JPH01196593A - Fuel assembly for boiling water reactor - Google Patents

Abstract

PURPOSE:To enhance the ratio of the number of hydrogen atoms. and the number of fuel atoms. without reducing fuel, by arranging fuel rods in a channel box so as to set an angle of 45 deg. with respect to the surface of said box. CONSTITUTION:In fuel aggregate 1 wherein a large number of fuel rods 3 are surrounded by a channel box 2, the fuel rods 3 are arranged so as to become an angle of 45 deg. with respect to the channel box 2. By this method, since the number of the fuel rods charged per one fuel aggregate can be increased, the part corresponding to the increased number of the fuel rods can be replaced with a water rod 4 and, therefore, the ratio of the number of hydrogen atoms. in water and the number of fuel atoms. can be increased without reducing a fuel wt. Since it is unnecessary to arrange the fuel rod to corner parts, output peaking can be lowered.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a fuel assembly for a boiling water nuclear reactor, and particularly to a fuel assembly suitable for increasing burnup.

(Prior Art) In recent years, with the increase in the amount of power generated by light water nuclear reactors, there has been a strict requirement to improve the economic efficiency of power generation, and various improvements have been made to fuel assemblies.

In addition, increasing the burn-up of fuel is effective in improving economic efficiency, and there is a demand for improvements in fuel to reduce the effects of higher burn-up on core operating characteristics.

FIG. 3 shows an example of a conventional fuel assembly used in a boiling water reactor (hereinafter referred to as BWR). It consists of fuel rods 3 arranged in a square grid of eight rows and columns, and two water rods 6 arranged at the center of the horizontal cross section inside the channel box 2.

In the BWR core, a water gap with a width of about 10 to 20 mm is provided between adjacent channel boxes 2, and a cross-shaped control rod 5 is inserted therein. In a BWR, light water flowing through the channel box 2 boils during operation and becomes a two-phase flow containing steam at an average volume ratio of about 40%. On the other hand, Channel Box 2
The light water flowing in the outer water gap region does not boil during operation. Due to the neutron moderating effect of light water in this water gap region, the horizontal thermal neutron flux distribution in the fuel assembly 1 tends to be large at the outer periphery and small at the center. For this reason, the fuel assembly 1 is provided with two water rods 6 near its center through which non-boiling water flows, and the deceleration effect of this water reduces the decrease in thermal neutron flux in the center. It plays the role of increasing the thermal neutron utilization rate by reducing power peaking and increasing thermal neutron flux.

By the way, when increasing burnup for the purpose of improving economic efficiency, the output mismatch between fuel assemblies increases and thermal restrictions such as maximum linear power density and minimum output ratio become stricter. As a result, the use of a 9-by-9 fuel assembly that increases the number of fuel rods is also being considered. Figure 4 is 9 rows 9
This is an example of a row fuel assembly, consisting of 72 fuel rods 3 and a large rectangular water rod 7 occupying 9 fuel rods.

(Problem to be solved by the invention) In order to achieve a high burnup, it is necessary to maintain long-term combustion by increasing the initial fissile material concentration (uranium-235 enrichment or pluton ram enrichment) of the fuel. There must be. However, increasing the initial fissile material concentration leads to hardening of the neutron spectrum, resulting in a decrease in the utilization rate of the fissile material.

Therefore, compared to conventional fuel, high burnup fuel has
It is known that the ratio between the number of hydrogen atoms in water that slows down neutrons and the number of fuel atoms (hereinafter referred to as H/U ratio) must be increased.

One way to increase the 1(/U ratio is to replace fuel rods with water rods, but this method reduces the fuel weight per fuel assembly,
Since the number of fuel replacement units per driving cycle increases, the fuel cost becomes unbalanced. Another method is to expand the water gap area without changing the number of fuel rods, but such fuel cannot be loaded into existing plants.

If a new plant is constructed that is compatible with such fuels, the core volume will be larger than that of conventional plants, leading to an increase in plant construction costs.

As mentioned above, it is important for high burnup fuel to improve the H/U ratio without expanding the water gap region and without reducing the amount of fuel material.

The present invention has been made in view of the above circumstances, and its object is to provide a fuel assembly for a boiling water reactor that can increase the H/U ratio without reducing fuel consumption.

