JP2004093141A - Nuclear reactor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nuclear reactor which eliminates the need for activities for monitoring the reactor in order to control the reactivity and requires no installations for controlling reactivity very minutely and responding to abnormalities in a reactivity controlling system. <P>SOLUTION: In-cassette cores 24 constituting fuel assemblies are provided in seven nuclear steam supply cassettes 21 laid out radially. In the in-cassette cores 24, the ratio of the quantity of loaded burnable poison contained in a core 24d in a peripheral cassette is higher than that in a core 24c in a central cassette so that the timing which makes it possible to keep the nuclear fission chain reaction in the core 24c in the central cassette and the core 24d in the peripheral cassette separately can vary. A control rod 32 is used as an installation in an emergency shutdown system of the reactor 20 and is kept fully pulled out upward during the normal operation of the reactor 20. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a structure of a nuclear reactor, and more particularly to a structure of a nuclear reactor that does not require operations for controlling the core reactivity.
[0002]
[Prior art]
In a conventional nuclear reactor, in order to suppress the excess reactivity of the core and maintain the effective multiplication factor of the core at 1.0, insertion and withdrawal of a control rod including a neutron absorber, or neutron absorber in the primary coolant Reactivity control operations such as changing the neutron absorbing material concentration by dissolving
Such operation for controlling the reactivity of the core is one of the most important technologies for safely operating the reactor, and at present, automated operation by computer control has been achieved.
[0003]
[Problems to be solved by the invention]
However, while such automated operation of the reactor is being performed, multiple operators with expert skills and knowledge constantly monitor the operating state of the reactor 24 hours a day in case of a failure of the reactor equipment. In addition, it is necessary to manually operate the reactor if necessary. For this reason, it is necessary to provide equipment that can be manually operated and also to train operators for manual operation.
Reactor monitoring activities by operators for such reactivity control account for a major proportion of the costs required to operate the reactor.
[0004]
On the other hand, the reactivity change during normal operation is very small, and in order to constantly detect the change and perform a delicate reactivity control operation, equipment having a highly reliable and precise structure and function is required. Further, in order to make the reactor highly reliable, it is necessary to assume that the reactivity input or loss occurs due to the abnormality of the reactivity control system. In such a situation, in order to safely stop the reactor, it is necessary to multiplex the facilities independently, for example, by providing a plurality of control rods for controlling the reactivity independently. Introducing equipment for such sophisticated reactivity control and abnormality handling of the reactivity control system accounts for a major proportion of the reactor construction cost.
[0005]
The present invention has been made to solve such a problem, and does not require a reactor monitoring operation for reactivity control, and has equipment for precise reactivity control and abnormality handling of the reactivity control system. An object is to provide an unnecessary nuclear reactor.
[0006]
[Means for Solving the Problems]
A nuclear reactor according to the present invention includes a sub-core having a plurality of divided cores, and at least two of the sub-cores have different amounts of burnable poisons.
Further, the plurality of sub-cores are divided into one central sub-core and a plurality of peripheral sub-cores radially arranged around the central sub-core, and the amount of burnable poison contained in the central sub-core is: The amount of burnable poison contained in the peripheral sub-core can be reduced.
A control rod is provided between the sub cores, and the control rod can be used in a withdrawn state during normal operation.
The sub-core can be constituted by a nuclear steam supply cassette including a pressure tube, a core made of a fuel assembly, and a steam generator inside the pressure tube.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a vertical sectional view of a nuclear reactor 20 according to an embodiment of the present invention, and FIG.
As shown in FIG. 2, seven nuclear steam supply cassettes 21 are radially arranged from the center in a moderator tank 31 filled with a moderator 30 of light water, heavy water, or graphite.
As shown in FIG. 1, each of the nuclear steam supply cassettes 21 includes a pressure tube 22 and steam generators 23 arranged circumferentially from the center of the inner wall to an upper part thereof. Is provided below the core in a cassette constituting a fuel assembly.
