Patent Number: 055132332
Section: claims

1. A boiling water nuclear reactor in which coolant is supplied to fuel assemblies in a pressure vessel from a lower plenum below the fuel assemblies comprising: means for improving flow stability of a two-phase flow of said coolant in said fuel assemblies of said boiling water nuclear reactor by providing an increased path length of said coolant flow, said flow stability means including a first passage connected to a coolant entrance of said lower plenum for effecting downward flow of said coolant, and a second passage communicating with said first passage for effecting upward flow of said coolant from the first passage, whereby said coolant is supplied to said fuel assemblies from said second passage and enabling flow stability of the two-phase flow of said coolant in said fuel assemblies.  means for improving flow stability of a two-phase flow of said coolant in said fuel assemblies of said boiling water nuclear reactor by providing an increased path length of said coolant flow, said flow stability means including a first passage connected to a coolant entrance of said lower plenum for effecting upward flow of said coolant, a second passage connected to said first passage for effecting downward flow of said coolant from said first passage and a third passage for effecting upward flow of the coolant from said second passage, whereby the coolant is supplied to said fuel assemblies and enabling flow stability of the two-phase flow of said coolant in said fuel assemblies.  means for improving flow stability of a two-phase flow of said coolant in said fuel assemblies of said boiling water nuclear reactor by providing an increased path length of said coolant flow, said flow stability means including a coolant passage of a spiral-form communicating with a coolant entrance of said lower plenum for mixing the coolant, before the coolant is supplied to said fuel assemblies from said coolant passage and enabling flow stability of the two-phase flow of said coolant in said fuel assemblies. 2. The nuclear reactor as defined in claim 1, wherein the flow stability means includes having the flow area of said first passage be approximately equal to the flow area of said second passage. 3. The nuclear reactor as defined in claim 2, wherein the flow stability means include having a number obtained by dividing the combined passage length of said first passage and said second passage in units of m by the flow area of the passages in units of m.sup.2 be 400 or more in units of m.sup.-1. 4. The nuclear reactor as defined in claim 1, wherein at least one of said first passage and said second passage has a spiral-form. 5. The nuclear reactor as defined in claim 1, wherein said second passage is surrounded by said first passage. 6. The nuclear reactor as defined in claim 1, wherein the first passage is connected to a coolant entrance located on an upper part of a fuel support means for said fuel assemblies, and said second passage has a coolant entrance located on the lower part of the fuel support means for connection with the first passage, the second passage having an exit located at a top of the fuel support means for supplying the coolant to said fuel assemblies. 7. A boiling water nuclear reactor in which coolant is supplied to fuel assemblies in a pressure vessel from a lower plenum below the fuel assembly comprising: 8. The nuclear reactor as defined in claim 7, wherein the flow stability means include having the flow area of said first passage, the flow area of said second passage and the flow area of said third passage be approximately equal. 9. The nuclear reactor as defined in claim 7, wherein said third passage is surrounded by said first passage and said second passage. 10. The nuclear reactor as defined in claim 7, wherein said first passage has a coolant entrance located on a bottom of a fuel support means for said fuel assemblies, said second passage being connected with said first passage and having a coolant entrance located on an upper part of the fuel support means, and said third passage being connected with said second passage and having a coolant entrance located on a lower part of the fuel support means and an exit located on a top of the fuel support means, said coolant being supplied to said fuel assemblies from the exit of said third passage. 11. A boiling water nuclear reactor in which coolant is supplied to fuel assemblies in a pressure vessel from a lower plenum below the fuel assemblies comprising: 12. The boiling water reactor as defined in claim 9, wherein said coolant passage of spiral-form has a coolant entrance located on a bottom of a fuel support means for said fuel assemblies and a coolant exit located on a top of the fuel support means, said coolant passage being a single pipe having the spiral-form and said coolant being supplied to said fuel assemblies from the exit. 13. The nuclear reactor as defined in claim 9, wherein said coolant passage includes a first passage of spiral-form having a coolant entrance located on an upper part of a fuel support means for said fuel assemblies and an exit located on a lower part of the fuel support means for effecting downward flow of said coolant. 14. The nuclear reactor as defined in claim 13, further comprising a second passage having a coolant entrance located on the lower part of the fuel support means for communicating with said first passage effecting downward flow and an exit located on the upper part of the fuel support means, said second passage effecting upward flow of said coolant, whereby said coolant is supplied to said fuel assemblies from said second passage.