JP3414561B2 - Jet pump - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a jet pump constituting a nuclear reactor recirculation system of a boiling water reactor, and more particularly to a jet pump having an improved beam portion structure for joining a transition piece and an elbow. is there.

[0002]

2. Description of the Related Art A conventional structure will be described with reference to FIGS.

FIG. 8 shows the entire reactor recirculation system. This reactor recirculation system consists of a reactor pressure vessel 1 core 2
A plurality of jet pumps 3 provided in the annular portion around the periphery of the
And a reactor recirculation pump 5 provided outside the reactor pressure vessel 1 via a recirculation pipe 4.

During operation of the reactor, the saturated water separated by the steam-water separator 6 and the steam dryer 7 above the core 1 is a core shroud provided around the reactor wall of the reactor pressure vessel 1 and the core 2. Flowing down through the annular portion between and, and is mixed with the feed water that enters the reactor pressure vessel 1 from the feed water sparger 8. A part of this is guided to the reactor recirculation pump 5 via the recirculation pipe 4 outside the reactor, the pressure is increased, and it is returned to the reactor pressure vessel 1 again and injected at high pressure in the nozzle portion of the jet pump 3. At that time, the cooling water in the reactor is entrained, mixed, discharged from the jet pump 3 to the lower core plenum 9, and then introduced into the fuel assembly of the reactor core 2 and heated (white arrow a in the figure). Indicates the flow of cooling water).

FIG. 9 shows the structure of the jet pump 3 in an enlarged manner. The jet pump 3 rises along the inner surface of the reactor wall 1a of the reactor pressure vessel 1, and riser elbow 10 and riser pipe 11 for raising the pressure water introduced from the reactor recirculation pump 5 side shown in FIG. , A pair of transition pieces 12 symmetrically connected to the upper end of the riser pipe 11, and a nozzle 14 and a mixer 15 sequentially connected to each transition piece 12 via an inverted U-shaped elbow 13. The diffuser 16 is connected to the lower end of the mixer 15.

In this structure, the riser elbow 10, the riser pipe 11, and the transition piece 12 are welded together, and the lower end of the riser elbow 10 is fixed to the reactor pressure vessel by welding. The riser pipe 11 is fixed to the pressure vessel by a riser brace 17 in order to prevent lateral vibration.

Also, the removable elbow 13 and nozzle 1
4 to prevent lateral vibrations of the parts of the mixer 4 and the mixer 15, brackets 18, supports 19 and wedges 20.
Is provided. The bracket 18 is welded and fixed to the riser pipe 11 to surround the mixer 15 in a ring shape, and the wedge 20 is driven into the gap between the bracket 18 and the mixer 15 to prevent lateral vibration. Wedge 20
Support 1 welded to the mixer 15 to prevent it from falling off
It is attached to 9.

By the way, in the jet pump 3, the most important structural and functional points are the elbow 13 and the nozzle 1.
4 and mixer 15. Assuming the possibility that the inner surface of the nozzle where high-temperature high-pressure water flows at high speed is abraded due to aging such as erosion and corrosion during operation and the inner diameter becomes large, the performance deteriorates and the required core flow rate cannot be obtained. It is designed so that it can be removed and replaced.

The elbow 13, the nozzle 14 and the mixer 15 are integrally assembled by welding, but the transition piece 12 and the diffuser 16 are provided.
The connection with is detachably connected by a mechanical method rather than welding so that it can be removed.

FIG. 10 is an enlarged perspective view showing the structure of the joint between the transition piece 12 and the elbow 13, and FIG. 11 is a vertical cross-sectional view of the main part of FIG.
FIG. 12 is a diagram showing an enlarged main part of FIG.

As shown in these figures, the transition piece 12 and the elbow 13 are provided at both ends of a groove 22 provided on the opposite surfaces of a pair of transition piece arms 21 that integrally rise from both sides of the transition piece 12. Is fixed to the upper surface 25 of the groove of the transition piece arm 21 by fixing the beam 23 into which the beam 23 has been inserted and the beam bolt 24 which is screwed downward at a substantially central portion of the beam 23 and pushes down the elbow 13 from the upper side. Upward reaction force is supported by the abutting upper surfaces 26 of both end portions.

That is, as shown in FIGS. 10 and 11,
Each of the transition piece arms 21 is configured such that the opening side of a substantially U-shaped plate body faces downward, and both ends thereof are welded and fixed to the upper end side surface of the transition piece 12,
The pair of elbows 13 are opposed to each other with both sides sandwiched therebetween.
Lateral grooves 22 are formed on both sides of opposite surfaces of each transition piece arm 21.