The pressure pipe 22 is an elongated cylindrical pipe having a diameter of about 50 cm and a length of several meters, for example, having closed upper and lower ends. The pressure pipe liner 25 is made of a metal material such as stainless steel. And a reinforcing fiber layer 26 reinforced with strength. Between the pressure tube 22 and the core 24 in the cassette, a downcomer portion 27 in which a primary coolant 28 sealed in the pressure tube 22 flows downward is formed. Above the core 24 in the cassette, a riser 29 is formed in which the primary coolant 28 heated by the core 24 in the cassette rises, and the primary coolant 28 flows through the pressure pipe 22 as shown by an arrow. It is configured to circulate.
[0008]
As shown in FIG. 2, among the cores 24 in the cassettes of the nuclear steam supply cassette 21 arranged radially, the core 24 in the cassette at the center constitutes a core 24c in the central cassette, and the circumference of the core 24c is formed around the core 24c. The core 24 in the cassette constitutes a core 24d in the peripheral cassette, and the reactor 20 as a whole constitutes a core in a package in which a plurality of the cores 24 in the cassette are combined.
That is, in the nuclear reactor 20, the core of the entire nuclear reactor is divided and arranged in a plurality of nuclear steam supply cassettes 21, the core 24c in the central cassette is a central sub-core, and the core 24d in the peripheral cassette is a peripheral sub-core. With a multi-sub core structure.
[0009]
Here, the core 24c in the center cassette contains a fissionable fuel such as uranium and plutonium having a loading amount c1 and a burnable poison such as boron having a loading amount c2. Further, the core 24d in the peripheral cassette contains a fissile fuel having a loading amount d1 and a burnable poison having a loading amount d2. The ratio of the amount of burnable poison loaded between the core 24c in the center cassette and the core 24d in the peripheral cassette has a relationship of c2 <d2, and the amount d2 of burnable poison in the core 24d in the peripheral cassette is larger than that of the center cassette. The burnable poisoning amount c2 of the inner core 24c is configured to be larger than the amount c2.
[0010]
As shown in FIG. 2, six control rods 32 are arranged on the circumference of the nuclear steam supply cassette 21 at the center so as to cover the core 24c in the center cassette. The control rod 32 is composed of three plate-like members extending between the adjacent nuclear power steam supply cassettes 21, and extends vertically along the core 24 in the cassette as shown in FIG. Is connected to a control rod drive unit 33 located at The control rod driving device 33 moves the control rod 32 up and down.
Here, the control rods 32 in the reactor 20 are used as emergency stop system equipment for the reactor. During the normal operation of the reactor 20, the reactor 20 is in a fully drawn state in which it is drawn upward. At the time of reactor scram when an abnormality occurs in the reactor 20, or at the time of reactor shutdown, the control rod 32 is inserted to the lower side so that the core in the package can always maintain subcriticality.
[0011]
A heat insulating material 34 is wound around the outer peripheral surface of the moderator tank 31, and a water shielding tank 35 is provided outside the heat insulating material 34. The water shielding tank 35 is filled with shielding water 36, which shields neutrons and gamma rays emitted from the cores 24 in the respective cassettes from the outside.
[0012]
Before the start of the nuclear reactor 20, the liquid level of the primary coolant 28 stored in the pressure pipe 22 is at the position H1, and the steam generator 23 is in a submerged state. Is formed with a gas phase portion 28a. Further, as a supply / discharge path of the secondary coolant 43 of the steam generator 23, a secondary coolant pipe 44 provided with an on-off valve 41 in the middle is arranged, and connected to a turbine and a pump (not shown). Further, a pipe 39 is arranged through the upper part 21 b of the nuclear-vapor-steam supply cassette 21 and is connected to a pressure reducing valve 40 in a storage container 42.
[0013]
On the other hand, a pipe 37 penetrates the bottom portion 21 a of the nuclear steam supply cassette 21 and penetrates the bottom surface of the moderator tank 31 to be connected to the emergency cooling water supply valve 38 provided in the water shielding tank 35. Have been.
When the gradual heating function of the nuclear steam supply cassette 21 is lost, such as when the heat exchange in the steam generator 23 cannot be performed, the shield water 36 in the water shield tank 35 is transferred from the emergency cooling water supply valve 38 to the nuclear steam supply cassette 21. The core 24 in the cassette is cooled, and the steam generated in the nuclear steam supply cassette 21 is depressurized by the pressure reducing valve 40 and discharged to the outside of the nuclear steam supply cassette 21.
