JPH06160571A - Reflector control type reactor - Google Patents

Abstract

PURPOSE:To surely apply negative reactivity at the time of a low flow without impairing the control capability of a reflector and secure the safety of a reactor and the healthiness of core fuel by dividing the neutron reflector, and arranging gas expansion containers changed with liquid levels of a coolant between them. CONSTITUTION:A reactivity controlling neutron reflector 9 surrounding a core 2 is divided, and low-flow gas expansion containers 30 are arranged between the divided reflectors 9. The reduction of the control capability caused by the division of the reflector 9 is offset when the length of the reflector 9 is increased. The Na level at the time of the 100% output is adjusted by the sealed gas in advance to set the Na liquid levels in the containers 30 above the top section of the core 2 at the time of operation. The dynamic pressure of a coolant Na is reduced and the Na levels in the containers 30 are reduced at the time of a low flow, a radiation leak from the core 2 is increased, the reactivity P of a reactor is surely reduced, the mis-match of the P/F ratio is reduced, and the healthiness of the fuel of the core 2 and the safety of a reactor are secured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflector-controlled nuclear reactor, and particularly, to increase the reactivity control capability of the neutron reflector without changing the length of the neutron reflector itself, thereby prolonging the reactivity life of the core. The present invention relates to a reflector-controlled nuclear reactor capable of extending the operating period without refueling.

[0002]

2. Description of the Related Art Among reflector control type nuclear reactors, a conventional general structure of a so-called external reflector type nuclear reactor will be described with reference to FIGS. Note that FIG. 11 schematically shows a conventional example only from the right half from the center, FIG. 12 shows a cross section of the nuclear reactor in FIG. 11, FIG. 13 shows the fuel assembly in FIG. 12, and FIG. The relationship between the core and the reflector in is shown enlarged.

That is, as shown in FIG. 11, inside the reactor vessel 1, there is a core 2 located at the central portion.
The neutron shield 3 is arranged at a position surrounding the circumference of each of the, and sodium (hereinafter, referred to as Na)
Is filled with a liquid metal coolant 4 such as.

As shown in FIGS. 12 and 14, the core 2 is composed of, for example, 18 hexagonal fuel assemblies 5, and the central portion of the core 2 is for controlling the reactivity of the core 2 and is in the upper position during operation. 6 for the neutron absorbing rod that is pulled out to
Are arranged and surrounded by the core barrel 7.

A partition wall 8 is arranged outside the core barrel 7 so as to divide the flow path of the coolant 4 at a predetermined distance. The space between the core barrel 7 and the partition wall 8 provides a space for the core 2. Moving region 10 of neutron reflector 9 used for operation of
Are formed.

Here, the coolant 4 flows from the bottom to the top in the inside of the partition wall 8 and enters the core 2 on the way to take away the heat generated by the nuclear fission to raise the temperature. Then, the coolant 4 whose temperature has risen flows into the inside of an intermediate heat exchanger (not shown), exchanges heat with the secondary system Na, and then flows out downward from the intermediate heat exchanger. The cooled coolant 4 after this heat exchange passes through the outside of the partition wall 8 and
It goes around to the lower part of and is introduced into the core 2 again.

The neutron shield 3 is for limiting the neutron irradiation dose of the reactor vessel 1 to a predetermined value or less over the entire life of the plant, and is arranged between the reactor vessel 1 and the partition wall 8. It is composed of a plurality of neutron shielding rods 11.

The structure of the neutron shield 3 includes, in addition to a structure made of stainless steel or the like, a pin accommodating a B ₄ C ceramic containing boron having a large neutron absorption capacity, hafnium, tantalum, or the like. The metal or a compound thereof can be included.

Further, the reactivity control capability of the neutron reflector 9 can be increased by disposing the neutron absorber. Reference numeral 12 is a guard vessel that surrounds the reactor vessel 1.

In the fuel assembly 5, as shown in FIG. 13, for example, a large number of fuel pins 14 are regularly arranged inside a hexagonal trumpet tube 13 made of stainless steel, and the upper part of the trumpet tube 13 is formed. And the neutron shields 15a and 15b are arranged in the lower part.

In FIG. 1, one of the many fuel pins 14 is taken out and shown.
A fuel portion 14a and a plenum portion 14b for containing gas components generated by nuclear fission are provided. Also, fuel pin 14
Is designed to promote the mixing of the coolant 4 by a wire wrap or a grid (not shown) and to be fixedly connected to the trumpet tube 13 at the lower end plug portion.

