EP0055679A2 - Container for under water storage of irradiated fuel assemblies, and method to realize such a container - Google Patents

Abstract

The casing for the underwater storage of fuel assemblies comprises a parallelepipedal metal envelope coated on the outside with a neutron absorbing material. The thickness of the metal envelope is comprised between 1.5 and 2.5 mm and the thickness of the coating is at the most equal to 2 mm The mass of boron carbide per cm<s2>s of surface of casing exceeds 0.146 g. The coating may be obtained by projection by a plasma torch of boron carbide grains completely coated with nickel or by gravity deposit of boron carbide fixed on the casing by electrolytic or chemical deposit of nickel. The invention applies to the storage of fuel assemblies in swimming pools.

Description

The invention relates to a case for the storage under water of irradiated fuel assemblies and to a method for producing such a case.

In the field of the operation of nuclear reactors, in particular water nuclear reactors, it is necessary to store or transport radioactive materials, such as irradiated fuel assemblies leaving the reactor core and to improve conditions storage or transport of these materials, it is necessary to have boxes whose walls absorb the neutrons emitted by these materials.

In particular, it is desirable to have boxes for storing irradiated fuel assemblies in the fuel pool, comprising walls made of a material having good neutron absorption capacity, in order to limit the radioactivity in the fuel pool and to its neighborhood.

In the case of nuclear water reactors, these housings are generally constituted by a parallelepipedic envelope of stainless steel with square section of sufficient dimensions to receive a fuel assembly, open at one of its ends and externally coated with a neutron absorbing material .

In the fuel pool, these boxes rest by their lower base on supports common to a set of boxes where they are arranged side by side so that the fuel assemblies can be stored under water, vertically, inside the pool of the combustible.

The square section fuel assemblies used in nuclear reactors have a very large height compared to their transverse dimensions, so that the assemblies storage boxes themselves have a very elongated shape.

It is necessary to maintain sufficient space between the boxes constituting a storage area or rack for ensuring adequate thermal contact between the water in the reactor pool and the box and for ensuring a sufficient moderating effect. To ensure that a sufficient sheet of water exists between the faces of the boxes, they must have perfectly controlled dimensions and geometry. Provisions are therefore generally made for boxes with high rigidity, making it possible to ensure sufficient mechanical strength and maintaining the gaps between the faces of the boxes. inside the storage rack. The metal envelope of these boxes is therefore of relatively large thickness, for example of the order of 3 to 4 mm.

On the other hand, the neutron absorbing coatings deposited on the external faces of the cases must contain a quantity of boron carbide per unit area of the case sufficient to ensure efficient absorption of the neutrons emitted by the irradiated assemblies.

To obtain effective absorption of neutrons by a coating deposited on the lateral external surface of the housing, it is necessary to have at least 20 mg of boron 10 which is the absorbent element contained in boron carbide, on a cm 2 of surface , which represents approximately 0.146 g of boron carbide for the same surface.

There are known boxes for storing irradiated fuel assemblies whose external neutron absorbing coating consists of boron carbide particles fixed to the wall to be coated with an organic binder such as a polymer resin. For long storage periods in the fuel pool, it is impossible to use boxes having such a coating on their external surface because the polymer resins risk being destroyed during a prolonged stay in the pool water. combustible. On the other hand, such polymer resins coating boron carbide constitute a thermal insulator between the assembly contained inside the case and the water of the fuel pool.

Also known are boxes for the storage of irradiated fuels constituted by a metal casing coated with debore carbide particles coated in a metallic material such as nickel.

Such a coating can be obtained by spraying at a high temperature a mixture of nickel powder and boron carbide, on the external surface of the housing, using a plasma torch.

However, to obtain a sufficient density of absorbent material on the surface of the housing, it is necessary to deposit relatively thick layers of coating, for example with a thickness of 3 to 6 mm. It is indeed necessary to use a relatively large proportion of nickel powder in the powder mixture, to ensure good adhesion of the coating to the substrate.

If smaller thicknesses of the coating are used, the latter no longer has sufficient absorbent properties.

Boron steel absorbent plates are also known but the thickness necessary to obtain a sufficient absorbent effect is of the order of 8 to 9 mm.

The boxes currently known therefore have relatively large wall thicknesses, which significantly increase their mass and size in the transverse directions.

The object of the invention is therefore to propose a case for the storage under water of irradiated fuel assemblies with a square section constituted by a metallic parallelepipedic envelope with a square section of sufficient dimensions to receive a fuel assembly, open to at least one with its two ends and externally coated with a neutron absorbing material constituted by particles of boron carbide coated in a metallic binder constituted by nickel, this casing having to have a rigidity and an adequate capacity for absorbing neutrons, while being a reduced mass and transverse size.

For this purpose, the thickness of the metallic envelope is between 1.5 and 2.5 mm and the thickness of the neutron absorbing coating is at most equal to 2 mm; the mass of boron carbide per cm2 of surface of the housing being greater than 0.146 g over the entire external surface thereof, with the exception of the areas adjacent to the edges.

In order to clearly understand the invention, there will be described by way of nonlimiting example, several embodiments of a case for the storage of irradiated fuel assemblies in the fuel pool of a nuclear water reactor.

