Patent Number: 
Section: claims

1. A method for removing decay heat from a nuclear reactor after shutdown, where said reactor has nuclear fuel rods and a cooling water input connected to deliver water that is supplied to said cooling water input to said fuel rods, comprising:providing a tank of water having an inlet and an outlet,providing a source of compressed gas,said source of compressed gas being connected to said inlet of said tank of water, said outlet of said tank of water being connected to said cooling water input and to said fuel rods of said reactor,allowing said compressed gas to flow into said inlet of said tank of water so that said compressed gas enters said tank and forces said water to leave said outlet of tank and flow to said cooling water input of said nuclear reactor and then to said fuel rods of said nuclear reactor, said compressed gas being the only force that urges said water to said cooling water input and said fuel rods of said nuclear reactor,whereby said compressed gas will force said water to flow into said nuclear reactor and cool said fuel rods to remove said decay heat from said reactor. 2. The method of claim 1 wherein(a) said source of compressed gas is compressed to at least 1000 pounds per square inch, and further including(b) providing a pressure reducing expansion valve to reduce the pressure of said gas to a relatively lower level of compression before it is allowed to flow into said inlet of said tank of water, and(c) conveying the water that said compressed gas forces from said tank of water to provide heat of expansion to said expansion valve to prevent freezing of the environment around said expansion valve before said water flows to said cooling water input of said nuclear reactor. 3. A passive safety method for cooling the fuel rods in a reactor pressure vessel in a nuclear power plant after a shutdown, where said reactor pressure vessel has a cooling water input, comprising:(a) providing a source of compressed gas at a relatively high pressure of at least 1000 pounds per square inch,(b) providing a reservoir of water,(c) conveying a stream of said highly compressed gas from said source to a location separate from said source,(d) expanding said stream of said highly compressed gas at said separate location to a relatively low pressure gas stream, resulting in lowering the temperature of said gas stream due to the expansion of said gas,(e) using said relatively low-pressure gas stream to force said water out of said reservoir in a stream to said separate location, such that said stream of water makes thermal contact with said relatively low pressure gas stream and supplies heat to said relatively low-pressure gas stream to prevent said relatively low-pressure gas stream from freezing the environment at said separate location,(f) conveying said stream of water from said separate location, after it supplies heat to said relatively low-pressure gas stream, to said cooling water input of said reactor pressure vessel of said nuclear power plant so as to cool said fuel rods in said reactor pressure vessel,whereby said low-pressure gas stream will force said stream of water of water to said separate location where it will first raise the temperature of said low-pressure gas stream to prevent freezing and thereafter to said reactor pressure vessel where it will second absorb heat from said fuel rods in said reactor pressure vessel to mitigate overheating thereof. 4. The passive safety method for cooling a reactor pressure vessel of claim 3 wherein:said location separate from said source contains an expansion valve having an input and an output and a heat exchanger having a water input, a water output, and a compressed gas output,said stream of said highly compressed gas is conveyed to said input of said expansion valve,said stream of water is conveyed to said water input of said heat exchanger so that, when water is forced out of said reservoir, it will flow to said water input of said heat exchanger,said expansion valve placed inside or connected to said heat exchanger so that said water flowing through said heat exchanger will transfer heat to said pressure-reducing expansion valve and to said compressed gas entering and leaving said expansion valve so that said compressed gas will not freeze its environment as it leaves said expansion valve,said output of said expansion valve being arranged to pass said expanded gas into said heat exchanger so that said compressed gas will flow through said heat exchanger to said compressed gas output of said heat exchanger,said compressed gas output of said heat exchanger is connected to said reservoir of water so that it will force said water out of said reservoir and to said water input of said heat exchanger,said water output of said heat exchanger being connected to said cooling water input of said reactor pressure vessel. 5. The passive safety method of claim 3 wherein a plurality of reservoirs of water is provided and said relatively low-pressure gas stream is arranged to force said water out of said plurality of reservoirs to said separate location. 6. The passive safety method of claim 3 wherein a plurality of said sources of compressed gas at said relatively high pressure is provided, and a plurality of streams of said highly compressed gas is conveyed from said plurality of reservoirs to said separate location. 7. The passive safety system of claim 3 wherein said source of compressed gas is contained within said reservoir of water and is surrounded by the water in said reservoir of water. 8. The passive safety method of claim 3 wherein said reservoir of cooling water and said source of compressed gas are portable. 9. The passive safety method of claim 8 wherein said reservoir of water and said source of compressed gas are mounted on at least one movable wheeled conveyance. 10. The passive safety method of claim 3 wherein at least one of said reservoir of water and said source of compressed gas is located at a place selected from the group consisting of above ground and underground. 11. The passive safety method of claim 3 wherein said source of compressed gas is a compressed-gas tank which stores said compressed gas at a pressure of at least 1000 pounds per square inch. 12. The passive safety method of claim 3, further including providing a compressed-gas valve arranged to deliver said stream of said highly compressed gas from said compressed gas tank when said valve is opened and to prevent delivery of compressed gas from said compressed gas tank when said valve is closed. 13. The passive safety method of claim 3 wherein a plurality of reservoirs of water are provided, and said relatively low-pressure gas stream is arranged to force said water out of said plurality of reservoirs to said separate location. 14. The passive safety method of claim 5 wherein a plurality of said sources of compressed gas at a relatively high pressure is provided, and a plurality of streams of said highly compressed gas is conveyed from said plurality of water reservoirs to said separate location. 15. A method for removing decay heat from a nuclear reactor after shutdown, said nuclear reactor having a cooling fluid inlet, said method comprising:providing a tank of water having an inlet and an outlet,providing a source of relatively highly compressed gas compressed to at least 1000 pounds per square inch,providing a pressure-reducing gas expansion valve having an inlet and an outlet and a heat exchanger having an inlet and an outlet,said source of highly compressed gas being connected to said inlet of said expansion valve, so that said highly compressed gas will expand to a relatively low pressure and drop in temperature,said outlet of said tank of water being connected to said inlet of said heat exchanger,said outlet of said expansion valve being connected to said inlet of said tank of water,said outlet of said heat exchanger being connected to said cooling fluid inlet of said reactor, andreleasing said highly compressed gas to said expansion valve so that said gas will expand to a relatively low pressure and flow to said tank of water and force said water to flow through said heat exchanger, said heat exchanger being in thermal contact with said expansion valve, so that said water in said heat exchanger will transfer heat to said gas to prevent said gas from freezing its environment, and then flow to said cooling fluid inlet of said reactor to absorb heat from said reactor to prevent overheating thereof. 16. The method of claim 15 wherein said tank of water and said source of compressed gas are buried underground. 17. The method of claim 15 wherein said tank of water and said source of compressed gas are portable. 18. The method of claim 15 wherein said tank of water and said source of compressed gas are mounted on at least one movable wheeled conveyance. 19. The method of claim 18 wherein said wheeled conveyance, said tank of water, and said source of compressed gas are concealed in a trench. 20. The method of claim 18 wherein a plurality of tanks of water and a plurality of sources of highly compressed gas compressed to at least 1000 pounds per square inch are provided and said plurality of tanks of compressed gas are connected to said plurality of tanks of water.