As shown in FIG. 11, the beam 23 has a substantially trapezoidal shape in which a central portion 27 is thick in the vertical direction and both end portions 28 are thin, and downwardly inclined surfaces 29 extend from the center to both ends. The thin end portions 28 are inserted into the groove 22 of the transition piece arm 21. A beam bolt 24 having a bolt head at the upper end is inserted from above into a mounting hole formed in the central portion of the beam 2 in the vertical direction.
The lower end portion of the is inserted into and abutted against the upper end portion of the elbow 13 via a retainer 31 fixed by a bolt 30 in a depressed state.

The contacting operation is performed by pulling the beam 23 upward (arrow b) with a handling tool (tensioner) (not shown) prepared in advance, and the pulling force is applied to the beam 23. The beam bolt 24 is tightened downward (arrow c).

As a result, the transition piece arm 2
Since the beam 23 inserted in the groove 22 of No. 1 is pushed upward by the reaction force, both ends 28 of this beam 23
The upper surface of the elbow 13 is pressed against the upper surface 25 of the groove portion of the transition piece arm 21, and the elbow 13 is pressed down and fixed by this pressing force.

[0016]

By the way, the beam 23
For the end portion 28, the transition piece arm 21
It is dimensionally restricted because it is necessary to be inserted into the groove 22 in the fitted state. Further, the beam 23 has a discontinuous portion in a shape at a portion connecting the thick central portion 27 and the inclined surface 29. Therefore, the contact portion between the beam 23 and the transition piece arm 21 (a part of the upper surface 26 of both end portions 28 of the beam 23)
Due to the shearing stress and the bending stress due to the load generated at 1, high peak stress occurs at both ends of the transition piece arm 21 and stress corrosion cracking or fatigue fracture may occur.

In this case, regarding the contact surfaces of the beam 23 and the transition piece arm 21, when the load is applied to the beam 23, the surface pressures of the two are evenly distributed.
Beam 2 needs to be machined into an accurate plane
The end portion 2 of the beam 23 in consideration of the mounting variation of
The flat upper surface of 8 needs to be formed longer toward the center side than the length required for contacting the groove portion upper surface 25 of the transition piece arm 21. Therefore, beam 2
Stress concentration is likely to occur in a portion between the inclined surface 29 from the central portion 27 to the end portion 28 and the upper end surface 26 of the end portion of the third portion 3 of the transition piece arm 21 from the inner surface to the specified length l. It is necessary to position it in the middle of the distance.

This means that the distance (L + 1) from the surface pressure acting portion L which is both contact surfaces to the shape discontinuity of the beam 23.
Is increased, which increases the stress due to the bending moment generated in the discontinuous portion, which is a factor of increasing the peak stress.

In order to solve this problem, a means of increasing the thickness of the beam 23 can be considered.
The retainer 31, the elbow 13, and the groove 22 and the beam 23 of the transition piece arm 21 located at the lower part of the
Due to the restrictions on the compatibility with, the increase in size is restricted.
Particularly in the operation of exchanging the beam 23 for an operating plant, it is difficult to replace the surrounding competing equipment, so that the margin of design change is small and it is difficult to significantly reduce the beam stress.

The present invention has been made in view of such circumstances, and a beam for mechanically tightening and fixing the transition piece and the elbow is contacted with both ends of the beam and the upper surface of the groove of the transition piece arm. An object of the present invention is to provide a jet pump capable of significantly reducing the stress generated by contact without substantially changing the shape of peripheral joint parts.

[0021]

In order to achieve the above object, the invention of claim 1 is a jet pump which is provided in a reactor pressure vessel of a boiling water reactor and constitutes a reactor recirculation system. A transition piece fixed to the upper end of the riser pipe that rises along the inner surface of the reactor wall of the reactor pressure vessel and raises the pressure water introduced from the reactor recirculation pump side outside the reactor, and this transition An inverted U-shaped elbow, one end of which is detachably joined to the upper end of the piece, a nozzle and a mixer which are integrally integrated with the other end of the elbow, and a lower end of the mixer, which is detachably attached to the furnace. A diffuser that is fixed and ejects the reactor water introduced by the nozzle downward, and the transition piece and the elbow are integrally raised from both sides of the transition piece. Of the transition piece arm is fixed by a beam having both ends inserted in grooves provided on opposite surfaces of the transition piece arm, and a beam bolt that is screwed downward at a substantially central portion of the beam and pushes down the elbow from above. Is one in which an upward reaction force is supported by the upper surfaces of both ends in contact with the upper surface of the groove portion of the transition piece arm, the beam has a concave recessed portion at the center position on the lower surface facing the upper surface of the elbow. Then, the lower surface can be lowered while avoiding the interference with the elbow, and the overall thickness is increased in the vertical direction, whereby the thickness of both ends of the beam that abuts on the upper surface of the groove of the transition piece arm is increased. Is set to be large.