Example 1: A tubular envelope of stainless steel was made with a length slightly greater than 4 meters and a diameter of 28 cm, from a sheet of thickness 2 mm.

Was placed along 4 generatrices of this tubular envelope arranged for two of them in a first plane passing through the axis of the tubular envelope and for the other two sections the plane perpendicular to the first plane passing through the axis of tubular envelope, covers making it possible to mask each an area with a width of approximately 20 mm on the lateral surface of the tubular envelope.

A deposit of boron carbide particles coated with nickel is then carried out on this tubular casing provided with its covers using a plasma torch ammented by coated powders.

To produce this neutron absorbing coating layer, a plasma torch with a power of 40 KW is used. ensuring the projection of a powder formed of particles of boron carbide coated with nickel.

Boron carbide is used in the form of a powder whose grains have a size of between 60 and 100 microns for the preparation of a coated powder with which the plasma torch will be fed. For the production of the coated powder, an initiation layer of palladium is deposited on the calibrated boron carbide grains by immersing the powder in a solution containing a few grams of sodium nitrite per liter, a few ppm of palladium per liter and a few drops of wetting agent. After immersion, the powder is drained and then dried for two hours at 110 °.

The boron carbide grains then have a very thin surface layer of absorbed palladium which is a practically monoatomic layer.

This powder is then introduced comprising the initiation layer into coating tubes closed at each of their ends by wire cloths making it possible to retain the powder inside the tube.

The coating tubes are then moved continuously in a chemical nickel plating bath of the Kanigen type.

During its stirring in the nickel-plating bath, the powder of boron carbide is covered with a layer of nickel which thickens over time. During various tests, the treatment was extended to obtain grains of boron carbide coated with nickel, in which the mass of the boron carbide relative to the weight of nickel represents from 20 to 50%.

At the end of the operation, the coating tubes are rinsed and the nickel-coated carbide powder is collected and dried in an oven for 2 hours at 120 °.

The powder is then ready to be used for projection in the plasma torch.

• - courant d'alimentation : 700 ampères sous 30 volts
• - débit d'argon : 30 m3 par heure
• - débit de poudre : environ 2 kg par heure.

A powder comprising, by mass, one third of boron carbide and two thirds of nickel, was used to feed the plasma torch which operates under the following conditions:

• - supply current: 700 amperes at 30 volts
• - argon flow: 30 m3 per hour
• - powder flow: approximately 2 kg per hour.

A coating with a thickness of 1 mm was made on the surface lateral side of the tubular casing which, taking into account the concentration of boron carbide in the powder, provides the desired density of neutron absorbing elements per cm 2 of the surface of the casing, with the exception of the areas masked by the caches.

During the entire coating operation, the tubular casing is rotated about its axis at constant speed and the torch moves in a direction parallel to the axis of the tubular casing. Several torches can also be used, each moving over a portion of the length of the tubular casing, to reduce the duration corresponding to the coating.

Prior to the plasma torch deposition operation, the tubular envelope can be preheated to a temperature allowing better adhesion of the particles at the time of their projection.

-Compared to previous techniques where a mixture of boron carbide powder and nickel was used, the grains of boron carbide entirely coated with nickel undergo no oxidation at high temperature at the outlet of the plasma torch and the deposition obtained has a very homogeneous composition.

In this way, it is possible to deposit a homogeneous and highly adhesive layer of a coating comprising a very high proportion of boron carbide. It is therefore possible to limit the thickness of the coating to a low value, for example 1 mm.

Each of the particles of boron carbide coated with high density nickel is projected onto the substrate at high speed with high kinetic energy and is welded onto it at the time of impact by temperature rise and by mechanical effect. The softening of the surface layer of nickel and its heating in fact allow very effective attachment to the substrate or the coating layer itself at the time of impact.

It has also been realized that a chemical deposition of nickel on the particles of boron carbide leads to better adhesion of the particles, during the formation of the coating than an electrolytic deposition of nickel, due to the melting point more bottom of the chemical deposit.

Another advantage of using coated boron carbide grains is that the powder can be stored and handled without fear of the separation of its constituent elements. The thickness of the nickel layer deposited on the grains of boron carbide has a thickness of between 2 and 10 microns for 80% of the particles which have been tested. We were able to make sure Check with these controls that the boron carbide grains are completely coated with a layer of nickel at the end of the treatment which has been described.

The coating process which has just been described makes it possible to obtain an extremely homogeneous coating, each of the grains of the powder brought to the torch having practically the composition of boron carbide and nickel of the deposit to be produced on the casing.

After coating, the tubular casing is cooled to room temperature and then put into the form of a parallelepiped with a square section of 22 cm side, by cold deformation.

The folding of the tubular envelope for the realization of the parallelepiped edges is done according to the generatrices located under the covers along which no coating deposition has been made. This prevents bursting of the coating during the final mechanical shaping of the parallelepiped.

The dimensions and the precise geometry of the casing envelope are obtained after coating, so that the possible deformations of the tubular envelope at the time of its preheating and at the time of the plasma deposition have no effect on the shape and the final dimensional accuracy of the case.

