Patent Number: 052727416
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a nuclear fuel assembly and, more particularly, to a nuclear fuel assembly for a boiling water reactor having spacer structure improved on heat transfer from fuel rods to the coolant. Fuel assemblies for nuclear reactors have been improved on spacer structures thereof to increase heat transfer from the fuel rods to the coolant. In a case of a pressurized water reactor (PWR) as disclosed in U.S. Pat. Nos. 3,395,077 and No. 3,379,619, spacer structures, used for holding the fuel rods to keep them laterally spaced from each other, are directed to improve the heat transfer. The structure is such that, in the center of four sides surrounding fuel rods, a grating type spacer and an obstacle serving as a vane are provided. In this structure, the coolant flows over the peripheries of the fuel rods so as to cover them due to suitable configuration and mounting positions of the vanes. As a result, the coolant is agitated, the heat transfer increases, thereby raising allowable power level of a reactor core. The above-described art relates to a PWR. If the spacer structure is used in a boiling water reactor, it is impossible to achieve the above-mentioned effect. In such cases, voids occur in the core of the BWR, and the coolant flows in a two-phase flow. Namely, a liquid film flow takes place on the surface of the fuel rod and a mixture of steam and liquid drops flow in a space region enclosed by fuel rods. Under this two-phase flow condition, if the above-mentioned conventional spacer is used, the coolant flows along the periphery of each fuel rod. Such a flow strips off the liquid film adhered to the fuel rod, thereby decreasing the amount of liquid adhered to the fuel rod in the form of liquid film. Namely, such a coolant flow is likely to cause boiling transition that nucleus boiling changes to film boiling. Therefore, power, that is, allowable power level at the boiling transition decreases. An example of a BWR fuel assembly having a plurality of vanes is disclosed in U.S. Pat. No. 4,698,204. The U.S. patent relates to a BWR fuel assembly having an intermediate flow mixing non-support grid. The grid does not support fuel rods to keep them spaced from each other and having vanes at all the fuel rod for mixing relatively cool coolant and relatively hot coolant. SUMMARY OF THE INVENTION An object of the invention is to provide a nuclear fuel assembly which has sufficiently high allowable power level and stability. Another object of the invention is to provide a nuclear fuel assembly which has sufficiently high allowable power level without causing an increase in pressure loss. The present invention resides in a nuclear fuel assembly for a BWR comprising a plurality of fuel rods, a polygonal channel box surrounding the fuel rods, a plurality of spacers axially spaced from each other and each keeping the fuel rods laterally spaced from each other, and a plurality of vanes disposed only in a region at and around a corner within the channel box, for generating swirling flows in the region to thicken a liquid film on each fuel rod in the region. According to an aspect of the present invention, the spacers are of round-cell type, and constructed of a plurality of cylindrical cells joined each other, and at least one of the spacers which is disposed in an upper region in which boiling transition is likely to occur comprises vane-formed cylindrical cells having vanes formed on the outer surface thereof and smooth-surface cylindrical cells having no such vanes as mentioned above, the vane-formed cells being disposed, in use, for holding fuel rods in a farther region from a control rod, and the smooth-surface cells being for holding fuel rods in a closer region to the control rod, whereby the thickness of liquid films on the fuel rod in the farther region is thickened and heat transfer from the fuel rods to the coolant increases effectively. When the above-mentioned vanes are disposed in two phase flow region in the nuclear fuel assembly of the BWR, the vanes generate swirling flows of steam and liquid drops in spaces enclosed by adjacent opposite fuel rods. The liquid drops in the steam are moved to the fuel rods by centrifugal force due to the swirling movement of the liquid drops and adhered to liquid films on the fuel rods, thereby to increase the thickness of the liquid film flows on the fuel rods. Therefore, thermal allowance to boiling transition is improved and the allowable power level increases. The thermal allowance increases by increasing more the intensity of the swirling movement of the coolant. The increase in the intensity of the swirling movement can be achieved by making the vanes larger in scale, however, the vanes of large scale cause an increase in pressure loss. It is necessary to increase the thermal allowance without increasing the pressure loss. The boiling transition does not occur at all the fuel rods, but it takes place locally. In general, the position where the boiling transition is likely to occur is one that power is high and thermal conditions are severe. That position is a corner at which fuel rods are positioned and which is farthest from a control rod and in the vicinity of the corner. A typical example of the corner is defined by two sides of the channel box and farthest from the corner defined by other two sides of the channel box which face two sides of a cruciform control rod. Therefore, when the spacer is constructed so that swirling flows take place only in spaces enclosed by adjacent opposite fuel rods and in spaces between channel box and the fuel rods facing the channel box in the corner region as mentioned above, the thermal allowance is increased without causing pressure loss since the number of vanes used there is small, even if vanes of large scale are employed. Use of the vanes of large scale increases an amount of liquid film flow flowing along the surfaces of the fuel rods. When a large number of vanes are used, projection area of the spacer increases, so that an area of flow passage defined within the spacer is made smaller, as a result, the pressure loss increases. In the present invention, the number of vanes can be reduced to 15 or less, for example, so that the pressure loss little increases. As mentioned above, the space where the boiling transition is apt to occur is at and around the corner of the insides of the spacer further from the control rod in a lateral plane. With respect to a vertical position, the boiling transition takes place at the upstream sides of first and second stage spacers from the top side. Usually, a fuel assembly comprises around 7 stage spacers. The boiling transition is apt to occur around the highest stage spacer. The liquid film thickness decreases from the lowest stage toward the highest stage because the liquid film is evaporated. However, at the spacers, the liquid film increase because the coolant is agitated by the spacers. An aspect of the invention is in that the vanes are provided on the second and third spacers, so that the thickness of liquid film on the fuel rods on which the liquid film is thin increases at the upstream sides of the spacers.