Patent Number: 047327294
Section: claims

1. A fast breeder reactor comprising: a reactor vessel, a reactor core constituted by a plurality of fuel assemblies and a plurality of control rod guide pipes disposed in said reactor vessel, a partitioning member disposed between said reactor vessel and said reactor core and serving to separate the inside space of said reactor vessel into an upper plenum and a lower plenum, a high-pressure plenum, a medium-pressure plenum whose pressure is lower than that of said high-pressure plenum but is higher than said upper plenum; and a means of introducing a coolant into said high-pressure plenum, wherein said high-pressure plenum is disposed below said reactor core, said medium-pressure plenum being disposed between said reactor core and said high-pressure plenum, and wherein there is provided a channel for introducing said coolant from said medium-pressure plenum into said control rod guide pipe, a partition plate separating said high-pressure plenum and said medium-pressure plenum, said partition plate supporting lower ends of said fuel assemblies and said control rod guide pipes and support members thereof, the lower ends of said fuel assemblies and said control rod guide pipes and said support members thereof extending across said medium-pressure plenum in a direction toward said high-pressure plenum without extending across said high-pressure plenum and ending in said partition plate, and means for introducing coolant from said high-pressure plenum to the lower ends of said fuel assemblies. 2. A fast breeder reactor comprising: a reactor vessel, a reactor core constituted by a plurality of fuel assemblies and a plurality of control rod guide pipes disposed in said reactor vessel, a partitioning member disposed between said reactor vessel and said reactor core and serving to separate the inside space of said reactor vessel into an upper plenum and a lower plenum, a high-pressure plenum, a medium-pressure plenum whose pressure is lower than that of said high-pressure plenum but is higher than said upper plenum, and a means of introducing a coolant into said high-pressure plenum, wherein said high-pressure plenum is disposed below said reactor core, said medium-pressure plenum being disposed between said reactor core and said high-pressure plenum, and wherein there is provided a first channel for introducing said coolant from said medium-pressure plenum into said control rod guide pipe and a second channel for introducing said coolant from said high-pressure plenum into the lower end portion of said fuel assemblies, a partition plate separating said high-pressure plenum and said medium-pressure plenum, said partition plate supporting lower ends of said fuel assemblies and said control rod guide pipes and support members thereof, the lower ends of said fuel assemblies and said control rod guide pipes and said support members thereof extending across said medium-pressure plenum in a direction toward said high-pressure plenum without extending across said high-pressure plenum and ending in said patition plate, and means for introducing coolant from said high-pressure plenum to the lower ends of said fuel assemblies. 3. In a fast breeder reactor comprising: a reactor vessel, a reactor core having a center zone and a peripheral zone surrounding said center zone and constituted by a plurality of core fuel assemblies arranged in said center zone, a plurality of blanket fuel assemblies arranged in said peripheral zone and control rod guide pipes in which control rods are guided therealong, said core blanket fuel assemblies each having an entrance nozzle in a lower end portion opened at lower end face thereof, and said control rod guides each having an entrance nozzle formed with at least one opening formed in a peripheral wall of a lower end portions thereof, a partitioning member disposed between said reactor vessel and said reactor core and delimiting the inside space of said reactor vessel into an upper plenum and a lower plenum; a high-pressure plenum laid below said reactor core; medium-pressure plenum whose pressure is lower than that of said high-pressure plenum but is higher than said upper plenum; and a means for introducing coolant into said high-pressure plenum, the improvement wherein said medium-pressure plenum is disposed between said reactor core and said high pressure-plenum, and said high and medium-pressure plenums are separated by a partition plate serving as a lower supporting plate for said core and blanket asssemblies and said control rod guide pipes, said partition plate being formed therein with through-holes receiving said entrance nozzles of said core and blanket fuel assemblies which extend across said medium-pressure plenum, said partition plate also being formed therein with recesses which receive said entrance nozzles of said control rod guide pipes which extend across the medium-pressure plenum, a partition wall having at least one orifice formed therein being disposed in said high-pressure plenum to define an annular plenum surrounding said high-pressure plenum, said annular plenum being communicated with said medium-pressure plenum through at least one opening formed in said partition plate while communicating with said high-pressure plenum through said orifice formed in said partition wall, whereby said core fuel asemblies are fed with coolant from said high-pressure plenum through said through-holes formed in said partition plate while said blanket fuel assemblies are fed with coolant from said annular plenum through said through-holes formed said partition plate, and said control rod guide pipes are fed with coolant from said medium-pressure plenum through the openings formed in the peripheral wall of said entrance nozzles of said control rod guide pipes. 