DESCRIPTION

Technical field of the invention

The present invention can be applied in the industrial field related to the implementation of components for the heat disposal in the thermo-nuclear fusion reactors, in particular for machines of Tokamak type.

More specifically, the invention relates to the implementation of junctions in the Plasma Facing Units (PFU) of the components of said machines assigned to the disposal of the plasma thermal power and called divertors. The present invention in particular relates to the construction of the PFUs through the known junction technology called Hot Radial Pressing (HRP).

Background

In a fusion reactor, in particular of Tokamak type, the divertor is placed in the lower portion of the fusion chamber area on which, thanks to the particular configuration of the magnetic field, the plasma energy is conveyed. The so- called “Plasma Facing Units” (PFU) indeed constitute the divertor portion which is facing the plasma and dissipates the heat thereof. Therefore, the PFUs are extremely stressed components, with thermal flows in the order of 20 MW/m ² Each unit mainly consists of a tube within which a cooling fluid circulates. The tube typically has a rectilinear path in the lower portion and a curved path at the upper area joining with the wall of the combustion chamber. Outside each tube parallelepiped blocks are welded, made of protection material resistant to high temperatures and to the erosion induced by the impact of the particles coming from plasma. Such blocks, generally made of tungsten, are perforated to allow the insertion, inside thereof, of the tube itself. Typically, a layer of pure copper is interposed between tube and block.

The above-mentioned block coating of the tube is generally called “reinforcement”. Each coated tube identifies one single PFU. This particular configuration of the PFUs - called ‘mono-block’ - demonstrated to be reliable and represents, in the state of art, the reference solution for the known ITER (International Thermonuclear Experimental Reactor) machine and even for the future reactors.

The structural integrity of the PFUs is fundamental for the purpose of operation effectiveness and safety of the reactor and then requires extremely reliable constructive techniques.

The critical portion of the PFU construction is the junction between the tube - typically made of copper alloy - and the - typically refractory - reinforcement blocks, since the different thermal expansion coefficient of the materials makes critical both the junction process - due to the high induced residual stresses - and the consequent resistance to thermal fatigue during the divertor operation.

Currently the most used techniques for implementing this junction are three, that is: Hot Radial Pressing (HRP), brazing and Hot Isostatic Pressing (HIPping).

The HRP technique has the advantage of not requiring thermal treatments for tempering the copper alloy of the tube, as it happens for brazing, nor subsequent mechanical processing, as in case of HIPping, which produces considerable permanent deformations in the piece due to the required high pressures.

However, the modes for implementing the PFUs with the HRP technique are still perfectible with respect to the reliability of the final product, especially in terms of compliance with project tolerances and absence of mechanical imperfections and to the possibility of performing highly accurate not destructive checks.

In particular, there is the need that the apparatuses, the control systems and the procedures for implementing the joining process by means of HRP technique allow to check the deformation of the portions of the PFU during the different phases of the process itself, by guaranteeing the integrity thereof and compliance with both size specifications and the thermo-physical properties of the materials.

Summary of the invention

The technical problem placed and solved by the present invention then is to provide an apparatus allowing to meet the requirements mentioned above with reference to the known art.

Such problem is solved by an apparatus according to claim 1 .

Preferred features of the present invention are set forth in the depending claims.

The present invention provides a special mechanical apparatus, or system, fixable or fixed inside a vacuum furnace to implement a junction between a tube of PFU and the related reinforcement blocks by means of the Hot Radial Pressing (HRP) technique

The apparatus of the invention allows to obtain a mechanically reliable junction, highly resistant to the stresses of thermal-mechanical nature. In particular, the apparatus is configured to check the deformations of the portions during the HRP process, which typically provides a thermal cycle with temperatures up to 600°C and an internal pressurization phase of the tube at 600 bar.

In other terms, the system of the invention limits the residual deformations of the portions to be joined by allowing a free expansion thereof during the several phases of the thermal cycle.

The permanent deformations then can be kept within the limits of the size tolerances currently provided for these components.

The apparatus of the invention then allows to obtain a junction between tube and reinforcement without imperfections, within wished size tolerances and reliably and effectively.

The system of the invention is applied in the most general case of PFUs having a rectilinear tract and a curved one and it is also suitable even in case of wholly rectilinear components.

Other advantages, features and use modes of the present invention will result evident from the following detailed description of some embodiments, shown by way of example and not for limitative purposes.