The fuel assembly 5 is constructed so as to be inserted into and fixed to the core support 17 through an entrance nozzle 16 having a small diameter, and is provided with a coolant inlet 18 and a coolant outlet 19. . In addition, the perforated trumpet tube in which a part of the trumpet tube of the fuel assembly is removed, the partial ductless or the ductless assembly has essentially the same configuration.

Then, in the moving region 10 between the core barrel 7 and the partition wall 8, as shown in FIG.
The neutron reflector 9 is
Suspended and supported at the lower end of the drive rod 20, this drive rod 20
It is configured to extend upward through a shield plug 21 that closes the upper end opening of the reactor vessel 1, and move up and down by a drive device 22 installed on the upper surface of the shield plug 21.

That is, as the drive unit 22 is driven, the drive rod 20 and, by extension, the neutron reflector 9 are moved to the core barrel 7 and the partition wall 8.
It is configured to move vertically in the moving area 10 between and. A cover gas space 23 filled with a cover gas is provided between the liquid surface 4 a of the coolant 4 and the shield plug 21.

As a result, the neutron reflector 9 is driven by the driving device 22.
It has been so arranged that the neutron leakage from the core 2 is adjusted by moving the reactor 2 in the up and down direction via the, and thereby the reactivity of the core 2 is controlled.

Also in the above-described reflector control type reactor, the primary system pump at the time of power loss even when the reflector and the neutron absorbing rod are inoperative, as in the conventional reactor in which the reactivity is controlled by using the control rod. When there is a decrease in the coolant flow rate due to the shutdown of the reactor, there is no damage to the fuel element and there is no damage to the fuel element, and so-called non-scram transient fluctuations (ATWS: AnticipatedT)
It is required that the safety of the reactor is ensured even under the ransient without Scram event.

In designing a nuclear reactor, the ATWS is designed by increasing the half-flow time of the flow rate of the primary system pump, optimizing the linear output of fuel, temperature and feedback reactivity coefficient.
Measures such as increasing resistance to events will be made.

The essential points of these core design ideas are:
A "negative" reactivity is injected by utilizing a physical phenomenon that occurs in the core when the flow rate of the primary system pump decreases, and "negative" reactivity feedback is required in a short time. If turned on, the output P of the reactor can be reduced to the decay heat level in a short time.

As a result, the value of P / F can be substantially reduced in relation to the flow rate half time that characterizes the flow rate F from the rated flow rate to the stoppage of the primary system pump of the nuclear reactor system. .

The P / F ratio is 1.0 during the rated operation, but if, for example, no negative feedback is input due to a flow rate decrease event, or if the slow entry of P compared to the reduction time of F is slow, P / F> 1.0. Therefore, the cladding temperature of the fuel element becomes higher than that at the time of rating, and the possibility of damage to the fuel element increases.

Therefore, it is important to reduce P in the shortest possible time so that P / F <1.0.
For example, it is possible to maintain the soundness of fuel elements and the like even if P / F> 1.0 continues for a limited time. However, it is generally desired to set P / F <1.0 as soon as possible.

The mechanism for lowering the furnace output P is to secure negative feedback. Conventionally, the Doppler effect associated with fuel temperature change, axial expansion / contraction, coolant temperature feedback, and structural material temperature feedback are used. , A negative feedback is introduced so that the control rod absorber is effectively inserted into the core portion more than the rated time by a device for transmitting the temperature change of Na to the drive shaft of the control rod absorber. It has been studied and proposed to introduce negative feedback by devising measures such as promotion of radial displacement of the.

As one of these measures, when the core flow rate is reduced, a gas is pre-filled in the vessel and arranged around the core, and when the Na flow rate is reduced, the Na in the vessel is expanded by the gas. Gas expansion assembly (hereinafter, referred to as GEM) in which negative reactivity is injected by accelerating the leakage of neutrons from the core and resulting in negative reactivity is proposed (for example, reference HEDL-SA-2957 Westinghouse). Hanford, April 9-13, 1
984).

FIG. 15 illustrates the principle of introducing the negative reactivity by the above GEM. FIG. 15 (a) is a conceptual diagram when the primary system coolant pump is operating normally, and the Na level in the GEM installed at the end of the core is held upward from the upper end of the core. On the other hand, FIG. 15 (b) is a conceptual diagram when the flow rate of the coolant is almost stopped by turning off the primary system pump.

That is, there is no dynamic pressure of liquid Na26 and GEM
From the communication port 27 of the liquid Na26 in 24, the liquid N in GEM24
a26 is discharged. This is because it balances with the pressure of the inert gas that has been enclosed. Note that reference numeral 25 indicates a sealed gas.

