JPH0644052B2 - Fuel assembly - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a fuel assembly, and in particular, a boiling water type that is suitable for controlling excess reactivity when the take-out burnup is increased and the operation cycle period is prolonged. The present invention relates to a fuel assembly of a nuclear reactor.

[Conventional technology]

FIG. 7 shows the structure of a conventional boiling water reactor fuel assembly. The fuel assembly is a square cylinder channel box 1,
It is composed of a fuel bundle 2 housed inside the channel box 1. The fuel bundle 2 includes an upper tie plate 3 and a lower tie plate 4 which are fitted in the upper and lower portions of a channel box 1, a plurality of spacers 5 arranged in the channel box 1 at intervals along the axial direction, and the spacers. 5 and a plurality of fuel rods 6 having both ends fixed to upper and lower tie plates 3 and 4 and a water rod 7.
It consists of and.

In order to operate the nuclear reactor for a certain period, it is necessary to have an excess reactivity at the initial stage of operation by an amount corresponding to the reactivity deterioration due to combustion of the fissile material during operation. Then, in order to keep the reactor critical, it is necessary to control this excess reactivity. As a conventional method, as shown in FIG. 8, a method of inserting a control rod 8 made of a neutron absorbing substance into the reactor core is used. And a method of using a fuel assembly including a special fuel rod 9 (hereinafter referred to as a gadolinia-containing fuel rod) in which a burnable poison having a large neutron absorption cross section such as Gd ₂ O ₃ is added to the concentrated fuel pellet. It was

In recent years, in order to improve fuel economy, it has been considered to increase the degree of enrichment to prolong the life of the fuel, but this has the following problems. First, since the average energy of neutrons is increased due to the increase in enrichment, the effect of controlling excess reactivity per fuel rod containing gadolinia is reduced. Second, the excess reactivity that must be controlled by extending the continuous operation period increases as shown in FIG. 9, and the number of conventional fuel rods with gadolinia must be increased. The fuel assembly shown in FIG. 8 has a take-out burnup of about 30 GWd / t, is continuously operated for 9 months, and has six gadolinia-containing fuel rods 9 arranged therein. For example, considering the case where the continuous operation period is 1.5 times as long as 13.5 months, the reactivity control amount decreases due to the gadolinia-containing fuel rods 9 being arranged close to each other, and further the enrichment increases as described above. Considering the decrease of the reactivity control amount,
It is considered that the number of necessary fuel rods 9 with gadolinia is about 13. However, an increase in the number of gadolinia-containing fuel rods 9 has an effect of increasing the local output peaking coefficient of the fuel assembly, and there is an upper limit to the number of fuel rods 9 due to restrictions on the location. Therefore, it is realistically expected that it will be difficult to set the continuous operation period to 15 months or more.

As a measure against such a problem, as described in JP-A-53-41696, a method of incorporating a control rod in which a burnable poison and a moderator are mixed in a fuel assembly, and a fuel pellet. There is a method of increasing the weight ratio of gadolinia to occupy so that gadolinia remains burned even in the beginning of the second cycle.

[Problems to be solved by the invention]

The above-mentioned conventional techniques have a problem in that the former sacrifices in terms of fuel economy because the uranium loading amount decreases and the latter decreases reactivity due to unburned gadolinia at the end of the cycle.

An object of the present invention is to provide a fuel assembly for a boiling water nuclear reactor capable of solving the problem of increased surplus reactivity with the prolongation of fuel life without deteriorating fuel economy.

[Means for solving problems]

In order to achieve the above object, the structure of the present invention is a fuel assembly composed of a large number of fuel rods arranged in a lattice, wherein the fuel rods are fissile materials in the upper and lower end regions of the fuel effective portion. Is smaller than that in other regions of the fuel effective portion, and some of the fuel rods have fuel pellets containing burnable poison in the upper and lower end regions. .

[Action]

The present invention has been made based on the following examination results. In the following explanation, uranium-
The case using 235 was taken up, but plutonium-2
The same applies when 39, 241 or uranium-233 is used.

