JPH04254791A - Fast breeder reactor - Google Patents

Abstract

PURPOSE:To ensure the proper streaming of a neutron and to stabilize the response of the neutron detector arranged to the upper part of a reactor core. CONSTITUTION:A neutron shield body 20 is arranged to the upper part of a reactor core 6 and a neutron detector is arranged to the upper part thereof. A control rod having a neutron absorbing part 24 is inserted in a fuel rod channel 22 from the upper part of the reactor core 6. The region length of the neutron shield body 20 of at least one fuel assembly among six fuel assemblies adjacent to the fuel rod channel 22 is reduced. By this constitution, even when the control rod 23 is pulled out, the neutron absorbing part 24 and the neutron shield body 20 do not overlap with each other and the proper streaming of a neutron can be ensured.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fast breeder reactor, and more particularly to a fast breeder reactor capable of improving the response stability of a neutron detector disposed in the upper part of the reactor core.

0002

2. Description of the Related Art FIG. 10 shows a general fast breeder reactor. In the figure, reference numeral 1 indicates a reactor vessel having a guard vessel 2 on the outer surface side, and the upper end opening of this reactor vessel 1 is , is closed by a furnace vessel cover 3, and this furnace vessel cover 3
A large rotation plug 4 and a small rotation plug 5 are attached to the.

On the other hand, inside the reactor vessel 1, there are a reactor core 6,
An intermediate heat exchanger 7, a primary main circulation pump 8, a core support structure 9, a core upper mechanism 10, a neutron detector 11, etc. are arranged, and a fuel exchanger 12 and a fuel exchanger 12 are arranged on the rotary plugs 4 and 5. A control rod drive mechanism 13 and the like are arranged.

By the way, in such a fast breeder reactor, a neutron shield is installed at the top of the reactor core in order to reduce the amount of fast neutron irradiation to the permanent structures of the reactor upper mechanism to below predetermined design standards and conditions. are doing.

In addition, from the perspective of improving maintainability and repairability of the reactor, the amount of neutron activation in the reactor vessel internal structures such as the primary main circulation pump 8 and the intermediate heat exchanger 7 is reduced, and the In order to reduce neutron streaming, etc., it is necessary to install neutron shields above and around the core.

Furthermore, an intermediate heat exchanger 7 using fast neutrons
Reducing fast neutrons is important to prevent and suppress the activation of sodium in the secondary system inside the reactor, and the neutron shield is required.

From these points of view, surrounding the reactor core 6,
A neutron shield and a radial neutron shield are arranged in the upper part of the core, and FIG. 11 shows an example thereof.

That is, an upper shaft blanket 14, a lower shaft blanket 15, and a diameter blanket 16 are arranged outside the core 6, and the upper shaft blanket 14
An upper gas plenum 17 is disposed above the shaft blanket 15, and a lower gas plenum 18 is disposed below the lower shaft blanket 15. A neutron shield 2 is connected to the upper part of the upper gas plenum 17 via an end plug and a coolant flow path 19.
0 is placed, and a radial neutron shield 21 is placed on the outer periphery.

Further, a control rod 23 inserted into the control rod channel 22 from above is provided with a neutron absorber portion 24 so as to overlap the neutron shield 20 in the axial direction at the end of the operation cycle.

By the way, from the perspective of reducing the cost of a nuclear reactor plant, it is necessary to make the reactor core 6 as compact as possible, and in order to make the reactor core 6 more compact, it is preferable to eliminate the neutron shields 20 and 21; Therefore, recently, B4C is used as the neutron shield 20, which is effective in making the reactor core 6 compact and also has excellent performance in neutron shielding.

On the other hand, in order to safely and smoothly perform normal startup, operation, and shutdown operations of the nuclear reactor, a neutron detector 11 is required to monitor the state of the reactor core 6.

Generally, it is effective to place the neutron detector 11 near the reactor core 6 in order to accurately grasp the state of the reactor core 6. The location of the neutron detector 1 is generally determined by the following factors: high neutron sensitivity, the ratio of gamma ray intensity to neutron flux intensity, local disturbances due to the movement of the control rods 23, and internal storage racks in plants with in-reactor storage racks. It is desirable to select a position that is not affected by the arrangement of spent fuel, etc., and to have a monotonous and simple relationship between the output level and the like of the entire reactor core 6.

