KR20050024378A - Cluster for adjusting a nuclear reactor core reactivity, absorber rod of the cluster and method for protecting the absorber rod against wear - Google Patents

Abstract

The present invention relates to a control cluster having a support 3 or a spider to which the absorbing rod 2 is attached. The absorbing rod consists of a hafnium tube welded to the hafnium or zirconium alloy cap at the bottom and welded to the titanium alloy cap 6 at the top. The spider 3 can likewise be made of a titanium alloy. The surfaces of the absorbent rod and the lower cap are protected against wear by oxidation. The upper cap 6 can likewise be protected.

Description

CLUSTER FOR ADJUSTING A NUCLEAR REACTOR CORE REACTIVITY, ABSORBER ROD OF THE CLUSTER AND METHOD FOR PROTECTING THE ABSORBER ROD AGAINST WEAR}

The present invention relates to clusters for controlling the reactivity of pressurized hard water cooled reactor cores and absorbing rods of such control clusters.

Reactors, such as pressurized water reactors, have a core consisting of fuel assemblies disposed adjacent to each other in a reactor vessel. The fuel assembly includes a bundle of fuel rods held within a support structure called a skeleton assembly having a frame for assembly. This skeletal assembly comprises in particular a guide tube arranged in the axial direction of the fuel assembly, connecting the upper and lower ends and supporting a spacer grid of the fuel rod. The purpose of placing these guide tubes is to ensure that the frame has satisfactory stiffness and to allow the assembly of the neutron absorbing rods used to control the reactivity of the reactor core.

Absorbent rods are connected together at their tops by a support, commonly referred to as a "spider", to form a bundle called a control cluster. The absorber rod set may move within the guide tube of the fuel assembly.

In order to control the reactivity of the reactor core during operation of the reactor, the vertical position of the control clusters within a particular assembly of the core is changed so that these clusters can be moved downwards after being inserted or withdrawn and the control clusters moved upwards to vary the length An absorbent rod is inserted into the core assembly. In other parts of the reactor, different types of control clusters are generally used to control the responsiveness of the core and the distribution of power in the core during operation of the reactor. In particular, black clusters which are high absorbance clusters and gray clusters which are low absorbance clusters are used.

Generally, the absorbent rod has a tube closed at the top by a first end plug called a top end plug and at the bottom by a second end plug called a bottom end plug. The absorbing rods are fixed to the fixed spider through their top plugs.

In general, in the case of black clusters, the absorbent rod assembly has an absorbent rod having a high neutron absorption capacity. These absorbent rods may comprise a cladding tube surrounding the absorbent pellets, such as boron carbide (B ₄ C), a neutron absorber tube without the absorbent pellets, or an absorbent tube surrounding the boron carbide (B ₄ C) pellets. In particular, it has been proposed to use hafnium tubes as absorber tubes for such absorbent rods in the control cluster. Thus, the cluster controlling the reactivity of the reactor may consist of an absorbent rod having a hafnium tube that may contain absorbent pellets such as boron carbide (B ₄ C) in whole or in part. In some circumstances it has been proposed to make only part of the absorbent rod, such as the bottom, from hafnium.

Gray clusters include both insertion rods and absorbing rods, which consist of simple tubes made of absorptive or almost absent materials that are closed at the ends with end plugs. The absorbent rod may comprise an absorbent tube such as hafnium.

Hafnium, compared with other absorbent materials, has excellent compatibility with primary fluids, shows little swelling during spinning, and exhibits good creep resistance at operating temperatures of pressurized water reactors. It has the advantage of having. Therefore, it can be used without any sheath.

However, hafnium can only be welded to alloys of the same group (Tinium, Zirconium, Hafnium) or alloys that form a continuous solid solution with Hafnium.

If hafnium is used in the top plug, the mechanical strength of the control cluster is not optimal because hafnium does not have sufficiently good mechanical properties to the stress experienced by the cluster in operation. In addition, the use of hafnium on the top of the absorbing rod is not really justifiable for neutron absorption because the top plug is only exposed to very low neutron flux because the top plug is held above the top of the core. Finally, the use of hafnium on the top plugs entails an increase in the mass of the cluster, which may constitute a serious operational constraint. The use of zirconium alloy in the top plug may be suitable for mass conditions without reducing the absorbency. However, the mechanical properties of these alloys are also inadequate. Conversely, the properties of titanium alloys are perfectly suited for the required performance.

