JPH0990076A - Fuel assembly and reactor core - Google Patents

Abstract

PROBLEM TO BE SOLVED: To properly control excess reactivity without degrading the core characteristics and improve fuel economics. SOLUTION: A zirconium alloy channel box 12 include boron-10 of ca. 0.2wt.%. A fuel assembly including the channel box 12 contains two kinds of fuel rod of No.1 and 2. Plutonium enrichment (a) of the fuel rod No.1 is lower by ca. 1wt.%. than that (b) of the rod No.2. Therefore, the amount of burnable poison in the fuel rods can be reduced owing to the channel box containing boron-10 and so the suppression of excess reactivity and suppress of local power peaking is possible and thus, the kind number of fuel rods with different plutonium enrichment can be greatly reduced. Especially, as the kind number of the fuel rods with different plutonium enrichment is two or less, the production process of the fuel assembly can be greatly simplified.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly and a reactor core, and more particularly to a fuel assembly and a reactor core suitable for application to a boiling water reactor (hereinafter referred to as BWR). Regarding

[0002]

2. Description of the Related Art Reactors continue to generate energy by a chain reaction in which neutrons emitted together with thermal energy generated by the fission of fissionable materials collide with other fissile materials to cause the next fission. The state in which the rate of occurrence of this chain reaction is in equilibrium is called critical, and the reactor that operates at a constant power continues to maintain this state. Also,
The state in which the chain reaction increases is called supercritical, while the state in which the chain reaction decreases is called subcritical.

Reactors must continue to operate for a period of time without refueling fuel material, including fissile material. Therefore, more fissile material than the amount required to maintain the criticality is loaded in the core of the nuclear reactor. Therefore, the reactor will be supercritical without the control material of neutron absorbing material. This excess reactivity is the excess reactivity. It is important to properly control this excess reactivity throughout the operation period of the reactor. It is well known to mix combustible poisons into fuel materials in order to control this excess reactivity throughout the operating period. Combustible poison is a neutron absorber that gradually burns and the amount of its substance decreases throughout the operation period. A typical example of a burnable poison is gadolinia that is mixed with a nuclear fuel material.

Next, the manner of suppressing the reactivity of the burnable poison will be shown with reference to the drawings. FIG. 6 shows an example of the change of the infinite multiplication factor of a fuel assembly having a plurality of fuel rods containing a fuel substance mixed with gadolinia, with respect to the burnup. In general, if the number of fuel rods containing combustible poisons increases, the infinite multiplication factor in the initial stage of combustion decreases. Further, if the concentration of the burnable poison mixed in is increased, the time when gadolinia burns out is delayed, and as a result, the maximum value of the infinite multiplication factor is suppressed. By utilizing these functions and appropriately selecting the combination of the concentration of burnable poison and the number of fuel rods containing burnable poison, the excess reactivity of the core can be appropriately controlled.

In this way, the reactor can properly control the excess reactivity by using the control rod and the burnable poison.

However, in recent years, a high burnup of the fuel assembly has been studied by increasing the enrichment. The excess reactivity of the core is further increased by using the high burnup fuel assembly. If the increase of the excess reactivity is dealt with by the insertion of the control rods, the increase in the number of the control rods inserted increases the radial power peaking of the core and reduces the thermal margin. Moreover, control rod operation is increased to compensate for large excess reactivity changes during operation. This leads to a reduction in the reactor operating rate and is not desirable from the viewpoint of fuel economy. On the other hand, if the increase in the surplus reactivity is dealt with by the increase in the concentration of the burnable poison, the heat conduction of the fuel pellet is reduced, which is not preferable in terms of fuel integrity. If the number of burnable poison-containing fuel rods is increased, the number of fuel rods with low output will increase, and local output peaking (the ratio of the maximum output to the average value of the fuel rod output in the fuel assembly) will increase.
Thermal margin is reduced.

In particular, a fuel assembly (MOX) having fuel rods filled with MOX fuel (nuclear fuel mixture of uranium and plutonium) containing several% of plutonium from the early stage of combustion (MOX)
In the fuel assembly), the thermal neutron absorption cross-section of plutonium is larger than that of uranium, so the proportion of neutrons with a relatively higher energy than the fuel assembly that does not contain MOX fuel but contains uranium as the fuel substance (uranium fuel assembly). There are many. As a result, in the core loaded with the MOX fuel assembly, the reactivity effect of the control rod (control rod value) and the reactivity effect of the burnable poison are reduced. As a result, the core loaded with MOX fuel assemblies must have a larger number of control rods and burnable poison-bearing fuel rods to control the excess reactivity, and the core loaded with uranium fuel assemblies must be In comparison, it becomes more difficult to control the excess reactivity.

