JPH0833474B2 - How to operate a nuclear reactor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an operation method of a light water moderated boiling water reactor, and particularly to an operation of a reactor suitable for achieving high burnup and uranium saving. Regarding the method.

[Conventional technology]

In a light water moderator reactor, in order to achieve a high burnup by a once-through method using enriched uranium and to effectively utilize uranium, for example, a high conversion burner core disclosed in JP-A-61-129594. Was developed. As shown in FIG. 2, this core is divided into radial regions to provide regions with different hydrogen to uranium atomic number ratios of fuel (hereinafter referred to as H / U). The core configuration is such that it is loaded in the high conversion area with a small U and the latter half is loaded in the burner area with a large H / U. That is, during the first half of the core stay period, H /
U is small and is loaded in the hard region of the neutron spectrum (high conversion region A) to convert the fuel parent material to fissile material. In the latter half of the core stay period, H / U is large and the neutron spectrum is soft region (burner region). It is intended to be loaded in B) and efficiently burn the fissile material.

Fig. 3 shows the burnup dependence of the infinite neutron multiplication factor at this time. The high conversion region where new fuel, which is a highly enriched uranium fuel, is loaded has a low neutron infinite multiplication factor, and the burner region where advanced combustion fuel is loaded has a high neutron infinite multiplication factor, reducing output mismatch. In addition, the excess reactivity of the fuel can be suppressed to a low level.

The control rod of the conventional boiling water reactor is plate-shaped and is installed so that it can move in the gap outside the channel box of each fuel assembly. On the other hand, in the high conversion burner furnace, the fuel rods 1 are densely arranged, and the control rod guide tubes are placed in place of the fuel rods at a part of the fuel rod positions arranged in a lattice pattern. A cluster type control rod in which the rod-shaped control rod 2 moves is used.

Now, in a boiling water reactor, boiling causes vapor bubbles (voids) in the lower part of the core, and the void volume ratio in the coolant increases toward the upper part of the core. In other words, the H / U of the fuel assembly is larger in the lower part of the core, so that the output peak occurs in the lower part of the core. Fig. 4 shows the relationship between infinite neutron multiplication factor and H / U for uranium fuel with enrichment of 6w / o. The change in H / U in each region of the high conversion and burner for the void ratio of 0 to 80% of the high conversion burner type core is 3.07 to 1.26 and 6.24 to 3.33, respectively. From FIG. 4, the corresponding change in the infinite multiplication factor of neutrons, that is, the reactivity change becomes larger in the high conversion region fuel assembly than in the conventional boiling water reactor in which the reactivity change is almost the same as in the burner region. I understand.

[Problems to be solved by the invention]

In the above-mentioned conventional technology, there is no consideration for the fact that the reactivity change due to the change of H / U in the high conversion region assembly in which the dense lattice fuel is arranged is larger than that of the conventional core, and the problem that the output peak in the axial direction becomes large I got it.

An object of the present invention is to provide a method for operating a nuclear reactor which reduces the change in H / U of a boiling water high conversion burner reactor using a dense lattice fuel assembly and flattens the axial power distribution. .

[Means for solving problems]

In order to achieve the above object, the present invention provides a dense lattice fuel using uranium as a fuel in a core in which regions having different ratios of the cross-sectional area occupied by a coolant per unit cross-sectional area and the cross-sectional area occupied by a fuel substance are mixed. Of the control rods to be inserted into the assembly, saturated water in the range of 1/6 to 1/2 of the heat generation length of the fuel rod was installed at the tip of the control rod from the core lower end in the guide tubes of all control rods not used for power adjustment. It proposes a method of operating a nuclear reactor by reducing the ratio of the moderator to the fuel atomic number of the fuel assembly by eliminating the water by the water removing unit.

Here, the dense lattice means a fuel rod lattice in a fuel assembly configured such that the ratio of the cross-sectional area occupied by light water as a moderator and the cross-sectional area occupied by fuel substances is 1.5 or less. Further, a substance having an absorption cross-sectional area smaller than that of the neutron absorbing material is enclosed in the water removing section.