As far as the bottom plug is concerned, the use of hafnium is not prohibited for reasons of mechanical strength, since the properties of this material are suitable for the mechanical stress applied to such components. It is useful to have neutron absorbing capacity at such sites where high neutron flux is present. Finally, because the volume of the bottom plug is small, the resulting increase in mass is small and suitable for the mass conditions of the control cluster. Therefore, the bottom plug may be made of hafnium or zirconium alloy while maintaining compliance with functional conditions.

1 is a perspective view of a control cluster for a pressurized water reactor inserted into a fuel assembly.

2 is an axial cross-sectional view of an absorbent rod according to the present invention.

3 is a partial cross-sectional view of the upper portion of the absorbent rod attached to the arm of the spider.

It is therefore an object of the present invention that a bundle of neutron absorbing rods each having a metal tube called cladding, which is sealed by a top plug at the top and a bottom plug at the bottom, and the absorbing rod is attached through their top plug. In a control cluster having radial supports or spiders, the cladding of at least a portion of the absorbent rod is a non-welded hafnium tube, and the upper plugs of the absorbent rod with the hafnium cladding are made of a titanium-based alloy and the upper portion of the hafnium cladding of the absorbent rod. And the lower plugs are made of lumped hafnium to be welded to the lower end of the hafnium cladding of the absorbing rod.

Preferably,

The upper plug of the absorbing rod with the hafnium cladding is made of TA6V or TA3V2.5 titanium alloy,

Protection against wear of the absorbent rods is provided by the flow of an oxidizing atmosphere in the traveling arrangement for the cladding welded to the bottom plug,

-Protection against wear of the top plugs made by titanium alloys is ensured in an oxidizing atmosphere under conditions which ensure that the properties of the alloys are maintained by a fixed furnace treatment;

Welding to at least one of the upper and lower plugs is carried out using one of friction welding, resistance welding and TIG welding processes,

Hafnium used to make cladding and bottom plugs contains more than 300 ppm oxygen.

The present invention also relates to an absorbing rod of a cluster for controlling a pressurized water reactor, the lower plug comprising a hafnium tube, a titanium alloy upper plug welded to the top of the hafnium tube, and a bulk hafnium welded to the lower end of the hafnium tube. It relates to a cluster absorbing rod comprising:

Finally, the present invention also includes a bundle of absorbing rods and a radial support called a spider, in which the absorber rods are attached through their upper plugs, the spider being a titanium-based alloy. A cluster is characterized by being manufactured.

Preferably, at least some of the absorbent rods in the cluster have a hafnium tube and a titanium top plug welded to the top of the hafnium tube.

Hafnium tubes or hollow bars draw perforated billets on the needles and then hot draw on deformable mandrels, which are cold drawn until they rupture. It is prepared according to a known method which is removed in the final work by. The advantage of this hot forming process is that it makes it possible to use metals with an oxygen content much higher than the oxygen content they can contain in the case of cold forming operations. In general, it is accepted that hafnium containing at least 300 ppm of oxygen can no longer be cold rolled. This process makes it possible to use billets containing 300 ppm and even 700 ppm or more of oxygen obtained after primary melting by electron impact in conventional manufacturing methods. Oxygen concentration makes it possible to improve the mechanical properties of the metal, which significantly reduces the sensitivity to surface and fabrication defects (marks, misalignment, etc.).

Zirconium or hafnium end plugs are obtained by machining solid bars of appropriate diameter. This structure makes it possible to satisfy neutron conditions, mechanical conditions and weight conditions.

