JPS61225690A - Stop device for nuclear reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a nuclear reactor shutdown device that quickly performs output control and startup/shutdown of a nuclear reactor, particularly emergency shutdown.

[Technical Background of the Invention II] Generally, the output control, starting and stopping of a nuclear reactor tl1m is performed by inserting or withdrawing a control rod into the reactor core.

At that time, if the coolant flow rate drops abnormally or the coolant temperature rises abnormally, the control rods are inserted securely into the reactor core and the reactor is brought to an emergency shutdown (hereinafter referred to as scram).
One operation is performed.

Such Scrum operations were generally performed as follows.

In other words, a coolant flow rate detector and a coolant temperature detector are installed inside the reactor core to detect abnormal changes in the coolant flow rate or coolant temperature, and based on the detection signals, mechanically and electrically control the control rods. was urgently inserted into the reactor core.

[Problems with the prior art] According to the above configuration, the reactor measurement system including the coolant flow rate detector and the coolant temperature detector, the electrical system that outputs the scram signal to the control rod drive mechanism, or the machine that performs the scram operation. There was a problem in that if a failure occurred in any of the systems, scram operations would not be performed.

[Objective of the Invention] The present invention has been made based on the above points, and its purpose is to ensure that the reactor is operated reliably even if a failure occurs in the reactor measurement system, electrical system, mechanical system, etc. An object of the present invention is to provide a nuclear reactor shutdown device that can perform an emergency shutdown and greatly improve reliability and safety.

[Summary of the Invention] That is, the nuclear reactor shutdown device according to the present invention includes a guide pipe that is arranged parallel to a fuel assembly installed in a reactor core and through which coolant flows in an upward flow, and a guide pipe that can be freely raised and lowered within this guide pipe. A control rod is housed in the control rod and has a handling head on the top, a control rod drive mechanism is located above the control rod and controls the position of the control rod, and a control rod drive mechanism is located between the control rod drive mechanism and the handling head. a latch mechanism located within the guide tube and below the control rod that rises by receiving a floating blade from the coolant to maintain the connection between the latch mechanism and the handling head; A float body that descends as the coolant 5iE1 decreases and releases the connection between the handling head and the latch mechanism, and a float body that is provided on the float body and operates to lower the float body when the coolant flow rate falls below a predetermined amount. The invention is characterized in that it is equipped with a valve mechanism for promoting.

In other words, a float body that descends as the coolant flow rate decreases is installed, and under normal conditions, the connection between the control rod and the control rod drive mechanism is maintained by the top of the float body via the latch mechanism and handling head, and the coolant flow rate is reduced. If it drops abnormally,
The float body descends to release the connection between the handling head and the latch mechanism, allowing emergency insertion of the control rod into the reactor core, and at this time, the valve mechanism opens to accelerate the descending speed of the float body. This allows the reactor to be brought to a more rapid emergency shutdown.

[Embodiment of the Invention] An embodiment of the present invention will be described by taking a fast breeder reactor as an example with reference to FIGS. 1 to 9. Reference numeral 1 in FIG. 1 is a core support plate that supports the reactor core from below in a reactor vessel (not shown), and a guide pipe 2 is disposed on this core support plate 1 via an entrance nozzle 19. This guide 1!2
A control rod 3 is housed inside. This control rod 3 is the third
As shown in the above, it has a so-called zero-handle structure, and a plurality of neutron absorption bins 4 are housed therein. The neutron absorption bin 4 is composed of a cladding tube 5 and an 84G beret 6 housed within the cladding tube 5.

These plurality of neutron absorption bins 4 are housed in the protection tube 8 with their upper and lower ends supported by a pair of support plates 7A and 7B. Further, a center rod 9 is disposed through the center of the control rod 3.

A handling head 10 is formed at the upper end of the control rod 3 . The control rod 3 is attached to this handling head 10
It is connected to the control rod drive mechanism side by a latch mechanism 11 of the control rod drive mechanism (not shown) located above it. That is, the reference numeral 13 in the figure is a latch finger,
This latch finger 13 is formed at the tip of an outer extension tube 14 extending from the control rod drive mechanism, and expands like a spring. The latch mechanism 11 has an inner extension 1 extending from the latch finger 13 and the control rod drive mechanism.
! A latch rod 16 is provided at the tip of the latch rod 15 so as to be movable up and down.