From the viewpoint of the behavior of neutrons in the core, Na
The effect of lowering the liquid level in the GEM from the upper end of the core to the lower end of the core is to enhance neutron leakage as described above.

This will be described in more detail with reference to FIG. When the radial distribution of neutron flux is viewed as an average value, it decreases from the core to the outside as shown in the upper part of Fig. 16.
This is dominated by the amount of leakage at the core boundary (Fig.
12 upper solid line). Figure 16 shows that there is a void area outside the core.
As shown by the dotted line in the upper row, there are many leaks and the gradient of the neutron flux becomes large.

Therefore, the reason why the position shown in the conceptual diagram of FIG. 15 is selected as the effective installation position of the GEM will be described below. FFTF
The reactor is a small reactor with an output of 400 MWth, but it is a homogeneous core with many control rods (not shown) arranged in the reactor.

FIG. 16 shows the shape of the substitution reactivity of Na and the void region as a function of the radial distance from the core center when the GEM is arranged radially from the core center. It has a positive reactivity at the core center, becomes zero to negative at the outermost periphery of the core, has a maximum value of "negative" at the core boundary, and becomes further reactive at a further distance.

As described above, it is conventionally known that it is most effective to arrange the GEM on the outer side in contact with the core where the "negative maximum effect" of Na void reactivity occurs.

As an effect of installing the GEM, FIG. 17 shows a comparison of furnace output changes by experiments in an FFTF furnace. This measured value is an example of a report in a flow rate decrease event experiment without scrum from the 50% output level (reduction to spontaneous circulation level).

Compared to the case without GEM installation (curve 1 in FIG. 17), the output decreases sufficiently quickly when GEM is installed (curve 2 in FIG. 17), and the negative reactivity input effect. Is larger, the output is stabilized at a level lower than that of the curve 1. It has been proved that the GEM is installed at the outermost position of the core and that the negative reactivity insertion by the GEM is performed rapidly.

FIG. 18 shows a schematic diagram of the operation during the functional verification test of the GEM using the FFTF furnace. In the figure, A is for refueling, B is for 100% outflow zero output, C is for rated operation, and D is for
The normal temperature state of 10% flow rate and the normal temperature state of 0% flow rate are shown in E.

The Na level changes depending on the operating state of the core (output, flow rate, system temperature conditions, etc.), but the key points are the balance between dynamic pressure due to flow rate, GEM gas temperature, and Na temperature. State C is 100% flow rate, 50
Corresponds to the rated operating condition of the% output test (change in output shown in Fig. 17).

[0035]

For a reactor having a reactivity control reflector in a reactor vessel, a control reflector having a space (cavity) above the neutron reflector as shown in FIG. 19 is provided. With the system (for example, Japanese Patent Application No. 4-203896), it is possible to construct a reactor core in which the void reactivity of the coolant during the life is negative for 10 years with no fuel exchange operation.

An example is the core equivalent diameter in the configuration of FIGS. 12 and 13.
83 cm, core fuel active length 4 m, fuel U-Pu-10
% Zr metal fuel was used, the fuel volume ratio was about 45% per assembly (assembly pitch about 16 cm), and the equivalent core diameter was about 83 cm.
Is. In this core example, the conversion ratio is about 0.6. In FIG. 19, reference numeral 28 is a box and 29 is a space.

The Na void reactivity inside the core barrel 7 in FIG. 12 is optimized to be negative during the life as shown in FIG. The reflector is made of 9Cr-1Mo steel, stainless steel, etc. and has an effective length of about 1.5 m. Figure 21 shows 10 years (output about 125MW
Fig. 3 shows an outline of the movement of the reflector during the operation in (th).

In this system, the reflector is slowly raised from the lower part of the core to burn as shown in FIG.
If the reflector is lowered from the rated operating position during the middle of combustion, the fission is reduced due to combustion, so the criticality of the reactor is always reduced (since the conversion ratio of the core is about 0.6, this Points are guaranteed). Of course, it is necessary to secure the amount of descent of the reflector in consideration of the amount of reactivity applied due to the temperature of the reactor system lower than that during operation.

The channel 6 in the center of the core shown in FIG. 12 is provided with a neutron absorbing rod that is pulled out above the core during normal operation. Furnace stopping means is provided.

Since the system has the independent two-system reactor stopping means like the conventional reactor, the safety of the reactor is ensured.

On the other hand, efforts have been made to further improve the reliability of the system by using an electromagnetic pump having no driving unit instead of a mechanical pump having a mechanical driving unit during the life of the plant. This pump has no equivalent to the inertial force of the torque of the mechanical pump in principle, assuming that the power supply is lost, so that the half-flow time is shortened.

