JP2886555B2 - Fuel assembly for boiling water reactor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a fuel assembly having a plutonium-enriched fuel rod, among fuel assemblies used for a boiling water reactor (BWR). About the body.

(Prior art) From the viewpoints of effective use of resources and energy security, plutonium using plutonium recovered by reprocessing of spent fuel as fuel in a light water reactor will be performed.

Plutonium has a high α-ray emission intensity, and it is necessary to prevent internal exposure to the human body. It also emits neutrons and gamma rays by decay and spontaneous fission. Therefore, the production and processing of the fuel containing plutonium must be performed in a more sealed state than in the case of uranium fuel. Many considerations must be given to equipment and processes, such as the need for more shielding equipment, and the need for strict attention to decontamination and maintenance. Therefore, regarding the production of plutonium-containing fuel pellets and fuel rods, it is economically disadvantageous to manufacture many kinds of fuel pellets having different enrichments.

In addition, since fuel assemblies containing plutonium are subject to strict conditions for transportation, metering control, and criticality control, the proportion of plutonium contained in one fuel assembly is increased as much as possible to reduce the number of fuel assemblies containing plutonium. It is desired.

In view of the above, in the fuel assemblies used as pluthermal, so-called "discrete MOX fuel" in which all fuel rods are enriched with plutonium is advantageous, and plans to use this type of fuel assembly are being considered. . This example is shown in FIG. 6 and FIG. These figures show the plutonium enrichment distribution (radial direction) for each fuel rod of the discrete MOX fuel, where 1 is a channel box, 2 is a fuel rod, and surrounded by fuel rods 2. W arranged is a water rod. Pi (i = 1-6, i =
Numerals 1 to 9) are plutonium-containing fuel rods, and their fissile plutonium enrichment is as follows. G is a fuel rod containing a burnable poison, and its uranium enrichment and poison concentration are as follows.

For Figure 6 _{P 1: 6.5%, P 2} : 5.0%, P 3: 3.6%, P 4: 3.3%, P 5: 2.4%, P 6: 1.3%, G: 235 U enrichment 4.9%, for poison concentration of 2.0% Figure 7 _{P 1: 5.5%, P 2} : 4.8%, P 3: 4.5%, P 4: 3.8%, P 5: 3.3%, P 6: 2.2%, P 7: 1.8% _{, P 8: 1.2%, P} 9: 0.7%, G: 235 U enrichment 4.9%, the poison concentration 1.5% BWR, the fuel assembly with a number arranged as shown in FIG. 8 constituting the reactor core. In FIG. 8, 3 is a fuel assembly,
4 is an LPRM (output area detector), 5 is an IRM (intermediate area detector), 6 is an SRM (neutron source area detector), and 7 is a control rod.

Between the fuel assemblies there is a water gap region with a certain width for placing cross control rods or instrumentation tubes. In the current BWR, a plant in which the width of the water gap in which the control rod is inserted and the width of the water gap in which the instrumentation pipe is provided are the same (referred to as a C grid type) and a plant in which the width of the water gap is different (D Grid type). The discrete MOX fuel assembly shown in FIG. 6 is used for a C lattice type plant, and the fuel assembly shown in FIG. 7 is used for a D lattice type plant.

The coolant in the channel box becomes a two-layer flow containing steam during operation, but no steam is generated in the water gap region because the coolant is not directly heated by the fuel rods. For this reason, the atomic number density of hydrogen in the water gap region is large, and as a result, the radial thermal neutron flux distribution in the horizontal section of the fuel assembly of the BWR is large at the periphery of the assembly as shown in FIG. Become. In order to reduce the output peaking coefficient in the inner diameter direction of the BWR fuel assembly, fuel rods with low enrichment or low enrichment are placed around the assembly as shown in FIGS. 6 and 7. The design to arrange is adopted. For this reason, it is necessary to use several types of fuel rods with low Pu enrichment in discrete MOX fuel assemblies,
These fuel rods use a small number of rods and are not economical to manufacture.

In the BWR, fuel rods containing gadolinia (Gd ₂ O ₃ ) mixed with fuel as burnable poison are loaded in the assembly in order to reduce the excess reactivity at the beginning of combustion. Since a small amount of gadolinia greatly affects the reactivity, fuel rods containing gadolinia must be manufactured on a separate line from ordinary fuel rods and must be strictly distinguished, including inspections. For this reason, when mixing gadolinia into a fuel containing plutonium, it is necessary to provide another plutonium fuel production line, which is economically disadvantageous.

