JPH03100491A - Lower plenum of reactor core - Google Patents

Abstract

PURPOSE:To prevent the in-pile structure from being damaged due to flow vibration by lowering a position of a reactor core shroud support leg from the lower end of a control rod guide tube, and expanding its breadth by that portion. CONSTITUTION:First of all, as for reactor water of a downcomer part 12 between a pressure vessel 4 and a wall of a reactor core shroud 5, a suction flow 9 is pressurized by an internal pump 6, becomes a discharge flow 10 from an opening part of a reactor core shroud support leg 5a and flows into a reactor. Also, a coolant flows as an almost orthogonal flow to the outermost peripheral part of a control rod driving mechanism housing 3, and thereafter, ascends along a control rod guide tube 2, and also, goes into a fuel assembly at the upper part. Subsequently, a position of the support leg 5a is lowered from the lower end of the guide tube 2, and also, the discharge flow from the pump is set to a downward flow, therefore, an edge shape is used for the support leg 5a shape roughly in parallel to the lower specular surface. In such a way, an interval of the support leg 5a is expanded by a portion for lowering the support leg 5a, and by setting an opening area being equal to a conventional one, an average flow velocity of the discharge flow 10 is held roughly uniformly in the same way as heretofore.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an ABWR nuclear reactor core body, and in particular, to a method for preventing internal reactor structures such as control rods and control rod guide tubes from flow vibration. This invention relates to a core shroud support leg structure suitable for.

[Conventional technology]

A conventional nuclear reactor or lower core plenum is shown in Figure 8.
As shown in FIG. 9, the core shroud support leg 5a
is installed at the lower end of the control rod guide tube 2, and as a unique effect of this structure, the pump discharge flow flows into the reactor internals as a cross flow, so that the control rod guide tube 2, which is the reactor internals, , and the flow vibration of the control rod drive mechanism housing 3 and the bending stress at the welded portion at the lowest end of the housing 3 act to a large extent, which may cause concerns about the reliability and safety of the equipment.

Therefore, JP-A-60-168087, JP-A-60-1
As described in No. 86790 and JP-A-61-114188, the leg shape of the core shroud support has a semi-cylindrical shape on the pump side, an inner arc shape on the pump side, and an airfoil shape to improve the pump discharge flow. There are things that point in the direction. The main purpose of the invention is to reduce the flow resistance of the pump discharge flow, and does not focus on reducing the vibration stress caused by the flow vibration of the reactor internal structure. In order to evaluate the vibration stress that actually acts on the reactor internals, it cannot be suppressed even if the cross-sectional shape of the core shroud support leg is changed as described above to lower the flow resistance. That is, in order to reduce the vibration stress that acts as bending stress on the lower end of the control rod drive mechanism housing, focus on the flow direction along the axial direction of the core lower plenum, and the height direction of the core shroud support leg, that is, Changes must be made to the longitudinal cross-sectional shape.

Therefore, all conventional inventions have focused only on reducing the flow resistance of the pump discharge flow, but have overlooked ensuring the safety and reliability of the reactor internals, which are the most important aspects of a nuclear reactor.

[Problem to be solved by the invention]

The above conventional technology deals with the flow vibration when the discharge flow from the internal pump enters from the opening in the lower part of the reactor core shroud and collides with the control rod drive mechanism housing and the control rod guide tube arranged in the reactor. This has not been taken into consideration, and there is a possibility that the equipment in the furnace will be damaged due to vibrations caused by the Karman vortices generated around the above-mentioned cylindrical rod.

SUMMARY OF THE INVENTION An object of the present invention is to provide a lower plenum in a reactor that prevents reactor internals from being damaged by flow vibration.

[Means to solve the problem]

In order to achieve the above object, the present invention lowers the position of the core shroud support leg from the lower end of the control rod guide tube, reduces the width of the support leg accordingly, and secures a sufficient opening area. Furthermore, in order to make the pump discharge flow downward, the support leg was formed into an edge shape almost parallel to the lower mirror surface of the pressure vessel.

