KR840002278B1 - Apparatus for limiting the effect of axial hydraulic pressure exerted on fuel assemblies of watercooled nuclear reactors - Google Patents

Abstract

The fuel rods are located between an upper plate and a lower plate; and an end piece is fixed to both the top and bottom ends of the fuel rods. On each end piece is a shock-absorber pot contg. an elastic sleeve made e.g. of a spring steel tube with a longitudinal slot. A centreing pin is located in the sleeve to provide the fuel rods with a limited amt. of axial movement. The frictional force of the sleeve on the pin prevents an axial movement of a fuel rod during normal changes in hydraulic pressure, but the pot has a radial hole via which the pressurised water can escape if the fuel rod expands axially along the pin. If the fuel rods expand axially, water can escape out of the shock absorber pot.

Description

Apparatus for effectively limiting the axial hydraulic pressure acting on the fuel assembly of a water-cooled reactor

1 is a schematic representation of a known component for inserting a fuel assembly into a furnace core of a light-water reactor.

2 is a longitudinal sectional view of the device for hydraulic damping and mechanical braking of the centering pin by a slit sleeve.

3 is a longitudinal sectional view of an apparatus for damping and braking a centering pin using a slit sleeve and a corrected opening.

4 is a longitudinal sectional view of the device for attenuating the centering pin by a closing sleeve associated with a calibrated opening in the housing.

TECHNICAL FIELD The present invention relates to a nuclear fuel assembly of a nuclear reactor, and particularly to an apparatus for effectively limiting the axial hydraulic pressure acting on the nuclear fuel assembly of a water-cooled reactor.

Nuclear fuel assemblies as known water-cooled reactors of French Patents 1,536,527 and 7,018,102 by Westinghouse Electric, Inc., are described.

In the case of the two French patents, the nuclear fuel of the reactor generally distributes a plurality of fuel rods located in the reactor core in the form of an adjacent aggregate in a rectangular section.

The angular fuel assembly has a rigid mechanical frame consisting of upper and lower end members, an orifice for passing cooling water circulating from the bottom to the government, and a plurality of guide tubes to secure the strength and stability of the device fixed to the end member. It is provided. There is also a support grid connected to the guide tube along each assembly and defining a cell having a rectangular cross section. They allow passage and transverse grip of the fuel rods and allow them to be suspended by friction in the grating without contacting the upper and lower end members.

Each fuel assembly extends over a few meters between an upper core plate and a lower core plate belonging to an internal device of the reactor vessel. It is placed on the lower core plate and screwed onto a centering pin which forms an integral part of the two aforementioned plates connected to the reactor vessel. Centering pins may be introduced somewhat into the hollow cylindrical recesses, two or four of which are generally provided in each of the upper and lower end members of each assembly. Under the action of the hydraulic pressure change caused by a change in the normal operating conditions of the cooling water circulation circulating from the bottom to the government within the reactor core, the upward vertical motion of each assembly between the two core plates is arranged on the upper end member of the fuel assembly. It is prevented by four springs. In figure 1 of French patent 1,536,257 this spring is shown by reference numeral 38. However, these springs are accidental transients during coolant circulation and are compressed against the top plate of the device in the reactor core during removal of the aggregate.

In particular, the hydraulic pressure is a function of the different levels of the reactor operating conditions and the fluctuations in the cooling water circulation under normal operating conditions lead to a change in the load on the aggregate to less than about 10% of its weight. But under the accidental conditions of transients, a strong and rapid impact is exerted on the assembly. For example, the impact can reach or exceed the weight of the assembly, which risks contacting this upper core with mechanical shock.

Even though the device of the type described above can be operated quite satisfactorily, the metal springs do not hop or damp the vertical displacement of the fuel assembly as well as the mechanical properties of the springs often operate at high temperatures of about 300 ° C. and under strong radiation. There is a problem that is seriously degraded as a function of.

The structure of this metal spring also needs to have a large dimension so that it can absorb large relative dimensional changes of the assembly with respect to the device in the container in order to effectively cope with the radiation change of radioactivity. Due to this large dimension, the springs severely squeeze the internal device under any operating condition.