Although the P / F ratio is reduced in order to ensure the soundness of the fuel even under the event that the primary system pump is stopped in this way, a means for ensuring a rapid decrease in the output P can be added. desired. Securing this means contributes to the improvement of overall reliability, such as increasing the selection range of the scrum signal processing time and the insertion delay of the above two reactor shutdown mechanisms and utilizing the means that has experience in applying to the plant. Can be something.

One method can be to devise a device such as a GEM assembly whose operation has been proved in the test of the FFTF furnace described in the section of the prior art.

On the other hand, F for controlling the reactivity by the control rod
In a core system such as an FTF reactor, it was effective to install a GEM on the outermost periphery of the core, but in the core of the reflector control system which is the object of the present invention, a gas expansion container at a low flow rate equivalent to GEM is installed. It is not preferable to install it on the outermost periphery of the core, and it cannot be discussed as an extension of the conventional knowledge regarding GEM installation.

The reason for this is, for example, as in the conventional knowledge, GE
Consider arranging several Ms so as to contact the outermost periphery of the core. In this case, it can be understood that the following points are not suitable for a reflector control type reactor.

(1) Na disappeared, inert gas and GEM
When the inside is replaced, the neutrons leaking from the core are returned more to the core due to the reflection effect by the reflector outside the GEM, which has the opposite effect of increasing the reactivity of the reactor. That is, the effective distance between the core and the reflector becomes closer to the void formation in the GEM, the neutron communication becomes good, and a positive reactivity is injected.

(2) The core size tends to increase substantially and the reflector control capability tends to decrease.

Since the positional relationship between the core and the reflector is closely related to the reactivity life, the optimum point is easily collapsed by arranging the GEM in the conventional arrangement adjacent to the core.

Further, in the conventional GEM installation, the level of the Na liquid level in the GEM is set in advance on the assumption that the nuclear reactor is constantly operated at the rated power. In the above-mentioned performance verification test of 100% flow rate and 50% output in the FFTF furnace, as shown in FIG.
It can be seen that the Na liquid surface level is determined assuming a 100% flow rate. This means that the ability to adjust the reactor power is significantly impaired.

Therefore, it is also important that the flow rate can be adjusted in a wide range as an output adjusting means even in a reactor equipped with a gas expansion vessel at a low flow rate.

The present invention has been made in order to solve the above-mentioned problems, and is of a reflector control system capable of reliably supplying a negative reactivity at a low flow rate without impairing the control ability of the reflector. To provide a nuclear reactor.

Further, it is another object of the present invention to provide a reactor of the reflector control system which can adjust the flow rate as a power adjusting means in a wide range and can secure the integrity of the core fuel.

[0053]

A first aspect of the present invention is to arrange a neutron reflector outside a reactor core immersed in a coolant, and move the neutron reflector in a vertical direction to remove neutrons from the core. In a reactor of a reflector control system for controlling the reactivity of the core by adjusting leakage, the neutron reflector is divided, and a gas in which the liquid level of the coolant changes between the divided neutron reflectors. It is characterized in that an inner container is arranged.

A second aspect of the invention is to arrange a neutron reflector outside a reactor core immersed in a coolant, and move the neutron reflector up and down to adjust the leakage of neutrons from the core to adjust the core. In the nuclear reactor of the reflector control system for controlling the reactivity of the above, a gas inclusion container is arranged behind the neutron reflector region.

In a third aspect of the invention, a neutron reflector is arranged outside the core of a nuclear reactor immersed in a coolant, and the neutron reflector is moved in the vertical direction to adjust the leakage of neutrons from the core, In a nuclear reactor of a reflector control system for controlling the reactivity of a core, a gas inclusion container is built in the neutron reflector.

[0056]

[Function] Even if the reflector and the neutron absorbing rod do not move when the flow rate of the primary system decreases, the Na in the gas expansion container is low at a low flow rate.
The reduced level increases neutron leakage from the core. Then, the reactivity of the nuclear reactor is surely lowered, the mismatch of the P / F ratio is reduced, and the soundness of the core fuel is secured.

That is, in addition to the two independent reactor shutdown systems of the neutron absorption rod (corresponding to the backup control rod), the system can be provided with the function as the reactor shutdown system based on the third different operation principle. The safety of can be further improved.

[0058]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of a reflector control type nuclear reactor according to the present invention will be described with reference to FIGS. It should be noted that in the figure, the same parts as those in FIGS. 11 to 15 are designated by the same reference numerals, and the description of the overlapping parts will be omitted. Only the main parts different from the conventional example will be described.