Brief description of figures

The figures of the enclosed drawings will be referred to, wherein:

■ Figure 1 shows a top perspective schematic view of a form of execution of a containment structure, or cage, partially exploded, according to a preferred embodiment of the apparatus of the present invention;

■ Figure 2 shows a top perspective schematic view of the cage structure of Figure 1 , partially exploded, during the implementation of a plasma facing unit (PFU);

■ Figure 3 shows a transparent or sectioned top view of a vacuum furnace including the cage structure of Figure 1 during the implementation of a plasma facing unit (PFU);

■ Figures 4A and 4B illustrate schematically an operating mode of the apparatus of the preceding figures, during a phase of increasing the temperature and a pressurization phase of a welding cycle performed by means of HRP technique, respectively;

■ Figure 5 shows a longitudinal section view of a form of execution of an antibreakage protection element of the tube called ‘spacer’ used in the apparatus of the preceding figures and Figure 5B shows a cross section view thereof performed according to the line B-B of Figure 5;

■ Figure 6 shows a front perspective partial view of the cage structure of Figure 1 and an embodiment of a respective guide on which the cage structure itself is supported;

Figures 7A to 7D show, each one, a perspective view of an embodiment of a component of the guide of Figure 6, a bearing support, a central body, a sliding shaft and attachment means, respectively;

■ Figure 8 shows a longitudinal section view of an embodiment of a locking system of the apparatus of the preceding figures, during the above- mentioned pressurization phase.

Detailed description of preferred embodiments

By firstly referring to Figures 2 and 3, an apparatus, or system, for implementing plasma facing units (PFU) according to a preferred embodiment of the invention is designated as a whole with 1 . The apparatus 1 , in use, is assembled within a vacuum furnace 100 configured to implement junctions according to the HRP (Hot Radial Pressing) technique.

The vacuum furnace 100, of known type on itself, mainly includes a more external vacuum chamber 101 and a heating chamber, or hot chamber, 102 arranged inside the vacuum chamber 101. The chambers 101 and 102 are configured to house a to-be-manufactured PFU 300 with the purpose of implementing the joining process between a tubular element, or tube, 301 thereof and blocks, or mono-blocks, 302 which define, in the finished component, a reinforcement outside the tube 301 .

The junction is obtained by diffusion welding caused by the above-mentioned HRP technique. The welding cycle provides two main phases, performed in time sequence. In a first phase the internal environment of the hot chamber 102 is heated until an operating, or regime, temperature, typically equal to about 600°C. In a second phase, called pressurization phase, within the tube 301 a predetermined pressure regime is applied, typically with maximum pressure equal to about 600 bar.

During the above-mentioned phases, as said the to-be-manufactured PFU 300 is received in the furnace 100 and in particular supported by means of the apparatus 1. The latter, according to the invention, allows a selective, or differentiated, check of the deformation of the components of the PFU in the above-mentioned two phases. In particular, as illustrated in greater details hereinafter, through the system of guides the apparatus 1 is free to expand, during the phase of temperature increase, both in the direction of the axis of the tube 301 with a length increase, and in the direction orthogonal to such axis with an increase in the radius of curvature, whereas such deformations are inhibited in the subsequent pressurization phase.

The vacuum furnace 100, according to notions known in the art, includes a plurality of actuation systems actuatable from outside.

In particular, the furnace 100 comprises:

■ a system for heating and checking the temperature, configured to guarantee within the hot chamber 102 the development of the thermal cycle required by the junction process by HRP; in particular, such system is configured to guarantee in the hot chamber 102, during the whole thermal cycle, a homogeneous temperature detected inside the tubes over the entire length of the PFU 300 to be implemented; preferably the maximum temperature of the tube is about 600°C, in particular with tolerance +5, -5°C with respect to a predetermined value; a pumping system to create in the chamber 101 a vacuum level adequate to obtain the junction during the above-mentioned pressurization phase, in particular about 600 bar.

The tube 301 of the to-be-manufactured PFU 300, in the present example, provides a substantially rectilinear tract 303, or a pair of substantially rectilinear tracts with longitudinal end, and a substantially curved tract 304, all formed in the hot chamber 102.

