JPH03215787A - Fuel assembly - Google Patents

Abstract

PURPOSE:To improve fuel economy by guiding a cooling material to outside of a water rod when neutron flux increases or into inner side the water rod when the neutron flux decreases. CONSTITUTION:The fuel assembly 20 contains fuel rods 22 arrayed in a square lattice shape and the water rod 23 as a cooling material guide member arranged in the center of them in a prismatic cylinder channel box 21. A flow rate control element 27 which guides the cooling material to outside of the water rod 23 when the neutron flux increases or into innerside of the water rod 23 when the neutron flux decreases and a heat generating body 36 which generates vapor by heat generation as the neutron flux increases are provided below the water rod 23. Further, a flow passage resistance control mechanism 40 which increases or decreases flow passage resistance as vapor increases or decreases is provided above the water rod 23. Consequently, neutron moderation effect increases at the upper end part of the fuel rods and plutonium accumulated at the upper end parts in the fuel rods in the beginning of an operation cycle can be burnt.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a fuel assembly for a light water reactor such as a boiling water reactor (hereinafter referred to as BWR), and particularly relates to a fuel assembly for a light water reactor such as a boiling water reactor (hereinafter referred to as BWR), and in particular a fuel assembly such as a water rod or a water cross. The present invention relates to a fuel assembly capable of performing spectrum shift operation by controlling the flow path resistance of a coolant guiding member.

(Prior Art) An example of a conventional fuel assembly loaded into a BWR core is one constructed as shown in FIG. It houses the fuel bundle 3.

In the fuel bundle 3, a plurality of fuel rods 4 are arranged in a square grid of, for example, 8 rows and 8 columns, and a water rod 5, which has a larger diameter than the fuel rods 4, is arranged in the center, and the water rods 4 are arranged in the axial direction. They are bound into a bundle by spacers 6 in the shape of flat rectangular cylinders arranged in multiple stages.

Further, an upper end plug 7 is fixed to the upper end of each fuel rod 4 and water rod 5, and a lower end plug 8 is fixed to the lower end of each fuel rod 4 and water rod 5. Furthermore, the upper end plug 7 is fixed to the upper tie plate 9, and the lower end plug 8 is fixed to the lower end of each fuel rod 4 and water rod 5. are supported by the lower tie plate 10, respectively.

The lower tie plate 10 introduces reactor water, which functions as both a moderator and a coolant, into the interior through the opening 10a as shown by the arrow in the figure, and the gap between each fuel rod 4 is directed upward from the bottom. While the heat released from each fuel rod 4 is removed at this time, the fuel is heated and flows to the upper part of the core, becoming a gas-liquid two-phase flow.

The water rod 5 introduces reactor water into the interior through the water inlet 5a at its lower end, guides it upward in the axial direction, and guides it axially upward to the water outlet 5.
b and guided to the upper end of each fuel rod 4. Here, the reactor water mainly acts as a moderator, flows slowly from the lower part of the core to the upper part, and joins and mixes with the gas-liquid two-phase flow in the upper part of the core. Incidentally, reactor water is also guided upward into the reactor core by the outer periphery of the channel box 2 and the water cross (not shown) in the same manner as the water rod 5.

By the way, in a BWR, the reactor output is controlled by the coolant flow rate (recirculation flow rate) and the insertion and removal of control rods (not shown). In recent years, control rods have been used relatively infrequently to control reactor power, and power is primarily controlled by controlling the flow rate of coolant.

This reduces the thermal impact on the fuel assembly 1,
It is well known that this is an excellent control means unique to BWR in ensuring the integrity of the fuel rods 4.

In addition, the voids (bubbles) in the channel box 2 increase as they move toward the upper part of the fuel assembly 1, and the void ratio may exceed 70% near the upper end of the heat generating part of the fuel assembly 1. Slightly above is the lowest point of void occurrence.

In order to ensure the integrity of the fuel rods 4, the conventional fuel assembly 1 changes its power distribution to the initial stage of the operation cycle (
(hereinafter referred to as BOC) to terminal stage (hereinafter referred to as EOC)
It was necessary to keep the condition as uniform as possible in the axial direction over the entire period.

However, recently, by providing a barrier layer on the inner surface of the fuel cladding tube of the fuel rod 4, the integrity of the fuel rod 4 has been significantly improved, so it is necessary to maintain the axial power distribution as constant as possible throughout the entire operating cycle. sex has decreased significantly.

