JPH0422893A - Fuel assembly - Google Patents

Abstract

PURPOSE:To quickly operate and control a reactor core by packing an upper area with a neutron moderator and flowing cooling water to a lower area from the lower part of the core to flow it out to a cooling area from an aperture part provided in the upper end part. CONSTITUTION:The upper area is packed with a neutron moderator 5. Light water is flowed from the lower part of the core in a lower area circular pipe structure and is flowed out from an aperture part 12 to a cooling area 8. Light water in the cooling area 8 is flowed in from the lower part of the core and is heated by heat generation of a fuel rod in a core part, and voids are generated by boiling. Then, the quantity of the neutron moderator 5 in a central moderation area 4 in the upper part of the core is not reduced and a proportion of light water in the cooling area 8 is increased to shift the void reactivity change to the negative side. Thus, the output distribution in a fuel assembly is flattened and the thermal margin is increased to quickly operate and control the core.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a heavy water-moderated, boiling, light-water-cooled pressure tube nuclear reactor, and in particular, achieves a long life and high burnup due to the thermal margin. Regarding fuel assemblies and reactor cores.

[Prior art] The fuel assembly of a heavy water-moderated, boiling light water-cooled pressure tube reactor is as follows:
It has a structure in which multiple fuel rods are bundled into a large number of rings. Usually, a water rond is placed in the center of the fuel assembly, in which light water flows inside the tube from the bottom of the core to the top, and the horizontal A cross-sectional view shows the configuration shown in FIG. By placing a large-diameter water rod with a larger diameter than the fuel rod in the center of the fuel assembly, the proportion of low-energy thermal neutrons is increased due to the neutron moderating effect of the light water in the large-diameter water rod.

Increasing the fission rate of the inner ring fuel rods in the center of the assembly flattens the power distribution within the assembly and makes the fissile material more effective (
Since each ring can be burned (on average), by lowering the enrichment of fissile material in the fuel rods of the inner ring, the average loading of fissile material in the aggregate can be reduced and fuel costs can be reduced. There is. In addition, the change in core reactivity (void reactivity change) caused by the increase in the boiling rate of light water, which is a coolant (increase in void ratio) due to the increase in core power, can be handled by installing large-diameter water rods. As a result, the average void rate of the aggregate can be relatively lowered (the contribution of non-voided water lots increases), so it becomes more negative compared to aggregates with small diameter water rondos and aggregates without water lots. known to migrate.

As a conventional technology for boiling light water reactors (BWRs), which have different types of reactors, Japanese Patent Laid-Open No. 62-250392 discloses a method in which the upper region of a large-diameter water rod is made into a double pipe structure, and light water is introduced from the lower part. There is one in which the coolant is guided to the upper end of the reactor core through a narrow inner tube in the upper region, and then flows downward through an outer tube portion to an external region through an opening provided at the lower end of the upper region. This technology has the same effect as the conventional technology for the above-mentioned heavy water-moderated, boiling, light-water-cooled pressure tube reactor in terms of flattening the power distribution within the assembly, but the effect on void reactivity changes is lower in the lower region. It is known that lowering the coolant void fraction by draining light water in addition to water rods and water lots in the upper region both have opposite signs (opposite effects). however,
In boiling light water reactors, the void reactivity change as a fuel assembly or core average is a large negative value, so it is difficult to operate and
There are no problems with controllability or safety.

[Problems to be Solved by the Invention] The above-mentioned conventional techniques do not have sufficient effect on the negative shift width of the void reactivity change in a region with a large void ratio such as the upper part of the reactor core.

The purpose of the present invention is to widen the negative shift width of void reactivity change even in a part of the core with a large void ratio, and furthermore, to achieve long-life, high-burnup heavy water moderation with excellent operability and controllability. The purpose of the present invention is to provide a fuel assembly and core for a boiling light water cooled pressure tube nuclear reactor.

[Means for Solving the Problems] The configuration of the fuel assembly according to the present invention for solving the above problems is to flow light water from the bottom to the top between a plurality of fuel rods arranged in a pressure pipe. Light water cooling pressure pipe that flows to cool the reactor core f! :! In the fuel assembly of a nuclear reactor, a moderation region divided into two regions, upper and lower, is provided in the center of the fuel assembly, the upper region is filled with a neutron moderator, and the lower region is filled with cooling water from the lower part of the core. The inflowing cooling water is configured to flow into the cooling area outside the deceleration area through an opening provided at the upper end of the lower area.