The object of the present invention is to eliminate the axial motion of the assembly during absorption or hydrodynamic damping of the rapid displacement of the assembly under normal operating conditions and transient operating conditions of hydraulic pressure and transient operating conditions by simple, effective and easy means, instead of removing the metal springs described above. It is to provide an apparatus for effectively limiting the axial water pressure on the fuel assembly of a water-cooled reactor that prevents severe mechanical impact with the aerocore.

The invention thus relates to a nuclear fuel reactor of a water-cooled reactor of the type connected to the upper and lower furnace cores of an internal device and fixed with any axial displacement by means of corresponding hollow cylindrical housings carried by assemblies or centering pins that slide in recesses. A device for effectively limiting the axial hydraulic pressure acting on an assembly, the apparatus or recess being adapted to grip a centering pin and to exert an appropriate friction thereon to prevent the axial movement of the assembly during a normal wave of hydraulic pressure. A cylindrical elastic sleeve disposed against the inner wall of the housing is provided therein, and the housing is perforated with an opening that allows for the controlled removal of water retained therein near the end member of the fuel assembly, thereby providing rapid When the centering pin is penetrated by excessive operation, it can be absorbed or absorbed. Relates to an apparatus for effectively limiting the axial hydraulic pressure acting on the fuel assembly of a water-cooled reactor. The opening for removing water is the corrected opening.

According to the first embodiment of the present invention, the opening allowing the removal of the controlled amount of water is a calibrated slot in the elastic sleeve which is a wide opening made in the housing which facilitates free removal.

In this case the means capable of countering the axial displacement hold the centering pins and the slit cylindrical arranged against the inner wall of the corresponding housing in order to exert an appropriate friction thereon to prevent axial movement of the assembly during the normal wave of hydraulic pressure. It is composed of an elastic sleeve.

The housing of the elastic sleeve is perforated with a calibrated opening controlling the removal of water in the vicinity of the end member of the fuel assembly, thereby preventing the penetration of the fin into the housing of the top plate and the fly-off of the fuel assembly during rapid transient operating conditions. As a result, the damping action occurs when the centering pins simultaneously start from the housing of the bottom plate.

In accordance with the first variant of this embodiment, the damping function is ensured by the elastic sleeve in which water in and out is controlled by the corrected opening in the sleeve. The relative dimensions of the sleeve and the fins ensure the sealing of the damping or absorbing member defined by the sleeve. In this variant, the inlet and outlet of the water is made wide.

According to the first embodiment and the second modification, the displacement countermeasure has a mixed structure of mechanical axial braking by friction and deceleration by hydraulic pressure damping. The axial displacement braking means consists of a slit cylindrical elastic sleeve. In the second variant the sleeve is secured in the housing by inserting a gasket at the corresponding housing end with respect to the inner wall of the housing for maintaining hermetic relationship with the centering pin. Similarly, the sleeve exerts a friction force on the pin. In this combination, a hydraulic damping effect is obtained due to the housing wall in the end member condition of the aggregate of the corrective opening for outflow of water and the gasket defines the damping or absorbing member.

In the latter variant, the packing washer may be provided at the end of the housing between the gasket and the slit sleeve so that at the end of the annular chamber defined by the housing and the sleeve the washer provides a circular ring-shaped surface of resin to the end fitting axis. .

In all the above-described modifications, the slit in the cylindrical elastic sleeve can have any phenomenon. It may in particular extend or spiral along the busbar of the cylindrical surface or in the form of a breaking line on the surface.

According to the second embodiment of the present invention, the weight displacement-countermeasure is mainly a hydraulic damping or absorbing means. Thus, this single function is suitable for controlling hydraulic problems in any reactor type. In particular, the weight of the fuel assembly is significantly higher or lower than the normal wave of hydraulic pressure, which makes it suitable for contact with one of the cores and under the action of rapid transient operating conditions. Thus, in this case the damping itself is sufficient to obtain satisfactory operation without sudden impact.

The hydraulic damping or absorbing means in this embodiment consists of a closed cylindrical sleeve whose inner diameter slightly exceeds the outer diameter of the centering pin and is arranged against the inner wall of the corresponding housing by the insertion of a gasket at the housing end, the corrected opening being It is provided to adjust the outflow of water upon introduction of the centering pin in the housing wall or the inflow of water on the exit of the centering pin to a value corresponding to the strength of the desired hydraulic damping.

