JPH01129193A - Control rod assembly - Google Patents

Abstract

PURPOSE:To reduce the influence of narrowing a gap caused by void swelling by providing a dash ram descending together with a control rod when the lower end of the control rod collide in a dash pot fixed on a lower guide tube. CONSTITUTION:Moderation and cushioning of a control rod 103 at the time of scram is performed by friction resistance of coolant which flows from a gap 107 between fluid floating dash ram 105 and a dash pot 106 to a gap 109 and further up to the gap 107 and by resistance caused by floating force of the fluid floating dash ram 105. After scram is completed, at the time of drawing the control rod 103 the coolant is introduced from a high pressure plenum 112, the fluid floating dash ram 105 is pushed up and returned to a prescribed position by the coolant regulated in proper pressure by a pressure reducing part 110. The positions of the fluid floating dash ram 105 and the dash not 106 are set at the position of low temperature of low neutron irradiation.

Description

[Detailed description of the invention] [Purpose of the invention] (Industrial application field) The present invention relates to a control rod assembly for a fast breeder reactor.

A control rod that avoids the effects of dimensional changes due to void swelling of the dashram installed at the lower end of the control rod and the dashpot installed near the lower end of the lower guide tube, and can exhibit excellent buffering function over a long period of time. Concerning aggregates.

(Prior Art) Generally, a control rod assembly of a fast breeder reactor is configured as shown in FIG. An entrance nozzle 2 is provided at the lower end of the lower guide tube 1 disposed within the JM sub-furnace vessel. This entrance nozzle 2 is inserted into a core support plate (not shown), and prevents the lower guide tube 1 from floating up by hydraulic hold down or the like.

A control rod 3 is inserted into the lower guide tube 1 so as to be vertically movable, and a handling head 4 is provided at the upper end of the control rod 3. An extension tube of a control rod drive mechanism (not shown) is connected to the handling head 4, and the control rod drive mechanism adjusts the degree of insertion of the control rods 3 into the reactor core, thereby controlling the output of the reactor. Note that FIG. 3 shows a state in which the control rod 3 is pulled upward.

A cylindrical dash ram 5 projects from the lower end of the control rod 3, and a bottomed cylindrical dash pot 6 is provided at the lower part of the lower guide tube 1.

When the control rod 3 is inserted into the core and descends, the fourth
As shown in the figure, the dash ram 5 is inserted into the dash pot 6, and in this case, a predetermined gap 7 is formed between the outer peripheral surface of the dash ram 5 and the inner peripheral surface of the dash pot 6. Therefore, when the control rod 3 is inserted during an emergency shutdown of the nuclear reactor (hereinafter referred to as scram), the dash ram 5 enters the dashpot 6 and the coolant in the dashpot 6 is pushed out from the gap 7. The control rod 3 is decelerated and buffered by the flow resistance at that time. Fig. 5 shows the insertion characteristics of the control rod 3 during this scram, that is, the scram characteristics. The horizontal axis shows time and the vertical axis shows stroke. The reference numeral B indicates an acceleration section in which the control rod 3 is accelerated downward by gas pressure or the urging force of a spring, and a descending section in which the control rod 3 descends at a constant speed determined by the weight and outer diameter of the control rod 3. Further, symbol C indicates a buffer section where the control rod 3 is decelerated and stopped by the dash ram 5 and the dashpot 6.

The gap 7 between the dash ram 5 and the dash pot 6 is designed to prevent the time required for the control rod 3 to be fully inserted, that is, the scram time, to exceed a predetermined time, and to prevent the impact at the time of stopping from exceeding a predetermined value. The dimensions have been set. Also, lower guide tube 1. The structural members of the control rod 3, the dash ram 5, the dash pot 6, etc. are usually made of austenitic stainless steel, which has excellent corrosion resistance and heat resistance.

In such a configuration, members within the reactor core generally undergo void swelling due to irradiation with fast neutrons, causing their volume to expand. Therefore, the outer diameter of the dash ram 5 and the inner diameter of the dash pot 6 gradually change, and the dimension of the gap 7 between them changes. As a result, the characteristics of the buffer section C of the scram characteristics shown in FIG. Since the dashram 5 is normally used in the withdrawn state, the dashram 5 is located at the center position 8 (third position) of the reactor core.
As shown in the figure), the temperature is high and the amount of neutron irradiation is high due to the fact that dash ram 5 has a larger amount of void swelling than dash pot 6. The amount of diameter increase is greater. As a result, the size of the gap 7 becomes smaller and the time of the buffer section C becomes longer, causing a problem that the scram time becomes longer. In order to solve this problem, it is conceivable to increase the gap 7 between the dashpot 6 and the dash ram 5 in advance in anticipation of the effect of void swelling described above.