FIG. 1 shows, for example, a first embodiment of the present invention applied to a Na-cooled small fast reactor of an internal reflector type having a core equivalent diameter of about 83 cm and an effective length of about 400 cm. It is made of stainless steel and has a length of about 150 cm and a thickness of 20 cm. The following points are different from the conventional example.

That is, the reactivity control neutron reflector 9 surrounding the reactor core 2 is divided, and a gas expansion container at a low flow rate (hereinafter referred to as a gas expansion container) 30 is provided between the divided neutron reflectors 9.
Are arranged.

Unlike the case described in the GEM example in FIGS. 14 and 15, the gas expansion container 30 does not have to have the same shape and size as the fuel assembly 5 in this embodiment. By dividing the neutron reflector 9, the reactivity control ability will be slightly reduced, but the length of the neutron reflector 9 is set to be smaller than that of the conventional example.
It is compensated by increasing it from 15 cm to 20 cm.

As shown in FIG. 2, the gas expansion container 30 has an upper shield 31 at the upper end and a lower shield 32 at the lower end. Fig. 2 shows the N in the gas expansion container 30 during operation and when the flow rate decreases.
The liquid level a is illustrated in comparison with the fuel assembly 5.
During operation, the Na liquid level is located above the top of the core.
The enclosed gas is adjusted in advance so that the Na liquid level is at this position when the flow rate is 100%. The length of L _{1 in} the figure is that after adjustment. When the flow rate is low, the dynamic pressure of the coolant decreases, and the Na liquid level in the gas expansion container 30 decreases. In some cases, the length of L _{2 in} the drawing need not be the lower end of the core.

In the example of the reactor of the reflector control system having the Na void reactivity characteristic, the effect of introducing the negative reactivity by the gas expansion vessel 30 is particularly necessary, for example, as shown in FIG. It is after 4 years of rated output operation when the frequency is close to zero.

In this case, the Na liquid level is lowered to a position about 2.5 m lower than the upper end of the core, and a great effect is brought about. This is because the Na temperature coefficient at a low flow rate is sufficiently negative even if the gas expansion container 30 is not assisted until the fourth year from the start of operation, so that the output is expected to decrease rapidly.

FIG. 3 shows an example in which the Na liquid level in the gas expansion container 30 is adjusted in advance so that it can be operated at a partial flow rate up to about 70%. Since the dynamic pressure of the coolant is large at a flow rate of 100%, the Na liquid level in the gas expansion container 30 is located at the highest position. How to set the partial output condition (mainly the flow rate condition) as one condition for plant operation is related to the setting of the distance from the upper end of the core to the Na liquid level as shown by the symbol L ₁ in FIG.

In FIG. 3, the reactor can be operated without significantly affecting the leakage of neutrons from the core even when the primary system pump speed is adjusted to about 70% flow rate. In this embodiment, the lengths indicated by the symbols l ₁ and l ₂ in FIG. 3 are set to be substantially the same. In addition,
In the figure, reference numeral 33 is an inert gas (encapsulated), 34 is the Na liquid surface level, and 35 is the cycle end position of the reactivity control reflector.

Next, the operation of the first embodiment will be described. First, a gas expansion vessel 30 is arranged between the divided neutron reflectors 9 in the system composed of the core of FIG. 1 and its operating reactivity control reflector. The effective size of the gas expansion container 30 is assumed to have a cross section of about 1/10 of the area of the neutron reflector 9.

When the liquid level of Na in the gas expansion container 30 is lowered from the upper end to the lower end of the core, the substitution reactivity of Na and the inert gas expanding in the container is about 50 ¢. That is, when the Na liquid level in the gas expansion container 30 changes from the core upper end level to the core lower end level, a negative reactivity of −50 is injected.

In the present embodiment, as shown in FIG. 3, FIG. 4 shows how the Na liquid level in the gas expansion vessel 30 fluctuates so that the partial power operation of the core can be up to 70% output. is there. The horizontal axis represents the time after the reactor flow rate starts to decrease, and the vertical axis represents the Na liquid level in the gas expansion vessel 30.

By adjusting to enable partial load operation,
Liquid level of Na in the gas expansion container during rated operation is about 4 m
When it decreases, the level has decreased to the upper end of the core.
It takes about 30 seconds for the Na liquid level corresponding to the rated power operation in the gas expansion vessel to drop to the upper end of the core.

FIGS. 5 and 6 show that when a power loss event occurs during rated power operation, at that time, the reflector is normally lowered and the neutron absorbing rod (see FIG. 1) is inserted to shut down the reactor. However, we investigated the behavior of the core at the time of scrum failure, which is an extremely low-frequency event that thinks that this large negative reactivity injection will not be performed.