[Operations] In the fuel assembly of the present invention loaded in the core of a heavy water moderation boiling light water cooling pressure tube reactor, light water flowing from the lower part of the core flows through the cooling region and moderation region in the assembly from the lower part of the core to the upper part. However, as the light water in the cooling region is heated by the heat generated by the fuel rods, boiling starts from the lower part of the core and voids are generated, so that the void distribution shown in FIG. 2 (transferred) is formed in the axial direction of the core. The void ratio of the coolant is zero up to about 1/4 from the bottom of the core, but increases with the height of the core and reaches about 80% at the top of the core. In the upper part of the core where the void ratio in the cooling region is high, normally the change in void reactivity is small.9 However, in the present invention, light water flows from the lower region of the deceleration region into the cooling region in the upper part of the core, thereby reducing the void ratio. As a result, the reactivity characteristic with a low void ratio, that is, the change in void reactivity shifts to the negative side.

[Embodiments] Examples of the present invention will be described below with reference to FIGS. 1 to 12.

FIG. 1(a) is a partial schematic diagram of a fuel assembly of a heavy water-moderated, boiling, light-water-cooled pressure tube reactor according to the first embodiment of the present invention, and FIG. 1(b) is a partial schematic diagram of a fuel assembly taken along the line A-A' FIG. 3 is a horizontal cross-sectional view of the cut.

In FIGS. 1(a) and (b), 1 is a fuel rod, 2 is
3 is a pressure pipe, 3 is a calandria tube, 4 is a moderation region, 5 is a neutron moderator, 8 is a cooling region, and 12 is an opening.

FIGS. 1(a) and 1(b) show a partial cross-section of a fuel assembly for a heavy water-moderated, boiling, light-water-cooled, pressure tube nuclear reactor according to the present invention, in which a plurality of fuel rods 1 are bundled into a multilayer ring shape. A deceleration region 4 is equipped in the radial center of the fuel assembly.
The upper region, which is divided into three regions in the axial direction, is filled with a solid or liquid neutron moderator 5, and the lower region has a circular tube structure, through which light water that has flowed in from the bottom of the core flows upward, and the opening is filled with solid or liquid neutron moderator 5. 12 into the cooling area 8 .

This fuel assembly is loaded into a pressure tube 2, and a heavy water moderation region filled with heavy water, which is a neutron moderator, is surrounded by a calandria tube 3 on the outside in the radial direction.

The light water in the cooling region 8 flows from the lower part of the core, but in the core, it is heated by the heat generated by the fuel rods and voids are generated due to boiling, and the void distribution in the axial direction of the core is as shown in FIG. 2. Void reactivity change, which is the core reactivity injected when the void ratio of the coolant changes (increases), is the neutron moderating function of light water, which is the coolant, when the void ratio is low (approximately 60% or less). It is known that the negative reactivity effect due to loss is dominant. However, as the void fraction increases, as shown in Figure 3, the thermal neutron flux distribution rises in the center of the fuel assembly, causing a positive reactivity effect of the inner ring fuel rods that are highly enriched in fissile material. is emphasized, so the negative effects mentioned above tend to be offset. Therefore, in order to shift the void reactivity change to a more negative side, it is sufficient to increase the proportion of light water in the cooling region of the core, which has a high void ratio. In this example, the proportion of light water in the cooling region is increased without reducing the amount of neutron moderator in the central moderation region at the top of the reactor core, and the void reactivity change is on the negative side without affecting other core performance. Achieving the transition. Figure 4 shows the change in void reactivity. That is, the fourth
From the figure, it can be seen that in the conventional example, when the void ratio is 60% or more, the void reactivity tends to be positive, but in the fuel assembly of the present invention, the negative reactivity tends to increase. In the present invention, the moderation region placed in the radial center of the fuel assembly is effective in flattening the radial power distribution, and together with the improvement of the cooling function by increasing the proportion of light water in the upper part of the core, the thermal margin can be greatly increased. Since the fuel burn-up can be increased, it is possible to improve economic efficiency such as reducing fuel cycle costs by increasing the burn-up of the fuel.