In a BWR, the void ratio originally increases as one moves upwards in the core, so while the BOC suppresses the power distribution at the upper end of the fuel assembly, it is distorted toward the lower end of the fuel assembly.

On the other hand, in EOC, the concentration of fissile nuclides at the lower end of the fuel assembly is depleted by combustion, and at the upper end of the fuel assembly, depletion is delayed due to voids, and more plutonium is accumulated due to spectrum hardening due to voids.
The behavior shows that the output decreases below the core and increases at the upper end of the fuel assembly.

Utilizing these inherent properties as much as possible is good for fuel economy, but in the past, in order to ensure or improve fuel integrity, the lower part of the fuel assembly was designed to absorb more depleting neutrons. Measures have been taken to deal with this problem, such as placing materials in the upper part of the fuel assembly and increasing the fuel enrichment at the top of the fuel assembly. These have led to a deterioration in the neutron economy, or a deterioration in the fuel economy due to unburned fuel.

This natural phenomenon can be adjusted over a fairly wide range by adjusting the recirculation flow rate, and in BOC, voids occur lower in the core, resulting in higher coolant pressure loss. , the coolant core flow rate tends to decrease, while EOC exhibits exactly the opposite behavior.

By the way, in the first half of the operation cycle, the density of water, which functions as a moderator, is reduced, hardening the neutron spectrum, thereby promoting the production and accumulation of plutonium, and using this plutonium in EOC to increase the density of the moderator (water). It is well known that if nuclear fission can be caused by this, nuclear fuel can be used effectively, and as a means of implementing this, there is a coolant flow rate control method in BWR.

This is achieved by lowering the coolant core flow rate with BOC to achieve high output in the lower part of the fuel assembly, while increasing the void ratio in the upper part of the fuel assembly to accumulate plutonium.

In addition, in EOC, by increasing the core flow rate, high output is achieved in the upper part of the fuel assembly, and the plutonium and residual uranium accumulated in BOC are burned. Such a driving method is called spectrum shift driving.

Spectral shift operation would be more effective if the water rod could be used as a void rod in the BOC of a BWR, i.e., to remove water from the water rod, and as a water rod in the EOC, which would greatly improve fuel economy. can be improved. These characteristics are exactly the same in pressurized water reactors (PWRs).

(Problems to be Solved by the Invention) The present invention has been made in consideration of the above circumstances, and its purpose is to provide a fuel assembly that can improve fuel economy.

[Structure of the invention]

(Means for Solving the Problems) The present invention includes a plurality of fuel rods filled with nuclear fuel, and a coolant guide member that guides a coolant from the lower part to the upper part of these fuel rods, and which also serves as a moderator. In a fuel assembly loaded in a reactor core immersed in fuel, at least when the neutron flux increases, guiding the coolant to the outside of the coolant guiding member,
A flow control element that guides the coolant into the coolant guide member at least when the neutron flux decreases, and a heating element that generates steam by heat generation as the neutron flux increases are provided at the lower part of the coolant guide member. A flow path resistance control mechanism is provided above the coolant guide member, which increases the flow path resistance when the steam increases and decreases the flow path resistance when the steam decreases.

(Function) In general, in a BWR, lowering the flow rate increases the lower peak, or inserting a control rod midway can increase the upper peak. In an operating mode in which power peaking occurs at the bottom of the core when the flow rate of coolant passing through the reactor core is reduced, the coolant flow control element guides the coolant to the outside of the coolant guide member, and the heating element directs the steam. generate a lot. Furthermore, the flow path resistance control mechanism increases the flow path resistance due to the increase in steam. Therefore, a lot of steam accumulates in the upper part 1 of the coolant guide member, and the coolant as a moderator decreases, so that a large amount of plutonium is generated and accumulated in the upper part of the fuel rod due to the spectral shift effect.

On the other hand, in operation modes in which the flow rate of coolant passing through the reactor core is increased to produce power peaking in the upper part of the core, or in operating modes in which control rods are inserted shallowly into the lower part of the core to suppress the heat generated by the heating elements, cooling The material flow control element directs the coolant into the interior of the coolant guide member and the generation of steam from the heating element is reduced. Additionally, the flow path resistance control mechanism reduces the flow path resistance due to the reduction in steam. Therefore, more moderator and coolant flows in the coolant guide member and burns off the plutonium accumulated in the upper part of the fuel rods.

(Example) Embodiments of the present invention will be described based on FIGS. 1 to 5.