On the other hand, fluctuations in the power distribution in the core 6, that is, the neutron flux distribution, due to combustion due to high burnup, long-term operation cycles, etc., occur due to the withdrawal of the control rods 23 due to a decrease in operating surplus reactivity. This trend tends to increase, including the fluctuations caused by the change. Therefore, the neutron detector 11 is determined taking these situations into consideration. This is because the control rod 23 at zero power at the beginning of the operation cycle, for example at the time of reactor startup.
It is necessary to install the neutron detector 11 so that a stable response can be obtained even during the extraction operation.

[0014] In a system in which a B4C shield is placed in the upper part of the reactor core to strengthen the shielding, the control rod channels 22 play an important role for neutrons passing into the upper part of the reactor core.

As described above, it is desirable to arrange the neutron detector 11 in the upper part of the reactor core 6 from the structural point of view of the nuclear reactor, and the arrangement of the neutron shields 20 and 21 around the reactor core 6 and the shielding It is necessary to satisfy both demands at the same time.

FIG. 12 shows the arrangement position of the neutron detector 11 in a general fast reactor core arrangement. That is, in this core 6, control rod assemblies 26 denoted by the symbol C are arranged in the core fuel assembly 25, and blanket fuel assemblies 27 denoted by the symbol B are arranged around them. A neutron shield 28 shown with a black circle is arranged so as to surround these. In the upper part of the blanket fuel assembly 27, a power range neutron detector is arranged at a position 11a marked with a circle, and a wide range neutron detector is arranged at a position 11b marked ◇. A near-critical neutron detector is arranged above the neutron shield 28 at a position 11c marked with an asterisk.

By the way, the neutron detector 11 installed in the upper part of the reactor core 6 and the neutron absorber section 24 of the operation control rod 23
Regarding the positional relationship between ”, the operation control rod 23
There are reports that the neutron flux distribution becomes complicated depending on the position of the neutron absorber section 24.
The positional relationship between the neutron shield 20 and the neutron shield 20 at the upper part of the fuel assembly surrounding it has not been considered.

It is desirable that the neutron detector 11 has high sensitivity. Further, it is desirable that the response of the neutron detector 11 to changes in the state of the reactor core, such as the power increase from zero power to the rated power at reactor startup and core combustion, be as simple and stable as possible. And this request is
The performance is satisfactory even if the reactor output needs to be changed, stopped, or restarted for some reason during the operation cycle.

[0019]

[Problems to be Solved by the Invention] In the conventional fast breeder reactor, for example, when the control rod 23 is withdrawn after the operation period has progressed to a certain extent during the operation cycle, the control rod 23 is withdrawn to increase the output by starting the reactor. Control rods 23 for output adjustment by extraction operation and combustion of the reactor core 6 during operation.
As shown in FIG. 11, in the process of pulling out the
The neutron absorber section 24 of the control rod 23 and the neutron absorber region arranged in the upper part of the core of the six fuel assemblies adjacent to the control rod channel 22 overlap in the axial direction, so that the control rod There will be less streaming neutrons from the top of 23. Therefore, when the power increases at the beginning of the operation cycle, and when the reactor output is changed or restarted during the operation cycle, the neutron detector 1
The problem is that the response (sensitivity and linearity) varies.

A possible way to solve this problem is to eliminate the neutron shield 20 at the top of the reactor core or uniformly reduce its thickness to prevent the control rods 23 from overlapping with the neutron absorber section 24. However, as mentioned above, this method increases the leakage of fast neutrons into the upper core mechanism, and
Activation of sodium in the secondary system due to fast neutrons will increase, which will conflict with shielding requirements.

[0021] The present invention has been made with these points in mind, and even in the process of withdrawing control rods for starting up and operating a nuclear reactor, it is possible to ensure appropriate neutron streaming and ensure that the upper part of the core The purpose of this invention is to provide a fast breeder reactor that can ensure the stability of the response of a neutron detector.

[0022]

[Means for Solving the Problems] As a means for achieving the above object, the present invention has a structure in which a control rod having a neutron absorber portion is inserted from the upper part of the reactor core, and a neutron shield is arranged in the upper part of the reactor core. , and in a fast breeder reactor in which a neutron detector is disposed above a neutron shield, the neutron absorber portion of at least some of the control rods and at least some of the plurality of fuel assemblies surrounding the control rod channel are The neutron shield area placed above the core of the fuel assembly is set in a positional relationship so that it does not overlap in the axial direction during the process of withdrawing the control rods for startup and operation of the reactor. It is characterized by what it did.

