Patent Number: 
Section: claims

1. A cooling structure for a control rod drive mechanism that is placed at an upper portion of a reactor vessel and by which a control rod is inserted into and withdrawn from a reactor core, the cooling structure for the control rod drive mechanism comprising:a housing that is fixed to the upper portion of the reactor vessel;an air intake unit that takes cooling air into the housing;a first exhaust duct that is arranged side by side with the air intake unit in a circumferential direction of the housing, into which cooling air in the housing is suctioned through a first inlet at a lower portion thereof, and that guides the cooling air upward;a second exhaust duct that is disposed below the air intake unit, into which cooling air in the housing is suctioned through a second inlet, and that guides the cooling air to the first exhaust duct; anda discharging unit that is attached to an upper portion of the housing and discharges cooling air in the first exhaust duct to exterior. 2. The cooling structure for the control rod drive mechanism according to claim 1, whereinthe housing includes a plurality of vertically disposed structures arranged side by side at predetermined intervals in a circumferential direction, andthe second exhaust duct is communicatively connected to the first exhaust duct by penetrating through the structures. 3. The cooling structure for the control rod drive mechanism according to claim 2, wherein each of the structures is formed in a hollow shape, has a third inlet through which cooling air in the housing is suctioned, and is communicatively connected to the second exhaust duct through a thorough hole. 4. The cooling structure for the control rod drive mechanism according to claim 1, wherein the second exhaust duct includes a projecting unit projecting inside the housing and the second inlet is formed on the projecting unit. 5. The cooling structure for the control rod drive mechanism according to claim 1, wherein the first inlet is provided with a flow rate adjusting member for adjusting an amount of cooling air flow. 6. The cooling structure for the control rod drive mechanism according to claim 1, wherein the second exhaust duct includes a flow rate adjusting member for adjusting an amount of cooling air flow. 7. The cooling structure for the control rod drive mechanism according to claim 1, wherein the housing has an openable and closable operation opening formed below the air intake unit, and the second inlet is formed below the operation opening. 8. The cooling structure for the control rod drive mechanism according to claim 1, wherein the discharging unit includes an exhaust fan. 9. A cooling method for a control rod drive mechanism that is placed at an upper portion of a reactor vessel and by which a control rod is inserted into and withdrawn from a reactor core by using a magnetic jack, the cooling method comprising:taking cooling air into inside of a housing in which the magnetic jack is housed along an upper side wall of the housing;extracting cooling air in the housing to an exhaust duct along an entire periphery of a lower side wall of the housing after cooling the magnetic jack while directing the cooling air downward; anddischarging the cooling air to exterior by an exhaust fan after directing the cooling air upward through the exhaust duct. 10. The cooling method for the control rod drive mechanism according to claim 9, wherein the exhaust duct is arranged side by side with an air intake unit in a circumferential direction of the housing, and cooling air that has cooled the magnetic jack is suctioned in through a first inlet at a lower portion of the exhaust duct, is suctioned in through a second inlet formed below the air intake unit, and is discharged by directing the cooling air upward through the exhaust duct. 11. A nuclear reactor comprising:a reactor vessel;a core barrel disposed in the reactor vessel;a reactor core disposed in the core barrel;a plurality of control rods that controls the reactor core;a control rod drive mechanism placed at an upper portion of the reactor vessel and by which the control rods are inserted into and withdrawn from the reactor core; anda control rod drive mechanism cooling device that cools the control rod drive mechanism with cooling air, whereinthe nuclear reactor produces electricity by generating steam by heat exchange between nuclear fuel and coolant, and driving a power generating turbine with generated steam, andthe control rod drive mechanism cooling device includesa housing that is fixed to the upper portion of the reactor vessel,an air intake unit that takes cooling air into the housing,a first exhaust duct that is arranged side by side with the air intake unit in a circumferential direction of the housing, into which cooling air in the housing is suctioned through a first inlet at a lower portion thereof, and that guides the cooling air upward,a second exhaust duct that is disposed below the air intake unit, into which cooling air in the housing is suctioned through a second inlet, and that guides the cooling air to the first exhaust duct, anda discharging unit that is provided to an upper portion of the housing and discharges cooling air in the first exhaust duct to exterior.