According to a second aspect of the present invention, there is provided a jet pump which is provided in a reactor pressure vessel of a boiling water reactor and constitutes a reactor recirculation system, the jet pump being provided along an inner surface of a reactor wall of the reactor pressure vessel. Rises and rises, and a transition piece fixed to the upper end of the riser pipe that raises the pressure water introduced from the side of the reactor recirculation pump outside the reactor and an inverted U whose one end is detachably joined to the upper end of this transition piece. The elbow and the nozzle and mixer which are integrated with the other end of the elbow and descend, and the lower end of this mixer is detachably fixed in the furnace and the reactor water introduced by the nozzle is jetted downward. A diffuser, and the transition piece and the elbow are provided on opposite surfaces of a pair of transition piece arms that integrally rise from both sides of the transition piece. A beam inserted at both ends only grooves,
The beam itself is fixed by a beam bolt that is screwed downward in the substantially central portion of the beam and pushes down the elbow from the upper side, and the beam itself has an upward reaction force on the upper surfaces of both ends that abut the upper surface of the groove of the transition piece arm. In which the inclined surface is formed from the center side to both end sides of the upper surface, and the inclined surface lower part of the upper surface of the beam is in contact with the groove part upper surface of the transition piece arm. It is characterized in that a depression having a curved surface lower than the upper surfaces of both ends of the beam is formed at a position near the central portion.

According to a third aspect of the present invention, there is provided a jet pump which is provided in a reactor pressure vessel of a boiling water reactor and constitutes a reactor recirculation system, the jet pump being provided along an inner surface of a reactor wall of the reactor pressure vessel. Rises and rises, and a transition piece fixed to the upper end of the riser pipe that raises the pressure water introduced from the side of the reactor recirculation pump outside the reactor and an inverted U whose one end is detachably joined to the upper end of this transition piece. The elbow and the nozzle and mixer which are integrated with the other end of the elbow and descend, and the lower end of this mixer is detachably fixed in the furnace and the reactor water introduced by the nozzle is jetted downward. A diffuser, and the transition piece and the elbow are provided on opposite surfaces of a pair of transition piece arms that integrally rise from both sides of the transition piece. A beam inserted at both ends only grooves,
The beam itself is fixed by a beam bolt that is screwed downward in the substantially central portion of the beam and pushes down the elbow from the upper side, and the beam itself has an upward reaction force on the upper surfaces of both ends that abut the upper surface of the groove of the transition piece arm. A notch portion for avoiding the generation of an upward reaction force with respect to the most distal end portion of the beam in at least one of the upper surface of the groove portion of the transition piece arm and the upper surfaces of both ends of the beam that abuts against the groove portion. Is formed.

The invention according to claim 4 is the jet pump according to claim 3, wherein the notch is an inclined surface or a step formed on either or both of the transition piece arm and the beam.

According to a fifth aspect of the present invention, in the jet pump according to any one of the first to fourth aspects, the position of the upper surface of the groove portion of the transition piece arm is increased by driving, and both end portions of the beam that abut on the upper surface of the groove portion. The feature is that the upper and lower thickness of the can be expanded.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a jet pump according to the present invention will be described below with reference to FIGS. Since the entire structure of the jet pump is the same as that of the conventional one, the above-mentioned FIGS. 8 to 10 and the description thereof are directly referred to, and the corresponding portions of FIGS. 1 to 7 are described using the same reference numerals. .

FIG. 1 is a sectional view of a main portion showing the first embodiment.

In the jet pump 3 according to this embodiment, the groove 22 of the pair of transition piece arms 21 that integrally rises from both ends of the transition piece 12.
Both ends of the beam 23 are inserted into the upper part of the beam 23, and the elbow 13 is pushed down by the beam bolt 24 screwed into the substantially central part of the beam 23.
Is in contact with the upper surface of the groove portion of the transition piece arm 21 to support the upward reaction force, which is the same as in the conventional case.

