GB1582933A

GB1582933A - Handling pot for a nuclear fuel assembly

Info

Publication number: GB1582933A
Application number: GB1983/78A
Authority: GB
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1977-01-26
Filing date: 1978-01-18
Publication date: 1981-01-14
Also published as: DE2803355A1; JPS5393297A; ES466106A1; IT1106974B; BE862530A; FR2379139A1; IT7867095A0; FR2379139B1

Description

(54) HANDLING POT FOR A NUCLEAR FUEL ASSEMBLY (71) We, COMMISSARIAT A L'EN ERGIE ATOMIQUE, of 29, rue de la Federa tion, 75752 Paris Cedex 15, France, a French Body Corporate, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a handling pot, specially adapted for transferring a nuclear fuel assembly from the core of a fast neutron nuclear reactor to a storage container or enclosure outside the core tank via a hod or the like, in order to load and unload such structures.

It is known that, in fast neutron nuclear reactors wherein the core is formed by the juxtaposition of a series of nuclear fuel assemblies containing a fissile and/or fertile material, these structures are cooled by the circulation of a suitable volume of a liquid metal, generally sodium, in a tank containing the core. It is also known that, when a spent assembly in the core is due to be replaced by a new assembly, after a specified period of operation, this spent assembly has appreciable residual power which makes it essential to leave the assembly in a separate container or storage enclosure when this power can decrease until it reaches a level which is sufficiently low for the assembly to be allowed to be transport.ed, under suitable con ditions of security, to a reprocessing plant.

In order to be moved from the core to the storage enclosure, the spent assembly is placed, in known manner, in a handling pot open at its upper end and filled with liquid metal, the said pot being designed to ensure.

effective transfer of the heat released by the assembly.

More precisely, in order to improve the transfer of heat, the idea of mounting a sleeve arranged coaxially with the nuclear fuel assembly inside the handling pot has already been implemented. The temperature of the liquid metal inside the sleeve, between the said sleeve and the assembly, is higher than that outside this same sleeve, between it and the inner wall of the pot Consequently, as the result of the thermal gradient created, the liquid metal is made to circulate by a thermosiphon effect, which improves the heat exchange between the assembly and the wall of the pot over its entire height. Moreover, the idea of providing fins of various forms over the outer surface of the handling pot, to increase its emissiveness and the surface area of exchange, has a;;o been used.

The present invention relates to an im- provement to a handling pot of the kind described hereinbefore, which substantially improves the desired heat exchange between the irradiated nuclear fuel assembly contained in the pot and releasing substantial residual power, and the outer atmosphere surrounding it.

For this purpose, the handling pot under consideration, which comprises an inner sleeve arranged coaxially with the nuclear fuel assembly and mounted vertically in the pot to define an outer space between said sleeve and the pot and to define an intermediate space between said sleeve and the said assembly for the circulation, by natural convection, of a liquid medium contained in the pot in contact with the nuclear fuel assembly, is characterised in that the said sleeve comprises two successive parts located in axial extension of each other, the lower part oonsisting of a heat conducting structure and the upper part consisting of a thermally insulating structure, whereby the natural convection of liquid medium is increased.

In a first embodiment of the invention, the upper part of the sleeve comprises two thin walls parallel to and coaxial with each other defining between them a sealed leakproof space filled with a thermally insulating material, particularly a woven metal-type material or the like. In a variant, the leakproof space may be filled with an inert gas or be under vacuum, whilst in the latter case the walls are prevented from caving in by the presence intercalated bracing means, for example of the nature of metal shot, especially cast iron.

Advantageously, the lower and upper parts of the inner sleeve are connected by a metal bellows so as to permit differential expansion of the inner and outer walls of the said upper part.

In another embodiment, the upper part of the sleeve is substantially thicker than the lower part, the two parts preferably being made of the same material, although they can be made of different materials.

Preferably, the sleeve is made integral at its bottom end with a transverse support bracket provided in the inner surface of the handling pot, and has lateral openings in its lower part for the passage of the liquid metal.

Finally, according to another feature, the handling pot consists of a cylindrical metal jacket and has longitudinal cooling fins in its outer surface.