It is said that this nuclear reactor has a primary system pump with a half-flow time of about 10 seconds. This ATWS event is
It is assumed that the Na density coefficient occurs near the end of life, which is the maximum during the life of the core, as shown in FIG.

FIG. 6 shows the components of the reactivity balance. The reactivity indicated by the symbols a, b, c and d is the case where no gas expansion container is used in the reflector region. The net reactivity of 1 is the sum of the above a, b, c and d. FIG. 5 shows changes in core power (P), change in flow rate (F), and output / flow rate ratio (P / F). (Furnace output) 1, (output / flow rate ratio) 1
Indicates that the gas expansion container is not used.

An example of using the gas expansion container will be described in comparison with the above example. Figure 6 shows a net reactivity of 2 for gas expansion vessels
The Na liquid level in 30 begins to drop from the upper end of the core, a negative reactivity is injected, and the output is substantially reduced.

In the present embodiment, the Doppler reactivity fuel slag shaft expansion, Na density reactivity, and core radial expansion feedback amount also slightly change from the above-mentioned symbols a, b, c, d, and the output decreases rapidly. After about 40 seconds, there is a slight shift to "the positive side" (not shown), but a net reactivity of 2 is obtained.

As shown in FIG. 5, after the elapse of about 30 seconds after the liquid level of Na in the gas expansion vessel is lowered from the upper end of the core due to the net reactivity of 2 as shown by the symbol (reactor output) 2, As a result, the reactor output decreases earlier than when the gas expansion vessel is not used, and the output / flow rate ratio decreases. This indicates that it has the effect of suppressing an increase in the maximum cladding tube temperature of the fuel element and greatly reducing the possibility of damage.

In the reflector-controlled nuclear reactor as described above, disposing the gas expansion vessel 30 in the reflector region is as follows.
It can be seen that it serves as the third reactor shutdown system for the ATWS event scrum failure flow down ULOF event. Furthermore, in this case, the coolant flow rate itself serves as an initiator, so that the coolant operates reliably.

Although not shown in FIGS. 1 to 3 in the above-mentioned embodiment, there is a structure in which the core fuel portion and the lower portion of the gas expansion vessel are communicated with each other through a common coolant inlet plenum. Furthermore,
This plenum section is optimally distributed to the high-pressure plenum and low-pressure plenum.

In this embodiment, the Na liquid level in the gas expansion container shown in FIG. 3 was adjusted so that the operation could be continued even at a partial output of about 70%. However, depending on the half flow time of the primary cooling system pump, May need to be rapidly charged with a negative reactivity so as to further reduce the peak value of (output / flow rate ratio) shown in FIG.

At this time, Na indicated by symbol L ₁ in FIG.
It is necessary to optimize the liquid level adjustment. When L ₁ is close to the upper end of the core, the negative reactivity that leads to a substantial decrease in power is injected rapidly.

Therefore, when the primary system pump of the plant is replaced during the life of the reactor, it is significant to adjust the internal Na liquid level of the gas expansion container by the half-flow rate characteristic of the pump. As a means for this, the pressure is adjusted from the outside by a tubular jig during pump replacement and repair through the lower plenum area.

As for the installation position of the gas expansion vessel, it is optimal to install it at the reflector position outside the core as described in FIG. 16, but widen the area between the divided parts of the reactivity control reflector. Causes a decrease in the reactivity control capability, that is, the reactivity life, and there is a trade-off relationship between the reactivity life and the negative reactivity effect that can be input.

Next, a second embodiment of the present invention will be described with reference to FIG. 7, the same parts as those in FIG. 1 are designated by the same reference numerals, and the description of the overlapping parts will be omitted. In the second embodiment, the gas expansion container 30 as a gas enclosing container is arranged outside the neutron reflector 9, and two reactors are arranged side by side inside the neutron shielding rod 11 to shorten the reactivity life. No, it is configured to ensure the required negative reactivity. The reactivity per unit volume is small at a position away from the core 2, but the volume can be increased in this region.

When the gas expansion container 30 is installed outside the neutron reflector 9 as described above, it is expected that the division of the neutron reflector 9 is changed from the conventional example shown in FIG. 14 in order to maximize the effect. To be done.

In the conventional example shown in FIG. 14, the dividing portion is narrowed in order to maximize the neutron reflection effect in consideration of structural restrictions. The neutron reflector 9 can have a shape having a curvature as shown in FIG. 14, a flat plate type, or a combination thereof.