In this embodiment, in the present embodiment, the beam 23 as a whole has a trapezoidal shape similar to the conventional one, but differs from the conventional one in the following two points.

The first different point is that the lower surface of the beam 23 is
At the center position facing the upper surface of the elbow 13, a concave recess 3 is formed.
2 has been formed. As shown in FIG. 1, the recess 32 is formed to have a wide concave shape that covers the entire upper surface of the elbow 13. This allows beam 2
The lower surface of 3 is configured so that it can be arranged at a lower position than the conventional one while avoiding interference with the upper surface of the elbow 13, particularly the retainer 31 provided on the upper surface of the central portion.

The second different point is that the lower surface of the beam 23 is lower than that of the conventional beam 23.
That is, the thickness of the both end portions 28 in the vertical direction is increased by the amount that the lower surface is lowered.

According to the jet pump 3 having such a structure, since the thickness of both ends of the beam 23 that abuts on the groove portion upper surface 25 of the transition piece arm 21 can be set to be large, the bending generated at both ends of the beam 23 can be set. It is possible to significantly reduce stress, shear stress and the like.
Moreover, there is no need to make any changes to the engagement relationship with other components such as the retainer 31, and when attaching or detaching the beam 23,
For example, with the beam 23 loosened downward,
Both ends 28 of the groove 2 of the transition piece arm 21
Work such as inserting into 2 and setting it just before tightening can be performed just like the conventional one.

FIG. 2 is a sectional view of the essential parts showing a second embodiment of the present invention.

In the jet pump 3 according to this embodiment, from the position below the inclined surface 29 formed from the center side to both end sides of the upper surface of the beam 23 and in contact with the groove upper surface 25 of the transition piece arm 21. Also at the position near the center, both ends 28 of the beam 23
A curved surface 33 that is lower than the upper surface 26 of the is formed in a dent shape.

The curved surface 33 is an arcuate surface having a radius R, and in comparison with the conventional structure shown in FIG.
The contact point S with 3 is displaced to the end side of the beam 23. Therefore, in the present embodiment shown in FIG. 2, the R center O point also comes closer to the transition piece arm 21 side as compared with the conventional one shown in FIG. As a result, the bending moment generated in the R-shaped portion of the beam 23 due to the pressing load acting from the groove upper surface 25 of the transition piece arm 21 to the upper surfaces 26 of both ends 28 of the beam 23 is the same as that of the conventional bending moment in this embodiment. It becomes possible to reduce it compared with the thing.

For example, if the R dimension is 19 mm and the depth of the recess is 0.5 mm, it is possible to shift R to the outside by about 4 mm, which reduces the bending moment acting on the shape discontinuity. In particular, when the beam 23 deviates from the regular mounting position and the bending moment acts excessively than the regular mounting position, according to the analysis by the finite element method,
It was revealed that the stress at the R portion, which is the peak of the stress, was reduced by 10% or more, and the effect of stress reduction was exhibited.

FIG. 3 is a sectional view of the essential parts showing a third embodiment of the present invention.

In the jet pump 3 according to this embodiment, the upper surface 2 of the groove portion of the transition piece arm 21 is used.
5 and the upper surface 2 of both ends 28 of the beam 23 that abuts on it
6, a notch 34 for avoiding the generation of an upward reaction force on the tip of the beam 23 is formed.

In the present embodiment, the notch 34 is a step 35 located on the bottom (back) side of the groove 22 of the transition piece arm 21 and having a stepped shape cut off the groove upper surface 26.

With such a structure, the range of the surface pressure acting on the end portion 28 of the beam 23 is narrowed toward the center of the beam 23. Therefore, the bending moment acting on the shape discontinuity can be reduced by this, and the generated stress can be reduced.

When such a structure is applied to the transition piece arm 21 of the existing plant, it can be performed by, for example, electric discharge machining.

FIG. 4 is a sectional view of the essential parts showing a fourth embodiment of the present invention.

In this embodiment, similarly to the third embodiment of FIG. 3 described above, the beam 23 is provided between the groove upper surface 25 of the transition piece arm 21 and the upper surfaces 26 of both ends 28 of the beam 23 that abuts against the groove. The notch 34 is formed so as to avoid the generation of an upward reaction force with respect to the most distal end, but the shape of the notch is different from that of the third embodiment.