[Structure of the invention]

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a fuel assembly in which a channel box surrounds a bundle in which fuel rods containing cylindrical fuel pellets are regularly arranged at regular intervals. , characterized in that the fuel rods in the cross section and the row of fuel rods adjacent to the fuel rods at positions closest to the fuel rods are arranged so as to form an angle of 45'' with respect to the plane of the channel box. It is something.

(Function) With such a fuel rod arrangement, when the fuel rod diameter is kept constant, it is possible to increase the number of fuel rods that can be loaded per fuel assembly, and this increase can be replaced with water rods. This makes it possible to increase the H/l ratio without reducing fuel weight.

In addition, in the conventional example, significant power beaking occurred in the fuel rods in a part of the corner where the deceleration effect was greatest, reducing the design margin, but in the fuel assembly according to the present invention, the fuel rod in the corner part had the greatest deceleration effect. Since it is not placed, the output peaking will decrease.

(Example) Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a plan view of one embodiment of the present invention. Note that the same components as those of the conventional example already described are given the same reference numerals, and detailed explanation thereof will be omitted.

As shown in the figure, in this embodiment, a cross-shaped water rod 4 that occupies 12 fuel rods is arranged in the center.
The area through which non-boiling water flows is increased compared to the conventional water rod, which is equivalent to nine fuel rods. However, although the fuel rod spacing is approximately 6% shorter than in the conventional example (fuel assemblies shown in FIGS. 3 and 4), the fuel rod spacing necessary for safety is still sufficiently secured. As a result, in this embodiment, compared to the conventional example, the H/U ratio can be increased by about 1% when the same weight of fuel is loaded. This improves fuel economy by 0.5-1%.

Regarding power beaking, if the initial fissile material concentration of all fuel rods is constant, the power beaking at 40% void rate in this example is about 8% compared to the conventional example.
% smaller. However, in normal design, power peaking is kept below a certain allowable value by distributing the initial fissile material concentration for each fuel rod.

FIG. 2 is a plan view of another embodiment of the present invention, in which the same parts as in the above embodiment are denoted by the same reference numerals.

In this embodiment, three types of fuel rods 1 to 3 (in this order have the highest initial fissile material concentration) are used as the fuel rods 3. On the other hand, conventionally, as shown in FIG. 5, four types of fuel have been used: I to II (in this order, the initial concentration of fissile material is highest). On the other hand, in this embodiment, as mentioned above, the output peaking can be suppressed to the same level as the conventional example shown in Fig. 5 using the three types of fuel rods I to ■. becomes easier.

As can be seen from the foregoing, the basis of the invention is to arrange the fuel rods at an angle of 45'' with respect to the channel box.

In addition, the shape of the water rod and the number of fuel rods arranged, etc.
Of course, various modifications may be made without being limited to this embodiment.

〔Effect of the invention〕

As explained above, according to the present invention, the H/U ratio can be increased without reducing the fuel weight per fuel assembly, so fuel economy is improved, especially in high burnup fuel. Further, since the types of fuel rods required to suppress power peaking to an allowable value can be reduced, manufacturing and management of fuel assemblies can be facilitated, and manufacturing costs can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a plan view of a fuel assembly according to an embodiment of the present invention, and FIG.
The figure is a plan view of a fuel assembly according to another embodiment of the present invention, FIGS. 3 and 4 are both plan views of a conventional fuel assembly, and FIG. 5 is a plan view of a conventional fuel assembly. . 1...Fuel assembly 2...Channel box 3...Fuel rod 4,6.7...Water rod 5...Control rod (8733) Agent: Yoshiaki Inomata, patent attorney (and 1 other person) ) Figure 1 Figure 2 Figure 3 Figure 4 Figure 5

Claims (1)

[Claims] (1) In a fuel assembly consisting of a channel box surrounding a bundle in which fuel rods containing cylindrical fuel pellets are regularly arranged at regular intervals, the fuel rod and the position closest to the fuel rod in its cross section. A fuel assembly for a boiling water reactor, characterized in that rows of fuel rods adjacent to each other are arranged at an angle of 45° with respect to the surface of the channel box.