At each one of the two ends of the to-be-manufactured PFU 300, the tube 301 has to continue for a length which is function of the geometrical and thermal features of the furnace 100. These longitudinal ends of the tube 301 are without mono-blocks, they project at the interface between the hot chamber 102 and the vacuum one 101 circumscribed to it and within the latter and they are called “cold tails”. The “cold tails” are designated with 306 and 307 in Figures 2 and 3. Generally, they are intended to be removed, in the factory, after the junction process and after having overcome a quality control.

The two “cold tails” 306 and 307 cross the thermal shields of the hot chamber 102 of the furnace 100 and end with two high-pressure joints 5. The function of the latter is to connect PFU with the above-mentioned pressurization, or pumping, system - outside the furnace - allowing to pressurize the inside of the tube 301 indeed in the pressurization phase. The cold tails 306 and 307, in their tract nearest to the hot chamber 102, guarantee a temperature drop in axial, that is longitudinal, direction of the tube 301 compatible with the mechanical resistance of the joints 5.

The joints 5 are configured, in the preferred embodiment, even to allow the passage of thermocouples and/or other sensors or transducers inside the tube 301. In particular, thermocouples of different lengths allow to check the temperature with high precision along the development of the tube 301 of the to-be-manufactured PFU 300.

In the present embodiment, the apparatus 1 mainly comprises:

■ a containment system of the to-be-manufactured PFU 300, based upon a cage-like structure 7 and associated spacers 4, the latter preferably of quick disassembly type;

■ a plurality of guides 3 thereon the cage structure 7 rests and the ends of the cold tails and which allow the deformation thereof during the heating phase;

■ the above-mentioned high-pressure joints 5;

■ in some embodiments, a system for the side locking and for locking the tails 6, to be activated before the pressurization phase.

Each one of such components will be now described in greater detail.

With reference to Figures 1 and 2, the containment system is configured:

- to position correctly all mono-blocks 302,

- to guarantee the final shape of the PFU;

- to prevent the collapse by internal pressure of the due longitudinal ends of the tube 301 arranged outside the hot chamber 102, that is the cold tails 306 and 307;

- to transmit to the guides 3 the mechanical forces generated by the process, in particular in the heating phase.

The cage structure 7 and the spacers 4 are advantageously made of metallic material, in particular steel AISI316L and/or an austenitic alloy with high mechanical resistance.

The cage structure 7 mainly consists of a lower housing 71 and a cover 72.

The lower housing 71 houses the to-be-manufactured PFU 300 and follows the three-dimensional profile thereof, then defining a substantially parallelepipedlike seat which is closed on the upper side by the cover 72.

The to-be-manufactured PFU 300 is locked mechanically inside the structure 7 by fixing the cover 72 to the housing 71 , for example by clamping bolts. With the purpose of locking the cover 72 on the housing 71 respective connecting means, or profiles, is provided designated with 721 and 722, respectively.

The configuration of housing and clamping is so that the cage structure 7 follows the deformations of the to-be-manufactured PFU 300 in the first portion of the joining process corresponding to the temperature increase in the hot chamber 102, that is in the above-mentioned heating phase.

To this purpose, the cage structure 7 is assembled on the system of guides 3 which will be described shortly. The spacers 4, shown in greater detail in Figures 5 and 5B, are configured to receive and contain the cold tails 306 and 307.

Each spacer 4 comprises two half-shells 41 and 42, each one with substantially semi-conical external profile, mutually coupled at respective longitudinal margins to define a cylindrical tubular seat. In particular, the half-shells 41 and 42 implement a longitudinal compartment, seat or housing with rectilinear axis, corresponding to the axis of symmetry of the cylindrical shape. Such compartment has diameter so as to be able to be closed, that is clamped, without yoke around the tube 301 at of the cold tails 306 and 307.

As said, externally the half-shells 41 and 42 have a side blanket with semi- conical longitudinal profile, that is with diameter which can axially vary linearly, to decrease towards outside the hot chamber 102.

The half-shells 41 and 42 are clamped around the tube of the cold tails 306 and 307 by means of a cylindrical body 43 circumscribed thereto, which has, too, internal tubular seat with tapered shape, that is with internal diameter which can axially vary correspondingly to the external blanket of the half-shells 41 and 42.

The conical shape of the above-described profiles allows to disassemble easily each component 4 after the process of implementing the PFU 300.

As mentioned above, a specific function of the spacers 4 is to contain locally the tube 301 during the pressurization phase, by preventing the excessive deformation and/or burst thereof.