As a refinement of the above embodiment the closed cylindrical sleeve has a peripheral cylindrical recess defining an annular chamber which exerts a calibrated opening in its base to the opposing wall of the end fitting and which is closed by a circular ring shaped surface perpendicular to the housing axis. .

In the second embodiment it is important to be able to have one or more openings for facilitating the passage of the displaced water by the introduction of a centering pin into the dripping housing of the fuel assembly at the end of the sleeve adjacent the corresponding end member. Do.

Non-limiting embodiments of the invention will now be described in detail with reference to the accompanying drawings.

2, 3 and 4 show the housing associated with the upper member of the fuel assembly not shown in detail in FIG.

However, the lower end members of the corresponding assemblies have the same but downward means, which work identically and internally. The following description deals with the case where the damping device is operated by the introduction of a centering pin and the outflow of water for simplicity, but this is not limitative and the symmetrical operation by the exit of the centering pin and the controlled inflow of water into the associated housing. It is also evident that they form part of the invention.

FIG. 1 shows a known arrangement of fuel assemblies in a water-cooled reactor in which the vessel is indicated by the example number 1. FIG. The internal device is suspended in the container 1. The internal device consists of upper and lower internal devices. The upper internal device consists of an upper plate 2 and a lower plate 4, which are interconnected by spacers. The lower inner device consists of a ferrule 3 and a lower plate 5 connected by welding. Example 6, which lies on the plate 5 between the two plates 4 and 5 and is held between the plates 4 and 5 at axial intervals by several centering pins 7 and The same fuel assembly is arranged. The structure of the fuel assembly 6 consists of an upper end member 8 and a lower end member 9 and is spaced apart and reinforced by a guide tube as shown in Example 10. The fuel rod 11 is held in place by a support grid 12 defining a cell having a quadrilateral cross section for passage of the fuel rod 11, which support grid 12 is connected to the guide tube 10. do. The guiding tube 10 is hollow and houses the absorbing rods 24 to examine the rapid shutdown and chain reaction of the reactor when necessary.

The system shown in FIG. 1 is immersed in pressurized hot water at a temperature close to 300 ° C. circulating in the furnace core between the inlet 13 and outlet 14. The different aggregates 6 are thus immersed in the upward cooling fraction, in particular to lead to "splashing" between the two plates 4 and 5 during a sudden and transient change in coolant flow conditions.

FIG. 2, which illustrates the first embodiment of the present invention, shows a centering pin 7 connected to an unillustrated plate of the upper internal device of a water cooled reactor. Example 8 designates an upper end member of the nuclear fuel assembly of such a reactor.

In the conventional construction the end member 8 has two housings or recesses 15, one of which is shown in FIG. 2. The housing 15 houses a centering pin 7 that slides in the axial direction therein. This makes it possible to keep the fuel assembly in place while allowing any axial movement of the fuel assembly to react to changes in hydraulic pressure caused by changes in the upward flow conditions of the coolant. This rather pronounced penetration of the centering pin 7 into the housing 15 should occur with a significant braking action, ie about 1/10 of the hydraulic scattering force, to prevent axial movement of the fuel assembly under normal operating conditions.

According to the invention this is by means of a cylindrical sleeve 16 made of an elastic material such as spring steel or Inconel and having a transverse slot 17 over its entire height along the cylindrical busbar. This sleeve 16 is fixed to the inner wall of the housing 15 by an upper widened portion 18 of the housing 15. And in the non-operating state, the inner diameter is smaller than the outer shape of the centering pin 7, and is gripped by friction during the axial introduction of the pin 7. A calibrated opening 19 is provided at the bottom of the housing 15 and the opposing slot 17 of the sleeve 16 to allow controlled removal of water in case of overpressure in the damping or absorbing member constituted by the housing 15. do.

In the case of scattering of the aggregate, the damping member of the upper end member is exposed to overpressure of water and the damping member of the lower end member is exposed to low pressure of water so that the effect is reversed during the downward motion of the aggregate.

Another dissolution according to the second variant is formed by the capacity of the sleeve 16 retained at the bottom by calculating the size of the slot 17 of the elastic sleeve 16 to form a corrected outlet of the damping member. . The damping member described above is closed by contact between the sleeve 16 and the pin 7. The opening 19 in the housing is made large enough so that water can flow freely.

