JPH01155292A - Fuel assembly - Google Patents

Abstract

PURPOSE:To intend an increment of a reactor shutdown margin, by making an average weight ratio of a fissile uranium contained in a down stream side region to the whole fuel materials in the region, to be smaller than that of an upper stream side region. CONSTITUTION:A fuel assembly under consideration has 120 fuel rods, for example, and is broken down to be 60 fuel rods 3 which are loaded with a plutonium fuel at an upper one third region and a uranium fuel at a lower two thirds region, and 60 fuel rods 4 which are loaded with a uranium fuel for the whole region. Therefore, in this fuel assembly, an average weight ratio of a fissile uranium to the whole fuel material at the upper region is smaller than that in the lower region and, on the contrary, an average weight ratio of a fissile plutonium is larger at the upper region. In this case, an infinitive multiplication factor of neutrons at an ambient temperature can be reduced as much as about 4%DELTAk/k compared to a fuel assembly consisting a uranium fuel only, consequently, a reactor shutdown margin can be increased by about 0.9%DELTAk/k.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a fuel assembly used in a boiling water nuclear reactor,
In particular, the present invention relates to a fuel assembly suitable for ensuring reactor shutdown margin.

[Prior Art] In a boiling water reactor, the density of water, which is a moderator, becomes lower than when the reactor is at room temperature because the water becomes high temperature and voids are generated during operation. Therefore, the potential reactivity of the core is generally greater at room temperature than during operation.

On the other hand, in nuclear reactor design, from a safety perspective, it is required that the reactor can be reliably shut down at room temperature even if the control rod with the highest reactivity value becomes unable to be inserted. That is, it is necessary to ensure a sufficient difference between the effective neutron multiplication factor in the state where the control rod with the highest reactivity value is not inserted and the effective neutron multiplication factor in the critical state of 1.0 (that is, reactor shutdown margin). The following methods have been mainly used to ensure margin for reactor shutdown.

(1) Changing the fuel assembly arrangement; (2) increasing the reactivity value of control rods; and (3) using fuel rods containing burnable poison.

In (1) above, the fuel is shuffled so that the reactivity values of the control rods are averaged. However, this method
The drawback is that fuel shuffling takes time and reduces plant capacity utilization.

(2) above secures reactor shutdown margin by increasing the reactivity control ability of insertable control rods.

However, surplus reactivity control during operation has drawbacks such as increased distortion in the output distribution due to the control rods.

The above (3) is to suppress the reactivity of the fuel by adding a neutron absorbing material such as gadolinia to the fuel, and to control the excess reactivity and secure margin for reactor shutdown. especially,
From the viewpoint of securing margin for reactor shutdown, a fuel assembly with a region containing many burnable poisons above the fuel was proposed in Japanese Patent Laid-Open No. 59-10218.
8 is presented. The reason why countermeasures for the upper part of the fuel are effective for reducing reactor shutdown margin is that (a) In a normal boiling water reactor, where water as a coolant flows in from below the fuel and creates voids, the output of the lower part of the fuel where the density of water is high is high. Since the upper part of the fuel burns more slowly than the lower part, a large amount of fuel remains in the upper part.

(0) In the upper part of the fuel where the void ratio is large, the difference in reactivity between operation and room temperature is larger than in the lower part.

For this reason.

On the other hand, Japanese Patent Application Laid-Open No. 51-89991 describes a fuel assembly in which the weight proportion of fissile uranium and the weight proportion of plutonium in the upper region are both larger than those in the lower region in a state in which combustion has progressed. There is.

[Problems to be solved by the invention] However, among the above-mentioned conventional techniques, the former (Japanese Patent Laid-Open No. 59-102
In No. 188), a large amount of burnable poison is added to the upper part of the fuel, so that during operation, the reactivity in the upper part of the fuel, which is lower than that in the lower part, becomes even smaller. This causes the power distribution to become increasingly distorted toward the downward direction of the fuel, which is undesirable from the viewpoint of flattening the power distribution.

Furthermore, in the latter (Japanese Patent Laid-Open No. 51-89991), more fissile uranium exists in the upper part of the fuel, where the atomic ratio of hydrogen to fuel changes greatly between operation and room temperature, than in the lower part. In other words, as a result, the upper part becomes more highly enriched fuel than the lower part. However, as schematically shown in FIG. 2, highly enriched uranium fuel has a larger difference in neutron multiplication factor between operation and room temperature than low enriched fuel. Therefore, there is a problem that the reactor shutdown margin becomes small.

An object of the present invention is to increase the reactor shutdown margin without deteriorating the axial power distribution during operation.

