JPH04102093A - Core internal structure of nuclear reactor - Google Patents

Abstract

PURPOSE:To contrive the improvement of reliability and safety of scram by integrating fuel assembles to open clearances one another, dividing into a plurality of fuel assembly blocks arranged as liquid tightness is held and introducing water capable of water level adjustment to the clearances among these fuel assembly blocks. CONSTITUTION:A reactor core 4 is divided into four fuel assembly blocks 21 by a cruciform control area 22. Thirty nine fuel assemblies are integrated in each block 21 and one control rod is inserted and removed among the four fuel assemblies except for the circumferential part of the reactor core 4. When the emergency shutdown of a nuclear reactor is performed in an output operation state, a valve 27 is opened. At this time water 29 of room temperature is pushed by high pressure gas from an injection tank 28 is rapidly introduced into the control area 22 through the valve 27 and a feed water tube 26. As a result, because steam filled in the control area 22 is cooled by the water 29 of room temperature to condense, the pressure of the control area 22 is lowered rapidly and coolant is quickly allowed to flow from a lower plenum 7.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a nuclear reactor core structure capable of performing a safe scram without using dynamic equipment such as control rods.

(Prior Art) FIG. 7 is a longitudinal sectional view of a pressure vessel 1 of a boiling water nuclear reactor with an electrical output of 520,000 kW. In the center of the pressure vessel 1,
A core 4 supported by a core support plate 3 is provided inside the cylindrical shroud 2, and an upper lattice plate 5 is provided above the core 4.
will be installed.

The reactor coolant (water) descends through the downcomer 6 outside the shroud 2 and reaches the lower plenum 7 below the reactor core 3. The coolant then rises and passes through the core 4, during which time it is vaporized and reaches the steam separator 9 via the upper plenum 8. In the steam/water separator 9, the coolant is separated into steam and water. After the steam is dried in the steam dryer 10, it is sent to a turbine (not shown), while the water is again introduced into the reactor core 4 via the downcomer 6 via the above-mentioned route.

FIG. 8 is a cross-sectional view of the reactor core 4. A large number of fuel assemblies 11 (about 4 m in height) are accommodated in the core 4, which has a circular cross section, and between the four adjacent fuel assemblies 11,
A control rod 12 having a cross-shaped radial cross section is moved in and out of the lower part of the reactor core 4 in the axial direction.

FIG. 9 is an enlarged view of a part of the fuel assembly 11 and control rod 12 shown in FIG. 8. The fuel assembly 11 has fuel rods 14 housed in a matrix in a channel box 13 having a rectangular cross section. It consists of Note that the reference numeral 15 is a water rod that flattens the output of the fuel rod 14 in the radial direction.

On the other hand, in the control rod 12, tubes 18 filled with B4C powder 17 are arranged in a sheath 16 having a cross-shaped cross section.

B (boron) in B4C used in the control rod 12 has a large neutron absorption cross section, so when the control rod 12 is inserted into the reactor core 4, the reactivity of the reactor core 4 is suppressed.

During output operation, several to ten or more control rods 12 are inserted into the reactor core 4 by a hydraulic drive device, and the output of the reactor is adjusted depending on the number and depth of insertion.

In addition, the control rods 12 play another important role in the event that any abnormality occurs within the reactor, by quickly inserting all control rods 12 into the reactor core and bringing the reactor to an emergency shutdown (scram). .

(Problem to be solved by the invention) By the way, the above-mentioned abnormalities in the nuclear reactor include increases in output and pressure changes, but fluctuations in these physical quantities are converted into electrical signals, and if the level of this electrical signal is within an acceptable level. When the level is exceeded, a hydraulic drive system that inserts control rods is activated to perform a scram.

Control rods move in space during a scram, but static equipment that does not move spatially is safer than equipment that moves spatially, such as dynamic equipment such as control rods. It is more reliable as a device, and in recent years there has been a demand for the introduction of such static equipment in nuclear reactors.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a nuclear reactor core structure that can perform more reliable and safe scram without using dynamic equipment such as control rods.

[Structure of the invention]

(Means for Solving the Problems) The present invention has been made to solve the above problems, and the present invention has been made in order to solve the above-mentioned problems. A nuclear reactor core structure is provided which is divided into fuel assembly blocks and introduces water whose level can be adjusted into gaps between these fuel assembly blocks.