Note that in FIGS. 1 to 5, common parts are given the same reference numerals, and explanations of the overlapping parts will be omitted.

FIG. 1 is a cross-sectional view of a first embodiment of the present invention, and a fuel assembly 20 of this embodiment has a rectangular cylindrical channel box 2.
1, a plurality of fuel rods 22 arranged in a square grid,
A water rod 23 serving as a coolant guiding member in a cylindrical shape with a lid and a bottom is disposed approximately in the center of these fuel rods 22 .

FIG. 2 is a longitudinal sectional view of the above embodiment, in which the fuel rods 22 and water rods 23 have their upper ends connected to the upper tie plate 24.
Further, the lower end portion thereof is supported by a fuel support portion 26 of a lower tie plate 25, and a rectangular cylindrical channel box 21 is fitted around the outer periphery of the fuel support portion 26 of the lower tie plate 25. A flow control element 27 is attached to a first spacer 28 at the lower end of the water rod 23.
The first coolant flow path 30 is installed through the first coolant flow path 30.
The inlet 31 penetrates the fuel support 26 and is exposed downward, and immediately adjacent thereto, a small hole 34 for the inlet 33 of the second coolant flow path 32 is inserted through the fuel support 26. It is installed through. In this embodiment, the first spacer 2
8 is a resistor. Directly above the flow control element 27 is a coolant outlet 35 . The lower end of the water rod 23 serves as a coolant outlet 35, and a heating element 36 is provided just inside thereof. A flow path resistance control mechanism 40 is provided at the upper end of the water rod 23 .

These are grouped together by a spacer 41 so as to always maintain a constant interval. In addition, in this example,
Considering improvements in core characteristics such as stabilization of axial power distribution through cycles, improvement of scram characteristics, and reduction of coolant pressure loss,
An example is shown in which the water rod is shortened and the upper part has a narrow diameter.

FIG. 3 is an enlarged view of only the water rod 23, flow control element 27, heating element 36, and flow path resistance control mechanism 40 of the above embodiment. Flow control element 27
A first coolant flow path 30 and a second coolant flow path 32 are provided together. The outlet 43 of the first coolant flow path 30 intersects the horizontal plane at an angle θ, and the outlet 43 of the first coolant flow path 30 intersects with the horizontal plane at an angle θ.
The outlet 44 intersects the horizontal plane at an angle φ. It is better for the value of θ to be as small as possible, but if it is too small, the flow rate control element 27 will have to be large and the pressure loss will become too large, so a value of about 30 degrees is optimal.

On the other hand, it is sufficient to set the value of φ to about 60 degrees for the purpose of slightly warping the direction of the power jet discharged from the outlet 43 of the first coolant flow path 30 to the inside of the water rod 23. It is. At the lowest end of the flow rate control element 27, the area of the inlet 31 of the first coolant flow path 30 and the inflow port 33 of the second coolant flow path 32 is larger, and at low flow rate, the area is as follows: The inlet 33 is designed so that the heat generating part starting from the inlet 33 described in 2 can sufficiently block the flow rate.
It can be determined by adjusting the area ratio between

The shaded portion 45 of the flow rate control element 27 is made of a material that has a large heat generation effect due to gamma rays, such as zirconium,
Hafnium metal or alloys such as Zircaloy with small amounts of tin, iron, chromium, etc. added to improve their corrosion resistance are suitable. This member is provided with an inlet channel with an internal thread 46, which leads to the outlet port 44 mentioned above. In the example shown in FIG. 3, the inlet passage with the female thread 46 is provided in a meandering manner to increase the pressure loss, but it can also be set in a straight line by adjusting the area ratio of the inlet 31 and the inlet 33. It is.

The upper end portion 47 of the flow control element 27 that is not shaded is preferably made of a material that has little heat generation effect due to gamma rays, such as metals such as titanium, silicon, and aluminum, or ceramics made from them.

The peripheral portion 48 of the structural material at the lower end of the water rod 23 is slightly bent inward, so that when the cooling water from the outlet 43 flows along the upper surface of the flow control element 27, it immediately flows from the coolant outlet 35. It exits the water rod 23 and serves to cool the fuel rods 22.

In this way, when the flow rate is low, the cooling water that has flown into the flow rate control element 27 does not flow into the water rod 23, and the flow of water in the water rod 23 is as shown by the solid line. Therefore, voids generated by the heating element 36 provided below the water rod 23 are trapped above the water rod 23.