[Effect]

When the core shroud support leg moves to the lower position and the leg opening area is sufficiently secured, the pump discharge flow will maintain the same average flow velocity as before at the leg opening. The cross flow does not directly hit the control rod guide tube; that is, it hits the control rod drive mechanism housing as a cross flow from the leg openings with a flow velocity distribution of transverse waves. Therefore, the effect of bending stress due to the fluid force on the guide tube and the lower part of the welding point of the housing, and the static and dynamic vibration stress due to these fluctuating flows are reduced.

Moreover, by making the lower end of the core shroud support leg edge-shaped parallel to the lower mirror of the pressure vessel, the flow direction of the pump discharge flow can be controlled to flow along the lower mirror, and the reactor internal structure can be controlled. Since the length of the point of action of the bending moment acting on the control rod guide tube and the drive mechanism housing can be reduced, vibration stress is further reduced.

This action reduces the flow vibration of reactor internals in the lower reactor plenum, improving reactor safety.

Ensure reliability.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 to 3. Figure 1 shows the structure inside the lower plenum of a new type of boiling water reactor (ABWR).

The ABWR recirculation system uses an internal pump to flow coolant into the reactor equipment instead of the traditional BWR jet pump. First, the overall structure of the ABWR will be omitted, and the internal structure of the lower blemish of the ABWR will be described. FIG. 1 is a partial cross-sectional view of the lower plenum of the present invention, and FIG. 3 is a cross-sectional view taken along the line -■ in FIG. In the structure of the lower plenum section 1, a plurality of control rod guide tubes 2 and control rod drive mechanism housings 3, which are internal reactor structures, are installed along a lower mirror section inside the pressure vessel 4. In addition, the core shroud 5 is arranged so as to circumscribe the outside of the two control rod guide tube bundles, and the pressure vessel 4
Ten or twelve internal pumps 6 are installed at equal intervals between the core shroud 5 and the core shroud 5. In addition,
A reactor internal structure consisting of a control rod guide tube 2 and a control rod drive mechanism housing 3 is supported by a core support 7 within an upper core 8. The lower mirror of the lower part of the pressure vessel 4 and the core shroud 5 are welded together by a core shroud support leg 5a, except for the coolant inflow holes at the front face of the pump and the middle part of the pump.

Next, its operation will be explained. First, the reactor water (coolant) in the downcomer section 12 between the walls of the pressure vessel 4 and the core shroud 5 is pressurized into a suction flow 9 by the internal pump 6, and is discharged from the opening between the core shroud support legs 5a. It becomes a stream 1o and flows into the furnace. The coolant 10 then flows in a cross-flow manner to the outermost periphery of the control rod drive mechanism housing 3, then rises along the control rod guide tube 2, and further enters the upper fuel assembly.

As described above, since the internal pump discharge position in the ABWR is located at a lower position than that of the jet pump in the conventional BWR, the flow in the furnace, that is, the pump discharge flow 10 is increased in speed. Therefore, the control rod guide tube 2,
Static vibrational stress due to these fluid forces or dynamic vibrational stress due to fluctuations in the pump discharge flow acts on the control rod drive mechanism housing 3. In particular, bending moments due to fluid collisions occur under the lower part of the pressure vessel 4. Since this acts on the welds at the base of each control rod drive mechanism housing 3 in the mirror section, there is a concern that a very large stress will be applied.

Therefore, in the present invention, as shown in FIG. 1, the position of the core shroud support leg 5a is lower than the lower end of the conventional control rod guide tube 2, and the discharge flow 10 from the pump is made to flow downward. An edge shape was used for the support leg 5a substantially parallel to the lower mirror surface. However, if the core shroud support leg 5a is lowered with the same width as the conventional one, the opening area of the leg will become smaller and the pump discharge flow will further increase. Therefore, as shown in FIG. By lowering shroud support 5a.

By widening the spacing between the core shroud support legs 5a and making the opening area the same as in the past, the average flow velocity of the pump discharge flow is kept similar to that in the past.

Next, effects specific to the embodiments of the present invention will be explained. FIG. 4 shows an explanatory diagram of the flow direction of the present invention.