(Function) The core structure of the nuclear reactor of the present invention has a structure in which a large number of fuel assemblies housed in the reactor core are grouped into a plurality of blocks and separated from each other, thereby creating a gap (a "control area") between these fuel assembly blocks. call). In order to keep this control area and each fuel assembly block liquid-tight, and to introduce water whose water level can be adjusted into this control area, one fuel assembly block is It is possible to move from one fuel assembly to another to adjust the amount of neutrons that cause nuclear fission, and to adjust the core power and perform scram without using dynamic equipment such as control rods.

(Example) The present invention will be described in detail below with reference to FIGS. 1 to 6.

FIG. 1 is a sectional view of a pressure vessel 20 of a small boiling water reactor with an electrical output of 100,000 kW, which employs the reactor core structure according to the first embodiment of the present invention. This reactor pressure vessel 20
Since the basic structure is not substantially different from that shown in FIG. 7, corresponding parts are given the same reference numerals and detailed explanation will be omitted.

In the reactor pressure vessel 20 of this embodiment, the reactor core 4 is divided into a plurality of fuel assembly blocks 21, and a control region 22 is provided between these fuel assembly blocks 21. Since the control region 22 and each fuel assembly block 21 are kept liquid-tight by the partition plate 23, coolant does not come and go between the two.

On the other hand, the bottom of the control area 22 is open to the lower plenum 7 and allows coolant to pass through, but the bottom of the control area 22 is
A heater 25 connected to a cord 24 extending from outside the pressure vessel 20 is installed. Further, one end of a water supply pipe 26 is inserted into the upper end of the control area 22 through the partition plate 23, and the other end of this water supply pipe 26 is connected to the outside of the pressure vessel 20.
It is connected to a water tank 28 via a valve 27. The water injection tank 28 is filled with water 29 at room temperature and a gas (for example, nitrogen gas) 30 at a pressure higher than that in the pressure vessel 20 .

FIG. 2 is a cross-sectional view of the reactor core 4 in this embodiment.

In this embodiment, the reactor core 4 is divided into four fuel assembly blocks 21 by a cross-shaped control region 22 . The width of the control region 22, ie, the distance between each fuel assembly block 21, is approximately 30c+n. Each fuel assembly block 21
Each has 39 fuel assemblies (approximately 2m in height)11
are grouped together, and one control rod 12 is inserted and removed between the four fuel assemblies except for the periphery of the core 4.

The core structure of the nuclear reactor of this example is operated as follows.

While the reactor is shut down, the control region 22 is filled with coolant and all the control rods 12 are inserted into the reactor core 4. When starting the operation of the nuclear reactor from this state, the valve 27 is closed and electricity is supplied through the cord 24 to operate the heater 25 and heat the coolant in the control area 22. Then, the coolant begins to evaporate within the control region 22, and water and steam become saturated. Then, as the coolant continues to be heated, the pressure due to water vapor increases in the control region 22,
Water vapor now occupies the entire control area 22.

Once this state is reached, it is time to gradually move the control rods 12 into the core 4.
The reactor is brought to pressure operating condition.

To make an emergency stop of the nuclear reactor in a power operating state, the valve 27 is opened. Then, from the water tank 28, water 29 at room temperature is pressed by high pressure gas and flows into the valve 27 and the water supply pipe 26.
and into the control area 22. In this case, the water vapor filling the control area 22 becomes water 29 at room temperature.
Due to cooling and condensation, the pressure in the control region 22 drops rapidly and coolant rapidly enters from the lower plenum 7.

Next, the reason why the reactivity of the core can be adjusted by the amount of water in the control region 22 will be explained.

FIG. 3 shows thermal neutron flux distributions (relative values) observed along directions that are each 45° from two adjacent ends of the cross-shaped control region 22 in FIG. The broken line shows the thermal neutron distribution when the control region 22 is filled with gas, and the solid line shows the thermal neutron distribution when the control region 22 is filled with water.

When the control region 22 is filled with gas, the thermal neutron distribution is largest and flat in the control region 22, and once it enters the fuel assembly block 21, it continues to decrease monotonically toward the outside of the core 4.