Coolant enters the fuel assembly through the holes in the lower tie plate and flows out through the holes in the upper tie plate. During that time, the coolant heated between the fuel rods becomes a two-phase flow, and the void fraction becomes about 70% in the upper part of the fuel assembly. As a result, the neutron diffusion coefficient in the upper part of the fuel assembly becomes large, and the leakage of neutrons from the upper part of the fuel assembly becomes significantly large. In order to improve the neutron economy and make effective use of fuel, the fuel with a high neutron infinite multiplication factor is placed in the region where the neutron importance is high in the operating state, and the fuel with a low neutron infinite multiplication factor is placed in the low neutron importance region. Should be placed. That is, the amount of fissile material in the region where the neutron importance is low at the upper and lower ends of the fuel assembly may be made smaller than in other regions.

FIG. 10 is a diagram showing the relationship between the enrichment of gadolinia-containing fuel rods and the excess reactivity control amount per gadolinia-containing fuel rod. Since the average energy of neutrons decreases as the enrichment decreases, the excess reactivity control amount per fuel rod with gadolinia increases. It also reduces the amount of gadolinia left over.

Therefore, since gadolinia was mixed in the region of low neutron importance, the neutron infinite multiplication factor can be further lowered, and the fuel economy can be further improved.
Further, since the linear power density does not become maximum in the low enrichment fuel region, it is possible to replace the fuel with gadolinia as much as necessary without worrying about the local power peaking coefficient.
Further, the burnup distribution in the fuel region (central fuel region) excluding the upper and lower ends of the fuel assembly can be flattened.

Further, as described above, since the voids are generated in the fuel assembly, the progress of combustion in the upper part is delayed, and the burnup distribution in the axial direction of the fuel assembly is not uniform, but flammability is present in the upper and lower ends of the fuel assembly. Since the poison is arranged, the burnup of the central fuel region in contact with the upper and lower ends can be controlled, and as a result, the burnup distribution can be flattened.

Further, the upper and lower core regions of the core where the amount of fissile material is reduced are regions where fuel economy is not deteriorated. FIG. 11 is a diagram showing the uranium saving effect when the upper and lower end regions of the fuel assembly are replaced with natural uranium. From FIG. 11, the regions having a length of 15% from the upper and lower ends of the active fuel length of the fuel assembly are defined as the upper and lower ends of the fuel assembly.

〔Example〕

The present invention will be described in detail below with reference to the embodiments shown in FIGS.

FIG. 1 is a horizontal cross-sectional view showing an embodiment of the fuel assembly of the present invention, in which the symbols (1) to (4) show fuel rods filled with fuel pellets having different enrichments. Is a water rod. FIG. 2 is an explanatory diagram of the axial enrichment distribution of various fuel rods in FIG. 1, in which a to h indicate the enrichment of fuel pellets, as shown in Table 1. . In Table 1, the concentration (W / O) Is U-23 with respect to the sum of U-235 and U-238.
5 shows a concentration of 5.

As shown in FIG. 2, the upper end of the fuel (2/24 of the active fuel length is approximately 8%) and the lower end of the fuel (1/24 of the active fuel length is approximately 4%) of each fuel rod to natural uranium. The gadolinia having a concentration of 4 W / O is mixed in the upper and lower ends of 24 of the fuel rods.

As a result, the following effects are obtained. G at the top and bottom of the fuel rod,
Compared to the case where the fuel assembly is composed of fuel rods consisting of only a to f pellets without h pellets, the required natural uranium amount per unit output is reduced by about 5% and the excess reactivity control amount is reduced. Increase by about 0.2% ΔK. In addition, the surplus reactivity control amount is approximately the same as in the case where the pellet h is not used and the upper and lower ends of the fuel rod are all configured with the pellet g.
Increase by 0.5% ΔK.

Note that in FIG. 1, the fuel assembly arrangement is changed from the current 8 × 8 lattice to the 9 × 9 lattice in consideration of the increase in the output in the region of high neutron importance and the increase in the linear power density. However, this is not limited to the 9 × 9 lattice, and 10 × 10 and 11 × 11 lattices may be used. Further, in recent years, a fuel assembly has been developed in which a thin film of Cu or Zr is adhered to the inner wall of the cladding tube to reduce the interaction between the fuel pellet and the cladding tube, and the linear power density can be increased. In the case of, an 8 × 8 grid can be used.