Other features and advantages of a handling pot constructed according to the invention will become apparent from the following description of two exemplarly embodiments, which are given as a guide and not in any limiting capacity, with reference to the accompanying drawings, wherein: Fig. 1 is a diagrammatic view of an axial section through a handling pot containing a nuclear fuel assembly, provided with an inner sleeve constructed according to a first embodiment of the invention, Fig. 2 shows a variant of a detail of the pot according to Fig. 1, Fig. 3 is a sectional view of an inner sleeve according to another embodiment of the invention, Fig. 4 shows a detail of a variant of the embodiment according to Fig. 3.

In the example illustrated in Fig. 1, the handling pot shown consists of a cylindrical jacket 1, made of stainless steel, in particular, and provided on its outer surface with means which facilitate the transfer of heat by radiation, these means being of the nature of longitudinal fins 2, triangular in section.

Of course, it would also be possible to pro vide other heat disperrive means on the out side of the jacket, for example by machining corrugations thereon to increase its surface area or by subjecting the surface to appro priate chemical treatment.

The jacket 1 of the handling pot contains a suitable volume 3 of a liquid metal, gen erally sodium, the level of which reaches a point below the open upper end of the jacket at 4. Inside this jacket is a nuclear fuel assembly 5 which is to be transported, whilst this irradiated assembly releases substantial residual calorific power which must therefore be transferred using means according to the invention.

The assembly 5 is made up, in known manner, of a casing 6 of polygonal section terminating at its lower end in a cylindrical base 7 provided with transverse apertures 8 enabling the liquid sodium contained in the pot to circulate partially inside the casing of the assembly in order to cool the nuclear fuel rods which it contains.

According to the invention, the nuclear fuel assembly 5 which is placed vertically in the jacket 1 of the handling pot is surrounded by a sleeve 9 in two successive parts 10 and 11. In particular, this sleeve comprises a thin-walled lower metal part 10 which has at its base a recessed portion 12 on which a spherical bearing surface 13 provided in the casing 6 of the assembly 5 abuts, this sleeve 9 itself being integral, particularly by means of a weld 9a or other fixing means, with a transverse bracket 14 provided in the inner wall of the cylindrical jacket 1 and comprising holes 14a for the liquid sodium flowing through them beneath the bracket 14 before returning through the holes 8 in the base 7 inside the assembly 5.The sleeve 9, together with the outer surface of the casing 6 of the assembly 5, defines an annular space 15 extending over the entire height of the sleeve and permitting the circulation of the liquid sodium penetrating inside the lower part 10 of the sleeve through wide transverse apertures or ports 16 provided in the latter. The upper part 11 is made up of two metal walls, 19 and 20 respectively, which are parallel to and coaxial with the jacket 1 of the handling pot, these two walls 19 and 20 defining between them a leakproof space filled with a suitable heat insulating material 21, generally a woven metal material, particularly of stainless steel. At its upper part, the space containing the insulating material is closed off by a crown 22 which is integral with the walls 19 and 20.As a variant, as illustrated in Fig. 2, the leakproof space contained between the walls 19 and 20 may be filled with an inert gas such as argon, or may be under vacuum; in the latter case, in order to prevent the walls caving in, this space may advantageously be filled with granulated shot 21a, particularly cast iron, or other appropriate intercalated means to brace the walls in a suitable manner.

Preferably, the bottom end of the upper part 11 is located at a level corresponding to the lower end of the fissile zone of the assembly 5. In order to permit differential expansion of the walls 19 and 20 of the upper part 11, this part 11 is connected to the lower part 10 in the manner described hereinafter. A metal bellows 18 is integral with the bottom end of the inner wall 19, on the one hand, and with the top end of the lower part 10, on the other hand. This latter also comprises a lateral extension 17 which is integral with the outer wall 20. Finally, the inner wall 19 is fitted with a flange 19a intended to keep the insulating material 21 in place, the said flange being directed towards the wall 20, leaving an annular clearance relative to the latter.

As a result of these arrangements, the inner sleeve 9 consisting of two parts 10 and 11 of different structures creates a thermal gradient between the inner space 15 on the one hand and the outer space 23 defined between this sleeve and the wall of the jacket 1 of the pot, this gradient being sufficient to cause the sodium to circulate by a thermosiphon effect. The sodium penetrates into the space 15 through the holes 16 provided in the lower part 10 of the sleeve, then rises up inside the sleeve, remaining in contact with the casing 6 of the nuclear fuel assembly 5, and finally leaves the sleeve to flow into the space 23 below the terminating crown 22.