The outside of the reactor core 4, that is, the vicinity of the inside of the shroud 2 and the downcomer 7 are filled with coolant and act as reflector regions that decelerate fast neutrons leaking from the fuel assembly block 21 and return them to the fuel assembly block 21. act. Immediately outside the fuel assembly block 21, the effect of this reflector area therefore increases the thermal neutron distribution.

On the other hand, the thermal neutron distribution when the control region 22 is filled with water has one peak at the center of the fuel assembly block 21, and decreases from this peak toward the control region 22 and the reflector region. The reflector region exhibits a pronounced peak just outside the fuel assembly block 21 due to the effect described above.

In addition, in the control region 22 filled with water, a peak occurs near the fuel assembly block 21 based on the same principle as in the reflector region where coolant flows, but the thermal neutron distribution becomes minimum at the center of the reactor core 4. From now on, control area 2
It can be seen that when the control area 22 is filled with water, the plurality of fuel assembly blocks 21 facing each other with the fuel assembly blocks 2 in between are blocked from each other by the control area 22 from transferring thermal neutrons.

As described above, in the divided fuel assembly block 21, there is a gradient in which the thermal neutron flux decreases from the center toward the outside of the core 4, so thermal neutrons leak out along this gradient. go. Although some of the thermal neutrons leaking from the fuel assembly block are pushed back into the fuel assembly block 21, they no longer contribute to the nuclear fission chain reaction within the fuel assembly block 21, so the effective multiplication factor decreases. .

Here, when the control region 22 is filled with water, a gradient of thermal neutrons also occurs in the direction toward the control region 22, so
The effect of lowering the effective multiplication factor is further increased. Note that the effect of such leakage of thermal neutrons is greater as the fuel assembly blocks 21 divided by the control region 22 are smaller.

In this embodiment, when the control region 22 is filled with gas (when the reactivity reducing effect is minimal) and when the control region 22
The difference in the effective multiplication factor of the core was approximately 8%Δ when the core was filled with water (when the reactivity reduction effect was at its maximum).
As a static device that does not use control rods, it has sufficient reactivity control ability even during scram. In this embodiment, the width of the control area is 30 cm, but if it is 20 cm or more, the nuclear reactor shutdown ability is sufficient.

In this embodiment, a control rod 12 is also installed and is used for adjusting output during operation and shutting down the reactor.

When the reactor is shut down, the temperature of the coolant decreases, and there is a risk that the previously decreased effective multiplication factor will increase again and reach criticality. can be suppressed. However, even in this case, there is no need for a mechanism for inserting the control rods 12 into the reactor core 4 at high speed, so the device for inserting the control rods can be much simpler than in the past.

FIG. 4 is a sectional view of a pressure vessel 30 employing a nuclear reactor core structure according to a second embodiment of the present invention.

The reactor is of the same boiling water type as the first embodiment. Since the basic structure of this reactor pressure vessel 30 is not substantially different from that shown in FIG. 1, corresponding parts are given the same reference numerals and detailed explanation will be omitted.

In this embodiment, the bottom of the control area 31 is closed, and a water supply pipe 26 connected to the water tank 28 is connected to the closed bottom.
Pull in. Further, at the top of the control area 31 extending through the upper grid plate 5 and the upper plenum 8, an air supply pipe 34 is provided which is drawn in from a pressurizer 32 outside the pressure vessel 30 via a valve 33.
It passes through the partition plate 23 and is drawn in. Therefore, water 29 from the water tank and gas sent from the pressurizer 26 are introduced into the control area 31 .

In this embodiment, when the reactor is shut down, the water injection tank 2
The water level of the water 29 supplied from the reactor 8 is above the upper grid plate 5, and all the control rods 12 are inserted into the reactor core 4.

When starting the operation of the reactor from this state, open the valve 33 of the pressurizer 32 with the valve 27 of the water injection tank 28 open, and supply gas such as nitrogen from the pressurizer 32 to the air supply pipe 3.
4 to the control area 31. Then the control area 31
Due to the introduced gas, the water level of the water 29 gradually decreases, and eventually the entire control area 3j, 1 is occupied by gas.

Once this state is reached, it is time to gradually move the control rods 12 into the core 4.
The reactor is then pulled out from the reactor to bring it into full power operation.