Further, in FIG. 2, the upper end of the effective fuel portion is about 8% of the effective fuel length and the lower end is about 4%, but these may be 15% or less of the effective fuel length, respectively.

FIG. 3 is a horizontal sectional view corresponding to FIG. 1 showing another embodiment of the present invention. In FIG. 3, is a fuel rod filled with fuel pellets having different enrichments, is a water rod, and FIG.
It is an explanatory view of the enrichment distribution of each fuel rod in the axial upward direction, and the enrichment of the fuel pellets a to h is the same as in Table 1. FIG. 3 shows the burn-up distribution flattened. The difference from the embodiment shown in FIG. 1 is that, as shown in FIG. 4, a pellet h is formed between the upper end and the lower end of the fuel rod. Is the difference in the number used. That is, the upper end has 12 lines and the lower end has 24 lines. As a result, the surplus reactivity control effect is halved compared to the previous embodiment, but it becomes possible to proceed with combustion in the upper part of the fuel assembly where the progress of burnup is delayed, and it is possible to realize a flat burnup distribution. There is an effect.

FIG. 5 is a horizontal cross-sectional view corresponding to FIG. 1 showing still another embodiment of the present invention. In FIG. 5, is a fuel rod filled with fuel pellets having different enrichments, is a water rod, and is a sixth rod.
The figure is an explanatory view of the axial enrichment distribution of each fuel rod in FIG. 5, and the enrichments of the fuel pellets a to h are the same as in Table 1. FIG. 5 is for realizing the same effect as FIG. 3, and the difference from the embodiment shown in FIG. 1 is that, as shown in FIG. 6, the fuel pellet h at the upper end of the fuel rod Is a part of the fuel pellet g without gadolinia. As a result, similarly to the embodiment shown in FIG. 3, it is possible to advance the combustion in the upper portion of the fuel assembly where the progress of the burnup is delayed. However, the effect of controlling the excess reactivity is almost half that of the embodiment shown in FIG.

〔The invention's effect〕

As described above, according to the present invention, the amount of the fissile material in the upper and lower end regions of the fuel effective portion of the fuel rod is smaller than that in the other regions, and at least a part thereof contains the burnable poison. Since the fuel pellets are arranged, the reactivity control effect can be increased without deteriorating the fuel economy, so that there is an effect that the surplus reactivity control amount during the long-term cycle operation can be increased.

[Brief description of drawings]

FIG. 1 is a horizontal sectional view showing an embodiment of the fuel assembly of the present invention, FIG. 2 is an explanatory view of axial enrichment distribution of various fuel rods in FIG. 1, and FIGS. 3 and 5 are A horizontal sectional view corresponding to FIG. 1 showing another embodiment of the present invention, and FIGS. 4 and 6 are explanatory views of axial enrichment distributions of various fuel rods of FIG. 3 and FIG. 5, respectively. FIG. 7 is a configuration diagram of a conventional boiling water reactor fuel assembly, FIG. 8 is a horizontal sectional view of a conventional fuel assembly, and FIG. 9 shows a relationship between a continuous operation period and an excess reactivity. Diagram, Fig. 10 is a diagram showing the relationship between enrichment and excess reactivity control per fuel rod with gadolinia, and Fig. 11 is a graph showing the relationship between the ratio of natural uranium region and the amount of saved uranium. It is a figure. ~, ~, ~ ... fuel rods, ah ... fuel pellets.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-60-242392 (JP, A) JP-A-61-84591 (JP, A) JP-A-61-147183 (JP, A)

Claims (4)

[Claims] 1. A fuel assembly comprising a large number of fuel rods arranged in a lattice pattern, wherein the fuel rods have an amount of fissile material in the upper and lower end regions of the fuel effective portion and other regions of the fuel effective portion. The fuel assembly is characterized in that the number of fuel rods is smaller and some of the fuel rods have fuel pellets containing burnable poison in the upper and lower end regions. 2. The fuel assembly according to claim 1, wherein the upper and lower end regions are made of natural uranium. 3. The fuel assembly according to claim 1, wherein a fuel pellet containing a burnable poison is arranged in a part of the upper end region. 4. The fuel assembly according to any one of claims 1 to 3, wherein the upper and lower end regions are 15% or less of the active fuel length.