JPH02268289A - Fuel assembly - Google Patents

Abstract

PURPOSE:To improve a characteristic of a spectrum shift operation by providing, at a cooling water inlet part of a water rod, a regulation mechanism to regulate a flowing-in rate of a cooling water at an inlet, according to a flow rate of a cooling water which flows through a channel box. CONSTITUTION:In case that a flow rate in an assembly of a cooling water is less than 100% of a rating one, a cylindrical body 14 is pushed downwardly and an inlet 17 goes down to a lower part of a long hole 11 of a water rod, because a flow resistance of a barrier plate 13 is much larger than an elastic force of a spring mechanism 15, and a flowing-in rate of the cooling water to the water rod decreases substantially. When the flow rate in the assembly exceeds 100% of the rating one, the flow resistance becomes smaller than the elastic force of the spring mechanism 15 and therefore the cylindrical body 14 is pushed upwardly, whole part of the inlet 17 rises into the long hole 11 and the flowing-in rate of the cooling water to the water rod, increases. In this situation, when a core flow rate is smaller compared to a rating one, no cooling water flows in the water rod and therefore an inside of the water rod turns almost into a void to decrease an average water density of the assembly. Otherwise, when the core flow rate is close to the rating one, no void is generated and the average water density of the assembly increases.

Description

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

(Prior art) In recent years, in water-moderated reactors, especially light water-moderated reactors, which are mainstream, fuel assemblies capable of long-term cycle operation and economical combustion have been developed in order to reduce the operating costs of nuclear couplants. The body is being examined.

FIG. 3 is a diagram showing the configuration of a fuel assembly conventionally used in a boiling water reactor (hereinafter referred to as BWR). There are a plurality of fuel rods 3 arranged in a manufacturing grid shape, for example, 8 rows x 8 columns, and a channel box 2.
It consists of two water rods 4 arranged at the center of the horizontal cross section.

This water rod 4 promotes effective moderation of neutrons,
It is designed to flatten the neutron flux distribution, and cooling water flows through it.

As shown in FIG. 4, the water rod 4 has a shape that is narrowed at the bottom, and the cooling water flowing through the water rod 4 flows from the bottom to the top. In such a water rod 4, cooling water flows into the inside of the water rod 4 from the inlet hole 5 bored at the lower end of the thin tube portion 4a below the water rod 4.
The water flows out of the water rod 4 from an outflow hole 6 formed at the upper end of the large pipe portion 4b above the water rod 4, mixes with the two-phase water after cooling the fuel rods, and flows out of the core. At this time, water rod 4
The cooling water flowing inside the water rod is heated by heat transfer from the outside through the water rod pipe wall and by heat generation due to neutrons and gamma rays in the water and inside the pipe material, but because the amount of heating is small, it does not boil and becomes a single-phase flow. The water flows out of Rod 4.

The diameters of the inflow holes 5 and outflow holes 6 of the water rods 4 are determined so as to maintain a flow rate close to the minimum at which the internal cooling water does not boil even under the most thermally severe operating conditions of the reactor core. It is being Under normal operating conditions of a conventional BWR,
The greater the number of hydrogen atoms relative to uranium atoms, the higher the reactivity, and the use of water rods with such dense cooling water can improve fuel economy.

Furthermore, as a means to improve fuel economy, the core flow rate is reduced from the early to middle stages of the cycle when the infinite fuel multiplication factor is high, increasing the void ratio within the assembly and hardening the neutron spectrum.
Accumulate 39. On the other hand, at the end of the cycle when the reactivity decreases, the core flow rate is increased to lower the void ratio within the assembly to increase the reactivity, and at the same time, the plutonium accumulated due to spectrum softening is burnt to further improve the reactivity. (hereinafter referred to as spectral shift operation) is also being considered.

(Problem to be Solved by the Invention) However, in the conventional spectrum shift operation for fuel assemblies, the difference in water density caused by 1 f for a change in flow m is small, and there is a possibility that sufficient improvement in fuel economy cannot necessarily be achieved. Ta. In other words, the lower limit of the core flow rate is limited from the viewpoint of the thermal integrity of the fuel assembly and the stability of the fuel core, while the upper limit is imposed by the head of the pump that circulates the core flow rate, so the rated core power state Under , the change in core flow rate remains at about 80-120% of the rated flow rate, so the change in water density is only about a few percent in terms of void fraction.