That is, in this embodiment, the notch 34 is formed by the inclined surface 36 in which the groove upper surface 26 located on the bottom (back) side of the groove of the transition piece arm 21 is gradually raised.

With this structure as well, the range of the surface pressure acting on the end portion 28 of the beam 23 is narrowed toward the center of the beam 23, so that the bending moment acting on the shape discontinuity can be reduced and the stress can be reduced. It is a thing.

5 and 6 are sectional views of the essential parts showing the fifth and sixth embodiments of the present invention.

The embodiment shown in these figures is also the same as the third embodiment.
And similarly to the fourth embodiment, the groove part upper surface 25 of the transition piece arm 21 and the beam 23 that abuts on the groove part upper surface 25.
The notch 34 for avoiding the generation of the upward reaction force to the tip of the beam 23 is formed between the upper surfaces of the both ends 28 of the beam 23, but the notch 34 is not on the groove side but on the beam 23 side. It was formed in.

In the fifth embodiment shown in FIG. 5, the notch 34
However, the upper surface 26 of both ends 28 of the beam 23 is formed by an inclined surface 37 which is gradually lowered on the most distal side.

Further, in the sixth embodiment shown in FIG. 6, the cutout portion 34 is formed by a step portion 38 in which the upper surface 26 of the both end portions 28 of the beam 23 is stepwise cut down on the most distal end side.

According to the fifth and sixth embodiments having such a configuration, the bending moment can be reduced by the same action as in the third and fourth embodiments.

FIG. 7 shows a seventh embodiment of the present invention.

In the jet pump according to this embodiment, the groove upper surface 25 of the transition piece arm 21 is used.
Is increased by being driven by a remote electric discharge machine, and the thickness of both end portions 26 of the beam 23 that abuts on the groove upper surface 25 can be increased. According to the present embodiment, the built-up portion 39 formed by electric discharge machining is formed in the inner part of the groove 22, and the built-up portion 40 is also formed on the upper surface of the beam 23.

According to this structure, the first to sixth
In the jet pump of the embodiment or the jet pump of the existing plant, for example, in the process of fitting the transition piece arm 21 from the state where the beam 23 is loosened, the mating surface on the transition piece side that engages with the beam end is remote-discharge machined. By carrying out the follow-up processing using, it is possible to further increase the plate thickness at the beam end and reduce the stress.

[0054]

As described in detail above, according to the jet pump of the present invention, the shape of peripheral fitting parts need not be substantially changed at the contact portion between the transition piece arm and the beam, and the beam can be operated during operation. It is possible to significantly reduce the stress generated at the time, and thereby it is possible to improve the operational reliability.

[Brief description of drawings]

FIG. 1 is an enlarged view of an essential part showing a first embodiment of a jet pump according to the present invention.

FIG. 2 is an enlarged view of essential parts showing a second embodiment of a jet pump according to the present invention.

FIG. 3 is an enlarged view of an essential part showing a third embodiment of a jet pump according to the present invention.

FIG. 4 is an enlarged view of an essential part showing a fourth embodiment of a jet pump according to the present invention.

FIG. 5 is an enlarged view of an essential part showing a fifth embodiment of the jet pump according to the present invention.

FIG. 6 is an enlarged view of an essential part showing a sixth embodiment of the jet pump according to the present invention.

FIG. 7 is an enlarged view of an essential part showing a seventh embodiment of the jet pump according to the present invention.

FIG. 8 is an overall configuration diagram showing a recirculation system of a boiling water reactor.

9 is an enlarged view of the jet pump shown in FIG.

10 is an enlarged view showing a part of the jet pump shown in FIG. 9. FIG.

FIG. 11 is a diagram showing a connection portion between a transition piece and a beam in a conventional jet pump.

FIG. 12 is an enlarged cross-sectional view showing an engagement portion between the transition piece and the beam shown in FIG. 11.

[Explanation of symbols]

1 Reactor pressure vessel 2 core 3 Jet pump 4 Recirculation piping 5 Reactor recirculation pump 6 steam separator 7 Steam dryer 8 Water supply sparger 9 Lower core plenum 10 Liza Elbow 11 riser tube 12 transition pieces 13 Elbow (reverse U shape) 14 nozzles 15 mixer 16 diffuser 17 riser brace 18 bracket 19 Support 20 wedges 21 transition piece arm 22 groove 23 beams 24 beam bolts 25 Groove top 26 Upper surface of both ends 27 Central part 28 Both ends 29 slope 30 volts 31 retainer 32 recess (concave) 33 curved surface 34 Notch 35 Step 36 slope 37 Inclined surface 38 both ends 39, 40 Overlay