The latch mechanism 11 is shown in FIG. An axially long slit 16A is provided at the upper end of the latch rod 16.
is formed 1, and a bottle 17 protruding from the inner extension tube is engaged with this slit 16A. That is, the latch rod 16 can move up and down within the range of this slit 16A. In addition, the latch rod 16 has a large diameter portion 16
The large diameter portion 16B allows the latch fingers 13 to be expanded from the inside and engaged with the handling head 10.

A heat-sensitive deformable metal 21 of the shape is attached (shown enlarged in FIG. 8). The heat-sensitive deformable metal 21 is a metal that has a higher coefficient of thermal expansion than the metal constituting the inner extension tube 15, for example, austenitic stainless steel, such as duralumin. Note that when a material having a lower coefficient of thermal expansion than the above-mentioned austenitic stainless steel (for example, 3341, Inconel, etc.) is used as the material of the structure, austenitic stainless steel can be used as the heat-sensitive deformable metal.

For example, when the coolant temperature rises abnormally, the heat-sensitive deformable metal fi 21 expands downward (upward expansion is regulated by the stepped portion 15A of the inner extension tube 15), causing the latch rod 16 to expand. forcibly lowering the latch finger 13 and the handling head 10, and
A control rod 3 is formed in the reactor core to effectively flow the coolant in the direction of the heat-sensitive deformable metal 21, thereby improving the responsiveness of the heat-sensitive deformable metal 21.

Below the control rod 3, a float body 31 is installed integrally with the center rod 9. When the coolant flow is normal,
The float body 31 is floated by the differential pressure between its upper and lower surfaces (
(The pressure on the bottom side is much larger than the pressure on the top side)
, biases the latch rod 16 upwardly via the center rod 9, thereby maintaining the connection between the latch finger 13 and the handling head 10. On the other hand, if the coolant flow mountain decreases abnormally, the float body 31
Since the pressure difference between the upper and lower surfaces of the float body 31 is lowered, the latch finger 13 is disconnected from the handling head 10, and the control rod 3 is urgently inserted into the reactor core.

This will be explained in detail below. As shown in FIG. 4, the float body 31 is provided with coolant passages 32 and valve mechanisms 33 alternately formed at equal intervals in the circumferential direction. As shown in FIG. 5, the valve mechanism 33 includes a flow sensing coolant flow path 34, a seat portion 35 formed within the flow path 34, and a valve mechanism 33 accommodated within the flow path 34. It consists of a spherical body 36 and a spring 37 that biases the spherical body 36 downward (that is, in the direction of opening the valve). That is, during normal operation, the sphere 36 is urged upward due to the pressure difference between the upper and lower surfaces of the float body 31, and as a result, the sphere 36 seats on the seat 35 from below against the urging force of the spring 37, and the valve is closed. are doing. On the other hand, if the coolant flow rate abnormally decreases and falls below a preset amount, the pressure difference between the upper and lower surfaces of the float body 31 becomes small, and the float body 31 descends. At the same time, the sphere 36 is lowered by the biasing force of the spring 37.

As a result, the descending speed of the float body 31 is accelerated, and a more rapid scram is achieved. Furthermore, the floating body 31 and the guide i! 1
A seal ring 41 is installed between the two.

The lower end of the float body 31 is a cylindrical dash ram 51 with a gentle taper, while a bottomed cylindrical dash bot 52 is formed in the i section of the guide tube 1. The inner diameter of this dash bot 52 is slightly larger than the outer diameter of the dash ram 51. In other words, during a scram, when the control rod 3 is inserted in an emergency, the float body 31
will also descend together. At this time, the dash ram 51 and the dash bot 52 absorb the impact of the fall.

Float body 31 on the inner peripheral surface of the guide tube 1 during normal operation
The inner diameter of the portion below the position is rounded. Reference numeral 53 in the figure indicates the stepped portion.

With this configuration, as shown in FIG. 6, when the float body 31 descends, a coolant flow path is secured between the float body 31 and the guide tube 1, and as a result, fluid resistance is reduced. ,
Faster - Scrum operations are possible. Reference numeral 54 in the figure is a guide tube handling head formed at the upper end of the guide tube 1, and the guide 1!1 is connected to a guide tube handling machine (not shown) via the guide tube handling head 54.