Next, a third embodiment of the present invention will be described with reference to FIG. In the third embodiment, as shown in FIG. 8A, the neutron reflector 9 is a gas expansion container as a gas inclusion container.
The installation area 36 for installing 30 is provided.
36 is also a space for Na. Note that FIG.
Corresponds to FIG. 8B to 8F show examples of various shapes of the installation region 36 provided in the divided neutron reflector 9.

That is, FIG. 8 (b) corresponds to FIG. 8 (a), and the both ends of the neutron reflector 9 are cut out in a triangular shape to form an installation region 36. Contrary to (b), (c) is an inverted triangular installation area 36, (d) is an equilateral triangle and inverted triangle installation area 36, and (e) is a quadrangle provided on both inner ends of the neutron reflector 9. In the installation area 36, (f) is the neutron reflector 9
It is a rectangular installation area 36 provided at both outer ends of. The above examples are optimized by various selections including combinations.

Next, a fourth embodiment of the present invention will be described with reference to FIG. In the fourth embodiment, as shown in FIG. 9 (a), the neutron reflector 9 is provided with a square-shaped built-in gas expansion vessel 37 at both ends thereof. FIG. 9B partially shows the neutron reflector 9 in a perspective view. The built-in gas expansion vessel 37 is in communication with the Na coolant at a lower part or a part thereof. The pressure of the gas to be sealed is adjusted so that the gas does not leak outside even at a low flow rate at the end of the reactivity life.

Next, a fifth embodiment of the present invention will be described with reference to FIG. In the fifth embodiment, the inert gas in the gas expansion vessel 30 in the first embodiment is used as a pressure control valve outside the reactor vessel 1.
The output is finely adjusted without moving the neutron reflector 9 by interlocking with an adjusting device (not shown) through 38. Reference numeral 39 in the drawing denotes a communication hole, which is provided at the lower end of the gas expansion container 30.

Normally, the Na liquid surface level is maintained above the upper end of the core 2, and the Na liquid surface level around the core rises due to the pressure change without using the pressure adjusting valve 38, and the reactivity enters. There is no. The pressure is set so that the Na surface level drops and only negative reactivity is introduced. It can also function as a third furnace shutdown system, like a manual scrum. When returning to the rated output operation, the Na liquid level in the gas expansion container 30 is restored.

Since the neutron leakage from the core is increased due to the decrease in the Na liquid level, the reactivity of the reactor is surely decreased, the P / F ratio mismatch is reduced, and the integrity of the core fuel can be secured. it can.

The embodiments according to the present invention are summarized as follows. (1) A reflector control method that controls the reactivity of the core by adjusting the leakage of neutrons from the core by vertically moving the neutron reflector arranged outside the core of the reactor immersed in the coolant In the nuclear reactor, the neutron reflector is divided, and a gas-containing container in which the coolant liquid level changes is arranged between the divided neutron reflectors.

(2) In the nuclear reactor of (1), a gas inclusion container is arranged behind the neutron reflector region. (3) In the nuclear reactor of (1), divide the neutron reflector and place the gas inclusion container behind the divided neutron reflector.

(4) In (3), the neutron reflector should have a cutout as a cross-sectional shape with the divided portion sandwiched therebetween. (5) In the reactor of (1), provide a gas enclosing container in the neutron reflector.

(6) In (1), a gas enclosing container is arranged in a region where the neutron reflector is divided, and a system is provided in which the enclosed gas region is connected to a gas pressure adjusting device outside the reactor container by a valve. , Close the valve during normal reactor operation and open the valve during partial power adjustment or scram.

(7) In (1) to (6), when the gas volume in the gas-containing container and the coolant liquid level are set, the coolant liquid is generated even if the primary coolant flow rate during partial load operation is reached. Adjust the surface level so that it does not affect the reactivity of the core.

(8) In (7), the amount of inert gas in the gas-containing container is sealed from outside the furnace container through a thin tube using the coolant communication hole of this container. (9) In (7) to (8), the gas filling amount is newly adjusted or the above is adjusted in accordance with the change of the flow rate half time by repairing the reactor pump and the change of the flow rate half characteristic by replacing the pump. Replace container.

[0098]

According to the present invention, the negative reactivity can be surely introduced at a low flow rate without impairing the controllability of the reflector, and the flow rate adjustment as the output adjusting means is performed over a wide range. By doing so, the integrity of the core fuel can be secured.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a first embodiment of a reflector-controlled nuclear reactor according to the present invention.

FIG. 2 is a graph showing the Na in the gas expansion container at a low flow rate in FIG.
The schematic diagram which shows a liquid level.

FIG. 3 is a schematic diagram showing the Na liquid level of the gas expansion container when the operation output is changed in FIG.

[Fig. 4] Fig. 3 shows Na in a gas expansion container at a low flow rate.
The curve figure which shows the fluctuation | variation of a liquid level.

