Patent Number: 059404639
Section: description

PREFERRED EMBODIMENTS OF THE INVENTION FIG. 1 shows an example of an in-vessel structure for a fast reactor according to the present invention. Since the basic reactor construction is substantially identical with that of the prior art described hereinbefore, the corresponding parts are designated by the same reference numerals so as to have the description of the present invention easily understood. In a fast reactor, a reactor core 12 is positioned in the interior of a reactor vessel 10, and an upper opening of the reactor vessel 10 is closed with a shielding plug 14. An upper core structure 16 is fixed to the shielding plug 14, and a cold leg piping 18 is introduced from an upper portion of the reactor vessel 10 into a high-pressure plenum 20, while a hot leg piping 22 is led out to the outside of the upper portion of the reactor vessel 10. A plurality of (for example, three) primary main cooling systems, each of which includes the cold and hot leg pipings 18 and 22, are usually provided, so that three cold leg pipings 18 and three hot leg pipings 22 are incorporated. In addition to these parts, four heat exchangers (DHX) of an auxiliary direct core cooling system (not shown) are provided. The coolant sodium is supplied from the cold leg piping 18 to a high-pressure plenum 20, and passes through a low-pressure plenum 26 to reach the core 12, in which it is heated. The heated cooling sodium flows out from a core outlet surface 12a into an upper plenum 24, and reaches an intermediate heat exchanger (not shown) outside the reactor vessel through the hot leg piping 22. A part of the coolant sodium passing through the low-pressure plenum 26 flows out to an intermediate plenum 28. According to the present invention, a plurality of (three in this embodiment) annular fins 40 are fixed horizontally, in a substantially equally spaced manner in the axial (i.e., vertical) direction, to both the portions of an outer circumferential surface of the upper core structure 16 and the opposite portions of an inner circumferential surface of the reactor vessel 10. These portions are under the free liquid surface formed when the reactor is in a rated operation. Each annular fin 40 in this embodiment is completely continuous over the whole circumference thereof, and set to a width around 10% of a distance between the inner circumferential surface of the reactor vessel 10 and the outer circumferential surface of the upper core structure 16. FIG. 2 shows the flow pattern in the fast reactor during the rated operation. The high-velocity coolant flowing out from the core outlet surface 12a impinges upon a lower surface of the upper core structure 16 to form a diagonal flow advancing toward the reactor vessel wall. Although this diagonal flow turns into an upward flow along the reactor vessel wall, a further upward movement thereof is stopped by the annular fins 40 provided on the reactor vessel 10. Thus the upward flow fails to further move up and turns into vortexes in a region below the free liquid surface 50. As a result, the inner flow cannot reach the free liquid surface 50, and the fluctuation of the liquid surface is thereby prevented. The vortexes comprising a surplus upward flow which cannot be stopped by the annular fins provided on the reactor vessel wall are restrained by the annular fins 40 provided on the outer circumferential surface of the upper core structure 16. Owing to a combination of these effects, the fluctuation of the liquid surface 50 is effectively minimized. FIG. 3 is an explanatory view showing a case where the emergency shutdown (plant tripping) of the reactor occurs. Although the low-temperature sodium flows out at a low flow velocity from the core outlet surface 12a, the heat transition to the primary main cooling system can be lessened since substantially the whole region of the interior of the upper plenum 24 can be utilized as an effective mixing space. When a thermal stratification phenomenon occurs in the reactor vessel due to a density-difference effect of the coolant as shown in FIG. 4, the early elimination of a thermal stratification interface 60 can be effected since substantially the whole region of the interior of the upper plenum 24 can similarly be utilized as an effective mixing space. FIG. 5 shows an in-vessel flow of the coolant in a case where the free liquid surface 50 lowers with an earthquake occurring. Even when the free liquid surface 50 lowers, some annular fins 40 in lower stages out of a plurality thereof are kept immersed in the coolant. Therefore, the fluctuation (sloshing) of the free liquid surface can be effectively lessened. In the above embodiment, the annular fins 40 are formed continuously over the whole circumference of the reactor vessel 10, i.e., they are formed like complete rings but they may not necessarily be completely continuous over the whole circumference of the reactor vessel. Namely, each of the annular fins may comprise a certain number of arcuate parts arranged in the same plane in a slightly space manner. When such discontinuous annular fins are used, the clearances among the arcuate parts in one stage shall not be aligned with those among the arcuate parts in an adjacent stage, e.g., the arcuate parts shall be arranged in a half interval staggered manner. EXAMPLE An analysis was made with an inner structure for a fast reactor vessel, having the following sizes of various parts thereof used. Diameter of a reactor vessel: 6 m PA1 Diameter of an upper core structure: 2 m PA1 Height of a free liquid surface: 4.5 m PA1 Positions of annular fins: PA1 Width of the annular fins: 0.26 m Fins in an upper stage: PA2 Fins in an intermediate stage: PA2 Fins in a lower stage: 1.5 m below the free liquid surface PA3 2.25 m below the free liquid surface PA3 3 m below the free liquid surface The numerical calculations were made by using a general purpose multidimensional thermal flow analysis code AQUA-VOF. As a result, it could be ascertained that this in-vessel structure was effective for lessening the sloshing of the liquid surface and very effective for preventing the gas entrainment in the coolant. The flows of sodium shown in FIGS. 2-5 and 7-9 are drawn schematically on the basis of the results of the analytic calculations. As being understood from the foregoing, according to the present invention, the inner flow of the coolant advancing upward along the reactor vessel wall is stopped, whereby the sloshing of the free liquid surface can be effectively prevented. This enables the gas entrainment in the coolant which is ascribed to the sloshing of the free liquid surface to be prevented, and the safety of an operation of the fast reactor to be secured. Also in the present invention, since substantially the whole region of the interior of the upper plenum can be utilized as an effective mixing space in a transition condition at the time of the emergency shutdown (plant tripping) of the reactor, the excessive heat transition to the primary main cooling system can be lessened and the early elimination of a thermal stratification interface at the time of occurrence of a thermal stratification phenomenon can be effected. Moreover, it is also possible to minimize the sloshing of the free liquid surface during an earthquake, prevent the fluctuation of the free liquid surface accompanied by the thermal shrinkage of the coolant (liquid level change) at the time of emergency shutdown of the reactor, and improve the plant operation controllability.