In that case, the gap 7 becomes too large in the initial state,
There is a problem in that it is not possible to provide sufficient cushioning during a scrum, and the impact during a scrum becomes large. Another method has been considered to reduce void swell by forming the dash ram 5 by cold working a bar material, but in this case, since the material used is a bar stock, the cold working is applied only to the surface. There is also a problem with reliability against cold working. Furthermore, since it is a bar material, it is difficult for heat to escape, and there is also the problem that the temperature rises due to one heat generation and the void swelling rate of the dash ram 5 becomes high, so this method could not be said to be an appropriate method.

(Problem to be Solved by the Invention) In the conventional case, as described above, there is a problem in that the gap between the dash ram and the dash pot changes (narrows) due to void swelling, and the optimal solution for this problem has been found. The present invention has been made based on this point, and its purpose is to reduce the gap between the dash ram and the dash pot due to void swelling. The object of the present invention is to obtain a control rod assembly that can provide stable scram operation over a long period of time while avoiding problems caused by narrowing.

[Structure of the invention]

(Means for Solving the Problems) That is, the control rod assembly according to the present invention includes a lower guide tube disposed in a reactor vessel, a control rod inserted into the lower guide tube so as to be movable up and down, and the lower guide tube. It is installed at the bottom of the guide tube, and when the control rod is inserted into the reactor core during an emergency shutdown of the reactor, it collides with the lower end of the control rod and then descends at the same time as the control rod.
In a control rod assembly equipped with a dashpot that decelerates and buffers the control rods due to the coolant resistance at that time, the dashram is able to return to the state before insertion when the control rod is pulled out, so that the reactor high pressure is The dash ram is raised by introducing and reducing the pressure of coolant in the plenum, and when the control rods are inserted, the control rods also exhibit a buffering effect.

In addition, the coolant for cooling the control rods is introduced through a low-pressure plenum, which has a flow path on the outer periphery of the dashram.

(Function) As mentioned above, since both the dash ram and the dash pot are installed at the bottom of the lower guide pipe, the amount of void swelling during the service life is small and does not affect scram performance.

In addition, even if there is void swelling at the lower end of the control rod,
The dimensions and shape are such that they do not affect scrum performance.

In the present invention, by providing the dashram and dashpot at the lower part of the lower guide pipe, the adverse effect on scram performance due to void swelling is avoided, and the dashram position return after the scram is completed and the buffering effect during the scram are provided in the reactor high pressure plenum. This is done by inserting a coolant and reducing the pressure.

(Embodiment) An embodiment of the present invention will be described below with reference to FIGS. 1 and 2. Figure 1 shows the dash ram 105 during normal operation. This is a diagram showing the dashpot 106 portion, and the reference numeral 101 in the diagram is a lower guide pipe.

A dashpot 106 is provided at the bottom of the lower guide tube 101 and includes a fluid floating dashram 105.
Furthermore, in the lower part, there is a high-pressure introduction section 111 into which high-pressure coolant flows from a high-pressure plenum 112 in order to float the fluid floating dash ram 105. Entrance nozzle 102 that incorporates a pressure reducing part 110 etc. that adjusts the appropriate pressure of the coolant that has flowed in.
is connected to.

Further, the tip of the entrance nozzle 102 is in contact with the low pressure plenum 113, and the coolant of the low pressure plenum 113 is introduced, and the coolant of the control rod is passed through the control rod coolant flow path 114, which is composed of the dashpot 106 and the lower guide tube 101. It's flowing.

Here, the gap 10f and the gap 109 between the fluid floating dashram 105 and the dashpot 106 are narrowed due to void swelling assumed during the life period, scram time of the control rod (not shown), control rod This is determined by taking into account that the impact force at the time of stopping does not exceed a predetermined value, and also taking into consideration the insertion resistance force due to the fluid floating dash ram 105, etc.