An air passage 45 for cooling the storage container 42 with air is formed on the outer periphery of the storage container 42.
[0014]
Next, the operation of the nuclear reactor according to the embodiment of the present invention will be described.
As shown in FIG. 1, before starting the reactor 20, the control rod 32 is inserted by the control rod driving device 33 below the moderator tank 31 so as to cover the core 24 c in the central cassette from the periphery, and the core in the package is controlled. Is in a subcritical state. Thereafter, the operation of the nuclear reactor 20 is started, the control rod 32 is pulled upward, and combustion is started when the control rod 32 is fully drawn.
FIG. 3 shows a graph of the change over time of the reactivity accompanying the combustion of the core 24c in the central cassette, the core 24d in the peripheral cassette, and the core in the package obtained by combining the core 24c and the core 24d in the peripheral cassette after the start of the combustion.
At the beginning of the combustion, the fission chain reaction is maintained only in the core 24c in the center cassette, and the reactivity of the core 24 in the center cassette is positive. However, since the amount of combustible poisons c2 in the core 24c in the center cassette is small and neutrons are small, neutrons generated in the core 24c in the center cassette are supplied to the core 24d in the peripheral cassette.
[0015]
The core 24d in the peripheral cassette has a large amount c2 of the burnable poison, so the reactivity of the core 24d in the peripheral cassette is negative. However, as the neutrons are supplied, the burnable poison reacts with the neutrons and disappears. Go. As the amount of burnable poison c2 decreases, the reactivity of the core 24d in the peripheral cassette increases, so that the fission chain reaction can be maintained by the core 24d in the peripheral cassette alone. Further, as the combustion proceeds, the reactivity of the core 24c in the central cassette alone decreases, but the fission chain reaction in the core 24d in the peripheral cassette is maintained.
[0016]
Next, looking at the change over time in the reactivity of the entire core in the package, it can be seen that even if the calorific value of the entire core in the package is fixed, it is maintained at around 0% and the reactivity can be controlled only by self-control. In range.
Here, the self-controllability means the Doppler effect that causes a negative reactivity change when the reactivity increases and the temperature of the core in the package furnace increases, or the self-controllability decreases when the reactivity increases and the temperature of the moderator 30 increases. The moderator density coefficient effect that causes a negative reactivity change due to a change in the density of No. 30 enables the reactivity to be maintained near 0% reactivity without exceeding the allowable range, and is also referred to as a reactivity feedback effect.
As described above, when the timing of the fission chain reaction of the core 24 in the cassette is shifted to maintain the reactivity within a range not exceeding the allowable range, the heating value of the entire core in the package does not greatly change and the self-control inherent in the core does not greatly change. The reactivity control can be performed only by the reactivity, and the reactivity control by inserting and extracting the control rod and the reactivity control by changing the neutron absorbing material concentration in the primary coolant become unnecessary.
[0017]
FIG. 4 shows an example of the evaluation of the change over time of the excess reactivity accompanying the combustion of the nuclear reactor according to the present embodiment, in a case where the calorific value of the entire core in the package is fixed.
In the plurality of nuclear steam supply cassettes 21 used for this evaluation, the core 24c in the center cassette contains 10 wt% of enriched uranium corresponding to about 860 fuel rods as the loading amount c1, and the loading of burnable poisons. The quantity c2 is 0 and no burnable poison is loaded. The core 24d in the peripheral cassette contains 10 wt% of uranium enriched as a loading d1 corresponding to about 770 fuel rods (about 90% of the loading c1). Further, the core 24d in the peripheral cassette contains natural boron corresponding to about 90 burnable poison sticks as the burnable poison with the loading amount d2.
[0018]
As shown by the solid line in FIG. 4, the entire combustion of the reactor 20 configured as described above, even when the high average burnup of 70 GWd / t is achieved, is obtained by the maximum excess reaction of the core in the package. The degree is about 5%, and the surplus reactivity change rate is 0.1% / (GWd / t). Therefore, these values are about 1/5 or less of the core of a normal nuclear reactor indicated by a broken line, and are very small.