4. In a fast breeder reactor comprising: a reactor vessel, a reactor core constituted by a plurality of fuel assemblies and control rod guide pipes in which control rods are guided therealong, said fuel assemblies each having an entrance nozzle in a lower end portion opened at a lower end face thereof, said entrance nozzle defining therein a flow passage, and said control rod guide pipes each having an entrance nozzle formed with at least one opening formed in a peripheral wall of lower end portions thereof, a partitioning member disposed between said reactor vessel and said reactor core and delimiting the inside space of said reactor vessel into an upper plenum and lower plenum; a high-pressure plenum disposed below said reactor core; a medium-pressure plenum whose pressure is lower than that of said high-pressure plenum but is higher than a pressure of said upper plenum; and a means for introducing coolant into said high-pressure plenum, the improvement wherein said medium-pressure plenum is disposed between said reactor core and said high-pressure plenum, and said high and medium-pressure plenums are separated by a partition plate serving as a lower supporting plate for said assemblies and said control rod guide pipes and support members thereof, said partition plate being formed therein with through-holes receiving said entrance nozzles of said fuel assemblies which extend across said medium-pressure plenum, said partition plate also being formed therein with recesses which receive said entrance nozzles of said control rod guide pipes which extend across said medium-pressure plenum, the lower ends of said fuel assemblies and said control rod guide pipes and said support members thereof extending across said medium-pressure plenum without extending across said high-pressure plenum and ending in said partition plate, means for introducing coolant from said high-pressure plenum to the lower ends of said fuel assemblies, said fuel assemblies being fed with coolant from said high-pressure plenum through said through-holes formed in said partition plate, and said control rod guide pipes being fed with coolant from said medium-pressure plenum through the at least one opening formed in the peripheral wall of said entrance nozzles of said control rod guide pipers, said flow passage in said entrance nozzle of each of said fuel assemblies having an upper section and a lower section with the upper section having a larger cross-sectional area than the lower section so that a pressure receiving area is obtained in said flow passage, at least one orifice being provided in said flow passage to enable a fuel assembly internal pressure which is effected upon said pressure receiving area, each of said through-holes in said partition plate being formed in a stepped shape to provide an annular shoulder part on which an annular bead is formed, the lower end face of said entrance nozzle abutting against said annular bead so that the pressure of said medium-pressure plenum acts upon a part of said lower end face outside of said annular bead while the pressure of said high-pressure plenum acts upon the remaining part of said end face inside of said annular bead, whereby a sufficient downward force is obtained by selecting a relationship among said fuel assembly internal pressure, the pressure of said medium-pressure plenum, said high-pressure plenum and said pressure receiving area, in order to prevent said entrance nozzle of said fuel assembly from lifting up from said annular bead. 5. A fast breeder reactor according to claim 2, wherein said high-pressure plenum and said medium-pressure plenum are located adjacent to each other with a supporting plate interposed therebetween, said supporting plate being provided with a multiplicity of recesses into which entrance nozzles that constitute the lower end portions of said fuel assemblies are inserted, and said second channel pierces through said supporting plate from the bottom surfaces of said recesses and constitutes an opening leading to said high-pressure plenum. 6. A fast breeder reactor according to claim 5, wherein an annular bead made in contact with the lower end surface of said entrance nozzle is disposed on the bottom surface of said recess concentrically with said second channel. 7. A fast breeder reactor according to claim 6, wherein a vertical section of said annular bead is a curved surface, and said curved surface is in contact with the lower end surface of said entrance nozzle. 8. A fast breeder reactor according to claim 6 or 7, wherein a transverse sectional area of said entrance nozzle radially outside of the point of contact between said entrance nozzle and said annular projection is larger than a transverse sectional area of said entrance nozzle within said point of contact. 9. A fast breeder reactor according to claim 2, wherein said high-pressure plenum and said medium-pressure plenum are communicated via a pressure-reducing means. 10. A fast breeder reactor according to claim 9, wherein said high-pressure plenum and said medium-pressure plenum are located adjacent to each other with a supporting plate interposed therebetween, said supporting plate being provided with a multiplicity of recesses in which entrance nozzles that constitute the lower end portions of said fuel assemblies are inserted, and said second channel pierces through said supporting plate from the bottom surfaces of said recesses and constitutes an opening leading to said high-pressure plenum. 11. A fast breeder reactor according to claim 10, wherein an annular bead made in contact with the lower end surface of said entrance nozzle is disposed on the bottom surface of said recess concentrically with said second channel. 12. A fast breeder reactor according to claim 11, wherein a vertical section of said annular bead is a curved surface, and said curved surface is in contact with the lower end surface of said entrance nozzle. 13. A fast breeder reactor according to claim 11 or 10, wherein a transverse sectional area of said entrance nozzle radially outside of the line of contact between said entrance nozzle and said annular projection is larger than a transverse sectional area of said entrance nozzle within said point of contact.