Patent Number: 
Section: claims

1. An emergency core cooling system for a reactor vessel using water as a coolant and a moderator, and receiving therein a reactor core on which nuclear fission occurs, the emergency core cooling system comprising:a containment structure surrounding the entire reactor system including the reactor vessel and condensing a vapor discharged from the reactor vessel to obtain water when emergency core cooling is performed;a reactor cavity that surrounds the reactor vessel and in which said water condensed in the containment structure is collected due to gravity;a first cavity pipe extending through the reactor vessel to an interior thereof, wherein said first cavity pipe has a first cavity pipe opening in said reactor cavity and outside of said reactor vessel that is lower than an upper end of the reactor cavity; anda cavity valve provided on the first cavity pipe to open the first cavity pipe when emergency core cooling is performed and thus discharge the vapor generated in the reactor vessel through the first cavity pipe opening to an exterior of the reactor vessel;wherein said first cavity pipe provides a recirculation loop of cooling water by discharging said vapor generated in the reactor vessel and supplying said water collected in said reactor cavity in opposite directions,wherein width of said reactor cavity is less than width of said containment structure, wherein upper most portion of said reactor vessel is located in the reactor cavity below upper most end of the reactor cavity, and wherein the upper most end of the reactor cavity is located below the containment structure. 2. The emergency core cooling system as set forth in claim 1, wherein the containment structure is formed of steel to condense, on a surface of an inner wall of the containment structure, the vapor discharged to the exterior of the reactor vessel from an interior of the reactor vessel. 3. The emergency core cooling system as set forth in claim 1, wherein a heat exchanger is provided in the containment structure to condense the vapor discharged from the reactor vessel on the heat exchanger. 4. The emergency core cooling system as set forth in claim 1, wherein the cavity valve is operated by an alternating current (AC) power supply, or is operated by a direct current (DC) power supply such as a battery when the AC power supply is unable to be used. 5. The emergency core cooling system as set forth in claim 1, wherein the first cavity pipe extends into the reactor vessel at an upper portion thereof. 6. The emergency core cooling system as set forth in claim 5, wherein the first cavity pipe is provided in plurality and the first cavity pipes are placed at the same height. 7. The emergency core cooling system as set forth in claim 1, further comprising:a second cavity pipe extending through the reactor vessel to said interior thereof to be placed at the same height as the first cavity pipe; anda rupture disk provided in the second cavity pipe that ruptures due to an increase in internal pressure of the reactor vessel when the cavity valve is not operated during emergency core cooling to thereby open the second cavity pipe between the interior of the reactor vessel and the exterior of the reactor vessel. 8. The emergency core cooling system of claim 1, wherein said first cavity pipe includes a plurality of pipes that allows said water after being condensed and said vapor from inside said reactor vessel to flow in opposite directions simultaneously. 9. The emergency core cooling system of claim 1, wherein said cooling system includes a path for condensation of said vapor extending from said containment structure to a location that is in said reactor cavity, outside of said reactor vessel and below said opening, wherein said reactor cavity maintains said water from said vapor after being condensed so that the water rises in said reactor cavity from below said first cavity pipe opening to enter said first cavity pipe opening. 10. The emergency core cooling system as set forth in claim 1, wherein an uppermost end of the reactor cavity opens to an upper area contained by the containment structure.