JPS5858036B2 - Light water reactor and its operation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a light water reactor in a nuclear power plant and a method of operating the same.

Economical power generation requires an increase in the power level of nuclear reactors.

In order to improve the power level of a nuclear reactor, it is possible to improve the power density (KW/J) of the nuclear reactor.

However, one of the factors that limits the upper limit of the power level of a nuclear reactor is the surface heat flux of the fuel rods.

Surface heat flux (Kcal/Tll: hr) is the amount of heat transferred per unit area and unit time on the surface of a fuel rod, and in nuclear reactors, the equivalent linear power density (KW/77Z) is conventionally used.

This linear power density is the calorific value per unit length in the axial direction of the fuel rod, and is obtained by multiplying the surface heat flux by the circumferential length of the fuel rod (2πr).

For example, consider a fuel assembly in which sintered uranium dioxide pellets placed in a Zircaloy-2 cladding tube are arranged in a 7x7 square lattice.

For example, 368 fuel assemblies are arranged in a shape close to a right cylinder, and a control rod having a cross-shaped cross section is inserted into the center of each of the four fuel assemblies.

Now, if each fuel assembly is loaded with a uniform average enrichment of 235 U of fissile material, there will be neutron leakage, so the neutron flux will be high at the center of the core and low at the outer periphery. .

Therefore, the output is high at the center of the furnace and low at the outer periphery.

In other words, the linear power density (
surface heat flux) is high at the center of the core and low at the outer periphery.

As a result, there is a risk that the linear power density, which is a thermal limit on reactor operation, may exceed the limit value, and there is a possibility that control rod operation during reactor operation may be difficult.

In order to improve this, various improvements have been made to fuel loading methods.

For example, efforts are being made to reduce the amount of fissile material (average enrichment) in the fuel assembly in the center of the reactor and increase the amount of fissile material in the outer periphery. Although this method has a great effect, since the fuel assembly with high reactivity is placed at the outer periphery where neutron leakage is large, it is bad in terms of neutron economy and reduces the acquired burnup.

In order to increase the obtained burnup, it is better to increase the amount of fissile material in the center of the reactor core.

It is also conceivable to increase the margin for thermal limitations by increasing the number of fuel rods constituting the fuel assembly and increasing the total heat transfer area of the fuel assembly.

For example, consider the entire reactor core as only 9x9 fuel assemblies.

In this case, thermal limitations are improved over current 8x8 fuel assemblies due to increased heat transfer surface area and lower surface heat flux (lower linear power density).

However, the number of fuel rods increases by about 27%, since 8×8 fuel requires 64 fuel rods and 9×9 fuel requires 81 fuel rods.

Since the manufacturing cost of a fuel assembly increases significantly as the number of fuel rods increases, it is disadvantageous in terms of fuel manufacturing cost to use 9x9 fuel until the fuel is located at a position where there is sufficient margin due to thermal limitations.

The present invention aims to reduce such disadvantages in neutron economy and fuel production cost as much as possible, satisfy thermal limitations, and improve burnup acquisition or output level.

According to the specific invention of the present invention, in a light water reactor, a large number of fuel assemblies with equal external dimensions constituting the core are composed of a square lattice arrangement of fuel rods having a small outer diameter and a large number of fuel rods. A square lattice array of fuel rods with a large diameter and a small number is used, and only the central region with a fuel assembly consisting of a large number of fuel rods in the former and the fuel assembly with a small number of fuel rods in the latter are loaded. By dividing the reactor core into two regions, a peripheral region and a peripheral region,

A first embodiment of the present invention will be described below.

In the arrangement of the fuel assembly described above, the 7x7 fuel in the center is replaced with the 8x8 fuel H, which has the same amount of 235U as the 7x7 fuel.
When replaced by , the power density of the fuel assembly (KW/l)
is the same for both 7x7 fuel and 8x8 fuel, so as the number of fuel rods increases, the linear power density of the 8x8 fuel rods decreases by about 12%, and the decrease in surface heat flux is slightly smaller. small), resulting in increased margin for thermal limitations.