FIG. 2 is a diagram showing only the main parts of the control rod 103 during a scram. The control rod 103 is arranged so that it can move up and down inside the lower guide tube 101, and during the scram, the lower end of the control rod 103 is placed on the upper surface of the fluid floating dashram 105. The control rod 103
It has a structure that causes it to fall downward.

The operation will be explained based on the above configuration.

The deceleration and buffering of the control rod 103 during scram is achieved by the gap 1 between the fluid floating dashram 105 and the dashpot 106.
07 to the gap 109 and then to the gap 107 due to the frictional resistance of the coolant flowing and the resistance by the floating blades of the fluid floating dashram 105. When the control rod 103 is withdrawn after the scram is completed, it is introduced from the high pressure plenum 112 and the pressure reducing part 1
The coolant adjusted to an appropriate pressure in step 10 pushes the fluid floating dash ram 105 upward and returns it to a predetermined position. Regarding the stability of the deceleration buffering effect of the control rod 103 by the fluid floating dashram 105 and the dashpot 106, as described above, the fluid floating dashram 105
Since the dashpot 106 and the dashpot 106 are located at a location where both the temperature and the neutron irradiation amount are low, the amount of voids and sea urchins is sufficiently small, and the gap is set in consideration of the amount of void swelling in the parentheses. It is possible to maintain stability and avoid the adverse effect of the narrowing of the gap 1079 on the deceleration buffering effect.

Regarding the cooling effect of the control rods 103, the coolant of the low pressure plenum 113 is introduced from the lower end of the entrance nozzle 102, passes through the control rod coolant flow path 114, and then cools the control rods 103.
This is done by letting it flow out in the direction of 3.

Furthermore, the dimensions of the lower end of the control rod 103 are smaller than those of the fluid floating dashram 105 and the dashpot 106 to allow sufficient clearance, so even if the lower end of the control rod 103 becomes larger in diameter due to void swelling, it will not affect the scram performance. It does not affect.

Therefore, even if the control rod assembly according to the present invention is used for a long period of time, the scram time can be maintained within a predetermined time, and the buffering effect during scram can also be ensured, thereby maintaining the health of the plant. In addition to improving safety and reliability, it is also possible to extend the life of the control rod assembly and reduce costs.

〔Effect of the invention〕

As detailed above, according to the control rod assembly according to the present invention, the deceleration of the control rods during scram is buffered by the fluid floating dash ram and dash pot installed at the bottom of the lower guide tube, so that the dash ram and the dash pot are Since the influence of narrowing of the gap between the dash ram and dash pot due to the difference in void swelling rate between them can be effectively reduced, stable scram operation can be provided over a long period of time.

[Brief explanation of the drawing]

1 and 2 are views showing one embodiment of the present invention, and FIG. 1 is a sectional view showing the main parts of a control rod assembly during normal operation;
Fig. 2 is a sectional view showing the main parts of the control rod assembly during scram, Figs. 3 to 5 are views showing conventional examples, Fig. 3 is a longitudinal sectional view of the control rod assembly, and Fig. 4 The figure is a sectional view showing the relationship between the dashram and dashpot.
FIG. 5 is a diagram showing scrum characteristics. 101...Lower guide pipe 105...Dash ram 106...Dash pot 107,109...
Gap 110...Pressure reducing part 111...High pressure introduction part 112...High pressure plenum 113...Low pressure plenum 114...Coolant channel 102...Entrance nozzle agent Patent attorney Noriyuki Chika Yudo Ken Daikomaru//3 Figure 1 Figure 2 Figure 3 Figure 4 Straw-7 Figure 5

Claims (2)

[Claims] (1) A lower guide pipe placed inside the reactor vessel, a control rod inserted into the lower guide pipe so as to be able to move up and down, and a control rod installed at the bottom of the lower guide pipe so that the control rod can be moved during an emergency shutdown of the reactor. After colliding with the lower end of the control rod when inserted into the reactor core, it is equipped with a buffer mechanism that decelerates and buffers the control rod using a dash ram that can be lowered simultaneously with the control rod and a dashpot fixed to the lower guide tube. control rod assembly. (2) A patent claim characterized in that the buffer mechanism includes a coolant introduction hole from a high-pressure plenum, a coolant pressure reduction mechanism, and a flow path that receives the coolant from the low-pressure plenum and bypasses it upward. The control rod assembly according to item 1.