The operation will be explained based on the above configuration. First, normal operation will be explained. During normal operation, the temperature and flow rate of the coolant are both within predetermined ranges. Therefore, the float body 31 is floated upward due to the pressure difference between its upper and lower surfaces, and urges the latch rod 16 upward via the center rod 9.

Therefore, the connection between the handling head 10 and the latch finger 13 on the control rod 3 side is maintained, and the control rod 3
and the control rod drive mechanism are connected, and the control rod 3
are inserted into a predetermined position within the reactor core (or withdrawn in some cases) by a control rod drive mechanism. Further, at this time, the valve mechanism 33 is closed. That is, due to the pressure difference between the upper and lower surfaces of the float body 31, the sphere 36 is floated upward against the biasing force of the spring 37, and as a result, the sphere 36 floats upward against the seat 3.
5 from below, the valve mechanism 33 is closed. At this time, the coolant rises through the coolant flow path 32 of the float body 31 and further flows into the control rod 3 through the hole 55 formed in the lower part of the control rod 3.

There, the neutron absorption bin 4, which is generating heat, is cooled down and flows into the reactor through the hole 56 formed in the upper part of the control rod 3.

Next, a normal scrum operation using a conventional configuration will be explained. In this case, first, when the coolant flow rate drops abnormally or the coolant temperature rises abnormally, each detector detects this and sends it to the control rods via the reactor measurement system, electrical system, and mechanical system. Outputs a scram operation signal to the drive mechanism. In response to the scram operation signal, the control rod drive mechanism lowers only the inner extension tube 15 while leaving the outer extension tube 14 as it is. The latch rod 16 is lowered by the lowering of the inner extension tube 15, and the expansion of the latch fingers 13 by the large diameter portion 16B of the latch rod 16 is released. As a result, the connection between the launch finger 13 and the handling head 10 is released, and the control rod 3 falls into the furnace due to its own weight. This causes a scram to occur and the reactor to be brought to an emergency shutdown. The shock absorbing effect of the dashram 51 and the dashbot 52 when the control rod 3 falls is as described above.

Next, a case will be described in which a situation such as a failure occurs in the above-mentioned nuclear reactor measurement system or the like and the scram operation is not performed by the normal mechanism. That is, for example, when the coolant flow rate decreases abnormally, the pressure difference between the upper and lower surfaces of the float body 31 becomes small, and as a result, the float body 31 descends.

At the same time, the valve mechanism 33 opens. In other words, the float body 3
When the pressure difference between the upper and lower surfaces of the ball 1 becomes smaller and becomes less than a predetermined value, the spherical body 36 is urged downward by the urging force of the spring 37. As a result, the valve mechanism 33 is opened, and the coolant flows upward through the coolant flow path 34.

This further reduces the differential pressure, reduces fluid resistance, and accelerates the descending speed of the float body 31. As the float body 31 descends, the latch rod 16 also descends as shown in FIG. 7, the engagement between the latch finger 13 and the handling head 10 is released, and the control rod 3 descends into the furnace due to its own weight. In addition, since the lower part of the guide tube 1 is expanded, a gap is formed between the float body 31 and the inner peripheral surface of the guide tube 1, and a coolant flow path is also formed here, which also causes fluid resistance. is reduced and the scrum speed is increased.

Next, a case where the coolant temperature rises abnormally will be explained. In this case as well, it is assumed that no scram operation is performed by the normal mechanism. When the temperature of the coolant rises abnormally, the heat-sensitive deformable metal 21 attached to the inner extension tube 15 of the control drive mechanism expands downward at a higher expansion rate than other structures. This downward expansion of the heat-sensitive deformable metal 21 forces the latch rod 16 downward as shown in FIGS. 8 and 9. This lowering of the latch rod 16 releases the engagement between the latch finger 13 and the handling head 10 as described above, and! The II m rod 3 falls into the furnace under its own weight, and a scram operation is performed. At this time, since a window 22 is formed in the outer extension tube 14 at the position of the heat-sensitive deformable metal 21, the coolant can easily flow into the vicinity of the heat-sensitive deformable metal 1121. A quick scrum action is taken.

Next, the reset operation after the scram operation will be explained.