[Means for Solving the Problems] In order to achieve the above object, the present invention consists of a bundle of a large number of fuel rods, and the flow material flows from the upstream side to the downstream side along the longitudinal direction of the fuel rods. In a fuel assembly in which voids are generated in the coolant as the temperature increases, the fuel assembly is divided into two regions in the direction of the flow of the coolant, and the fissile uranium contained in the downstream region is divided into two regions. the average weight proportion of the fissionable plutonium contained in the downstream region to the total fuel material contained in the region is smaller than that in the upstream region, and the average weight proportion of fissile plutonium to the total fuel material contained in the downstream region It is characterized by being configured to be larger than that in the upstream area.

[Function] According to the present invention, the difference in neutron multiplication factor during operation and at normal temperature becomes small, and the above object can be achieved.

In the following, a boiling water reactor in which fuel assemblies are arranged vertically and coolant flows from bottom to top along the fuel assemblies will be described as an example. Further, in the following, uranium fuel refers to a fuel that contains only uranium and does not contain plutonium, and plutonium fuel refers to a fuel made by enriching depleted uranium, natural uranium, or slightly enriched uranium with plutonium.

The operation of the present invention will be explained using FIG. In this diagram,
The relationship between void fraction and infinite neutron multiplication factor for a fuel assembly with a water-to-fuel volume ratio of about 1.3 is shown. Concentration level 6. When using uranium fuel consisting of 0wt% uranium oxide (1102), the difference in the infinite neutron multiplication factor between the state with a void ratio of 70% during operation and the state at room temperature is about 23%Δ/N. On the other hand, for example, when using plutonium fuel made by enriching depleted uranium with plutonium oxide (PLI02) at an enrichment level of 10.0 wt%, the neutron The difference in infinite multiplication factors is about 7% Δ/. In other words, by using plutonium fuel instead of uranium fuel in the upper part of the fuel, where the void ratio is large during operation and the difference in hydrogen to fuel atomic ratio between operation and room temperature is large, as mentioned above, The difference in the infinite neutron multiplication factor can be reduced.

Therefore, the reactor shutdown margin can be increased without reducing the reactivity during operation.

The same can be said of platonium fuel, which is made by enriching natural uranium or slightly enriched uranium with plutonium instead of depleted uranium.

On the other hand, plutonium fuel has a macroscopic fission cross section for fast neutrons that is about 1.3 times that of uranium fuel. For this reason, plutonium fuel tends to have higher output than uranium fuel. Therefore, by using plutonium fuel instead of uranium fuel in the upper part of the fuel, which has lower output than the lower part, it is possible to increase the output of the upper part, increase the reactor shutdown margin, and at the same time flatten the axial power distribution. can.

[Example] The following example is an example of a boiling water reactor in which fuel assemblies are arranged vertically and coolant flows from bottom to top.

FIG. 1 shows a first embodiment of a fuel assembly according to the present invention. As shown in Figure (a), the fuel assembly 1 has a hexagonal shape, with a channel box 2, fuel rods 3.4 arranged in a hexagonal close-packed grid, and control guide tubes 5 (indicated by black circles in the figure). consists of things). The water to fuel volume ratio of the present fuel assembly is approximately 1.3. Fuel rod 3 (indicated by a hatched circle in the figure) has plutonium fuel 6, which is enriched with depleted uranium and 10.0 wt% plutonium, in the upper 1/3 region, and plutonium fuel 6 in the lower 2/3 region. Concentration level 6. Owt%
uranium fuel 7 (see Figure 1(b)). On the other hand, fuel rod 4 (indicated by a white circle in the figure) is
It is loaded with uranium fuel 7 with an enrichment of 6.0 wt% (see Fig. 1(C)).

By the way, when no control rod is inserted, the inside of the control rod guide tube 5 is filled with non-boiling water. Therefore, around the fuel rods surrounding the control rod guide tube 5, a state of locally good neutron moderation occurs. - On the other hand, plutonium fuel has a macroscopic fission cross section for thermal neutrons that is approximately 2.0 times that of uranium fuel. In contrast, as mentioned above, the cross-sectional area of plutonium fuel for fast neutrons is about 1.3 times that of uranium fuel. That is, compared to uranium fuel, plutonium fuel tends to have higher output in a region where neutron moderation is good. Considering this point and from the viewpoint of flattening the power distribution in the radial direction, in this embodiment, fuel rods 4 loaded with uranium fuel are arranged around the control rod guide tubes 5.