[Fig. 5] Core power in core behavior when scrum fails,
FIG. 6 is a characteristic diagram showing changes in flow rate and changes in output / flow rate ratio.

FIG. 6 is a characteristic diagram showing a reactivity balance when scrum fails.

FIG. 7 is a cross sectional view showing a second embodiment of the present invention.

FIG. 8 (a) is a cross-sectional view showing an essential part of a third embodiment of the present invention, and FIGS. 8 (b) to 8 (f) are views showing the installation area of the gas expansion container provided in the reflector in FIG. The partial cross section figure which shows an example.

FIG. 9A is a cross-sectional view showing an essential part of a fourth embodiment of the present invention, and FIG. 9B is an enlarged perspective view showing a reflector in FIG. 9A.

FIG. 10 is a vertical sectional view schematically showing only the right half of the fifth embodiment of the present invention.

FIG. 11 is a vertical cross-sectional view schematically showing a conventional reflector-controlled nuclear reactor only in the right half.

12 is a cross-sectional view of the nuclear reactor shown in FIG.

13 is a vertical cross-sectional view showing the relationship between the fuel assembly and the core support in FIG.

FIG. 14 is a cross-sectional view for explaining an example in which the reflector is divided and the GEM is arranged in FIG.

15 (a) is a conceptual diagram when the primary system pump is operating normally for explaining the principle of negative reactivity injection by GEM in FIG. 14, and FIG. 15 (b) shows the pump in (a). Conceptual diagram when not in operation.

FIG. 16 is a radial distribution diagram of the liquid Na void reactivity when the GEM assembly is arranged in the radial direction from the center of the furnace.

FIG. 17 is a characteristic diagram showing a change in output at the time of a decrease flow rate without scrum in an FFTF reactor as an effect of installing a GEM.

FIG. 18 is a schematic diagram showing an operation during a GEM verification test in an FFTF furnace.

FIG. 19 is a vertical cross-sectional view showing a nuclear reactor in which a reactivity control reflector is installed in a nuclear reactor vessel.

FIG. 20 is a characteristic diagram showing changes in Na void reactivity with operation.

FIG. 21 is a schematic diagram showing a reflector position during driving.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Reactor vessel, 2 ... Reactor core, 3 ... Neutron shield, 4 ... Coolant, 5 ... Fuel assembly, 6 ... Channel for neutron absorbing rod, 7 ... Core barrel, 8 ... Partition wall, 9 ... Neutron reflector ,Ten
… Movement area, 11… neutron shielding rod, 12… guard vessel,
13 ... Trumpet tube, 14 ... Fuel pin, 14a ... Fuel part, 14b ... Plenum part, 15a ... Upper neutron shield, 15b ... Lower neutron shield, 16 ... Entrance nozzle, 17 ... Core support, 18
... Coolant inlet, 19 ... Coolant outlet, 20 ... Drive rod, 21 ... Shield plug, 22 ... Drive device, 23 ... Cover gas space, 24 ... GE
M, 25 ... Enclosed gas, 26 ... Liquid Na, 27 ... Communication port, 28 ... Box, 29 ... Space, 30 ... Gas expansion container at low flow rate, 31 ... Upper shield, 32 ... Lower shield, 33 ... No Active gas (filled), 34 ...
Na liquid level, 35 ... End of cycle of reactivity control reflector, 36 ... Installation area, 37 ... Built-in gas expansion container, 38 ... Pressure adjusting valve, 39 ... Communication hole.

Claims (3)

[Claims] 1. A reaction of the reactor core, wherein a neutron reflector is arranged outside a reactor core immersed in a coolant, and the neutron reflector is moved in the vertical direction to adjust the leakage of neutrons from the reactor core. In a reactor of a reflector control system for controlling the degree, the neutron reflector is divided, and a gas-containing container in which the liquid level of the coolant is changed is arranged between the divided neutron reflectors. Characteristic reflector controlled nuclear reactor. 2. A neutron reflector is arranged outside the core of a nuclear reactor immersed in a coolant, and the neutron reflector is moved vertically to adjust the leakage of neutrons from the core to adjust the reactivity of the core. In the reactor of the reflector control system that controls
A reflector control type nuclear reactor, wherein a gas-containing container is arranged behind the neutron reflector region. 3. A reaction of the core by arranging a neutron reflector outside the core of a nuclear reactor immersed in a coolant, and moving the neutron reflector up and down to adjust the leakage of neutrons from the core. A reflector control type nuclear reactor for controlling the degree, wherein the neutron reflector has a gas inclusion container built therein.