In the lower core plenum, the fluid force of the pump discharge flow acts noticeably as an excitation force mainly in the control rod guide tubes 2.
, and the outermost periphery of the reactor internal structure arrangement such as the control rod drive mechanism badge 3. According to the invention, the flow direction 1 when flowing from the opening between the core shroud support legs 5a
0 and the flow velocity distribution 11 become a downward flow velocity distribution along the lower mirror of the pressure vessel 4 as shown in FIGS. 4 and 5.

Therefore, the lowest point of the control rod drive mechanism housing 3
The bending moment M acting on the welded part is given by the following equation.

M=FQ...(1)
Here, if we consider that the fluid force F acts on the position Ω as a concentrated load, the bending stress σ5 at the point ■ is small regardless of the increase in the flow velocity distribution V va a
is small.

On the other hand, in the conventional case, as shown in FIGS. 6 and 7,
Since the fluid force F' of the flow velocity distribution 11 with a -like flow velocity distribution 11 acts as a concentrated load at the position Ω', the bending moment M' acting on the welding part at point ■ is calculated by the following formula % formula % (2) In this case , the flow velocity distribution is almost uniform and V, □press Vmax
Furthermore, the position Ω' where the fluid force acts as a concentrated load is larger than in FIG. 3.

In addition, the bending stress σh also increases.

Therefore, due to the difference in the shape of the core shroud support leg 5a, a comparison of the bending moment M and the bending stress σb at point (3) of the present invention and the conventional example is linear.

MUM...(3)
σbkuσ5 ...(4
) Therefore, the bending stress acting on the root welds of the control rod guide tube 2 and the control rod drive mechanism housing 3 is reduced by the present invention.

Although it is effective to use the above support leg structure only in the middle part of the internal pump to improve the flow velocity distribution,
If the strength of the support leg is permitted, it can also be adopted for both the front part and the middle part of the internal pump.

〔Effect of the invention〕

According to the present invention, the core shroud support leg is lowered lower than the lower end of the conventional control rod guide tube.

By widening the width to keep the opening area constant and making the support edge-shaped, bending stress and flow vibration that act on the control rod guide tube and control rod drive mechanism housing, which are internal reactor structures, are reduced. It is possible to provide a nuclear reactor with high safety and reliability.

[Brief explanation of drawings]

FIG. 1 is a partial sectional view of one embodiment of the present invention, and FIG.
Figure 3 is a cross-sectional view taken along the arrows ``-■'' in the figure, Figure 3 is a view taken along the mm-m arrow in Figure 2, Figure 4 is an explanatory diagram of the flow direction of the present invention, and Figure 5 is the flow velocity distribution and bending shown in Figure 4. Stress relationship diagram, Figure 6 is an explanatory diagram of the conventional flow direction, Figure 7 is a diagram of the relationship between the flow velocity distribution of Figure 6 and bending stress, Figure 8 is a conventional longitudinal cross-sectional view, and Figure 9 is an illustration of the conventional flow direction. Figure IX
-IX arrow sectional view. 1... Lower plenum, 2... Control rod guide tube, 3...
・Control rod drive mechanism housing, 4...pressure vessel, 5.
... Core shroud, 5a...! Core shroud support leg. 6... Internal pump, 7... Core support,
8...Reactor core, 9...Internal pump suction flow,
10...Discharge flow of internal pump, 11...Speed distribution, dream 2 Figure #3 8 formations (Nikuhata Sekishu's Hoguchi Yoshiaki 4 figure, brown figure 5)

Claims (1)

[Claims] 1. In a nuclear reactor in which a core shroud is provided in a reactor pressure vessel and several internal pumps are installed between the reactor pressure vessel and the core shroud, the support for the core shroud is provided. A core lower plenum characterized by ensuring a sufficient opening area between the legs by lowering the position of the legs below the lower end of the control rod guide tube and narrowing the support width accordingly. 2. The lower core plenum according to claim 1, wherein the support legs of the core shroud have an edge shape facing inward.