As a result, it becomes possible to increase the output level by the margin.

Further, since the number of fuel assemblies with a large number of fuel rods is limited to the fuel in the central region, an increase in fuel manufacturing cost can be alleviated.

This effect is not limited to the combination of 7x7 fuel and 8x8 fuel, but the same can be said for 8x8 fuel and 9x9 fuel.

Next, a second embodiment of the present invention will be described with reference to the drawings.

First, in the light water reactor of the present invention as shown in FIG. 1, the reactor core 1 is divided into a central region 2 (shaded area downward to the left), an intermediate region 3 surrounding the central region 2 (shaded area downward to the right),
It is divided into an outer peripheral area 4 (blank area), and fuel assemblies 5, 6.7 with different fuel rod arrangements are loaded therein.

The fuel assembly 5 has an 8×8 square lattice arrangement as shown in FIG. 2, and the fuel assemblies 6 and 7 have a 7×7 square lattice arrangement as shown in FIG.

Although fuel assemblies 5, 6, and 7 are arranged in an 8x8 arrangement and a 7x7 arrangement, the total amount of nuclear material (for example, 235U and 238g) is 187 hours for 7x7, and 183 for 8x8. As such, not much has changed.

Note that here, the amount of fissile material (for example, 235u) in the fuel rods in the fuel assemblies 5 and 6 is large, and the amount of fissile material in the fuel assembly 7 is small.

In order to increase the amount of fissile material (235U), it is sufficient to increase its enrichment.

In the case of light water reactors, slightly enriched fuel with an average fuel assembly enrichment of about 2% is used, so simply increasing the enrichment to about 3% will reduce the amount of fissile material to 3% by 2%-1, 5, which is a 50% increase.

In such a core, the central and intermediate regions contain 23
The fuel assembly with a large number of 5U is also
This is advantageous in terms of neutron economy since it requires less fuel.

Further, since the fuel in the central region is 8×8 fuel with a large number of fuel rods, the thermal limit can be satisfied even if the amount of '35U is large.

Since the output peaking is low in the intermediate region, the thermal limit can be satisfied even with 7x7 fuel with a large amount of 235U.

Here, 7x7 new fuel (unirradiated fuel) is placed in the middle area, and in the next cycle, this fuel is moved to the outermost area, and 7x7 new fuel is loaded into the middle area. If the core operation method is adopted, it will be the same as loading a small amount of fuel, 235U, on the outermost periphery, and the same effect as in the embodiment described above will be obtained.

Here, instead of the combination of 7x7 fuel and 8x8 fuel, a combination of 8x8 fuel and 9x9 fuel may be used.

Next, a third embodiment of the present invention will be described with reference to the drawings.

As shown in FIG. 4, fuel assemblies 5, 6 in the central region
is loaded so that the control rod 9 is located at the center.

At this time, four fuels 5 and four fuels 6 are loaded in a checkered pattern around the control rod 9.

Furthermore, fuel assemblies 6 are loaded in groups of four in the intermediate region 3 so that the control rods 9 are located at the center, and fuel assemblies 7 are loaded in the outer peripheral region 4 only at the outermost portions. There is.

In such a fuel-loaded core, the amount of 235U loaded in the entire reactor is constant, and the amount of 235U loaded in the 8x8 fuel assembly is
By increasing the amount and decreasing the amount of 235U in the 7×7 fuel assembly, the power density (KW/l) of the 8×8 fuel assembly becomes greater than that of the 7×7 fuel.

Since 8x8 fuel has more fuel rods, the linear power density is correspondingly lower, so there is more margin than 7x7 fuel before the operating limit for linear power density, so this margin is ×
It is possible to lower the power density (KW/l) of the 7x7 fuel and increase the power density of the 8x8 fuel so that the 7x7 fuel and the 8x8 fuel have the same power.

Furthermore, if the 8x8 fuel is loaded in the central region, the fuel containing a large amount of fissile material will be placed in a position where neutron leakage is small and reactivity is high, increasing efficiency in terms of neutron economy and reactor efficiency. The effective multiplication factor increases, resulting in an increase in the obtained burnup.