The fuel assembly used for the BWR adopts a design in which the fuel enrichment or the amount of burnable poisons is changed in the axial direction in order to flatten the power distribution in the axial direction. When manufacturing a fuel assembly in which the amount of fuel enrichment or burnable poison is changed in the axial direction, the types of fuel pellets increase.
Therefore, discrete type with sufficient axial change
When manufacturing MOX fuel assemblies, the types of MOX fuel pellets are further increased, which is not economical in manufacturing.

(Problems to be Solved by the Invention) The present invention has been made in view of the above circumstances, and has an economical effect on production by reducing the number of types of plutonium-containing fuel pellets in a discrete MOX fuel assembly. It is intended to enhance the performance and to make the handling advantageous.

[Constitution of the Invention] (Means and Action for Solving the Problems) That is, the present invention relates to a fuel cell for a boiling water reactor in which fuel rods enriched with plutonium are bundled in a square lattice. Part of the fuel rods in the region consisting of the outermost peripheral position of the rod arrangement and the corner position of the second round are replaced with fuel rods containing no plutonium, and at least the fuel rod at the outermost peripheral corner position on the control rod insertion side must be plutonium. The present invention relates to a fuel assembly for a boiling water reactor, wherein the entire outermost row does not include a plutonium-free fuel rod.

As can be seen from FIGS. 6 and 7, many types of fuel rods with low plutonium enrichment are used for the outermost peripheral position and the second round corner position of the square lattice fuel rod arrangement. . Therefore, by making a part of this region a uranium fuel rod containing no plutonium, the type of MOX fuel can be reduced while minimizing the decrease in plutonium loading per fuel assembly.

In the above fuel assembly, by using uranium fuel that does not contain plutonium for the fuel rods containing the burnable poison, the production cost of the MOX fuel can be reduced by reducing the number of MOX fuel production lines. An increase in the number of types of MOX fuel pellets can be prevented by preventing the enrichment distribution in the axial direction from being generated in the fuel rods containing.

(Example) An example of the present invention will be described with reference to the drawings.

FIG. 1 is an example of a pluthermal fuel assembly according to the present invention used in a BWR C lattice plant. In this embodiment, as compared with the conventional discrete MOX fuel assembly shown in FIG. 6, 20 fuel rods near the corner of the assembly are uranium fuel rods.

In the figure, U _j (j = 1 to 3) is uranium fuel, P _i (i = 1 to
3) is a fuel containing plutonium. The plutonium enrichment and uranium enrichment are as follows.

Fissile plutonium enrichment _{P 1: 6.2% P 2:} 5.0% P 3: 3.6% 235 U enrichment _{U 1: 3.5% U 2:} 2.8% U 3: 2.0% The G in burnable poison containing fuel rods , ²³⁵ U enrichment 4.5
%, Poison concentration 2.0%.

These 20 fuel rods have the lowest enrichment in the conventional discrete MOX fuel assembly, and are less than half the enrichment of the fuel rods at the center. Therefore, when the fuel rods at these positions are uranium fuel rods, the decrease in the amount of plutonium loaded per fuel assembly can be minimized.

FIGS. 2 (a) and 2 (b) are examples of a pluthermal fuel assembly according to the present invention for use in a BWR D lattice plant. In the embodiment of (a), ten fuel rods near the corner of the assembly are uranium fuel rods as compared with the discrete MOX fuel assembly shown in FIG.
In the embodiment of (b), one more uranium fuel rod is added at the corner position of the second round. In these cases, since the width of the water gap on the side where the control rod enters and the width of the water gap on the side where the instrumentation pipe enters are different, the arrangement of the uranium fuel rods is also asymmetric on the control rod side and the instrumentation pipe side.

The fissile plutonium enrichment and uranium enrichment in FIGS. 2 (a) and (b) are as follows.

_{P 1: 5.5% P 2:} 4.8% P 3: 4.5% P 4: 3.4% P 5: 2.7% P 6: 1.9% U 1: 3.8% U 2: 2.8% U 3: 2.0% The burnable poison containing Fuel rod G has a uranium enrichment of 4.5%,
Poison concentration is 2.0%.

In the embodiment of the present invention shown in FIGS. 1 and 2A and 2B, plutonium-containing fuel and plutonium-free uranium fuel are used as fuel rods containing burnable poisons. Can be. The use of uranium fuel as a base is advantageous in reducing the cost of producing plutonium fuel. This reduces the plutonium loading per fuel assembly, so if emphasis is placed on large plutonium loading,
It is conceivable to mix burnable poison with fuel containing plutonium.

FIG. 3 shows another embodiment of the present invention.
The invention of the present invention is applied to the fuel assemblies (9 × 9 fuel) bundled in rows. The plutonium enrichment and uranium enrichment are as follows.

Fissile plutonium enrichment _{P 1: 6.2% P 2:} 5.0% P 3: 3.6% 235 U enrichment _{U 1: 3.5% U 2:} 2.0% combustible ²³⁵ U enrichment of 4.5% poison containing fuel rods G, Poison concentration is 2.0%.