Next, a method of selecting axial positions for dividing the deceleration region into three regions, an upper region and a lower region, will be explained. Figure 2 shows the void ratio and power distribution of the coolant in the axial direction of the core for a large reactor with an electrical output of 600,000 kW class. In this core, the fuel assembly is divided into three regions in the axial direction, and the enrichment of fissile material in the upper and lower fuel rods is higher than that in the center, thereby flattening the axial power distribution. There is. Therefore, the output distribution has three peaks as shown in Figure 5. The lower limit of the point at which the deceleration region is divided into upper and lower parts is that the opening provided at the upper end of the lower region must be located above the void starting point (void ratio is zero on the lower side). As can be seen from FIG. 2, a position approximately 1/4 of the core height from the lower end of the core is selected. Next, as seen from the output distribution shown in Figure 2, the upper limit of the point that divides the upper and lower parts is as follows:
In the second part of the core, which exceeds approximately 415 mm of the total length of the core from the bottom of the core, the core power decreases rapidly, and even if openings are provided in this part to lower the void ratio in the cooling region, there is no effect on changes in void reactivity. This point was chosen because it is small. In other words, the dividing point between the upper and lower regions of the deceleration region ranges from about 1/4 to about 41% of the total length of the core from the lower end of the core.
It can be seen that an appropriate selection can be made between 5 and 5 depending on the operating conditions and temperature conditions of the core.

In addition, as the neutron moderator to be filled in the upper region of the moderation region, a material with a small thermal neutron absorption cross section and a large neutron moderation cross section is suitable, and solid materials such as ZrHz, TiH
Metal hydrides such as 2 are preferred, and liquids such as light water and deuterium water are used.

FIG. 5 is a partial bird's-eye view of a fuel assembly according to a second embodiment of the present invention. The symbols are the same as in the first embodiment.

In this example, light water is used instead of a solid moderator as the neutron moderator in the upper region of the moderation region 4, and the 5-part region has a double pipe structure, and the light water is supplied from the lower part of the reactor core to the upper region through the inner pipe. It is a structure that leads to. The light water flowing from the lower part of the reactor core through the outer pipe of the lower region flows into the cooling region 8 from an opening 12 provided at the upper end of the lower region.

This embodiment is different from the first embodiment described above in that light water can be used as the neutron moderator in both the upper and lower regions. The operation of the invention is the same as that of the first embodiment, and has improved fuel economy.

Similar effects of the invention, such as improved driving and controllability, can be obtained.

Next, a third embodiment will be described based on FIGS. 6 to 8. The symbols are the same as in the first embodiment.

This embodiment relates to the position and arrangement of the opening that allows light water to flow from the lower area of the deceleration area to the cooling area. FIG. 6 shows an example in which a plurality of openings are also provided in the upper region in the first embodiment. FIG. 7 is similar, but is an example in which the shape of the region in which the neutron moderator is filled in the upper region is devised, and the light water flow path is provided closer to the outer periphery. FIG. 8 shows an example in which a plurality of openings 12 are provided in the upper region in the second embodiment.

This embodiment aims to strengthen the cooling capacity by equalizing the proportion of light water flowing into the cooling region in the upper part of the core in the axial direction. The effects of the invention are similar to those of the embodiments described above.

Next, a fourth embodiment will be described based on FIGS. 9 to 11. All symbols are the same as in the first embodiment,
11 is a lower support plate.

This example relates to a method of introducing light water into the lower and upper regions of the deceleration region. Figure 9 shows the first and third
In this embodiment, the deceleration region 4 is fixed to the lower support plate 11 of the fuel assembly, and the opening is provided on the upper side of the support plate. FIG. 10 similarly shows openings 12 for admitting light water to the lower and upper regions in the second and third embodiments.
is provided on the upper side of the support plate 1]. FIG. 11 shows an example in which the inner pipe in the lower region is shortened and the light water intake D in the upper region is arranged upward.

In this embodiment, the lower structure of the deceleration region is simplified, and the effects of the invention are similar to those of the previous embodiments.

