Patent Number: 
Section: claims

1. A safety injection tank with a gravity-driven fluidic device, comprising:a cylindrical pressure vessel configured to contain emergency core cooling water therein;a tubular vertical pipe uprightly placed inside the pressure vessel and provided with at least one first flow inlet port formed in a side surface of a lower portion of the vertical pipe communicating with a lower surface of the pressure vessel and a second flow inlet port defined in an upper end of the vertical pipe which is open to an inner space of the pressure vessel;a gravity-driven fluidic device comprising: a spring placed in the upper end of the vertical pipe; and a vertically movable water tub movably placed on the spring so as to be movable in a vertical direction, and configured to contain the emergency core cooling water therein; anda guide cylinder mounted to the upper end of the vertical pipe and arranged to entirely surround a side surface of the vertically movable water tub and a portion of an upper surface of the vertically movable water tub, and wherein a plurality of through holes are formed in a surface of the guide cylinder;wherein, when the emergency core cooling water contained in the pressure vessel is externally discharged through the vertical pipe and an emergency core cooling water level is reduced lower than a vertical position of the vertically movable water tub, the vertically movable water tub elastically biased by the spring is moved in a downward direction such that a lower surface of the vertically movable water tub comes into contact with the vertical pipe to close the second flow inlet port. 2. The safety injection tank with the gravity-driven fluidic device according to claim 1, wherein the lower portion of the vertical pipe is provided with a lower discharge nozzle having the first flow inlet port therein and a diameter, which is ½ of a diameter of the second flow inlet port. 3. The safety injection tank with the gravity-driven fluidic device according to claim 1, wherein a diameter of the second flow inlet port, a diameter of the first flow inlet port and a number of the first flow inlet ports is described by a following relational expression according to a predetermined turndown ratio,      d    2    =            1      N        ×          1      TDR        ×          (                        D          2                +                  Nd          2                    )      Where d is the diameter of the first flow inlet port, N is the number of the first flow inlet ports, TDR is the turndown ratio between a first flow rate and a second flow rate, and D is the diameter of the second flow inlet port. 4. The safety injection tank with the gravity-driven fluidic device according to claim 1, wherein a spring constant of the spring is described by a following relational expression            W      T              H      S        ≤  k  ≤                    W        w            +              W        T                            y        *            +                        1          2                ⁢                  H          S                    Where Ww is a weight of emergency core cooling water contained in the vertically movable water tub, WT is a weight of the vertically movable water tub, k is the spring constant of the spring, HS is a vertical thickness of the spring, and y* is a vertical spacing distance of the vertically movable water tub. 5. The safety injection tank with the gravity-driven fluidic device according to claim 1, wherein the vertically movable water tub is provided with at least one drain port formed in a side surface thereof. 6. The safety injection tank with the gravity-driven fluidic device according to claim 5, wherein the at least one drain port is sized to have a total flow area, which can completely discharge the emergency core cooling water contained in the vertically movable water tub after the emergency core cooling water contained in the pressure vessel has been completely discharged to the outside through the vertical pipe. 7. The safety injection tank with the gravity-driven fluidic device according to claim 1, wherein the spring comprises at least one arc-shaped plate spring placed in a circumferential direction outside the second flow inlet port, the plate spring having a height equal to a vertical spacing distance of the vertically movable water tub. 8. The safety injection tank with the gravity-driven fluidic device according to claim 7, wherein the arc-shaped plate spring comprises three plate springs having a circumferential length corresponding to a 90° angle of circumference when they are orthogonally projected relative to the upper end of the vertical pipe, the three plate springs being spaced apart from each other at 120° angular intervals. 9. The safety injection tank with the gravity-driven fluidic device according to claim 1, wherein an outer diameter (ODw) of the vertically movable water tub is 1.5 times an inner diameter (D) of the vertical pipe, a height (H) of the vertically movable water tub is 1.5˜2.0 times the inner diameter (D) of the vertical pipe, and an inner diameter (IDc) of the guide cylinder is 1.6 times the inner diameter (D) of the vertical pipe. 10. The safety injection tank with the gravity-driven fluidic device according to claim 4, wherein the vertical spacing distance (y*) of the vertically movable water tub is 0.5˜1.0 times the inner diameter (D) of the vertical pipe. 11. The safety injection tank with the gravity-driven fluidic device according to claim 1, wherein a total flow area of the through holes, which are formed in the guide cylinder within a range corresponding to the vertical spacing distance of the vertically movable water tub, is set as 1.2˜1.5 times a flow area of the second flow inlet port. 12. The safety injection tank with the gravity-driven fluidic device according to claim 1, wherein the first flow inlet port is formed at a location around a lowermost surface of the pressure vessel.