Here, since the output peaking is not so large in the intermediate region, 235U of 7x7 fuel is used instead of 8x8 fuel.
A large amount of fuel may be used.

Furthermore, it is more efficient in terms of neutron economy to load 7×7 fuel with a small amount of 235U on the outermost periphery where neutron leakage is large.

As a result, the 8×8 fuel is used only for fuel with high power density, so the number is smaller than in the second embodiment described above, which is effective in reducing manufacturing costs.

Here, instead of the combination of 7x7 fuel and 8x8 fuel, a combination of 8x8 fuel and 9x9 fuel may be considered.

In addition, new fuel (unirradiated fuel) with a small number of fuel rods is loaded into the fuel in the intermediate area, and in the next cycle, fuel loading positions with a small number of fuel rods in the center area and outer peripheral area (fuel 6 in Fig. 4, Even if four (7G) fuels are loaded, the effects of the arrangement of the fuels 6 and 7, etc., can be obtained.

Next, an example of the operating state of the light water reactor according to the present invention in which fuel assemblies are loaded as described above will be described.

FIG. 5 shows a control rod operation pattern during operation of the light water reactor according to the present invention.

In the figure, the x mark indicates deep insertion, the * and 10 marks indicate shallow insertion, and the blank area indicates full withdrawal.

As is clear from this figure, in the central area, 8×8
The control rods are operated in a deeply inserted state into the fuel assembly.

The control rods of this 8x8 fuel assembly are mainly used to compensate for reactivity changes caused by combustion, and combustion progresses during the cycle (thus they are withdrawn, reducing the insertion depth and number of rods inserted). At the end of the cycle, the control rods are completely withdrawn.

Fuel with control rods inserted deeply for a long period of time during the cycle does not burn as well as other fuels, so
At the time the control rods are withdrawn, the output is greater than that of other fuels.

Therefore, it is more advantageous in terms of thermal limitations to use 8×8 fuel, which has a low linear power density, as a deep insertion rod.

Furthermore, for the shallowly inserted control rods, 8×8 fuel is used in the central region, and 7×7 fuel is used in the middle and outer peripheral regions.

Shallow insertion control rods are used during the cycle to adjust the power distribution.
Since the degree of insertion changes frequently, the output distribution changes significantly before and after the control rod withdrawal operation.

In the central region where the power peaking is large, the range of this change is large, so placing the 8x8 fuel with low linear power density in the shallowly inserted control rod position will reduce the change in linear power density.
This is advantageous in terms of the integrity of the fuel rod cladding.

That is, if the increase in linear power density is large, the mechanical interaction between the pellets of the fuel rod and the cladding becomes excessive and the fuel cladding may be damaged, so it is desirable to reduce this increase in power as much as possible.

When the first cycle is completed by loading the fuel assembly shown in FIG. 4 and inserting the control rods shown in FIG. 5G, the fuel assembly shown in FIG. N), i.e. the fuel assembly 5 in the central region.
The unirradiated fuel N of 8x8 fuel is replaced with the unirradiated fuel N of 7x7 fuel, and the fuel assembly 6 in the intermediate region adjacent to the outside thereof is replaced with unirradiated fuel N of 7x7 fuel, and the remaining part remains as it was in the first cycle. Perform the second cycle operation.

In this case, the control rod pattern is returned to the state shown in FIG.

When approaching the end of the second cycle, the control rod, which is deeply inserted in FIG. 5, is again pulled out.

After the completion of the second cycle, as shown in Figure 7, the fuel at the position indicated by (N) in the center area is replaced with 8x8 fuel, and the fuel in the middle area is replaced with unirradiated fuel at 7x7 fuel. do.

As a result, the unirradiated fuel loaded in the second cycle, indicated by (n) in the figure, becomes a sphere, and the fuel assembly indicated by (N) in the figure is the fuel assembly in the center region and the fuel assembly in the intermediate region. Almost all of the fuel assemblies in the body and intermediate regions will be replaced with unirradiated fuel.