It will be noted that, with the jacket of the handling pot filled with liquid sodium, circulation by the thermosiphon effect can occcur only if weight of the column of sodium contained in the space 23 is greater than the weight of the column of sodium contained inside the sleeve. The heat transferred from the nuclear fuel assembly by the circulation of the sodium is then conducted through the jacket 1 of the handling pot and is dissipated into the outer atmosphere by radiation and convection.

In another embodiment illustrated in Fig.

3, the inner sleeve 9 is again made up of two parts, 30 and 31 respectively, but in this case they are made of the same material, namely a thin sheet of stainless steel for the lower part 30 and a thicker steel sheet for the upper part 31, welded to the first sheet at 32. Advantageously, the twcspart inner sleeve has a thickness of stainless steel of the order of 7mm in its lower part 30, whereas in its upper part 31, at its thickest point, the radial dimension of the sleeve is of the order of 32 mm. It should be noted that the parts 30 and 31 could also be made from different materials, with the lower part 30 made from a metal which is a good conductor of heat and the upper part 31 made from a material which is a good insulator.However, in practice, taking the conditions of use into account, it is more advantageous to construct the pot in two parts 30 and 31 made from stainless steel.

The welded joint 32 between the parts 30 and 31 may be made more easily if the part 31 is given the form shown in Fig. 4, wherein the part 31 terminates in a conical end 3 la prolongated by a cylindrical endpiece 31b which in turn is welded at 32 to the part 30.

In each of the above embodiments, the rate of flow of the liquid sodium inside the sleeve and in contact with the nuclear fuel assembly is of the order of 1000 g of sodium per second, whilst the flow rate inside the assembly represents only 0.5 % of the total flow rate, the majority of which thus passes to the outside of the casing. It should be noted that this low internal flow is nevertheless sufficient to reduce the radial temperature gradient inside the assembly between a peripheral rod and a central rod in the bundle of rods inside this assembly.

In this way, a handling pot is obtained which gives a better and more reliable performance compared with the known solutions.

In particular, a pot of this kind permits suitable transfer of heat released by the nuclear fuel assembly in cases where the pot becomes fixed, during handling, in a position such that its lower part is immersed, or not, as the case may be, in the sodium contained in the core tank, whilst its upper part emerges totally or partially into the gaseous atmosphere above the free surface of the sodium.

WHAT WE CLAIM IS: 1. Handling pot for a nuclear fuel assembly, comprising an inner sleeve arranged coaxially with the said assembly and mounted vertically in the pot to define an outer space between said sleeve and the pot to define an intermediate space between said sleeve and the said assembly for the circulation, by natural convection, of a liquid medium contained in the pot in contact with the nuclear fuel assembly, characterised in that the said sleeve comprises two successive parts located in axial extension of each other, the lower part consisting of a heat conducting structure and the upper part consisting of a thermally insulating strucrure, whereby the natural convection of liquid medium is increased.

2. Handling pot according to claim 1, characterised in that the upper par.t of the sleeve comprises two thin walls parallel to and coaxial with each other, defining between them a sealed leakproof space.

3. Handling pot according to claim 2, charcterised in that the leakproof space is filled with a thermally insulating material.

4. Handling pot according to claim 2, characterised in that the leakproof space is filled with an inert gas.

5. Handling pot according to claim 2, characterised in that the leakproof space is under vacuum, the two walls being braced against each other by means of metal shot.

6. Handling pot according to claim 2, characterised in that the lower part and upper part of the timer sleeve are connected by a metal bellows permitting differential expansion of the said parallel and coaxial thin walls of the upper part.

7. Handling pot according to claim 1, characterised in that the upper part of the sleeve is substantially thicker than the lower part, these two parts being made of the same material.

8. Handling pot according to any of claims 1 to 7, characterised in that the sleeve is integral, at its bottom end, with a transverse

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

inner sleeve 9 consisting of two parts 10 and

11 of different structures creates a thermal gradient between the inner space 15 on the one hand and the outer space 23 defined between this sleeve and the wall of the jacket 1 of the pot, this gradient being sufficient to cause the sodium to circulate by a thermosiphon effect. The sodium penetrates into the space 15 through the holes 16 provided in the lower part 10 of the sleeve, then rises up inside the sleeve, remaining in contact with the casing 6 of the nuclear fuel assembly 5, and finally leaves the sleeve to flow into the space 23 below the terminating crown 22.