The member 49 between the heating element 36 and the upper end portion 47, which has little heat generation effect due to gamma rays, is made of a material that has little heat generation effect due to gamma rays in order to insulate the space between them, and at the same time, for the purpose of improving the cooling effect. The member 49 is provided with thread grooves or fins.

A horizontal outlet 51 is provided on the side of the upper end of the water rod 23.
An orifice plate 52 is fixed in parallel to the horizontal direction in the water rod 23 on the upstream side (lower side in FIG. 3) of the discharge port 51 in the flow direction of the reactor water and in the vicinity. .

The orifice plate 52 has a plurality of orifices 53 penetrating through it in the thickness direction, and these orifices 53 have a hole diameter that has a large flow resistance against voids (bubbles) in the reactor water, but on the other hand, it has a large flow resistance against the liquid phase. It is set so that it is small.

Therefore, the orifice plate 52 continuously increases its flow path resistance as the void of reactor water passing through the orifice 53 increases. ,, While reducing the amount of reactor water flowing through the water rod 23, the flow path resistance is continuously reduced as the voids decrease, increasing the amount of reactor water flowing through the water rod 23. It is configured in the flow path resistance control mechanism 40.

Next, the effect when this embodiment is loaded into the core of a BWR type nuclear reactor, for example, will be explained.

The axial power distribution of the fuel assembly 20 loaded in the core of a BWR reactor is distributed as shown in FIG. , the lower end) to around 1/4 L, but as the operating cycle progresses from the middle of the operating cycle (hereinafter referred to as MOC) to EOC, it gradually moves to the upper part of the fuel assembly 20.

The main cause of such axial distribution is due to the change in void fraction shown in FIG.
It gradually decreases as it progresses to OC. This is because the core flow rate of reactor water is minimum at BOC and increases as it progresses sequentially to MOC and EOC, increasing the reactor water pressure and improving the efficiency of crushing voids.

Here, in an operation mode in which the flow rate of coolant passing through the reactor core is reduced and output peaking is produced in the lower part of the core, the substance constituting the second coolant flow path 32 of the coolant flow rate control element 27 is neutron The amount of heat generated by gamma rays due to the increase in flux also increases, and a void is generated in the female thread 46 provided at the inlet 33 of the second coolant flow path 32, resulting in a large pressure drop and the flow path is closed. become. On the other hand, the inlet 31 of the first coolant flow path 30 of the coolant flow rate control element 27
has a large opening, allowing the coolant to flow in under any operating conditions, and since the coolant outlet 43 faces outward at an angle of approximately 30 degrees with respect to the horizontal section of the water rod 23, The coolant exiting from the narrow outlet 34 forms a jet and constantly flows out of the water rod 23 along the upper surface of the coolant flow control element 27. At the same time, the heating element 36 provided at the center of the lower end of the water rod 23 similarly generates a large amount of steam in the water rod 23 due to the heating effect of gamma rays caused by the increase in neutrons.

Therefore, due to the synergistic effect of the flow rate control element 27 and the heating element 36, reactor water is discharged from the inside of the water rod 23 to the outside from the BOC to the MOC, and at the same time, many voids are generated inside the water rod 23 and the orifice plate 52
As the liquid accumulates in the orifices 53 on the lower surface side, the void ratio increases there, the flow path of liquid phase reactor water narrows, and the flow path resistance of each orifice 53 increases continuously, causing the liquid to flow through these orifices 53. The amount of water flowing through the phase decreases, and the amount of reactor water that flows out from the discharge port 51 and is guided to the upper end of the fuel rod 22 decreases.

Therefore, the neutron moderating effect of liquid-phase reactor water is reduced at the upper ends of these fuel rods 22, and more plutonium is generated and accumulated at the upper ends of the fuel rods 22.

On the other hand, in operating methods that increase the flow rate of coolant passing through the core, power peaking occurs at the top of the core, or
In an operation mode in which the control rods are inserted shallowly into the lower core to suppress the heat generated by the heating elements, the second
Due to the reduction in heat generation in the coolant flow path 32, the second coolant flow path 32 is opened due to the decrease in voids generated in the flow path and the increase in the flow rate of the coolant, and the coolant is spouted out. come.

Therefore, the direction of the jet stream jetting out of the water rod 23 from the outlet 43 of the first coolant flow path 30 is bent inside the water rod 23, and the water - Steam accumulated in the rod 23 is pushed out from the flow path resistance control mechanism 40 at the upper end, and the inside of the water rod 23 is filled with coolant.