In order to achieve larger changes in water density and improve fuel economy through spectral shift operation, the water density can be adjusted according to the combustion state of the core in water rods located within the assembly to flatten the power distribution. One effective means is to add a function to adjust the

The present invention has been made in view of the above-mentioned conventional circumstances, and by changing the flow rate of cooling water flowing in the water rod according to the core flow rate and adjusting the amount of internal void generation, the average water content of the fuel assembly is improved. We have created a fuel assembly that maintains the core density at a low level until the end of the cycle, but increases the water density when the core flow rate increases towards the end of the cycle, improving spectrum shift operation performance and further improving fuel economy. The purpose is to provide

[Structure of the Invention] (Means for Solving the Problems) The fuel assembly of the present invention includes a large number of fuel rods housed in a channel box, and a fuel rod arranged in the group of fuel rods and arranged downwardly in the channel box. In a fuel assembly equipped with a water rod having an inflow hole for introducing cooling water and an outflow hole for flowing it upward, the cooling water flowing inside the channel box is provided at the cooling water inflow hole portion of the water rod. The present invention is characterized in that an adjustment mechanism is provided that adjusts the amount of cooling water flowing into the inflow hole in accordance with the flow rate of the cooling water.

(Function) By adjusting the amount of voids generated in the water rod, a higher void state is realized during the low flow period until the end of the cycle, and plutonium-239 is generated and accumulated, and during the high flow period at the end of the cycle when the reactivity decreases. By burning plutonium-239 under low void ratio conditions, it is possible to increase reactivity and improve fuel economy.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. Incidentally, the same parts as in FIGS. 3 and 4 are given the same reference numerals, and the explanation of the overlapping parts will be omitted.

FIG. 1 is a diagram showing the configuration of a water rod according to an embodiment, in which a pair of elongated holes 11 that are long in the axial direction are bored in the circumferential wall of the narrowed narrow tube portion 4a at the bottom of the water rod 4. Fluid elements 12 are arranged on the inner and outer peripheries of the elongated hole 11 .

As shown in FIG. 2, this fluid element 12 consists of a resistance plate 13, which is a ring body, which is disposed at an angle almost perpendicular to the flow path with a slight gap from the outer periphery of the thin tube portion 4a, and a water rod thin tube. A cylindrical body 14 is inserted into the portion 4a so that it can be raised and lowered, and an elastic mechanism, such as a spring mechanism 1, is provided at the lower end of the cylindrical body 14.
The main part consists of 5th magnitude.

The resistance plate 13 has a two-part structure in which semicircular rings 13a and 13b are combined, and each resistance plate 13a and 13b is connected to a support rod 16 radially erected from the outer periphery of the cylindrical body 14 through a long hole 11. It is fixed with screws, etc.

Further, at a position corresponding to the elongated hole 11 below the cylindrical body 14,
A plurality of cooling water inlet holes 17 for introducing cooling water are provided, for example, two in the axial direction. This cylindrical body 14 and the inner wall of the thin tube part 4a are connected to O
Watertightness is maintained by seal members 18a and 18b such as rings.

A pair of notches 19 are provided at symmetrical positions in the wall of the circular tube body below the lower seal member 18b, and these notches 19 and a convex portion (not shown) provided on the inner periphery of the narrow tube portion are connected to each other. They are configured so that rotation of the cylindrical body 14 can be prevented by fitting.

The lower end of the spring mechanism 15 is fixed to the end plug at the lower end of the water rod, and its spring constant is such that the cylindrical body 14 resists the flow resistance of the resistance plate 13 until the flow rate in the assembly approaches 100% of the rated value. The minimum value is set so that the inflow hole 17 does not protrude above the lower end of the long hole 11 of the water rod, while the flow rate in the aggregate is
If it exceeds 0%, the maximum value is set so that the cylindrical body 14 rises so that all of the inlet holes 17 are accommodated in the elongated holes 11. Furthermore, spring members are selected that have material properties that are less likely to deteriorate due to radiation irradiation, but since the spring is located near the bottom of the reactor core and the neutron flux is relatively small, and neutrons are thermalized, irradiation will have an effect. is small, and there is little need to consider the effect on neutron absorption, so there is a wide range of materials to choose from. Furthermore, the shape, size, and spring constant of the resistance plate are optimally determined through experiment and analysis. It is preferable that the spring mechanism 15 is not affected by cooling water, and in this embodiment, a cylindrical seal member prevents cooling water from flowing into the spring mechanism 15.