When making an emergency stop of the nuclear reactor in the power operating state, the valve 27 is fully opened. Then, water 29 at room temperature is rapidly introduced from the water tank 28 into the control area 22 through the valve 27 and the water supply pipe 26 under pressure with high-pressure gas. As a result, the water level of the water 29 in the control area 31 reaches above the upper grid plate 5, which is the level at the time of reactor shutdown. At this time, the valve 33 of the pressurizer 32 may be closed or open.

In this embodiment, even during normal output operation, the pressurizer 3
The water level of the water 29 in the control area 31 can be freely adjusted by adjusting the opening degrees of both the valve 33 of the water injection tank 28 and the valve 27 of the water filling tank 28. Therefore, according to the above-described principle, the effective multiplication factor of the core can be increased by lowering the water level in the control region 31, and the effective multiplication factor of the core can be decreased by raising the water level in the control region 31.

That is, according to this embodiment, the output can be adjusted in place of the control rod even during output operation.

In this case, if the space of the control region 31 is enlarged and the space of the fuel assembly block 21 is divided into small parts, a large reactivity control ability can be obtained, so that it becomes possible to not equip any control rods at all.

FIG. 5 is a cross-sectional view showing a core structure 35 of a nuclear reactor according to a third embodiment of the present invention.

The reactor core structure 35 of this embodiment is applied to a small nuclear reactor with an electric output of 100,000 kW class, and the reactor core 4 is arranged concentrically from the center to a fuel assembly block 36a, a control region 37. It is divided into fuel assembly blocks 36b. That is, by providing two circular fuel assembly blocks 36a and 36b and arranging the control area 37 between them,
The above-described mechanism allows the control region 37 to perform the core power adjustment function and the scram function. Note that the control rod is omitted in FIG. 5.

FIG. 6 is a cross-sectional view showing a core structure 40 of a nuclear reactor according to a fourth embodiment of the present invention.

The core structure 40 of this embodiment is applied to a nuclear reactor with an electric output of 520,000 kW, but in the core structure shown in FIG. 5, the inner and outer fuel assembly blocks are each further shaped into a cross shape. Divided into eight fuel assembly blocks 41
a to 41h are provided, and each fuel assembly block 41a to 41
The area between h is defined as a control region 42. Also in Figure 6,
Control rods are omitted.

When applied to a large-power reactor core like this example, the larger the space of the control region and the smaller the space of each fuel assembly block, the greater the reactivity control ability. As a result, this core structure 40 allows the control region 42 to fully exhibit the core power adjustment function and scram function by the above-described mechanism.

〔Effect of the invention〕

As explained above, the core structure of the nuclear reactor of the present invention controls the reactivity by adjusting the amount of water introduced in the gaps (control areas) between the plurality of fuel assembly blocks that are constructed by dividing the core. Because of the adjustment, a safe and reliable scram can be performed without using dynamic equipment such as control rods.

[Brief explanation of drawings]

FIG. 1 is a vertical cross-sectional view of a pressure vessel including a core structure of a nuclear reactor according to a first embodiment of the present invention, and FIG. 2 is a cross-sectional view of a core structure of a nuclear reactor according to a first embodiment of the present invention. FIG. 3 is a distribution diagram of thermal neutron flux in the reactor core, FIG. 4 is a vertical cross-sectional view of a pressure vessel including the core structure of a nuclear reactor according to a second embodiment of the present invention, and FIGS. 5 and 6 are in accordance with the present invention. FIG. 7 is a vertical cross-sectional view of a conventional pressure vessel, FIG. 8 is a cross-sectional view of a conventional reactor core structure, and FIG. 8th
It is a partially enlarged view of the figure. 4... Core, 11... Fuel assembly, 21... Fuel assembly block, 22... Control area, 23... Partition plate, 26... Water supply pipe, 28... Water injection tank . Applicant's representative Hisashina Reactor Core + Distance from the heart N (crn)

Claims (1)

[Claims] The core of a nuclear reactor is divided into a plurality of fuel assembly blocks, which are arranged so that the fuel assemblies are grouped together and maintained liquid-tight with gaps between them, and the gaps between these fuel assembly blocks are filled with water whose water level can be adjusted. The core structure of a nuclear reactor that introduces