Patent Number: 
Section: claims

1. A nuclear power plant spent fuel negative pressure unloading system, comprising: a fuel element transport pipe and a gas transport pipe,wherein:the fuel element transport pipe comprises a fuel element output pipe, a fuel element lifting pipe, and a fuel element unloading pipe connected in series, the fuel element unloading pipe is arranged obliquely downward in the direction of movement of a fuel element, and the distal end of the fuel element unloading pipe is connected sequentially to a fuel loading apparatus and a transfer apparatus;the gas transport pipe is connected to either end of set positions of the fuel element output pipe and the fuel element unloading pipe respectively, a gas driving mechanism is connected to the gas transport pipe, and an inlet of the gas driving mechanism is arranged at one end in proximity to the fuel element unloading pipe for sucking gas in the fuel element transport pipe, the fuel loading apparatus and the transfer apparatus, and then release the gas to the fuel element output pipe via the gas transport pipe so as to drive the movement of the fuel element. 2. The nuclear power plant spent fuel negative pressure unloading system of claim 1, wherein an iodine adsorber is installed between the end of the gas transport pipe in proximity to the fuel element unloading pipe and the gas driving mechanism. 3. The nuclear power plant spent fuel negative pressure unloading system of claim 1, wherein a dust filter is installed between the end of the gas transport pipe in proximity to the fuel element unloading pipe and the gas driving mechanism. 4. The nuclear power plant spent fuel negative pressure unloading system of claim 1, wherein an outlet pipe in parallel with the gas transport pipe is connected to an outlet of the gas driving mechanism for connecting the atmosphere. 5. The nuclear power plant spent fuel negative pressure unloading system of claim 1, wherein the fuel elements comprise spent fuel elements and graphite pebble elements. 6. The nuclear power plant spent fuel negative pressure unloading system of claim 5, wherein a radiation measuring instrument is installed at the inlet of the fuel element output pipe, the fuel loading apparatus and the transfer apparatus comprise a graphite pebble loading apparatus and a graphite pebble transfer apparatus connected sequentially and a spent fuel loading apparatus and a spent fuel transfer apparatus connected sequentially, an element dispenser is installed at the distal end of the fuel element unloading pipe, and two outlets of the element dispenser are connected with the graphite pebble loading apparatus and the spent fuel loading apparatus through pipes, respectively. 7. The nuclear power plant spent fuel negative pressure unloading system of claim 6, wherein ball valves are installed on the pipes through which the two outlets of the element dispenser are connected with the graphite pebble loading apparatus and the spent fuel loading apparatus respectively, for controlling opening and closing of the pipes. 8. The nuclear power plant spent fuel negative pressure unloading system of claim 1, wherein the fuel loading apparatus comprises a feeding pipe, a fuel loading pipe, a lifting slider and a driving motor; a canister plug claw is installed in the fuel loading pipe, a cylinder is installed above the fuel loading pipe, a telescopic rod of the cylinder is connected with the canister plug claw, and a fuel discharging port is fixed to the lower end of the fuel loading pipe; one end of the feeding pipe is connected with the distal end of the fuel element unloading pipe, and the other end is connected with the fuel loading pipe; the cylinder is fixedly connected to the lifting slider, and the driving motor is used to drive the lifting slider to move up and down. 9. The nuclear power plant spent fuel negative pressure unloading system of claim 1, wherein the transfer apparatus comprises a storage canister, a shield top cover, a shield cylindrical body, a movable bottom plate which realizes opening and closing of the shield cylindrical body, a moving mechanism which drives the storage canister to move in two vertical directions, and a hoisting system for hoisting the storage canister, wherein the shield top cover and the shield cylindrical body are connected and fixed, the movable bottom plate is located at the bottom of the shield cylindrical body for supporting the storage canister, and a shield fuel loading port is installed on the shield top cover. 10. The nuclear power plant spent fuel negative pressure unloading system of claim 9, wherein a vent which is connected with an air intake pipe is set on the movable bottom plate; and a vent which is connected with an air exhaust pipe is set on the shield top cover.