To this purpose, the material of the spacers 4 advantageously can have a lower thermal expansion coefficient than that of the tube 301 of the to-be- manufactured PFU 300.

As mentioned above, the apparatus 1 further comprises a plurality of guides 3 which support locally the cage containment structure 7 within the hot chamber 102. Such guides are shown in detail in Figures 6 to 7D.

The guides 3 allow to sustain the containment structure 7 in horizontal position, on a work surface of the furnace 100, and they implement mobile devices capable of modifying the type of mechanical constraint depending upon the ongoing process phase. In particular, the guides 3 allow a longitudinal and radial deformation of the tube 301 in the heating phase and they can be locked to inhibit it in the subsequent pressurization phase. More specifically, the guides 3 allow the above-mentioned free thermal expansions of the containment structure 7, and then of the to-be-manufactured PFU housed inside thereof, during the temperature increase and decrease, in particular during the heating phase. In fact, during the temperature increase and cooling phases of the welding cycle, the containment structure 7 has to be simply sustained and free to expand thermally. During these phases, the to-be- manufactured PFU 300 has not to be subjected to reaction forces of any type. In particular it has to be allowed:

- an increase in the radius of curvature in the curved area 304 (if present);

- an elongation in the rectilinear tract(s) 303.

The allowed deformation lines are highlighted in Figure 3 and herein designated with 8 and likewise illustrated in Figures 4A and 4B respectively for the heating and pressurization phases.

The guides can be locked at the end of the heating phase to prevent additional deformations of the containment structure 7, and then of the to-be- manufactured PFU 300, in the pressurization phase.

Each guide 3, in the preferred embodiment, is of linear type and comprises four supports with linear bearings, each one designated with 31 and one thereof shown singularly in Figure 7A.

Each guide 3 further comprises a central body 32, shown in detail in Figure 7B, which is sustained on the supports 31 at its own base 321. The latter has two through cross seats 322 and 323, which receive, each one, a respective sliding shaft 33, one thereof is shown in detail in Figure 7C. At its own upper portion, the body 32 has a connecting profile 324 for an attachment 34 to the containment structure 7, the latter shown in detail in Figure 7D. The attachment 34 has a cradle seat 341 engaging a lower portion of the housing 71 of the containment structure 7.

The attachment 34 also has an arm or side appendix 342 which provides a seat 343 for a locking rod 6, as explained in greater details hereinafter.

In the present embodiment, the apparatus 1 indeed comprises a plurality of locking rods, or abutments, 6 operating in the pressurization phase and which then are movable between a rest and a locking position.

In fact, to avoid collapse and/or permanent deformation, the to-be-manufactured PFU 300 has to be locked in its final position, that is all mechanical deformations deriving from pressurization have to be prevented, in particular:

- elongation in the rectilinear tracts 303;

- increase in the radius of curvature in the curved area 304, if present;

- lateral instability (peak load).

With reference to Figure 8, the locking is obtained by means of the above- mentioned movable rods 6, preferably placed in the curved portion and/or at the high-pressure joints 5. The joints 5 can even act as resting plane for such locking rods 6 during the pressurization phase.

As said, the herein considered locking system has the main purpose of opposing the axial elongation of the tube di PFU during pressurization. In case of PFU with curved portion, the locking system assists the cage structure 7 in resisting to the pressure load which would tend to vary the radius of curvature thereof.

The locking system, for example, can include servo-mechanical actuators connected to the rods 6 and arranged outside the vacuum chamber 101 which, through the rods 6 passing in the vacuum chamber 101 and in the hot chamber 102, lock the ends of the tube 301 (and, in case of curved portion, the motion of the radial guides) inserting in the compartments 343 present in the guides 3.

A suitably planned sufficiently stiff cage structure 7 may not require a dedicated locking system as the just described one.

Incidentally, it is reported that an apparatus implemented according to the teachings of the invention allowed to produce PFUs of a prototype of an ITER machine diverter. The so-implemented PFUs passed all, not destructive, size tests, and the cyclic thermal fatigue tests 10s ON - 10s OFF (5000 cycles with absorbed thermal flow of 10 MW/m ² and 300 cycles of 20MW/m ² ), allowing to qualify the productive process as suitable for the above-mentioned ITER machine.

The present invention has been sofar described with reference to preferred embodiments. It is to be meant that other embodiments belonging to the same inventive core may exist, as defined by the protective scope of the herebelow reported claims.