In the MOX fuel assembly, if there is no record of mixing burnable poisons in the fuel rods filled with MOX fuel and there is a restriction on the use of burnable poisons, the increase in the number of burnable poison-bearing fuel rods is MOX. The number of fuel rods will decrease and the plutonium loading will decrease. In addition, the increase in local power peaking is mitigated by increasing the number of types of fuel rods with different plutonium enrichment (hereinafter simply referred to as enrichment) in the fuel assembly, and by increasing the enrichment of MOX fuel rods to a high power position. Requires placement. The increase in the number of fuel rods with different enrichment is
The manufacturing process of the OX fuel assembly is complicated. In order to simplify the manufacturing process, it is desirable that the number of types of fuel rods having different enrichment be as small as possible.

As described above, in the high burnup of the fuel assembly and the use of the MOX fuel assembly, depending on the increase of the control rod operation and the number of burnable poison-bearing fuel rods,
Constraints tend to increase in terms of thermal margin and fissile material loading.

An example of a conventional initially loaded high burnup core is described in Japanese Patent Laid-Open No. 5-249270. This core has a high enrichment fuel assembly with an average enrichment of 3.4%, a medium enrichment fuel assembly with an average enrichment of 2.3%, and an average enrichment of 1.
It is composed of three types of fuel assemblies, 1% low enrichment fuel assemblies. In order to effectively use fissile material, fuel assemblies with lower average enrichment are taken out of the core earlier.
Fuel assemblies with a higher average enrichment stay in the core for longer periods. In order to further increase the ejection burnup of the fuel assemblies loaded in the initially loaded core, it is necessary to further increase the average enrichment of the core and prolong the stay period of the fuel assemblies in the reactor. As a method of increasing the average enrichment of the core, there are a method of increasing the average enrichment of the fuel assembly and a method of increasing the number of highly enriched fuel assemblies among the fuel assemblies constituting the core. According to the former method, the average enrichment of the replacement fuel assembly newly loaded in the core at the time of refueling differs from the average enrichment of the high enrichment fuel assembly loaded in the initially loaded core. Further, according to the latter method, the effect of the fuel rod mixed with the burnable poison is greatly exerted, which causes a difficulty that the excess reactivity becomes higher during the cycle.

As an example of a conventional MOX fuel assembly, as described in JP-A-63-108294, a plutonium-enriched fuel rod and a plutonium-free gadolinia-containing fuel rod are arranged in a square lattice. There is a fuel assembly in which the concentration distribution or the gadolinia concentration distribution is changed in the axial direction in the gadolinia-containing fuel rod. If the plutonium enrichment is further increased for higher burnup in this fuel assembly, it is necessary to increase the number of gadolinia-containing fuel rods, which reduces plutonium loading in the fuel assembly and differs in plutonium enrichment. An increase in the number of types of fuel rods occurs.

[0012]

When the uranium enrichment or plutonium enrichment is increased to increase the burnup of the fuel assembly, the excess reactivity of the core increases. In contrast,
If the number of control rods inserted and the concentration of combustible poisons are increased, problems such as a decrease in thermal margin, deterioration of fuel characteristics, a decrease in plutonium loading, and an increase in the number of types of fuel rods with different enrichment Occurs. Further, it is desirable that the number of enrichment types for reducing local output peaking is as small as possible.

An object of the present invention is to provide a fuel assembly and a core of a nuclear reactor capable of appropriately controlling the excess reactivity without impairing the core characteristics and improving fuel economy.

[0014]

Means for Solving the Problems The features of the present invention that achieve the above object are that a channel box made of a zirconium alloy contains a substance having a larger neutron absorption cross section than that of the zirconium alloy, The number of types of fuel rods having different uranium enrichment or plutonium enrichment in the horizontal section is 2 or less.

Since the channel box made of zirconium alloy contains a substance having a larger neutron absorption cross section than that of the zirconium alloy, for example, boron 10, the filling amount of the burnable poison (eg gadolinia) in the fuel rod is reduced. it can. The channel box containing a substance having a larger neutron absorption cross section than the zirconium alloy enables suppression of excess reactivity and suppression of local power peaking. As a result, the number of types of fuel rods having different uranium enrichments or plutonium enrichments can be significantly reduced.

[0016] Preferably, the channel box containing the neutron absorbing material is replaced with a conventional zirconium alloy-containing channel box containing no neutron absorbing material at the stage where the reactivity of the fuel assembly is lowered during the life of the fuel. As a result, it is possible to prevent unnecessary absorption of neutrons.

