump including an inlet mixer pipe connected to a riser pipe, and a diffuser pipe connected to the inlet mixer pipe to cause a forced circulation of coolant water in the reactor pressure vessel, the jet pump comprising:a slip joint structure connecting the inlet mixer pipe and the diffuser pipe to each other by inserting the inlet mixer pipe into an upper end opening of the diffuser pipe with a clearance left therebetween; anda self vibration damping structure configured such that when the clearance defined by an outer pipe wall of the inlet mixer pipe and an inner pipe wall of the diffuser pipe is widening or narrowing due to vibration of the inlet mixer pipe or the diffuser pipe, a flow path resistance inside a clearance flow path for pumped coolant water defined by the clearance is not smaller than a fluid inertia force all over the clearance flow path. 2. The jet pump according to claim 1, wherein the self vibration clamping structure includes a narrowing clearance flow path configured to gradually narrow the clearance flow path for pumped coolant water defined by the outer pipe wall of the inlet mixer pipe and the inner pipe wall of the diffuser pipe toward an upper end of the diffuser pipe. 3. The jet pump according to claim 2, wherein the narrowing clearance flow path is defined by the outer pipe wall of the inlet mixer pipe configured to gradually increase an outer diameter thereof with distance away from a lower end of the inlet mixer pipe, and the inner pipe wall of the diffuser pipe configured such that an inner diameter thereof is uniform. 4. The jet pump according to claim 2, wherein the narrowing clearance flow path is defined by the inner pipe wall of the diffuser pipe configured to gradually increase an inner diameter thereof with distance away from the upper end of the diffuser pipe, and the outer pipe wall of the inlet mixer pipe configured such that an outer diameter thereof is uniform. 5. The jet pump according to claim 2, wherein the narrowing clearance flow path is defined by the outer pipe wall of the inlet mixer pipe configured to gradually increase an outer diameter thereof with distance away from a lower end of the inlet mixer pipe, and the inner pipe wall of the diffuser pipe configured to gradually increase an inner diameter thereof with distance away from the upper end of the diffuser pipe. 6. The jet pump according to claim 2, further comprising a slip joint clamp provided so as to cover an opening edge of the diffuser pipe, and inserted into the clearance flow path for pumped coolant water defined by the outer pipe wall of the inlet mixer pipe and the inner pipe wall of the diffuser pipe;wherein the narrowing clearance flow path is defined by an inner wall of the slip joint clamp configured to gradually decrease an inner diameter thereof with distance away from a lower end of the slip joint clamp, and the outer pipe wall of the inlet mixer pipe. 7. The jet pump according to claim 2, further comprising a slip joint clamp provided so as to cover an opening edge of the inlet mixer pipe, and inserted into the clearance flow path for pumped coolant water defined by the outer pipe wall of the inlet mixer pipe and the inner pipe wall of the diffuser pipe;wherein the narrowing clearance flow path is defined by an outer wall of the slip joint clamp configured to gradually increase an outer diameter thereof with distance away from a lower end of the slip joint clamp, and the inner pipe wall of the diffuser pipe. 8. The jet pump according to claim 1, wherein the self vibration clamping structure has a configuration in which the clearance flow path for pumped coolant water defined by the outer pipe wall of the inlet mixer pipe and the inner pipe wall of the diffuser pipe has a minimum clearance flow path width on a downstream side in a coolant water pumping direction, and a maximum clearance flow path width on an upstream side in the coolant water pumping direction, as well as is configured such that a value of “(the maximum clearance flow path width−the minimum clearance flow path width)÷(the minimum clearance flow path width)” is less than 1. 9. The jet pump according to claim 1, wherein the self vibration damping structure comprises a labyrinth structure provided on any one side of the outer pipe wall of the inlet mixer pipe and the inner pipe wall of the diffuser pipe, the labyrinth structure forming a turbulent flow in a clearance flow flowing through the clearance flow path for pumped coolant water defined by the outer pipe wall of the inlet mixer pipe and the inner pipe wall of the diffuser pipe. 10. The jet pump according to claim 9, wherein the labyrinth structure is provided on any one of the outer pipe wall of the inlet mixer pipe and the inner pipe wall of the diffuser pipe. 11. The jet pump according to claim 1, wherein the self vibration damping structure is provided on any one of the outer pipe wall of the inlet mixer pipe and the inner pipe wall of the diffuser pipe, and comprises a ridge structure that projects so as to block a clearance flow flowing through the clearance flow path for pumped coolant water defined by the outer pipe wall of the inlet mixer pipe and the inner pipe wall of the diffuser pipe. 12. The jet pump according to claim 11, wherein the ridge structure is provided on any one of the outer pipe wall of the inlet mixer pipe and the inner pipe wall of the diffuser pipe. 13. A jet pump disposed in a reactor pressure vessel of a boiling water reactor, the jet pump including an inlet mixer pipe connected to a riser pipe, and a diffuser pipe connected to the inlet mixer pipe to cause a forced circulation of coolant water in the reactor pressure vessel, the jet pump comprising:a slip joint structure connecting the inlet mixer pipe and the diffuser pipe to each other by inserting the inlet mixer pipe into an upper end opening of the diffuser pipe with a clearance left therebetween; anda self vibration damping structure including a groove portion provided on any one side of an outer pipe wall of the inlet mixer pipe and an inner pipe wall of the diffuser pipe, and a convex portion provided on the other side and being fit into the groove portion with a minute clearance left therebetween. 14. A jet pump disposed in a reactor pressure vessel of a boiling water reactor, the jet pump including an inlet mixer pipe connected to a riser pipe, and a diffuser pipe connected to the inlet mixer pipe to cause a forced circulation of coolant water in the reactor pressure vessel, the jet pump comprising:a non-slip joint structure connecting the inlet mixer pipe and the diffuser pipe to each other by abutting an opening edge of the inlet mixer pipe against an opening edge of the diffuser pipe. 15. The jet pump according to claim 14, wherein any one side of the opening edge of the inlet mixer pipe and the opening edge of the diffuser pipe is formed into a convex sphere, and the other side is formed into a concave sphere that receives the convex sphere. 16. A method for restraining vibration of a jet pump disposed in a reactor pressure vessel of a boiling water reactor, in which an inlet mixer pipe and a diffuser pipe are connected to each other by inserting the inlet mixer pipe into an upper end opening of the diffuser pipe with a clearance left therebetween, to cause a forced circulation of coolant water in the reactor pressure vessel, the method including:controlling a flow of a clearance flow such that when the clearance defined by an outer pipe wall of the inlet mixer pipe and an inner pipe wall of the diffuser pipe is widening or narrowing due to vibration of the inlet mixer pipe or the diffuser pipe, a flow path resistance inside a clearance flow path for pumped coolant water defined by the clearance is not smaller than a fluid inertia force all over the clearance flow path.