However, the longitudinal and orbital motion of the cluster is likely to cause abrasion (wear at the tip) in the cluster guide (continuous guide and guide card) and the fuel assembly. In fact, it is known that these materials (titanium, zirconium and hafnium) do not withstand abrasion well. One way to protect these materials against wear is to conduct high temperature oxidation in an oxidizing atmosphere. Such treatment produces an oxygen diffusion layer that provides protection against wear and an oxide layer that is difficult to prevent from forming due to the very low equilibrium pressure of the oxide in an oxidizing atmosphere. The depth of the oxygen diffusion layer needed to ensure wear resistance is about 20 micrometers. Thus, the minimum target depth for this operation is 35 to 50 μm.

Performing the furnace oxidation process of 3.5 to 4.0 m requires a furnace of sufficient dimensions to operate in an oxidizing atmosphere of 800-1000 ° C. Therefore, the present invention also ensures oxygen diffusion to a depth sufficient to provide abrasion resistance while maintaining a constant temperature—a measure of equalization of oxidized bars—without introducing straightness or mechanical heterogeneity. To an oxidation treatment in a higher temperature traveling arrangement. Diffusion of oxygen exceeding 50 μm can be achieved by induction heating at a temperature of 1300-1700 ° C. and a moving speed of 50-250 mm / min in an oxidizing atmosphere consisting of argon and oxygen. Heating to higher temperatures is likely to result in phase changes in the metal (1725-1775 ° C.) or oxide (˜1700 ° C.). The transfer oxidation is carried out on absorbing rods welded to the bottom plug.

In addition, the bottom plug of the welded tube is treated to ensure continuity of protection against wear in the conical bottom plug portion. However, it is undesirable to process the finished absorbent rod (with a welded top plug). Indeed, for some absorbent rods, the presence of packing pieces, columns of B ₄ C pellets and support structures hinders heating and restricts the choice of packing pieces and support devices (eutectic at processing temperatures). (共融: materials prone to induce molten eutectics) should be excluded. In addition, changes in the heating conditions at the hafnium-titanium connection are difficult to control without the risk of overheating the titanium, which impairs the persistence of the mechanical properties of the titanium.

The treatment in the moving structure also makes it possible to avoid oxidation of the portion to be welded to the top plug to avoid contamination of the weld.

The top plug of the absorber rod is protected against wear by treatment in an oxidizing atmosphere under conditions which ensure that the properties of the alloy are ensured in a static furnace. The treatment in the fixed furnace is generally carried out for 2 to 12 hours at temperatures between 550 ° C and 850 ° C. For example, the treatment can be done at 730 ° C. for 4 hours.

Finally, the present invention also relates to a control cluster in which the spider is made of a titanium-based alloy. This structure makes it possible to take advantage of the better mechanical properties of these alloys and their lower density. Therefore, the design of the cluster is easier because the mass portion of the spider can be assigned to the absorber rod.

The spider supporting the absorbent rod in the cluster may be of the same shape and dimensions as the support spider for the absorbent rod of the control cluster according to the prior art. However, under certain conditions, depending on the shape and dimensions of the titanium alloy top plug, the shape and dimensions of the portions of the spider to which the absorbing rods are attached may change.

The use of titanium-based alloy spiders, instead of steel support spiders for absorbing rods, may benefit from higher grade mechanical properties of steel. Therefore, these improved mechanical properties can increase the reliability and life of the spider. In addition, when the spider is made of a titanium-based alloy of good mechanical properties, it is also possible to slightly reduce the lateral dimension of the spider supporting the control cluster. Therefore, the loss of head when the control cluster is lowered into the core of the reactor is reduced, and the fall time is also shortened.

The spider can be constructed by cutting it from a piece of titanium alloy in which the metallurgical integrity is checked. Therefore, the risk of defects is reduced, and the number of welded or brazed connections between spider components is reduced. Cutting may be performed by mechanical, chemical or electrical processing or water jet cutting.

Titanium alloys are not affected by erosion in the reactor vessel as expected above. Therefore, less activatable product is delivered to the primary circuit.

Finally, the metallurgical integrity of the material and the simplicity of fabrication of spiders from titanium-based alloys make it possible to reduce manufacturing costs and operational abnormalities and to improve productivity in the manufacture of control clusters.

Titanium-based alloy spiders may be used in any control cluster, with or without absorbing rods having hafnium tubes.