The operation of the present invention will be specifically described below based on specific examples. The fuel assembly shown in FIG. 8 includes fuel rods 16 and water rods 13, and a bundle of fuel rods 16 is surrounded by a channel box 17 made of a zirconium alloy. Boron 10, which is a neutron absorbing substance, is added to the channel box 17, and the infinite multiplication factor of the fuel assembly and the local output peaking boron 1 when the addition amount is changed.
The 0 concentration dependence is shown in FIG. Channel box 17
The infinite multiplication factor and local power peaking of the fuel assembly are both reduced by the addition of boron 10 therein. Preferably, by setting the concentration of boron 10 to be 0.3% or more, the infinite multiplication factor and local power peaking of the fuel assembly can be significantly reduced. In the characteristic of FIG. 7, a reduction of about 20% in the infinite multiplication factor corresponds to a reduction of about 10% in the local output peaking.

Since the infinite multiplication factor is reduced by the present invention, the filling amount of the fuel rod of gadolinia, which is a burnable poison, is reduced as compared with the conventional one, and the control of the core excess reactivity, the securement of the fuel characteristic margin, and the plutonium loading are carried out. The quantity can be increased. Further, since the local output peaking is reduced, it is possible to secure a thermal margin and reduce the number of types of fuel rods having different plutonium enrichments. In particular, since the number of kinds of fuel rods having different uranium enrichment or plutonium enrichment in one horizontal section in the axial direction of the fuel assembly is 2 or less, the number of fuel rods is 2 or less.
Since it is only necessary to manufacture various types, the manufacturing process of the fuel assembly can be significantly simplified.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below.

(Embodiment 1) FIG. 1 shows a fuel assembly which is an embodiment of the present invention. The fuel assembly of this embodiment includes a fuel rod 11, a water rod 13, and a channel box 15, and is a MOX fuel assembly. Channel box 15
Is made of a zirconium alloy and contains about 0.2% by weight of boron 10, which is a neutron absorbing substance. The BWR core loaded with the fuel assemblies has one fuel assembly for every four fuel assemblies.
The cross-shaped control rod 14 is inserted in proportion to the body. The control rod 14 is filled with boron as a neutron absorbing material, and is inserted between the fuel assemblies.

In this core, the width of the water gap formed between the fuel assemblies on the side where the control rods are inserted is formed between the fuel assemblies on the side opposite to the side where the control rods are not inserted. Equal to the width of the water gap (C lattice core). As another core, there is one called a D lattice core in which the width of the former water gap is wider than the width of the latter water gap.

The fuel rods 11 constituting the fuel assembly are shown in FIG.
There are two types of fuel rods 1 and 2, as shown in FIG. As shown in FIG. 2, the plutonium enrichment of the fuel rod 1 is a,
The plutonium enrichment of the fuel rod 2 is b. These fuel rods are arranged in the cross section of the fuel assembly in the channel box 12 as shown in FIG. The fuel rod 1 and the fuel rod 2 have different plutonium enrichments. The fuel rod 2 has a plutonium enrichment lower than that of the fuel rod 1 by about 1% by weight (b
<A).

In this embodiment, the fuel rods 2 having a low plutonium enrichment are arranged only at the corners of the outermost fuel rod array. There are four fuel rods 2 and the other fuel rods are fuel rods 1.
It is.

The change in the infinite multiplication factor of the fuel assembly with respect to the burnup in this embodiment is shown in characteristic C of FIG. A change in local output peaking with respect to burnup in the fuel assembly of this embodiment is shown in characteristic C of FIG.

The change of the infinite multiplication factor and the local power peaking with respect to the burnup in a fuel assembly using a channel box not containing boron 10 and having only one kind of fuel rod 1 as a fuel rod is shown in FIGS. 9 and 10. Characteristic A
It is. Further, in the fuel assembly using the channel box including the boron 10 and having only one type of fuel rod 1 as a fuel rod, the infinite multiplication factor and the change in the local output peaking with respect to the burnup are the characteristics B of FIGS. 9 and 10. Is. FIG. 5 shows the structure of a fuel assembly using only one type of fuel rod 1.

Channel box 1 containing boron 10
2. In the present embodiment having the fuel rods 1 and 2, an infinite multiplication factor of 20%, a local output peaking of about 25% with respect to the characteristic A, and a reduction effect of about 15% with respect to the characteristic B can be obtained.

The fuel assembly of the present embodiment, in which the channel box made of a zirconium alloy contains boron 10 and two kinds of fuel rods 1 and 2 having different plutonium enrichments are used, the infinite multiplication factor and the local power peaking are obtained. A large reduction effect can be obtained.

(Embodiment 2) FIG. 3 shows a fuel assembly which is another embodiment (Embodiment 2) of the present invention.