In other words, in EOC, the recirculation flow rate is increased compared to BOC, and the core flow rate of reactor water is also increased, so as shown in Figure 4 (
Since the axial output distribution moves upward as shown in A), the gamma ray heating effect by the flow rate control element 27 and the heating element 36 is relatively reduced. Therefore, the orifice plate 5
Since the voids accumulating on the lower surface side of 2 are reduced, the flow path resistance of each orifice 53 is continuously reduced accordingly. Moreover, since the core flow rate is increasing, when reactor water passes through the small holes 34 made in the fuel rod support part 26 and flows into the inlet 33 of the flow control element 27, voids are formed in the inlet 33 due to the decrease in heat generation. The reactor water easily flows through the second outlet 44.
The water is discharged from the inside of the water rod 23, and the first
The direction of the power jet exiting from the outlet 43 of the water rod 23 is deflected to the inside of the water rod 23. This jet stream easily passes through the orifice 53 where the flow path resistance is continuously reduced, and discharges the voids in the water rod 23 that have been accumulated up to the MOC, and at the same time, the upper end of the fuel rod 22 The amount of reactor water guided to the reactor increases.

Therefore, at the upper ends of these fuel rods 22, the neutron moderation effect due to the liquid phase reactor water increases, and the fuel rods 22 during BOC.
The plutonium accumulated in the upper end of 2 can be burned. As a result, fuel economy can be improved by the amount of plutonium that can be burned.

FIG. 5 shows a longitudinal sectional view of a second embodiment of the invention.

In this embodiment, the fuel support portion 26 of the lower tie plate 25 functions as a resistor. The first of the flow control elements 27
Not only the coolant inlet 31 but also the second inlet 33 are exposed at the lower part of the fuel support 26 . therefore,
The small hole 34 in FIG. 3 becomes unnecessary. Since the neutron flux is small in this vicinity of the fuel assembly 22, almost no heat generation effect due to the gamma rays of the flow rate control element 27 can be expected. The flow rates entering the first and second channels are controlled. Alternatively, the flow rate can also be controlled by providing an orifice at the inlet 33. The heating element 36 is B
It is effective to install it near or slightly below where the axial output distribution peaks in the OC.Here, the first spacer 2
It's a little above 8. Naturally, in this embodiment, the first spacer 28 has a structure with little pressure loss, just like the other spacers 41.

Since the connecting portion 49 that holds the heating element 36 to the flow control element 27 is sufficiently long, no fins or the like for heat removal are required.

[Brief explanation of drawings]

Fig. 1 is a cross-sectional view of a first embodiment of a fuel assembly according to the present invention, Fig. 2 is a longitudinal sectional view of the first embodiment, and Fig. 3 is a water rod, a flow rate control element, and a heat generating element in the first embodiment. Enlarged view showing body and flow path resistance control mechanism, Fig. 4 (A), (
B) is a graph showing the axial power distribution and void distribution of a typical fuel assembly, FIG. 5 is a vertical cross-sectional view of the second embodiment, and FIG. 6 is a vertical cross-sectional view of a conventional fuel assembly. . 20... Fuel assembly, 21... Channel box, 22... Fuel rod, 23... Water rod, 24
... Upper tie plate, 25... Lower tie plate, 26... Fuel support section, 27... Flow rate control element, 2
8... first spacer, 30... first coolant channel,
31... Inflow port, 32... Second coolant channel, 33
...Inlet, 34...Small hole, 35...Outlet, 3
6... Heating element, 40... Channel resistance control mechanism, 43.
44... Outlet, 46... Female thread, 48... Peripheral part, 51... Outlet, 52... Orifice plate, 53
...orifice.

Claims (1)

[Claims] A fuel assembly that has multiple fuel rods filled with nuclear fuel and a coolant guide member that guides coolant from the bottom to the top of these fuel rods, and is loaded into a reactor core that is immersed in a coolant that also serves as a moderator. a flow control element that guides the coolant to the outside of the coolant guide member when at least the neutron flux increases and guides the coolant to the inside of the coolant guide member when at least the neutron flux decreases; A heating element that generates steam by heat generation is provided at the lower part of the coolant guide member, and when the steam increases, the flow path resistance is increased, and when the steam decreases, the flow path resistance is decreased. A fuel assembly characterized in that a mechanism is provided above the coolant guide member.