First, at the end of the scram operation, the control rod 3 is located at the inner bottom of the guide tube 1. In order to raise the control rod 3, a control rod drive mechanism is used to raise the outer extension tube 14 and the inner extension tube 1.
5 together, and lower the handling head 10 until the tip of the latch rod 16 comes into contact with the upper end of the center rod 9.
Insert inside. Next, the inner extension tube 15 is left as it is, and only the outer extension tube 14 is further lowered. As a result, the latch finger 13 is expanded by the large diameter portion 16B of the latch rod 16. In this state, the inner extension tube 15 is raised until the bottle 17 engages with the upper end of the slit 16A. Next, the inner extension tube 15, the latch rod 16 and the outer extension tube 14
and rise at the same time. Thereby the latch finger 1
3 engages with the handling head 10 and the control rod 3 is raised. When the control rod 3 is located at the upper end of the guide tube 1, the control rod drive mechanism is stopped. At this time, since the coolant flow rate has returned to the normal flow rate, the float body 31 is raised due to the pressure difference between the upper and lower surfaces, and urges the latch rod 16 upward. In this state, insert the inner extension tube 15 into the slit 1.
Tori Sera 1-0 is completed by lowering it by the length of 6A.

According to this embodiment, the following effects can be achieved. In other words, if the coolant flow rate drops abnormally or the coolant temperature rises abnormally, scram operation can be performed instantly without going through the existing reactor measurement system, electrical system, or mechanical system. As a result, the safety of nuclear reactors can be significantly improved.

Furthermore, even if some kind of failure occurs in the reactor measurement system, electrical system, or mechanical system, scram operation can be performed reliably.

Note that it is of course possible to use existing equipment to adjust the reactivity during normal operation, or to stop the system by generating a scram signal during an operation test.

Next, a second embodiment will be described with reference to FIGS. 10 and 11. In this second embodiment, in addition to the configuration of the first embodiment, a fluid element 61 is installed at the lower part of the guide tube 1. The configuration of this fluid element 61 will be explained below. Reference numeral 62 in the figure is a supply nozzle section of the fluid element 61, and this supply nozzle section 62 communicates with the entrance nozzle 19 installed at the bottom of the guide tube 1. A first output section 64 and a second output section 65 are formed above the supply nozzle section 62 in the drawing, respectively.

The first output section 64 is a flow path for circulating the coolant above the guide tube 1, whereas the second output section 65 is a flow path for returning the coolant into the furnace below the guide tube 1. A flow path adjustment chamber 6 is provided between the first output section 64 and the supply nozzle exterior 62.
6 is formed.

Further, a main flow section 67 is continuously formed above the first output section 64 .

According to the above configuration, first, during normal operation, sufficient flowing coolant is ensured, so the coolant flowing out from the entrance nozzle 19 is jetted out from the supply nozzle part 62 at a high flow velocity, and as a result, the coolant on the exit side becomes static. The pressure decreases, and the static pressure within the flow rate adjustment chamber 66 also decreases. Therefore, the coolant ejected from the supply nozzle portion 62 is sucked by the negative pressure in the flow path adjustment chamber 66, the direction of the flow path is tilted, and the coolant flows into the first output portion 64.

Then, it flows into the lower side of the float body 31 via the main flow section 67. Therefore, the float body 31 is floated upward by the same action as in normal times as described above, and the control rod 3 is held at a predetermined position.

Next, a case where the coolant flow rate decreases for some reason will be explained. In this case, the flow velocity of the coolant jetted from the supply nozzle section 62 decreases, and as a result, the static pressure on the outlet side increases. Therefore, the influence of the negative pressure of the flow path adjustment v66 is reduced, and as a result, the coolant flows into the second output part 65 and flows into the furnace through the opening 68 of the guide tube 1, as shown by the broken line arrow in the figure. leaks to. Therefore, the flow rate of the coolant flowing in the direction of the float body 31 is reduced. As a result, the downward movement of the float body 31 shown in the first embodiment is further promoted.
Scrum, action is made more quickly.

In other words, this second embodiment uses a two-stage configuration to detect a reduction in waste, and as described above, it is possible to improve reliability and safety by allowing a scram operation to be performed more quickly.

Next, a third embodiment will be described with reference to FIGS. 12 and 13. In this third embodiment, an outlet 11 joint 81 is installed at the coolant outlet 56 at the upper part of the control rod 3.
This outflow l regulator 81 is made of bimetal. The upper surface of the outflow amount regulator 81 is fixed to a protrusion 56A projecting from the edge of the outflow port 56.