That is, in order to achieve the above-mentioned object, the present invention provides a fuel assembly in which a plurality of fuel rods are arranged in an aligned manner, and a cross-sectional area occupied by a coolant and a cross-sectional area occupied by a fuel substance per unit cross-sectional area. In a method of operating a nuclear reactor having a high conversion burner core in which regions of different ratios are mixed and light water flows upward from below, and a plurality of control rods inserted from the lower part of the core into the core region Of the control rods to be inserted in the high conversion region where the ratio of the area and the cross-sectional area occupied by the fuel substance is 1.5 or less, the control rod not used for output adjustment is closer to the tip side than the neutron absorber region, and is more than the neutron absorber. Install a water removal unit that encapsulates a substance with a small absorption cross-section, and insert the water removal unit from the lower end of the reactor core into the range of 1/6 to 1/2 of the fuel rod heat generation length for operation. It proposes a method of operating a nuclear reactor.

[Action]

When the saturated water in the control rod guide tube of the dense lattice fuel assembly is removed by the water removing unit (hereinafter referred to as follower) provided at the tip of the control rod, the moderator to fuel atomic number ratio (here, hydrogen) of the fuel assembly is removed. The ratio of the number of uranium atoms (H / U) is reduced.

FIG. 5 shows that in the high conversion area assembly of the high conversion burner core, when the control rod guide pipe is filled with saturated water,
And the relationship between the hydrogen-to-uranium atom number ratio of the assembly and the void volume ratio of the coolant (light water) when the saturated water in the guide tube is removed by the follower. From this figure, it can be seen that when the follower is inserted, H / U is reduced by about 0.4 for the same void volume ratio.

As already described with reference to Fig. 4, the effect of changes in H / U on changes in reactivity increases as H / U decreases.
The effect of the follower is more significant in a dense lattice fuel assembly in which / U is less than 3.0, that is, the ratio of the sectional area occupied by the moderator light water to the sectional area occupied by the fuel substance is less than 1.5.

Now, the axial output peak of the dense lattice fuel assembly to be solved by the present invention mainly results from the H / U of the fuel assembly changing with the axial position. Therefore,
The magnitude of the axial output peak depends on the axial position of the follower from the lower end of the core.

FIG. 6 shows the relationship between the axial insertion position of the control rod follower from the lower end of the core and the axial output peak value of the dense lattice fuel assembly. From this figure, it can be seen that the output peak is most reduced when the follower is inserted at the 6/24 (= 1/4) node position from the lower end of the core. In addition,
The insert range of the follower that can suppress the axial output peak to about 2.2 is 4/24 to 12/24 (1/6 to 1/2) nodes from the core bottom end.

Fig. 7 shows the operating cycle of the dense lattice fuel assembly fuel when the follower is inserted in the range of 6/24 (= 1/4) node from the core lower end of all the dense lattice fuel assemblies of the high conversion burner reactor. It shows the axial output distribution in the final stage. For comparison, the output distribution without a follower is also shown. Inserting a follower will reduce the output peak in the follower injection range by about 33%.
As a result of suppressing the output peak in the lower part of the core, the output in the region above the tip of the follower increased relatively,
The power distribution in the core axis direction is flattened.

〔Example〕

 Hereinafter, the present invention will be described in more detail with reference to Examples.