In the fluid cord 12 having such a configuration, when the flow rate in the cooling water assembly is 100% or less of the rated value, the flow resistance of the resistance plate 13 is greater than the elastic force of the spring mechanism 15, so that the cylindrical body 14 is moved downward. The inflow hole 17 descends below the elongated hole 11 of the water rod, and the flow of cooling water into the water rod is significantly reduced.

Additionally, as the flow rate within the cooling water assembly exceeds 100%, the flow resistance becomes smaller than the elastic force of the spring mechanism 15, so the cylindrical body 14 is pressed upward, and all of the inflow holes 17 are filled into the elongated holes 11. The amount of cooling water flowing into the water rod increases.

In this way, according to this example, when the core flow rate is small compared to the rated value, almost no cooling water flows through the water rod, so the interior becomes almost void due to heating, and the aggregate average water density decreases. On the other hand, when the core flow rate approaches the rated value, sufficient cooling water flows through the water rods, so no voids are generated inside, and the aggregate average water density can be increased.

Further, in this embodiment, there are two cooling water inlet holes 17, but
Furthermore, by arranging a large number of inflow holes in the axial direction of the cylindrical body 14, more precise adjustment of the inflow amount becomes possible.

Incidentally, the diameter and number of the inlet holes 17 in this embodiment are set to be the same or smaller than those of a conventional water rod having the same diameter. This setting is possible because the inflow hole diameter of the water rod 2 was previously set with a margin to ensure the minimum flow rate that would not cause voids in the rod even at low core flow rates and high power. This is because in the embodiment, since it is necessary to prevent the generation of voids only in the high flow rate state, the flow rate in the water rod can be made smaller. As a side effect, this action increases the amount of fuel cooling water, thereby improving the limit output of the fuel assembly. When using a large diameter or super diameter water rod,
The flow rate in the water rod can reach nearly 10% of the total flow rate of cooling water in the assembly, so if a margin of 20% of the core flow rate is created, it is expected that the flow rate in the assembly will increase by about 2%. This corresponds to an improvement of over 1% in terms of marginal output.

[Effects of the Invention] As explained above, according to the fuel assembly of the present invention, the flow rate of the cooling water flowing in the water rod is changed according to the core flow rate, and the amount of internal void generation is adjusted. It is possible to keep the aggregate average water density low until the end of the core cycle, and then increase the water density when the core flow rate increases towards the end of the cycle, improving spectral shift operational performance and further improving fuel economy. Moreover, since it is possible to improve the critical output, the thermal margin of the fuel can be improved, and this can greatly contribute to improving the economic efficiency and safety of the nuclear reactor.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the configuration of the water rod of the fuel assembly of the embodiment, Fig. 2 is a diagram showing the configuration of the fluid element in Fig. 1, and Fig. 3 is a plan cross section showing the configuration of the conventional fuel assembly. 4 are diagrams showing the structure of a conventional water wood. 1...Fuel assembly 3...Fuel rod 4...Water rod 11...Long hole 12 ......Flow element 13...Resistance plate 14...Cylindrical body 15...Spring mechanism 17...・・・・・・Inflow hole 18・・・・・・Seal member applicant Japan Atomic Energy Corporation
Toshiba Corporation

Claims (1)

[Scope of Claims] A large number of fuel rods are housed in a channel box, and an inflow hole is arranged in the group of fuel rods at the bottom to introduce the cooling water in the channel box, and the inflow hole is arranged at the bottom to introduce the cooling water in the channel box. In a fuel assembly equipped with a water rod having an outflow hole, the cooling water inflow amount of the cooling water inflow hole of the water rod is adjusted according to the flow rate of cooling water flowing in the channel box. A fuel assembly characterized by being provided with an adjustment mechanism for adjustment.