It will be noted that, with the jacket of the handling pot filled with liquid sodium, circulation by the thermosiphon effect can occcur only if weight of the column of sodium contained in the space 23 is greater than the weight of the column of sodium contained inside the sleeve. The heat transferred from the nuclear fuel assembly by the circulation of the sodium is then conducted through the jacket 1 of the handling pot and is dissipated into the outer atmosphere by radiation and convection.

In another embodiment illustrated in Fig.

3, the inner sleeve 9 is again made up of two parts, 30 and 31 respectively, but in this case they are made of the same material, namely a thin sheet of stainless steel for the lower part 30 and a thicker steel sheet for the upper part 31, welded to the first sheet at 32. Advantageously, the twcspart inner sleeve has a thickness of stainless steel of the order of 7mm in its lower part 30, whereas in its upper part 31, at its thickest point, the radial dimension of the sleeve is of the order of 32 mm. It should be noted that the parts 30 and 31 could also be made from different materials, with the lower part 30 made from a metal which is a good conductor of heat and the upper part 31 made from a material which is a good insulator.However, in practice, taking the conditions of use into account, it is more advantageous to construct the pot in two parts 30 and 31 made from stainless steel.

The welded joint 32 between the parts 30 and 31 may be made more easily if the part 31 is given the form shown in Fig. 4, wherein the part 31 terminates in a conical end 3 la prolongated by a cylindrical endpiece 31b which in turn is welded at 32 to the part 30.

In each of the above embodiments, the rate of flow of the liquid sodium inside the sleeve and in contact with the nuclear fuel assembly is of the order of 1000 g of sodium per second, whilst the flow rate inside the assembly represents only 0.5 % of the total flow rate, the majority of which thus passes to the outside of the casing. It should be noted that this low internal flow is nevertheless sufficient to reduce the radial temperature gradient inside the assembly between a peripheral rod and a central rod in the bundle of rods inside this assembly.

In this way, a handling pot is obtained which gives a better and more reliable performance compared with the known solutions.

In particular, a pot of this kind permits suitable transfer of heat released by the nuclear fuel assembly in cases where the pot becomes fixed, during handling, in a position such that its lower part is immersed, or not, as the case may be, in the sodium contained in the core tank, whilst its upper part emerges totally or partially into the gaseous atmosphere above the free surface of the sodium.

WHAT WE CLAIM IS: 1. Handling pot for a nuclear fuel assembly, comprising an inner sleeve arranged coaxially with the said assembly and mounted vertically in the pot to define an outer space between said sleeve and the pot to define an intermediate space between said sleeve and the said assembly for the circulation, by natural convection, of a liquid medium contained in the pot in contact with the nuclear fuel assembly, characterised in that the said sleeve comprises two successive parts located in axial extension of each other, the lower part consisting of a heat conducting structure and the upper part consisting of a thermally insulating strucrure, whereby the natural convection of liquid medium is increased.
2. Handling pot according to claim 1, characterised in that the upper par.t of the sleeve comprises two thin walls parallel to and coaxial with each other, defining between them a sealed leakproof space.
3. Handling pot according to claim 2, charcterised in that the leakproof space is filled with a thermally insulating material.
4. Handling pot according to claim 2, characterised in that the leakproof space is filled with an inert gas.
5. Handling pot according to claim 2, characterised in that the leakproof space is under vacuum, the two walls being braced against each other by means of metal shot.
6. Handling pot according to claim 2, characterised in that the lower part and upper part of the timer sleeve are connected by a metal bellows permitting differential expansion of the said parallel and coaxial thin walls of the upper part.
7. Handling pot according to claim 1, characterised in that the upper part of the sleeve is substantially thicker than the lower part, these two parts being made of the same material.
8. Handling pot according to any of claims 1 to 7, characterised in that the sleeve is integral, at its bottom end, with a transverse

support bracket provided in the inner surface of the handling pot and comprises, in its lower part, lateral openings for the passage of the liquid metal.
9. Handling pot according to any of claims 1 to 8, characterised in that it comprises a cylindrical metal jacket and has longitudinal cooling fins on its outer surface.
10. Handling pot according to claim 1, characterised in that the materials consisting the upper and lower parts of the sleeve are different