Claims (5)

(57) [Claims] 1. A jet pump which is provided in a reactor pressure vessel of a boiling water reactor and constitutes a reactor recirculation system, which rises along an inner surface of a reactor wall of the reactor pressure vessel,
A transition piece fixed to the upper end of the riser pipe for raising the pressure water introduced from the reactor recirculation pump outside the reactor, and an inverted U-shaped one end of which is detachably joined to the upper end of this transition piece. An elbow and a nozzle and a mixer that are integrated with the other end of the elbow and descend, and a diffuser that is fixed to the lower end of the mixer in a detachable state in the furnace and ejects the reactor water introduced by the nozzle downward. A beam having the transition piece and the elbow, both ends of which are inserted into grooves provided on opposite surfaces of a pair of transition piece arms rising integrally from both side portions of the transition piece, and a substantially central portion of the beam The elbow is fixed by a beam bolt that pushes the elbow downward from above, and the beam itself is fixed by the beam bolt. In the one in which the upper surface of both ends of the piece arm which is in contact with the upper surface of the groove portion is supported with an upward reaction force, the beam has a concave recessed portion at a central position on the lower surface facing the upper surface of the elbow. The overall thickness of the upper and lower parts of the beam is set to be large in the vertical direction so that the lower surface can be lowered while avoiding interference with the beam. A jet pump characterized by that. 2. A jet pump which is provided in a reactor pressure vessel of a boiling water reactor and constitutes a reactor recirculation system, which rises along an inner surface of a reactor wall of the reactor pressure vessel,
A transition piece fixed to the upper end of the riser pipe for raising the pressure water introduced from the reactor recirculation pump outside the reactor, and an inverted U-shaped one end of which is detachably joined to the upper end of this transition piece. An elbow and a nozzle and a mixer that are integrated with the other end of the elbow and descend, and a diffuser that is fixed to the lower end of the mixer in a detachable state in the furnace and ejects the reactor water introduced by the nozzle downward. A beam having the transition piece and the elbow, both ends of which are inserted into grooves provided on opposite surfaces of a pair of transition piece arms that integrally rise from both sides of the transition piece; and a substantially central portion of the beam. The elbow is fixed by a beam bolt that pushes the elbow downward from above, and the beam itself is fixed by the beam bolt. Is intended to be supported upward reaction force in contact with both end portions the top surface to the groove top surface of the piece arm,
And in the one in which the inclined surface is formed from the center side of the upper surface to both end sides, at a position closer to the central portion than the portion in contact with the groove upper surface of the transition piece arm at a lower position of the inclined surface of the upper surface of the beam, A jet pump characterized in that depressions having curved surfaces that are lower than the upper surfaces of both ends of the beam are formed. 3. A jet pump which is provided in a reactor pressure vessel of a boiling water reactor and constitutes a reactor recirculation system, which rises along an inner surface of a reactor wall of the reactor pressure vessel,
A transition piece fixed to the upper end of the riser pipe for raising the pressure water introduced from the reactor recirculation pump outside the reactor, and an inverted U-shaped one end of which is detachably joined to the upper end of this transition piece. An elbow and a nozzle and a mixer that are integrated with the other end of the elbow and descend, and a diffuser that is fixed to the lower end of the mixer in a detachable state in the furnace and ejects the reactor water introduced by the nozzle downward. A beam having the transition piece and the elbow, both ends of which are inserted into grooves provided on opposite surfaces of a pair of transition piece arms that integrally rise from both sides of the transition piece; and a substantially central portion of the beam. The elbow is fixed by a beam bolt that pushes the elbow downward from above, and the beam itself is fixed by the beam bolt. In the one in which an upward reaction force is supported by the upper surfaces of both ends that are in contact with the upper surface of the groove of the piece arm, at least one of the upper surface of the groove of the transition piece arm and the upper surfaces of both ends of the beam that abuts the same A jet pump characterized in that a notch is formed so as to avoid the generation of an upward reaction force against the tip of the beam. 4. The jet pump according to claim 3, wherein the notch is an inclined surface or a step formed on either one or both of the transition piece arm and the beam. 5. The jet pump according to any one of claims 1 to 4, wherein the position of the upper surface of the groove portion of the transition piece arm is increased by driving in, and the vertical thickness of both ends of the beam abutting the upper surface of the groove portion is increased. A jet pump characterized by being expandable.