The diameters for the fins 7 and the sleeves 16 are a function of the elasticity so that the radial frictional forces generated by the sleeves 16 on the fins 7 are axial in the fuel assembly due to the lack of safety with respect to the coolant flow. It is dimensioned enough to prevent vibration. For example, this is sufficient for the axial frictional force due to the grip of the transportation sleeve 16 on the centering pin 7 to be about 1/10 of the hydraulic pressure so that the movement of the fuel assembly is prevented. A recess 31 is provided on the upper surface of the end member 8 of the assembly, which rounds off the purpose to be introduced to the dotted line by the centering pin 7 and in the figure the dashed line 20 indicates the best introduction position.

3 shows a third variant of the first embodiment of the present invention in which mechanical braking and hydraulic damping effects are combined.

For this purpose, a gasket 25 in the form of a circlip is provided between the housing 15 and the centering pin 7 to close the housing end by supporting it against the side wall of the pin 7. The sleeve 16 is slit laterally so that the slot 17 is provided over its entire height. An opening 26 for a controlled withdrawal of water is made in the housing wall 15 facing the slot 17. In this embodiment the inner diameter of the sleeve 16 is smaller than the outer diameter of the pin 7 and the sleeve made of spring steel or inconel can hold the pin 7 by the frictional force acting thereon.

And via the opening 26 corrected to only one length to withdraw water in the housing 15 to induce some supplementary hydraulic pressure attenuation. As a supplementary device there is a packing washer 30 on the apex of the housing 15 which provides a circular ring-shaped surface 29 perpendicular to the housing axis at the end of the annular chamber between the housing 15 and the sleeve 16. . The force acting on the annular surface 29 by the pressurized water as necessary raises the sleeve 16 slightly and then removes the water retained therein towards the corrected opening 26.

In the case of all the variants described above, the slot 17 in the cylindrical elastic sleeve can have any shape. It can thus extend along the busbar of the cylindrical surface or spiral or in the form of a break line on the surface.

4 shows the pattern applicable to a special case of a reactor in which the assembly remains in contact with any one of the cores under the effect of a steady wave of hydraulic pressure and the device acts only to monitor rapid and powerful impacts under transient conditions. A second embodiment of the invention is shown.

FIG. 4 has a centering pin 7, an end member 18 and a housing 15 as in FIG. 2, but the embodiment of FIG. 4 has the effect of attenuating the centering pin 7 by pure water pressure means. In this case the unslit sleeve 16 is thus kept sealed in the housing 15 by the gasket 25 as described above in connection with FIG. The bottom of the housing 15 is provided with an opening 26 corrected to limit the outflow of water retained in the housing 15 to a predetermined value during the introduction of the centering pin 7 in which progress has been significantly slowed down in order to obtain a certain accurate damping. Is provided. Water in the housing 15 can naturally be withdrawn by passing under the end of the sleeve 16 adjacent the end member 8. In other embodiments, openings 27 may be provided at the bottom of sleeve 16 to facilitate removal of water. Finally a possible improvement is shown in the embodiment of FIG. 4 with peripheral cylindrical recesses at the bottom of the sleeve 16 which define the annular chamber with the opposing wall of the housing 15.

This annular chamber is closed at a circular ring-shaped surface 29 perpendicular to the axis of the housing 15. The device acts with an upward force on the annular surface 29 which assists the outflow of water towards the corrected opening 26 by slightly raising the sleeve 16 when the pressure water can spread evenly into the annular chamber described above. It is important.

Claims (1)

Axial, acting on the fuel assembly of the water-cooled reactor, connected between the upper and lower core plates of the internal device and fixed at any axial displacement by a centering pin that slides in a hollow cylindrical housing carried by the assembly. In a device for effectively limiting water pressure, A cylindrical elastic sleeve disposed against the inner wall of the housing to grip a centering pin and to exert an appropriate frictional force to prevent axial movement of the assembly during normal fluctuations in water pressure; An opening formed in the housing to withdraw the water retained near the end member of the assembly in a controlled amount, A device for effectively limiting the axial hydraulic pressure acting on a nuclear fuel assembly of a water-cooled reactor, characterized in that the hydraulic damping or absorption is effected upon introduction of the centering pin under rapid transient conditions.

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