[0023]

[Operation] In the fast breeder reactor according to the present invention, the neutron absorber portion of at least some of the control rods and the upper part of the core of at least some of the plurality of fuel assemblies surrounding the control rod channels are provided. The positional relationship with the arranged neutron shield region is set so that they do not overlap in the axial direction during the process of withdrawing the control rods for startup and operation of the nuclear reactor. Therefore, appropriate streaming of neutrons is ensured through the upper structure of the control rod absorber section and the coolant layer, and response stability of the neutron detector is ensured.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained below with reference to the drawings.

FIG. 1 is a sectional view showing the positional relationship between the control rods and the neutron shield in the upper part of the reactor core at the end of the operating cycle of a fast breeder reactor according to the first embodiment of the present invention. It is the reactor core. Outside the core 6, an upper shaft blanket 14, a lower shaft blanket 15, and a diameter blanket 16 are arranged.
An upper gas plenum 17 is disposed above the shaft blanket 15, and a lower gas plenum 18 is disposed below the lower shaft blanket 15. A neutron shield 2 is connected to the upper part of the upper gas plenum 17 via an end plug and a coolant flow path 19.
0 is placed, and a radial neutron shield 21 is placed on the outer periphery.

Further, control rods 23 having neutron absorber portions 24 are inserted into the control rod channel 22 from above, and at least some of the control rods 23 are adjacent to the control rod channel 22. Neutron shield 2 on the upper part of at least some of the six fuel assemblies
The 0 region has a short axial length. As a result, in the process of withdrawing the control rod 23 for startup and operation of the reactor, the neutron absorber portion 24 of the control rod 23
and the neutron shield region do not overlap in the axial direction to ensure proper neutron streaming.

That is, six fuel assemblies are arranged around the control rod channel 22, as shown in FIGS. 2 and 3, but in the case of FIG.
Only this fuel assembly is configured with a short axial length in the region of the neutron shield 20, and the other five fuel assemblies 31 are configured with normal fuel assemblies. On the other hand, in the case of FIG. 3, all six fuel assemblies 30 have a short axial length in the region of the neutron shield 20.

In the above configuration, starting the reactor and increasing the output are normally performed by gradually withdrawing the control rods 23. For example, in a 1 million KWe class oxide fuel fast breeder reactor, in order to increase the output from zero output to rated output, a control rod withdrawal operation corresponding to a reactivity of about 1% ΔK is required. The withdrawal amount for this control rod operation is 1 million K, which has 18 operating control rods.
In the case of the We core, approximately 10c at the beginning of the operating cycle.
m, but when the reactor is temporarily stopped and restarted at the end of the operating cycle, it is approximately 20 cm.

FIG. 4 shows the response of the neutron detector in the upper part of the core when the power is increased from zero power to rated power at the beginning of the operation cycle. The response of the neutron detector is highly linear and stable.

FIG. 5 shows the response of the neutron detector in the upper part of the reactor core when the reactor is temporarily stopped and then restarted and the output is increased from zero output to rated output near the end of the operating cycle.

Conventionally, as shown by the broken line in FIG. 5, when the control rods are near the fully withdrawn state, such as near the end of the operating cycle, the control rods are withdrawn for reactor startup and operation. As the neutron absorber part of the control rod and the neutron shield area located at the top of the core of the six fuel assemblies adjacent to the control rod channel overlap, neutrons from the top of the control rod overlap. As the streaming decreases and the control rods approach the fully withdrawn state (rated output), the neutron detection sensitivity of the neutron detector placed at the top of the core decreases slightly, and linearity is sometimes lost.

[0032] The neutron detector must be able to accurately monitor the state of the reactor core from the viewpoint of smooth operation of control rods and safety during startup, normal operation, and shutdown of the reactor. To this end, it is desired that the response of the neutron detector be as simple and stable as possible. Therefore, in the conventional case, there is a problem in this respect.

On the other hand, in the case of this embodiment, even when the reactor is temporarily stopped and then restarted during the operation cycle and the output increases from zero output to the rated output, the output is increased as shown by the solid line in FIG. As a result, the response of the neutron detector is improved, linearity is better maintained, and stability is ensured.