Fuel rod 1 constituting the fuel assembly of this embodiment
1 is also two types of fuel rods 1 and 2. 32 fuel rods
The number of fuel rods 2 is 28. The fuel rod 2 has a plutonium enrichment degree lower than that of the fuel rod 1 by about 1% by weight. Fuel rod 2
Are arranged on the outermost periphery of the fuel rod array adjacent to the channel box 12. The fuel rods 1 are arranged inside the outermost periphery of the fuel rod array. The channel box 12 made of a zirconium alloy contains boron 10 in an amount of about 0.2% by weight like the channel box of the first embodiment.

The change in infinite multiplication factor with respect to burnup in the fuel assembly of this embodiment is shown in characteristic D of FIG. Further, the combustion change of the local output peaking in the fuel assembly of this example is shown in the characteristic D of FIG.

In this embodiment, the infinite multiplication factor is reduced to the same extent as in the first embodiment, and the local output peaking is further reduced by about 5%.

(Embodiment 3) A fuel assembly according to another embodiment of the present invention is shown in FIG.

As in the first embodiment, two types of fuel rods 1 and 2 are used as the fuel rods 11 constituting the fuel assembly. There are 24 fuel rods 1 and 36 fuel rods 2.
The fuel rod 2 has a plutonium enrichment degree lower than that of the fuel rod 1 by about 1% by weight. The fuel rods 2 are arranged adjacent to the channel box 12, the outermost periphery of the fuel rod array, and the position adjacent to the water rod 13, respectively. The fuel rod 1 is arranged at a position other than the position where the fuel rod 2 is arranged. The channel box 12 made of a zirconium alloy contains boron 10 in an amount of about 0.2% by weight like the channel box of the first embodiment.

The present embodiment can obtain the same effect as that of the second embodiment, and the local output peaking is reduced more than that of the second embodiment especially at the time when combustion progresses.

[0035]

According to the present invention, even when the uranium enrichment or plutonium enrichment for increasing burnup is increased, the excess reactivity of the core can be appropriately controlled and the thermal margin can be secured. It is possible to secure a fuel characteristic margin, increase plutonium loading, and reduce the number of types of fuel rods with different plutonium enrichments.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a fuel assembly which is a preferred embodiment of the present invention.

FIG. 2 is an explanatory view showing a distribution of plutonium enrichment of fuel rods used in the example in FIG.

FIG. 3 is a cross-sectional view of a fuel assembly which is another embodiment of the present invention.

FIG. 4 is a cross-sectional view of a fuel assembly which is another embodiment of the present invention.

FIG. 5 is a cross-sectional view of a fuel assembly in which one type of fuel rod filled with plutonium is arranged in a conventional fuel assembly.

FIG. 6 is an explanatory diagram showing the influence on the infinite multiplication factor by changes in the concentration of burnable poison and the number of fuel rods containing burnable poison.

FIG. 7 is a characteristic diagram showing changes in infinite multiplication factor and local peaking with respect to the concentration of boron 10 contained in the channel box.

8 is a cross-sectional view of a fuel assembly which is a subject of study for obtaining the characteristics of FIG.

FIG. 9 is a characteristic diagram showing changes in infinite multiplication factor with respect to burnup in various fuel assemblies.

FIG. 10 is a characteristic diagram showing changes in local power peaking with respect to burnup in various fuel assemblies.

[Explanation of symbols]

11 ... Fuel rod, 12, 15 ... Channel box, 13
… Water rod, 14… God stick.

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical display location G21C 3/30 GDBW GDBD GDBX GDBT (72) Inventor Katsumasa Urikawa 3-1-1, Saiwaicho, Hitachi-shi, Ibaraki Stock company Hitachi Ltd.Hitachi factory

Claims (5)

[Claims] 1. A fuel assembly having a plurality of fuel rods filled with a fuel substance inside and a channel box made of a zirconium alloy surrounding these bundles, wherein the channel box has a neutron absorption insulation more than that of the zirconium alloy. A fuel assembly comprising a substance having a large area, and the number of kinds of the fuel rods having different uranium enrichment or plutonium enrichment in one horizontal cross section in the axial direction is 2 or less. 2. The fuel assembly according to claim 1, wherein among the fuel rods, a fuel rod having a low uranium enrichment or a low plutonium enrichment is disposed adjacent to the channel box and near a corner portion of the channel box. 3. A fuel rod having a low uranium enrichment or a plutonium enrichment among the fuel rods, which is disposed adjacent to the channel box, near a corner portion of the channel box, and in a central portion of the fuel assembly. The fuel assembly of Claim 1 disposed facing a rod. 4. The fuel assembly according to claim 1, wherein the fuel rod containing burnable poison is not arranged. 5. A reactor core comprising a fuel assembly according to any one of claims 1 to 4 loaded therein.