The configuration of the fuel assembly shown in FIGS. 4A and 5B and FIGS. 5A and 5B is an embodiment of the present invention with respect to the axial composition. Both FIG. 4 and FIG.
Shows the fuel arrangement in the radial direction, and (b) shows the fuel arrangement in the axial direction.

These embodiments show that the distribution of axial fissile material (enrichment and enrichment) or burnable poisons suitable for BWR fuel can be obtained by using the uranium fuel rods and uranium containing burnable poisons introduced by the present invention. It is composed of fuel rods. The embodiment shown in FIG. 4 changes the enrichment and the amount of burnable poisons in two regions in the axial direction, and corresponds to the design of the uranium fuel assembly shown in FIG. Compared with the design of the uranium fuel assembly shown in FIG. 10, the number of fuel rods having the composition in the axial direction is sufficient, and an appropriate axial distribution for the BWR fuel can be achieved. In the embodiment shown in FIG. 5, the enrichment and the amount of burnable poison are changed in three regions in the axial direction, and a natural uranium bracket is arranged at the upper and lower ends.

The Pu enrichment and U enrichment in FIGS. 4 and 5 are as follows.

Figure 4 fissionable Pu enrichment _{P 1: 6.2% P 2:} 5.0% P 3: 3.6% 235 U enrichment _{e 1: 4.0% e 2:} 3.0% e 3: 3.5% e 4: 2.3% e 5 : 2.6% e ₆ : 1.8% ²³⁵ U enrichment of fuel rod G containing burnable poison e _g1 : 4.1% e _g2 : 4.9% Poison concentration of fuel rod G containing burnable poison g ₁ : 3.5% g ₂ : 4.5% Figure 5 fissionable Pu enrichment _{P 1: 6.2% P 2:} 5.0% P 3: 3.6% 235 U enrichment _{e 1: 4.2% e 2:} 3.9% e 3: 3.4% e 4: 3.9% e 5 : 3.4% e ₆ : 2.5% e ₇ : 3.4% ²³⁵ U enrichment of fuel rod G containing burnable poison e _g1 : 2.8% e _g2 : 3.0% e _g3 : 2.8% Poison concentration of fuel rod G containing burnable poison _{g 1: 3.5% g 2:} 4.5% g 3: 2.5% as is apparent from the effect of the invention] above, according to the present invention, so-called discrete type MOX fuel enriched in plutonium conventional all fuel rods In the above, the fuel rods in the predetermined area outside the low Pu enrichment were replaced with uranium fuel rods that do not contain plutonium. By doing so, the reduction rate of the amount of plutonium loaded per fuel assembly can be reduced as much as possible to reduce the type and number of plutonium fuel rods, thereby reducing the economic burden and production burden of plutonium fuel rod production. .

[Brief description of the drawings]

FIGS. 1 to 3 are fuel rod arrangement diagrams showing an embodiment of the present invention, and FIGS. 4 (a), (b) and 5 (a), (b) show an embodiment in the case of axial distribution in the present invention FIGS. 6 and 7 show examples of fuel rod arrangements, and FIG.
FIG. 8 is a layout diagram of a fuel assembly of MOX fuel, FIG. 8 is a layout diagram of a fuel assembly and control rods in a conventional BWR core, and FIG. 9 is a diagram showing a thermal neutron flux distribution in a horizontal cross section of the conventional fuel assembly. 1 ... channel box 2 ... fuel rod 3 ... fuel assembly 4 ... LPRM (output area detector) 5 ... IRM (intermediate area detector) 6 ... SRM (neutron source area detector) 7 ... Control rod

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G21C 3/30

Claims (2)

(57) [Claims] 1. A boiling water reactor fuel assembly comprising a plurality of fuel rods bundled in a square lattice, wherein the plurality of fuel rods include a first fuel rod containing no plutonium and a burnable poison, and a flammable fuel rod. A second fuel rod containing toxic poisons and no plutonium; and a third fuel rod enriched with plutonium, wherein the first fuel rod has at least two different uranium enrichments.
At least one third fuel rod is arranged in each of the outermost rows of the square grid fuel rod arrangement, and at least the outermost corner position on the control rod insertion side and this corner position The first fuel rod is disposed at the outermost peripheral position adjacent to the first fuel rod, and the first fuel rod is disposed at the corner position.
Wherein the uranium enrichment of the fuel rod is set lower than the uranium enrichment of the first fuel rod located at the outermost peripheral position adjacent to the corner position. 2. The fuel assembly for a boiling water reactor according to claim 1, wherein the axial reactivity distribution is given by the amount of the burnable poison or the uranium enrichment.