In the fuel assembly of this example, the number of fuel rods is 120, of which 60 fuel rods 3 are loaded with plutonium fuel in the upper 1/3 region and uranium fuel in the lower 2/3 region. There are 60 fuel rods 4 loaded with only uranium fuel. Furthermore, the average weight ratio of fissile uranium to the total fuel material in the upper third of the fuel assembly is approximately 3
．． 1 wt%, the average weight fraction of fissile plutonium in the region is about 5.9 wt%. On the contrary,
The average weight fraction of fissile uranium in the lower two-thirds of the fuel assembly is 6. Owt%, the weight proportion of fissile plutonium in this region is 0. Therefore, the present fuel assembly has a lower average weight proportion of fissile uranium to total fuel material in its upper region than in its lower region;
Moreover, the average weight proportion of fissile plutonium is increasing.

Figure 4 shows the void fraction and infinite neutron multiplication factor that can be induced in this fuel assembly. In the fuel assembly of this embodiment, the number ratio of the fuel rods 3 having plutonium fuel in the upper third and the fuel rods 4 comprising uranium fuel is 1:1, so that voids are eliminated during operation. The neutron infinite multiplication factor at room temperature is lowered by approximately 4% Δ/I compared to the comparative example fuel assembly composed only of uranium fuel, without changing the neutron infinite multiplication factor in the state of 70%. be able to. As a result, the reactor shutdown margin can be increased to about 0.9%Δ.

Furthermore, in this example, as described above, by loading plutonium fuel in the upper region, the output in this region can be made higher than in the case of only uranium fuel, which is a comparative example. As a result, as shown in FIG. 5, the output distribution in the axial direction can be flattened at the same time.

A second embodiment of the fuel assembly according to the present invention is shown in FIG. The fuel assembly of this example (FIG. 8(a)) has an enrichment level of 9. 10% plutonium fuel, enrichment 6.0% in the lower 2/3 region. A fuel rod 8 (Fig. 6(b)) loaded with uranium fuel 7 of
% plutonium fuel 11 (Fig. 6)
C)). As mentioned above, from the viewpoint of flattening the power distribution in the radial direction, fuel rods 9 loaded with plutonium fuel 11 with a low enrichment degree are arranged around the control rod guide tube where the power becomes high. In this case, while achieving a flattening of the axial power distribution that is almost the same as in the previous example, the infinite neutron multiplication factor at room temperature is reduced to about 4.5%Δ compared to a fuel assembly made of only uranium fuel. It is possible to increase the reactor shutdown margin to about 1.0%Δ/.

FIG. 7 shows a third embodiment of the fuel assembly according to the present invention. In this embodiment, 19/2 of the effective fuel length from the bottom of the fuel
A fuel rod 12 (FIG. 7(b)) loaded with plutonium fuel 6 with an enrichment of 10.0 wt% in the region 4 to 21/24 and uranium fuel with an enrichment of 6 Owt% in the other regions,
Concentration level 6. Fuel rod 4 loaded with Owt% uranium fuel 7
(FIG. 7(C)) constitute a fuel assembly (FIG. 7(a)). This fuel assembly is loaded with uranium fuel containing gadolinia instead of plutonium fuel 6.
Compared to the fuel assembly described in No.-102188, it is possible to increase the reactor shutdown margin by almost the same amount without reducing the reactivity in that region.

Big! (*4 Figure 8 shows the fourth embodiment of the fuel assembly according to the present invention. In this embodiment, in order to reduce the leakage of neutrons and aim for effective use of neutrons, an area of 1/24 of the upper and lower Fuel rods 13 loaded with natural uranium fuel 15, plutonium fuel 6 with an enrichment of 10.0 wt% in the area from 15724 to 23/24 from the bottom of the fuel, and uranium fuel 7 with an enrichment of 6.0 wt% in other territories. (Fig. 8(b)) and a fuel rod 14 loaded with natural uranium fuel 15 in the upper and lower 1/24 f regions and uranium fuel 7 with an enrichment of 6°Owt% in the other regions (Fig. 8(C) )) constitute a fuel assembly (FIG. 8(a)). In this embodiment as well, it is possible to increase the reactor shutdown margin and flatten the axial power distribution as in the first embodiment.

Example 5 A fifth example of the fuel assembly according to the present invention is shown in FIG. The fuel assembly of this example (FIG. 9(a)) has an enrichment of 10.0w in the region from 15/24 to 23/24 from the bottom of the fuel.
t% plutonium fuel 6.1/24 to 15/24 enrichment 5. Fuel rods 16 loaded with Owt% plutonium fuel 17 and natural uranium fuel 15 in other areas (
9(b)) and the same fuel rod 14 as in FIG. 8(c) (FIG. 9(C)). In this embodiment, from the viewpoint of flattening the axial power distribution, plutonium fuel with a low enrichment degree is used in the lower part of the fuel where the void ratio is low and the power tends to be high.