FIG. 1 is a diagram showing a first embodiment of a method for operating a nuclear reactor according to the present invention. The core of the present embodiment is a combined high conversion burner core in which a burner area having an H / U of the same degree as that of the current core is arranged around the high conversion area of the dense lattice fuel assembly. The fuel assembly itself in each region is the same as that shown in FIG. In the high conversion region fuel assembly A, the cluster consisting of the fuel rods and the control rod guide tubes is surrounded by the channel box 6, and the voids generated in the lateral direction due to the boiling of light water that also serves as a moderator and a coolant float in the lateral direction. Is being prevented. In the burner area assembly B, a water rod is placed instead of the fuel rod in order to increase H / U. In the high conversion burner furnace, since the fuel rods or water rods are densely arranged, the control rod is the 8th.
Use cluster control rods as shown. The length of the neutron absorber region 4 of this cluster control rod is made equal to the heat generation length of the fuel rod, and the inside is filled with B ₄ C. Further, the follower portion 3 is provided in a region 1/6 to 1/2 of the heat generation length of the fuel rod on the tip side of the neutron absorbing material region, and has a substance having a smaller absorption cross section than the neutron absorbing material, such as carbon. To fill.
One cluster control rod drive device is installed for each of the three fuel assemblies. In the high conversion region of the high conversion burner furnace configured as described above, as shown in FIG. 1, in order to suppress the excess reactivity at the initial stage of the operation cycle, in this example, 7 sets (denoted by A ₀₎ ( (21 bodies) all the control rods of the fuel assemblies are inserted, and in all the remaining fuel assemblies indicated by A ₁ , only the follower section 3 at the tip of the control rods is inserted into the lower part of the core for operation.

In a boiling water reactor, a void distribution occurs in the axial direction of the core during operation, and the void fraction is 0% at the lower end of the fuel, but is about 70% at the upper end. Due to this void distribution, a difference in H / U above and below the fuel occurs, and thus an infinite neutron multiplication factor also varies, resulting in an output peak below the fuel. H / U is 3.0
As shown in FIG. 4, in the dense lattice fuel assembly below, as compared with the conventional light water reactor with H / U close to 5.0, or with the fuel assembly in the burner region B of the above-mentioned high conversion burner reactor, In comparison, the infinite neutron multiplication factor with the change in H / U is large and the peak value due to the void distribution is also large.

On the other hand, according to the operating method of the present embodiment, the change in the core axial direction of H / U in the lower core of the low void fraction is reduced to suppress the output peak generated in the dense lattice fuel assembly,
The output distribution can be flattened. As described above, in the present invention, the output distribution is flattened without taking any special measures such as the enrichment distribution in the height direction of the fuel assembly.

FIG. 9 shows a second embodiment of the method for operating a nuclear reactor according to the present invention. The follower portion 3 is inserted in the lower part of the core of all the fuel assemblies in the high conversion region A, and the burner region B operates the core in the fully drawn state. This operation method is effective mainly at the end of the operation cycle where the combustion of fuel has advanced. That is, at the end of the operation cycle, the excess reactivity is small, and this can be suppressed only by the decrease in reactivity due to the insertion of the follower 3. Since the neutron absorption cross section of the follower part 3 is smaller than the absorption cross section of the neutron absorbing material, the operation method of the core of this example is more advantageous in neutron economy than the conventional method of suppressing the excess reactivity by only the neutron absorbing rod. Yes, the life of the fuel can be extended.

FIG. 10 shows a third embodiment of the method for operating a nuclear reactor according to the present invention. This example is directed to a high conversion burner furnace in which a Y-shaped control rod designated by 9 is inserted into the gear water region 10. In this embodiment, a follower portion is provided at the tip of the neutron absorbing material region of the Y-shaped control rod, and this is inserted into the high conversion region assembly to operate the nuclear reactor.

In addition to the present embodiment, also in a high conversion burner furnace using a cruciform control rod, a method of providing a similar follower and operating the core can be considered.

In the first embodiment of the present invention, the control rod for suppressing the excess reactivity (the control rod to be inserted into the high conversion area assembly A ₀ )
As a conventional control rod without follower. Alternatively, if a control rod with a follower is also combined as a control rod for suppressing the excess reactivity and a control rod drive plan that also considers neutron absorption in the follower portion is executed, the output distribution can be flattened throughout the operation period.

Furthermore, if gadolinia fuel rods or short fuel rods are used in the initial stage of operation, the output distribution can be further flattened.

