Patent Number: 055286410
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a fuel assembly, and more particularly to a fuel assembly suitable for the boiling water reactor. 2. Description of the Prior Art Uranium 238 contained in the nuclear fuel material changes into plutonium 239 by absorption of the high-speed neutron. With an increase in void fraction in the core, a quantity of high-speed neutron increases. Plutonium 239 causes fission by absorbing thermal neutron. With a decrease in the void fraction in the core, a quantity of the thermal neutron increases, too. Spectral shift operation utilizes the above characteristics. The spectral shift operation has the following features: (1) At the beginning of the fuel cycle, plutonium 239 is produced by increasing the void fraction in the core. PA0 (2) At the end of the fuel cycle, fission of plutonium 239 is increased by lowering the void fraction. The nuclear fuel material is used efficiently by the spectral shift operation. EP-A-0282600 shows the fuel assembly with the spectral shift rod which is one example to realize the spectral shift operation. This spectral shift rod is a water rod which comprises a coolant descending path that leads cooling water rising inside of a coolant ascending path to the lower part of the core. The coolant ascending path has a cooling water inlet port. This cooling water inlet port communicates with the lower part which is located at a portion lower than the fuel supporting portion of the lower tie-plate. The coolant descending path has a cooling water outlet port that communicates with the cooling water passage formed among the fuel rods in the upper part of the fuel supporting portion. The above water rods have a function of changing a first condition of forming a steam area inside the coolant ascending path (a coolant surface is formed in the coolant ascending path) to a second condition of filling inside of the coolant ascending path with cooling water (a coolant surface is not formed in the ascending path), by setting the resistance value of the fuel supporting portion to the prescribed value, as shown in EP-A-0282600, according to the increase of the cooling water flow rate that is supplied in the fuel assembly. The spectral shift operation can be achieved by the operation of forming the steam area inside water rods and not forming it inside the water rods. This spectral shift operation can provide a great improvement of a fuel economy. The resistance value at the fuel supporting portion is defined as a rate of the total cross-sectional area of the plural penetrating holes to the cross-sectional area of the cooling water passage formed among the fuel rods. SUMMARY OF THE INVENTION An object of this invention is to provide a fuel assembly that can suppress the decrease in the liquid level in the coolant ascending path that corresponds to the flow rate of coolant in the normal state and that can suppress the change speed of the liquid level at the transition period as well. The feature of this invention resides in setting a ratio of the minimum cross-sectional area B of the coolant ascending path to the maximum cross-sectional area A to be the range of 0.2-20%. After studying the characteristics of the above water rods in detail, the inventors discovered that the following problem might arise. The problem is that between the lower part area and the upper part area of the fuel supporting portion, the pressure difference arises by resistance of the fuel supporting portion where there are formed plural penetrating holes of the lower tie-plate. The liquid level that is formed in the coolant ascending path of the water rods are settled by balance with the sum of the above pressure difference and the pressure loss of the static water head and the coolant inlet port that arises with the liquid level in the coolant ascending path. It was revealed that there is a region where the liquid level in the coolant ascending path abruptly drops, when the ratio (hereinafter referred to as cross-sectional area ratio) of the cross-sectional area of the coolant inlet port to the coolant ascending path is small, even if the above pressure difference is the same. This is because of the following reason. When the cross-sectional area ratio is sufficiently large, coolant flows in the coolant ascending path from the coolant inlet port to compensate an amount of steam that occurs per unit time in the coolant ascending path of the water rod. At this time, since the coefficient of pressure loss in the coolant inlet port is small, the local pressure loss that occurs by the coolant inlet port is small. Hence, the above static water head balances with the above pressure difference occured by the fuel supporting portion. However, when the cross-sectional area of the coolant inlet port is small, the local pressure loss in the coolant inlet port increases sharply, and the static water head necessary for balance with the above pressure difference decreases. Therefore, the liquid level in the coolant ascending path is formed at the lower part than the level of the liquid level that is formed by the flow rate of cooling water. According to the present invention, the above mentioned problem is solved. Because the ratio of the crossing area is 0.2% or more, the liquid level formed in the coolant ascending path can suppress the decrease in the liquid level formed corresponding to the coolant flow rate in the normal operation. In addition, since the ratio of the cross-sectional area is 20% or less, the change speed of the liquid level in the coolant ascending path at the time of the transition period can be suppressed remarkably.