FIG. 12 is a partial plan view of a core made of fuel assemblies according to the present invention, and is a fifth embodiment.

In FIG. 12, 10 is one heavy water moderation region, 13 is a calandria tank, and 15 is five fuel assemblies of the present invention.

FIG. 12 shows an example of a 1/4 reactor core when the fuel assembly 15 of the present invention is loaded in the heavy water staging area 10 in which the calandria tank 13 is filled with heavy water.

When all the fuel assemblies 15 of the present invention explained in the first to fourth embodiments are loaded, the performance effect of the reactor core is maximized, but even when they are loaded only in a part of the core, a corresponding effect is achieved. It is expected that the core performance will improve.

[Effects of the Invention] According to the present invention, in a heavy water-moderated, boiling, light-water-cooled pressure tube reactor, the void reactivity change in the core is shifted to the negative side, while flattening the power distribution and thermal margin within the fuel assembly. This allows for rapid operation and operation of the reactor core.
It is possible to provide a fuel assembly and a nuclear reactor core that are highly economical and are suitable for increasing fuel burnup and reducing fuel cycle costs in the future.

[Brief explanation of drawings]

1(a) and 1(b) are partial cross-sectional views of a fuel assembly according to the first embodiment of the present invention, and FIG. 2 is a diagram of the relationship between the reactor axial height, void ratio, and power distribution of a conventional reactor. Fig. 3 is a relationship diagram between radial distance and thermal neutron flux, Fig. 4 is a void reactivity characteristic diagram of fuel assemblies of the present invention and conventional fuel assemblies, and Fig. 5 is a diagram of the relationship between the radial distance and thermal neutron flux. Partial bird track diagram of fuel assembly, No. 6
8 are partial sectional views of a fuel assembly according to a third embodiment of the present invention, FIGS. 9 to 11 are partial sectional views of a fuel assembly according to a fourth embodiment of the present invention, and FIG. 12 is a partial sectional view of a fuel assembly according to a fourth embodiment of the present invention. FIG. 13, a partial plan view of a reactor core comprising a fuel assembly of the invention, is a horizontal sectional view of a fuel assembly of a conventional light water-cooled pressure tube type nuclear reactor. <Explanation of symbols> 1... Fuel rod, 2... Pressure tube, 3... Calandria tube, 4... Moderation region, 5... Neutron moderator, 6... Fuel pellet, 7... Covering material, 8. Cooling area, 9. Carbon dioxide gas, 10. Heavy water moderation area, 1]. Lower support plate, 12.
Opening, 13. Calandria tank, 1. 1. Water rod, 15. Fuel assembly of the present invention.

Claims (1)

[Claims] 1. In a fuel assembly for a light water-cooled pressure tube nuclear reactor in which light water flows from the bottom to the top between a plurality of fuel rods arranged in a pressure tube to cool the reactor core. A moderating region divided into two upper and lower regions is provided in the center of the fuel assembly, the upper region is filled with a neutron moderator, and the lower region has a structure in which cooling water flows from the lower part of the core. A fuel assembly characterized in that cooling water can flow out from an opening provided at an upper end of the lower region to a cooling region outside the deceleration region. 2. In the fuel assembly according to claim 1, a boundary portion dividing the deceleration region into two upper and lower regions is provided at a position of 1/4 to 4/5 of the axial height from the lower end of the deceleration region. Characteristic fuel assembly. 3. The fuel assembly according to claim 1, wherein light water is used as a neutron moderator in the upper region, and the light water for cooling is guided from the lower region to the upper region, and an opening is provided at the upper end of the lower region. 1. A fuel assembly characterized in that light water can flow from the deceleration region to a cooling region outside the deceleration region. 4. The fuel assembly according to claims 1 to 3, wherein a plurality of openings for allowing light water to flow out from the lower region to the cooling region are provided on the side surface of the upper region, and the light water is guided from the lower region. A fuel assembly characterized in that it is also provided. 5. The fuel assembly according to claim 1, wherein an opening for light water to the lower region of the deceleration region is provided above a lower support plate of the fuel assembly. 6. A nuclear reactor core, characterized in that it is loaded with at least one fuel assembly according to claims 1 to 5.