Patent Number: 
Section: claims

1. A neutron detection apparatus configured to detect a neutron flux distribution inside of a pressurized water reactor, the neutron detection apparatus comprising:a neutron detector configured to detect the neutron flux distribution inside of the reactor;a thimble guide tube configured to be inserted into the reactor from the outside of the reactor, for inserting the neutron detector inside of the reactor;a drive apparatus which is connected to the thimble guide tube and which inserts the neutron detector into the thimble guide tube or extracts the neutron detector out of the thimble guide tube;a vacuum unit which controls a vacuum state in the thimble guide tube;a supply unit which supplies carbon dioxide gas;a gas purge unit which is connected to the supply unit and conducts gas purge in the thimble guide tube via the flow of carbon dioxide gas into the thimble guide tube;a gate valve which is provided between the thimble guide tube and the drive apparatus and opens or closes to allow or prevent communication between the thimble guide tube and the drive apparatus; anda control apparatus which controls the gate valve, the drive apparatus, the vacuum unit, the supply unit, and the gas purge unit, so that when the gate valve is closed, first flow of carbon dioxide gas flows inside the thimble guide tube, and when the gate valve is opened, a second flow of carbon dioxide gas, smaller than the first flow, flows inside the thimble guide tube. 2. The neutron detection apparatus according to claim 1, whereinthe control apparatus is configured to close the gate valve and control the vacuum unit to make the inside of the thimble guide tube have a vacuum atmosphere. 3. The neutron detection apparatus according to claim 2, whereinthe control apparatus is configured to, after controlling the vacuum unit to make the inside of the thimble guide tube have a vacuum atmosphere, adjust carbon dioxide gas flowing from the gas purge unit to perform gas purge in the thimble guide tube. 4. The neutron detection apparatus according to claim 3, further comprising an exhaust pipe configured to discharge gas from the inside of the drive apparatus, whereinthe control apparatus is configured to, after the gas purge in the thimble guide tube, open the gate valve, and adjust carbon dioxide gas flowing from the gas purge unit to perform gas purge in the thimble guide tube and in the drive apparatus, andthe exhaust pipe discharges carbon dioxide gas to outside of the drive apparatus. 5. The neutron detection apparatus according to claim 2, whereinthe control apparatus is configured to open the gate valve and insert the neutron detector into the thimble guide tube via the drive apparatus. 6. The neutron detection apparatus according to claim 2, whereinthe control apparatus is configured to open the gate valve and insert the neutron detector into the thimble guide tube via the drive apparatus. 7. The neutron detection apparatus according to claim 4, whereinthe control apparatus is configured to, after the gas purge in the thimble guide tube and in the drive apparatus, insert the neutron detector into the thimble guide tube via the drive apparatus. 8. The neutron detection apparatus according to claim 1, wherein the position of the gate valve is configured to be higher than a top portion of the reactor.