An experiment was conducted on the mechanical strength of the control cluster according to the present invention under conditions that reproduced the conditions in the operating reactor.

Wear tests were also performed on various other parts of the absorbent rods to confirm the effectiveness of the anti-wear treatment using oxidation.

The experiments performed were designed to check the abrasion resistance of the absorber rod end plugs and in particular the top plug, the hafnium tube of the absorber rod and the portion connecting the top plug with the control cluster spider. Durability experiments were carried out, which showed that the control cluster according to the present invention can operate within the reactor without premature failure during the expected lifetime of the reactor according to the current technology.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the present invention may be understood, control clusters and absorbing rods according to the invention will be described by way of example with reference to the accompanying drawings.

In FIG. 1, the control cluster for a pressurized water reactor is shown generally at 1 .

The control cluster 1 comprises a bundle of absorbent rods 2 and a spider 2 for holding and fixing these absorbent rods 2 in the form of bundles in which the absorbent rods are parallel to each other and arranged laterally in the same arrangement as the fuel assembly guide tube. ).

The spider 3 has a cylindrical hub 3a having an inner groove so that the control cluster can be coupled to the absorbing rod and move the absorbing rod in a vertical direction within the core. An arm 3b to which these absorption rods 2 are attached is provided by the upper end plug of the absorption rod.

At least some of the absorbing rods 2 in the control cluster 1 have tubular bodies, including hafnium tubes.

If the cluster 1 is a black cluster, the tubes of all absorbing rods 2 in that cluster may be hafnium.

In the case of gray clusters, only some of the absorbing rods 2 have tubes made of hafnium, and the tubes of the other absorbing rods are any other non-absorbing material which satisfies the operating conditions in the steel or reactor.

Figure 2 shows another absorbing rod of the present invention in a black cluster that can be used, for example, in a pressurized water cooled reactor operating at an output of 1300 MWe.

The absorbing rod 2 shown in FIG. 2 surrounds a stack of highly absorbent boron carbide (B ₄ C) pellets 5 and is surrounded by titanium alloy plugs 6 at the top and conical hafnium or zirconium at the bottom. The hafnium tube 4 sealed by the alloy plug 7 is provided. On the tube welded to the bottom plug, oxygen diffusion 11 is effected to protect the tube against wear. The top plug may or may not be protected by oxygen diffusion 12.

Hafnium used may contain more than 300 ppm oxygen.

The stack of boron carbide (B ₄ C) pellets 5 is secured in a hafnium tube 4 by a spring or any other anchoring device 6, and the bottom of the column of pellets has a strut 7a. It bears against the bottom plug 7 through. The lower end plug 7 of the hafnium absorbing rod 2 is integrated with the lower end of the hafnium tube 4 via a welding line 7b in which welding is performed by, for example, laser beam, electron beam, TIG, friction or resistance welding. The welds thus obtained are perfectly sound and perfectly strong.

According to the invention, the upper plug 6 of the absorbing rod 2 is made of titanium or a titanium alloy, for example Ti-6Al-4V (TA6V) alloy or TA3V2.5 alloy, which is a tube through the weld 9. (4) is securely and tightly fixed to the top. Testing has shown that it is possible to weld between the titanium alloy plug 6 and the hafnium tube 4, for example using laser beam, electron beam, TIG, friction or resistance welding. The welds thus obtained are perfectly sound and perfectly strong. In the case of TIG or friction welding, the fracture zone of the hafnium / titanium or hafnium / zircaloy specimen is outside the weld zone. Breaking occurs under load that corresponds to the ultimate strength of the solid material.

As can be seen in FIG. 3, a titanium alloy upper plug 6 which secures the absorbent rod 2 to the arm 3b of the spider 3 of the control cluster is provided with that of the upper plug of the absorbent rod according to the prior art. It may be the same shape and dimensions. The upper plug 6 having a spiral for fixing the absorbing rod to the arm 3b of the spider 3 may be screwed to the spider arm or placed in an orthogonal position and fixed by the upper nut 10, the upper part of which is The nut 10 also guides the cluster as it is being raised.

As can be seen in FIG. 3, the upper plug 6 of the absorbent rod has a small cross section 3c which provides the flexibility required for the absorbent rod.