〔The invention's effect〕

According to the present invention, there is provided a method of operating a core by inserting a control rod follower into the lower part of the core of a dense lattice fuel assembly of a high conversion burner reactor, so that H / U can be reduced, and therefore the reactivity can be reduced. Can also be reduced, so that the axial power distribution is flattened.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of a method of operating a nuclear reactor according to the present invention, FIG. 2 is a diagram showing a configuration of a reactor core constructed on the basis of a once-through system, and FIG. 3 is a diagram showing FIG. Fig. 4 is a diagram showing the combustion variation of the neutron infinite multiplication factor of the conventional fuel assembly used in the core of Fig. 4, and Fig. 4 is the neutron infinite multiplication factor of the uranium fuel loaded in the reactor constructed based on the once-through method. FIG. 5 shows the relationship with H / U, and FIG. 5 shows the relationship between the H / U of the fuel assembly loaded in the high conversion region of the core shown in FIG. FIG. 6 shows the case of using fuel and the case of not using fuel, and FIG. 6 shows the relationship between the core axial output peak of the fuel loaded in the high conversion region of the core shown in FIG. 2 and the insertion position of the follower according to the present invention. Fig. 7 shows the operating size of the fuel loaded in the high conversion region of the core. Fig. 8 is a diagram showing relative power distribution in the axial direction of the reactor core in the final stage, with and without the follower according to the present invention. Fig. 8 is a cluster control with follower used for operating the reactor of the first embodiment according to the present invention. A sectional view and a side view of the rod, FIG. 9 is a diagram showing a second embodiment of the method for operating a nuclear reactor according to the present invention, and FIG. 10 is a diagram showing a third embodiment of the method for operating a nuclear reactor according to the present invention. is there. A: High conversion area assembly, A _1: High conversion area assembly with follower control rod inserted, B: Burner area assembly, A ₀
...... High conversion area assembly in which all control rods are inserted, 1 ... Fuel rod, 2 ... Control rod, 3 ... follower part, 4 ... Neutron absorber, 5 ... Water rod, 6 ... Channel box , 7
...... High conversion core, 8 ...... Burner core, 9 ...... Y-shaped control rod, 10 ...... Gear water area.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hiromi Maruyama 1168 Moriyama-cho, Hitachi City, Hitachi, Ibaraki Pref., Energy Research Laboratory, Hitachi, Ltd. (72) Tomoyuki Matsumoto 1168 Moriyama-cho, Hitachi City, Ibaraki, Hitachi, Ltd. (56) References JP-A-61-86676 (JP, A) JP-A-56-8592 (JP, A) JP-A-62-62286 (JP, A)

Claims (2)

[Claims] 1. Light water in which a fuel assembly in which a plurality of fuel rods are arranged in an array is mounted, and regions having different ratios of the cross-sectional area occupied by a coolant and the cross-sectional area occupied by a fuel substance per unit cross-sectional area are mixed. With a high conversion burner core in which
In a method of operating a nuclear reactor having a plurality of control rods inserted from below the core in the core region, a high conversion region in which the ratio of the cross-sectional area occupied by the coolant and the cross-sectional area occupied by the fuel substance is 1.5 or less. Among the control rods to be inserted into, the tip side of the neutron absorbing material region of the control rod that is not used for output adjustment, a water removal unit is installed in which a substance having an absorption cross section smaller than the neutron absorbing material is sealed inside. A method for operating a nuclear reactor, comprising: inserting the water removing unit into a range of 1/6 to 1/2 of a heat generation length of the fuel rod from a lower end of a core of the nuclear reactor. 2. The water removing section according to claim 1, wherein the water removing section is inserted from a core lower end of a nuclear reactor into a range of 1/6 to 1/2 of a fuel rod heat generation length, and at the beginning of an operation cycle, A method for operating a nuclear reactor, comprising: fully inserting a control rod having no water removing section into some sets of the fuel assemblies to operate.