FIG. 6 shows a second embodiment of the present invention, in which the excess reactivity of the core is increased by increasing the plutonium enrichment of the core fuel, and during operation at the end of the operation cycle, the control rods 23 The core is controlled so that the neutron absorber section 24 does not overlap the neutron shield 20 at the top of the six fuel assemblies adjacent to the control rod channel 22.

In other respects, the structure is the same as that of the first embodiment, and the operation is also the same.

Even with this arrangement, the same effects as in the first embodiment can be expected.

FIG. 7 shows a third embodiment of the present invention, in which the length of the upper gas plenum 17 of the fuel pins disposed inside the fuel assembly is increased, the core 6 is disposed downward, and the control Even when the rod 23 is completely withdrawn, the neutron absorber section 24 of the control rod 23 and the neutron shield 20 in the upper part of the core do not overlap.

In other respects, the structure is the same as that of the first embodiment, and the operation is also the same.

Even with this configuration, the same effects as in the first embodiment can be expected.

FIG. 8 shows a fourth embodiment of the present invention, in which the neutron absorber portions 24 of at least some of the control rods 23 and the neutron absorbers 24 of the 63 fuel assemblies adjacent to the control rod channels 22 are The neutron shield 20 above at least some of the fuel assemblies is removed.

In other respects, the structure is the same as that of the first embodiment, and the operation is also the same.

Even with this configuration, the same effects as in the first embodiment can be expected.

FIG. 9 shows a fifth embodiment of the present invention, in which six fuel assemblies 3 adjacent to the control rod channel 22
A part of the upper neutron shield region of No. 2 is removed, for example, in the shape shown with diagonal lines in FIG.

In other respects, the structure is the same as that of the first embodiment, and the operation is also the same.

[0045] Even with this configuration, it is possible to ensure appropriate neutron streaming as in the first embodiment.

[0046]

As explained above, in the present invention, the neutron absorber portion of the control rod and the neutron shield region in the upper part of the reactor core are connected to each other during the process of withdrawing the control rod for startup and operation of the reactor. Since the positions are set so that they do not overlap in the axial direction, appropriate neutron streaming is always ensured, and the stability of the response of the neutron detector can be improved.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing the positional relationship between a control rod and a neutron shield in the upper part of the reactor core at the end of the operation cycle of a fast breeder reactor according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a case where only the neutron shield area of one of the six fuel assemblies around the control rod channel is changed.

FIG. 3 is an explanatory diagram showing a case where all neutron shield areas of six fuel assemblies around a control rod channel are changed.

FIG. 4 is a graph showing the response of a neutron detector when the output increases from zero output to rated output at the beginning of the operation cycle.

FIG. 5 is a graph comparing the response of a neutron detector with a conventional one when the reactor is temporarily stopped and then restarted and the output is increased from zero output to rated output near the end of the operating cycle.

FIG. 6 is a diagram corresponding to FIG. 1 showing a second embodiment of the present invention.

FIG. 7 is a diagram corresponding to FIG. 1 showing a third embodiment of the present invention.

FIG. 8 is a diagram corresponding to FIG. 1 showing a fourth embodiment of the present invention.

FIG. 9 is a diagram corresponding to FIG. 3 showing a fifth embodiment of the present invention.

FIG. 10 is a schematic cross-sectional view showing an example of a fast breeder reactor.

FIG. 11 is a cross-sectional view showing the positional relationship between the control rods and the neutron shield at the top of the reactor core at the end of the operating cycle of a conventional fast breeder reactor.

FIG. 12 is an explanatory diagram showing an example of a core arrangement of a fast breeder reactor.

[Explanation of symbols]

6 Reactor core 11 Neutron detector 20 Neutron shield 22 Control rod channel 23 Control rod 24 Neutron absorber section

Claims (1)

[Claims] Claim 1: A high-speed reactor having a structure in which a control rod having a neutron absorber portion is inserted from the upper part of the reactor core, a neutron shield is arranged in the upper part of the reactor core, and a neutron detector is arranged in the upper part of the neutron shield. In a breeder reactor, a neutron absorber section of at least some of the control rods and a neutron shielding region disposed above the core of at least some of the fuel assemblies of a plurality of fuel assemblies surrounding a control rod channel are provided. A fast breeder reactor, characterized in that the control rods are set in a positional relationship such that they do not overlap in the axial direction during the process of withdrawing control rods for starting and operating the reactor.