In addition, the weld 9 between the top of the titanium alloy top plug 6 and the top of the hafnium 4 (FIG. 2) is observed for any additional erosion at the connecting weld 9 of the top plug 6 in the reactor environment. It has been found to withstand mechanical, thermal, and chemical stresses.

In addition, when the control cluster is used in the core of the reactor, the plug 6 is placed in a zone which is not subjected to strong neutron flux obtained in the reactor core, on the top surface of the core. Therefore, the top plug made of titanium alloy is not in a condition that results in swelling or loss of mechanical properties under irradiation. Therefore, the top plug with high grade of mechanical properties retains its properties over long life in the reactor core.

In addition, the top plug of the hafnium absorbing rod in the control cluster according to the invention made of titanium alloy with high grade mechanical properties can be configured to have the largest possible length suitable for the use of the control cluster. Therefore, the length of the hafnium tube is reduced, making it possible to reduce the cost and to control the mass of the absorbing rod.

The invention applies to any control cluster for a hard water cooled reactor that includes an absorber rod with a titanium tube.

Claims (12)

A bundle of neutron absorbing rods 2 each having a metal tube 4 called cladding, which is sealed at the upper end by an upper plug 6 and at the lower end by a lower plug 7; In the pressurized water reactor control cluster having a radial support or a spider attached through these upper plugs 6, At least a part of the absorbing rods is a non-welded hafnium tube, and the upper plug 6 of the absorbing rod 2 having the hafnium cladding is made of a titanium-based alloy, and the hafnium cladding of the absorbing rod 2 4) welded to the upper end of the control cluster, characterized in that the lower plug (7) is made of block hafnium and welded to the lower end of the hafnium cladding of the absorbing rod (2). The control cluster according to claim 1, wherein the upper plug (6) of the absorbing rod (2) having a hafnium tube (4) is made of a TA6V or TA3V2.5 titanium alloy. The protection of the absorbing rods according to claim 1, wherein the protection against wear of the absorbing rod is at a temperature of 1300 ° C to 1700 ° C in an oxidizing atmosphere moving at a rate of 50-250 mm / min over the cladding 4 welded to the bottom plug 7 A control cluster, characterized in that provided by oxidation. The protection against wear of the top plug 6 made by the titanium alloy is treated in an oxidizing atmosphere under conditions which ensure that the properties of the titanium alloy are maintained in a static furnace. Control cluster, characterized in that secured by. 5. The control cluster of claim 4, wherein said treatment in a fixed furnace is performed for a time between 2 and 12 hours at a temperature between 550 ° C and 850 ° C. The control cluster according to claim 1, wherein at least one of the upper plug (6) and the lower plug (7) is performed using at least one of friction welding, resistance welding, and TIG welding processes. 2. The control cluster according to claim 1, wherein the hafnium used to make the cladding and the bottom plug contains more than 300 ppm oxygen. In the absorption rod of the pressurized water reactor control cluster, A hafnium cladding 4, a titanium alloy upper plug 6 welded to the upper end of the hafnium cladding 4, and a bulk hafnium lower plug 7 welded to the lower end of the hafnium cladding 4. Absorbing rod, characterized in that it comprises a. In a pressurized water reactor control cluster comprising a bundle of absorbing rods (2) and a radial support called spider (3), to which the absorbing rods (2) are attached through their upper plugs (6). , Control cluster, characterized in that the spider (3) is made of a titanium-based alloy. 10. The method of claim 9, wherein at least some of the absorbing rods (2) in the cluster (1) have a hafnium tube (4) and a titanium alloy top plug (6) welded to the top of the hafnium tube (4). Control cluster, characterized in that. A method for protecting an absorbent rod according to claim 8, wherein the cladding (4) of the absorbent rod (2) is oxidized at a high temperature in an oxidizing atmosphere. The method according to claim 11, characterized in that the oxidation of the cladding (4) welded to the lower plug (7) is carried out in a moving structure at a temperature of 1300 ° C to 1700 ° C and a speed of 50 to 250 mm / min. How to protect absorbent rods.