According to the above configuration, during normal operation, the outflow m regulator 81 is in a state as shown in FIG. 12, so that a coolant flow path is secured. On the other hand, if the temperature of the coolant rises abnormally, the bimetal 81 is deformed as shown in FIG.

Thus, the coolant flow area is reduced, resulting in a significant reduction in the coolant flow layer. Such a decrease in the coolant flow rate is equivalent to a decrease in the coolant flow rate flowing out into the guide tube 1 through the entrance nozzle 19, so that the float body 31 descends due to the decrease in differential pressure. Thereafter, a scram is performed by the same operation as described in the first embodiment. In other words, by converting the abnormal temperature rise of the coolant into a decrease in the coolant flow rate,
A scram operation is performed via the float body 31.

According to this embodiment, an abnormal rise in coolant temperature is caused by a heat-sensitive deformation mechanism such as the heat-sensitive deformable metal 21 or the bimetal 71! Since it is detected in a two-stage configuration with the regulator 81,
It is possible to reliably detect an abnormal rise in coolant temperature and perform a scram, thereby further improving reliability.

Next, a fourth embodiment will be described with reference to FIGS. 14 and 15. In this fourth embodiment, a thermosensitive expansion member 92 is housed in a bellows 91 as the outflow 1 m1 bar 81 in the third embodiment. In this case, due to the abnormal rise in coolant temperature, the heat-sensitive expansion body 2 expands as shown in FIG. 15, and the outflow port 56 is pinched, so that the same effect as in the third embodiment can be achieved.

Next, a fifth embodiment will be described with reference to FIGS. 16 to 18. FIG. 16 shows the fuel assembly 10 adjacent to the control rod 3.
1, the guide tube 1 on the control rod 3 side and the trumpet tube 102 on the fuel assembly 101 side are in contact at three places: an upper spacer pad 1o3, an intermediate spacer pad 104, and a lower spacer pad 105. are doing. This improved spacer pad 103 and intermediate spacer pad 10
Holes 106 and 107 are formed in each of the holes 106 and 107, respectively, as shown in FIG.

According to the above configuration, under normal conditions, as shown in FIG. 17, the hole 106 formed in the trumpet tube 102 and the hole 107 formed on the guide tube 1 side are axially shifted from each other, so that the coolant can flow. Never.

On the other hand, if the coolant temperature rises abnormally, the trumpet tube 102 expands and the holes 106 and 107 are brought into alignment as shown in FIG. At this time, since the pressure of the coolant in the trumpet tube 102 is higher than the pressure of the coolant flowing in the guide tube 1, the coolant flows from the trumpet tube 102 side into the guide tube 1. Due to the flow of the coolant, the pressure on the upper surface of the float body 31 increases, the pressure difference between the upper and lower surfaces of the float body 31 decreases, and the float body 31 descends. This results in a scrum operation.

Next, a sixth embodiment will be described with reference to FIGS. 19 and 20. In this sixth embodiment, the hole 107 on the guide tube 1 side of the fifth embodiment has a larger diameter, and a bimetal 11 is placed inside the hole 107.
1 was installed.

Therefore, when the coolant temperature rises abnormally, the bimetal 111 deforms and a coolant flow path is formed as shown in FIG. Thereafter, by the same action as in the fifth embodiment, the float body 3
1 falls and a scram operation is performed.

Therefore, the same effects as in the fifth embodiment can be achieved, and whereas in the case of the fifth embodiment, the accuracy of the holes 106 and 107 is a problem, in the case of the present embodiment, There is no such problem, and reliability is further improved.

Next, a seventh embodiment will be described with reference to FIG. 21. This is because the hole 106 of the tube 102 is filled with a low melting point metal (such as aluminum) 121, and when the coolant temperature reaches a dangerous temperature (about 600°C), the low melting point metal 121
21 is melted and a coolant flow path is secured. Therefore, the same effects as in the fifth and sixth embodiments can be achieved.

FIG. 22 shows an eighth embodiment, in which a blind 1131 made of a low melting point metal is installed by brazing inside the hole 107 on the guide tube 1 side (numeral 132 in the figure indicates the brazing part). ), when the coolant temperature reaches a dangerous temperature, the blind lid 131 melts and the coolant flow path is secured, and the same effects as in the seventh embodiment can be achieved.