In this way, by making the amount of burnable poison loaded between the core 24c in the central cassette and the core 24d in the peripheral cassette different, the timing in which each core 24 in each cassette can independently maintain the fission chain reaction is shifted. The surplus reactivity of the core in the package can be maintained at almost 0% only by the inherent self-controllability.
[0019]
As described above, for example, the amount of burnable poisons in each cassette core 24 is adjusted, and for several years from the start of operation of the reactor 20, the fission chain reaction is maintained alone in the central cassette core 24c, For several years, the nuclear reactor 20 can be operated for a long time without refueling by maintaining the fission chain reaction alone for the plurality of cores 24d in the peripheral cassette.
Further, since the divided sub-cores can be provided in the nuclear steam supply cassette 21, it is easy to make the amount of burnable poison loaded for each sub-core different. Further, a desired power generation amount can be obtained according to the number of the nuclear steam supply cassettes 21.
[0020]
The present invention described above is not limited to the above, and can be implemented with appropriate modifications.
For example, in the above-described embodiment, a plurality of nuclear steam supply cassettes 21 are used to divide the entire reactor into a plurality of cores. However, the present invention is not limited to this, and one core is provided in the reactor vessel. In a conventional nuclear reactor having only a single core, the core may be divided into a plurality of parts, and the amount of burnable poison contained in the divided core may be different. That is, such a multi-sub core structure is applicable to a conventional nuclear reactor.
Also, as shown in FIG. 1, the liquid level of the primary coolant 28 filled in the nuclear steam supply cassette 21 was at the position H1, but the liquid level was changed to the position H3, All the generators 23 may be exposed. In this case, heat exchange with the secondary coolant 43 is performed by the primary coolant 28 that has become the boiling steam. That is, as in the conventional boiling water type light water reactor, by utilizing the coolant boiling in the nuclear steam supply cassette 21, the reactivity control only by the self-controllability inherent to the core is further facilitated.
[0021]
【The invention's effect】
As described above, according to the present invention, the core of the entire nuclear reactor is divided into a plurality of sub-cores, and at least the amount of burnable poison contained in the two sub-cores is different. By shifting the time at which a single fission chain reaction can be maintained, the reactivity of the entire reactor can be maintained within an allowable range that can be controlled only by self-control. Therefore, it is possible to obviate the need for reactor monitoring work for reactivity control, precise reactivity control, and equipment for responding to abnormalities in the reactivity control system.
[Brief description of the drawings]
FIG. 1 is an elevational sectional view showing a structure of a nuclear reactor according to an embodiment of the present invention.
FIG. 2 is a plan sectional view of the nuclear reactor of FIG.
FIG. 3 is a graph showing a concept of a change in reactivity of a reactor core accompanying combustion of a nuclear reactor according to the embodiment.
FIG. 4 is a graph showing an evaluation example of a change in excess reactivity of a reactor core accompanying combustion of a nuclear reactor according to the embodiment.
[Explanation of symbols]
Reference numeral 20: package type reactor, 21: nuclear steam supply cassette, 22: pressure tube, 23: steam generator, 24: core in cassette, 24c: core in central cassette, 24d: core in peripheral cassette, 32: control rod.

Claims (4)

A sub-core with a core divided into a plurality
At least two nuclear reactors having different amounts of burnable poisons contained in the sub-cores. The plurality of sub-cores are divided into one central sub-core and a plurality of peripheral sub-cores radially arranged around the central sub-core,
The reactor according to claim 1, wherein the amount of burnable poison contained in the central sub-core is smaller than the amount of burnable poison contained in the peripheral sub-core. A control rod is provided between the sub cores,
The reactor according to claim 1, wherein the control rod is used in a drawn state during normal operation. The nuclear reactor according to any one of claims 1 to 3, wherein the sub-core comprises a nuclear reactor steam supply cassette including a pressure tube, a core made of a fuel body, and a steam generator inside the pressure tube. .

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