A sixth embodiment of the fuel assembly according to the present invention is shown in FIG. The fuel assembly of this example (FIG. 10(a)) has an enrichment level of 1 (1,
Fuel rod 18 loaded with 0wt% plutonium fuel 6 and natural uranium fuel 15 in other areas (Fig. 10(b))
There are 12 fuel rods, the same fuel rods 13 as in Fig. 8(b) (Fig. 10(
There are 48 fuel rods (C)), the same fuel rods 14 (first
Figure 0(d)) consists of 60 fuel rods. Similarly to the previous embodiment, this is also done from the viewpoint of flattening the axial output distribution.
The number of plutonium-enriched fuel rods at the bottom of the fuel tank is reduced.

A seventh embodiment of the fuel assembly according to the present invention is shown in FIG. 11.The fuel assembly of this embodiment (FIG. 11(a)) is shown in FIG. Concentration level 1 in the area. O
Plutonium fuel enriched with 9.5 wt% plutonium in wt% enriched uranium 20, 1/24-15/2
Concentration level 6.4 in the area. It consists of fuel rods 19 loaded with Owt% uranium fuel 7 and natural uranium fuel 15 loaded in other areas. In this embodiment as well, the reactor shutdown margin can be increased to the same extent as in the above-described embodiment.

FIG. 12 shows an eighth embodiment of the fuel assembly according to the present invention. In this embodiment, in order to suppress excess reactivity at the initial stage of combustion, fuel rods 21 loaded with uranium fuel 22 mixed with gadolinia, which is a reburning poison, are arranged. Gadolinia can also be mixed with plutonium fuel, but plutonium fuel has a high conversion ratio compared to uranium fuel, and when reprocessing technology advances further in the future, plutonium fuel can be mixed with recycling methods and uranium fuel. Considering that the plutonium fuel will be used once-through, the plutonium fuel to be reprocessed was designed to contain as few impurities as possible.

[Effects of the Invention] As explained above, according to the present invention, while flattening the axial power distribution, the difference between the infinite neutron multiplication factors during operation and at room temperature is reduced, and the reactor shutdown margin is reduced to 0. It can be increased to/over 9%Δ.

[Brief explanation of the drawing]

FIGS. 1(a), (b), and (C) are the first embodiments of the present invention.
Figure 2 is a diagram showing the relationship between the hydrogen to fuel atomic ratio of enriched uranium fuel and the neutron multiplication factor, Figure 3 is a diagram showing the relationship between the void ratio and the neutron infinite multiplication factor, and Figure 4 The figure shows the relationship between the void fraction and the infinite neutron multiplication factor in the first embodiment, Figure 5 shows the output distribution in the axial direction, Figures 6 (a), (b), (C), Figure 7(a). (b), (C), Figure 8 (a), (b), (c),
Figures 9(a) and (b). (C), Figure 10 (a), (b), (c),
(d), FIG. 11(a). (b), Figure 12 (a), (b), (c),
(d) is a diagram showing a second embodiment to an eighth embodiment of the present invention, respectively. 1...Fuel assembly 2...Channel box 3...Fuel rod 4...Fuel rod 5...Control rod guide tube 6...Plutonium fuel with enrichment of 10.0+yt% 7
...Concentration level 6. Owt% uranium fuel 8...fuel rods 9...fuel rods 10...enrichment 9.5
wt% plutonium fuel 11...enrichment 8. Owt
% plutonium fuel 12...fuel rods 13
...Fuel rod 14...Fuel rod 15...Natural uranium fuel 16...Fuel rod 17...Enrichment level 5. Owt% plutonium fuel 18
...Fuel rod 19...Fuel rod 21...Fuel rod fuel Figure 1 (α) (b)
(C fuel supply 3 Inhalation camphor 4 Hydrogen to fuel atomic ratio - Inu-hoyt conflict (%) Hoyt ratio (%) Phase number output Fig. 6 (b) (C) Fuel frame 8 Fuel rod 9 Fig. 7 <a> (b) , (C) Fuel rod) 2 Fuel rod 4 Fig. 8 (b) CC) Fuel rod 13 Fuel rod 14 Fig. 9 (α) (b) (C) Fuel rod 16 Fuel rod 14th Figure 10 (b) (C) (d) Fuel rod 18 Fuel fence 13 Fuel rod 1
4 in Figure 11) (b) Fuel rod 19 Figure 12

Claims (1)

[Claims] 1. In a fuel assembly consisting of a bundle of a large number of fuel rods, in which voids are generated in the coolant as the flow material flows from the upstream side to the downstream side along the longitudinal direction of the fuel rods, the fuel assembly is It is divided into two regions in the direction of coolant flow, and the average weight ratio of fissile uranium contained in the downstream region to the total fuel material contained in the region is smaller than that in the upstream region. , and characterized in that the average weight ratio of fissile plutonium contained in the downstream region to the total fuel material contained in the region is larger than that in the upstream region. fuel assembly.