By appropriately selecting the second to eighth embodiments described above and incorporating them into the first embodiment, the reliability of the reactor shutdown system can be greatly improved, and the abnormal flow rate of coolant can be reduced. Alternatively, it is possible to reliably detect an abnormal rise in coolant temperature and perform a desired scram operation.

Although the first to eighth embodiments are explained using a fast breeder reactor as an example, the present invention is not limited to this, and can be similarly applied to a structure in which control rods are inserted and used from above. , the same effects as in the first and second embodiments can be achieved.

[Effects of the Invention] As detailed above, according to the nuclear reactor shutdown device according to the present invention, even if some kind of failure occurs in the existing scram operation mechanism, the scram operation can be performed reliably, and the The operation is quick, and the reliability of the reactor shutdown device can be greatly improved, and the safety of the reactor can be improved.

[Brief explanation of drawings]

1 to 9 are diagrams showing a first embodiment of the present invention,
Figure 1 is a sectional view showing the state of the reactor shutdown system during normal operation, Figure 2 is a diagram showing the latch mechanism, and Figure 3 is the
-■ Cross-sectional view, Figure 4 is the IV-IV cross-sectional view of Figure 1, and Figure 5 is
6 and 6 are cross-sectional views showing the operating state of the float body, and FIG.
The figure is a cross-sectional view showing the entire reactor shutdown device when the float body is activated, and Figure 8 is a cross-sectional view showing the state when the heat-sensitive deformable metal is expanded.
The ninth factor is a cross-sectional view showing the entire reactor shutdown device when heat-sensitive deformed metal expands, FIGS. 10 and 11 are views showing the second embodiment, and FIG. 10 is a cross-sectional view of the reactor shutdown device. FIG. 11 is a sectional view showing the configuration of the fluidic element in detail, FIGS. 12 and 13 are sectional views showing the third embodiment, and FIGS. 14 and 15 are sectional views showing the fourth embodiment. , FIGS. 16 to 18 are diagrams showing the fifth embodiment, and FIG. 16 is a sectional view of the adjacent ilJ tit 1m and the vicinity of the fuel assembly,
FIGS. 17 and 18 are cross-sectional views of the butt part, FIGS. 19 and 20 are cross-sectional views of the butt part showing the sixth embodiment, and FIG. 21 is a cross-sectional view of the butt part showing the seventh embodiment. 22 are cross-sectional views of the butt portion showing the eighth embodiment. DESCRIPTION OF SYMBOLS 1... Core support plate, 2... Guide tube, 3... Control rod, 9... Center rond, 10... Handling head, 13... Latch finger, 16... Latch rod, 51... Float body, 33... Valve mechanism. Applicant's agent Patent attorney Takehiko Suzue No. 5!21 Figure 6 Figure 12 @ Figure 13 Figure 14 < I! 15va@17vIj
Figure 18 Figure 19 fjja Figure 20 Figure 21 Figure 22

Claims (2)

[Claims] (1) A guide tube that is placed parallel to the fuel assembly installed in the reactor core and through which the coolant flows in an upward flow, and a guide tube that is housed in the guide tube so as to be able to rise and fall freely, and is equipped with a handling head at the top. a control rod; a control rod drive mechanism located above the control rod for controlling the position of the control rod; a latch mechanism located between the control rod drive mechanism and the handling head for attaching and detaching the two; It is installed in the guide tube and below the control rod, and rises by receiving the floating blade from the coolant to maintain the connection between the latch mechanism and the handling head, and descends as the coolant flow rate decreases to connect the handling head. The float body is characterized by comprising a float body that releases the connection between the float body and the latch mechanism, and a valve mechanism that is provided on the float body and operates when the coolant flow rate falls below a predetermined amount to promote the descent of the float body. Nuclear reactor shutdown equipment. (2) The valve mechanism includes a coolant channel formed in a float body and having a seat inside, a spherical body attached to the coolant channel, and a force that urges the spherical body in a direction to separate from the seat. A nuclear reactor according to claim 1, further comprising a spring, wherein when the coolant flow rate falls below a predetermined amount, the biasing force of the spring overcomes the fluid pressure and causes the sphere to separate from the seat. Stop device.