Further, the unreplaced fuel in the center area and the fuel assembly 7 in the outermost peripheral area are replaced by the fuel assembly in the outer peripheral area one space inside, and the fuel in the position (N) and the fuel assembly 7 in the central area (N) are replaced. , (II), and fuel in the outer peripheral area where combustion has progressed sufficiently is the unirradiated 7x7
In exchange for fuel, the outer circumferential area G one space inside is loaded.

In this way, the third cycle of operation is carried out, and at the end of the cycle, the deeply inserted control rod is withdrawn as shown in FIG.

After the third cycle is completed, the operation returns to the first cycle, and thereafter, the operation is repeated in the order of the first to third cycles.

During operation, the deeply inserted control rods shown in Figure 5 are operated with fully inserted until a predetermined output is obtained, and when it becomes difficult to maintain the output, they cannot be pulled out. The amount of fissile material in the fuel assembly in region O is sufficient even at the end of the first cycle.
At the end of the third cycle (also at the end of the core cycle), there is a sufficient amount of fissile material at the end of the core cycle, the neutron economy is good, and the effort of heat exchange can be saved.

As explained above, the present invention not only reduces the disadvantage of fuel production cost as much as possible, but also satisfies thermal limitations, improves burnup acquisition, increases output level, or lowers output level. You can make improvements.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of the core area, Figure 2 is an 8x fuel rod
FIG. 3 is a plan view showing the loading state of fuel assemblies in 8 arrays; FIG. 3 is a plan view showing the loading state of fuel assemblies in 7×7 array; FIG. The figure shows a control rod operation pattern, FIG. 6 is an explanatory diagram of fuel loading in the second cycle, and FIG. 7 is an explanatory diagram of fuel loading in the third cycle. 1...Core, 2...Central region, 3...
...Middle area, 4...Outer area, 5, 6,
7... Fuel assembly, 8... Poison curtain, 9... Control rod.

Claims (1)

[Scope of Claims] 1. In a large number of fuel assemblies with the same external dimensions constituting the core, there are two types: one consisting of a square lattice arrangement of fuel rods with a small outer diameter and a large number, and the other consisting of a square lattice arrangement of fuel rods with a large outer diameter and a small number. Using a square lattice arrangement, the former has two areas, the central area containing fuel assemblies consisting of a large number of fuel rods, and the latter area containing only fuel assemblies consisting of a small number of fuel rods. A light water reactor characterized by having a separate core. 2. A large number of fuel assemblies with the same external dimensions constituting the reactor core are divided into two types: one consisting of a square lattice arrangement of fuel rods with a large amount of fissile material and a large number of fuel rods, and the other consisting of a square lattice arrangement of fuel rods with a large amount of fissile material and a small number of fuel rods. A fuel assembly consisting of a square lattice array, a square lattice array of fuel rods with a small amount of fissile material and a small number of fuel rods, and a fuel assembly consisting of fuel rods with a large amount of fissile material and a small number of fuel rods, When fuel assemblies consisting of fuel rods with a small amount of material and a small number of fuel rods are loaded in an alternating arrangement in the central region of the core (
The intermediate region surrounding the central region is loaded with a fuel assembly consisting of a large amount of fissile material and a small number of fuel rods, and the outer region is loaded with a fuel assembly consisting of both fissile material and a small number of fuel rods. A light water reactor loaded with aggregates and operated. 3. A large number of fuel assemblies with equal external dimensions constituting the reactor core, one consisting of a square lattice arrangement of fuel rods with a small outer diameter and a large number, and the other consisting of a square lattice arrangement of a large number of fuel rods with a large outer diameter. The reactor core is divided into two regions: the former has fuel assemblies with a large number of fuel rods, and the latter has a peripheral region where only fuel assemblies with a small number of fuel rods are loaded. In the light water reactor, after the first cycle is operated in this state, a part of the fuel assembly in the central region and an intermediate region between the central region and the peripheral region is replaced with a fuel assembly made of unirradiated fuel. A light water reactor operating method 0 characterized in that a second cycle operation is performed using