If the deposit had been made directly on a metal envelope of parallelepiped shape, we could not have guaranteed obtaining, with a wall thickness of 2 mm, a precise geometry and dimensions of the case, because of the deformations of thermal origin.

The method described therefore made it possible to obtain a thin-walled housing comprising an effective absorbent layer but of small thickness.

The fact that the area near the edges of the case is not covered with a coating has practically no effect on the neutron absorption capacity of the latter. In any case, the increase in the thickness of the water layer made possible by the small thickness of the housing more than compensates for this small reduction in the neutron absorption capacity of the housing.

To improve the wear and corrosion resistance of the housings, the coating process can be completed using a plasma torch, by supplying the torch with nickel or stainless steel powder so as to produce a layer with a thickness close to 200 microns above the coating layer comprising boron carbide, which eliminates the roughness of the coating due to B4C. The extremely smooth and continuous nickel or stainless steel surface layer plays a role protective for the coating layer.

Example 2: A stainless steel box of parallelepiped shape was produced.

The unit which has undergone chemical degreasing and then electrolytic degreasing is subjected to a chemical depassivation treatment in hydrochloric or fluonitric bath. The box is then placed in a large electrolysis tank filled with a bath containing NiCl2 at the rate of 250 g per liter and hydrochloric acid at the rate of 130 g per liter. Then performs processing of _R electrolytic assivation in two stages on the outer surface of the housing. During the first phase, or anode phase, lasting 15 seconds, the box constitutes the anode and the electrolysis current density is 1 to 2 amperes per dm2.

_During the second phase or cathodic phase, lasting 2 minutes, the box constitutes the cathode and the electrolysis current is 3 amperes per dm2.

The four faces of the case are then pre-nickeled, inside the electrolysis tank containing a working bath comprising NiS04 at the rate of 280 g per liter of NiCl2 at the rate of 45 g per liter, H3B03 at a rate of 45 g per liter, as well as a few milliliters of a wetting agent. The duration of this treatment is half an hour and the current density is 4 amperes per dm2. Then carried out, successively on each of the external faces of the box placed in a horizontal position in the electrolysis tank, a set of operations aimed at obtaining an absorbent coating, by rotating the box a quarter of a turn .between each deposition of boron carbide powder, after fixing the last deposited layer.

These operations successively include the constitution of a regular layer of particles of boron carbide B4C on one face of the housing, the electrolytic deposition of nickel through the layer of powder of B4C until the particles are perfectly attached to the substrate and joined together, then the deposition of a new layer of B4C particles followed by the deposition of electrolytic nickel for the attachment of these particles, the number of successive layers required being determined by the quantity of boron carbide to be deposited per unit of surface of the case.

The entire housing is placed in the electrolysis tank so that the nickel layers grow at the same time on all four sides sides of the case.

A powder was used consisting of B4C particles with a maximum dimension of 200 microns and each of the layers of boron carbide deposited by gravity on the faces of the housing had a thickness identical to the maximum dimension of the particles, ie say 200 microns. In this way, unwanted overlaps of the particles are avoided and the regularity / thickness / neutron efficiency of the coating is improved and improved. The coating therefore consists of a superposition of monolayers, that is to say of layers of powder constituted by grains arranged side by side with a minimum of superposition of several grains, these monolayers being linked together by the deposition of nickel.

It is easy to determine the quantity of powder necessary for the production of a monolayer on a determined surface and corresponding to the area of each of the faces of the housing.

The nickel deposits between the boron carbide particles are obtained using an electrolysis bath identical to that used for pre-nickel plating and a current density of 2 amperes per dm2. Each of the successive electrolysis operations is continued for half an hour.

When the density of boron carbide deposited per cm2 of face of the housing is greater than 0.146 g, the coating operation is terminated by an electrolysis of 2 hours with a current density of 2 amperes per dm2 in order to

to permanently fix the coating particles and to produce a continuous layer of nickel on top of the layers of boron carbide particles coated with nickel.

The box is then removed from the rinsed and dried electrolysis tank.

In all cases, it was possible to obtain a density of boron carbide, that is to say an efficiency of the case with regard to the absorption of neutrons, sufficient, with a deposit of a total thickness less than 2 mm .

The various layers of particles of B4C trapped in the nickel matrix deposited one above the other are practically continuous insofar as one carries out a regular distribution of these particles to constitute the successive layers. However, it is possible to use particles of a smaller size, for example 60 microns in admixture with particles of 200 microns. The particles of smaller size are interposed between the particles of 200 microns to produce a continuous layer of boron carbide.

The superimposition of several monolayers makes it possible to avoid the presence of coating zones containing a very small amount of B4C. The repair tition of the particles is therefore extremely homogeneous and on the other hand the proportion of B4C particles relative to the nickel matrix is large, if we compare it with what was obtained by the previously known methods. This proportion is for example, in the case which has just been described, 50% by mass. A coating is therefore obtained easily by the process according to the invention comprising the desired quantity of boron carbide and therefore of boron 10 per cm 2 of substrate, while having a coating layer with a total thickness of less than 2 mm.

Example 3: Operations similar to those which have been described in connection with Example 2 are carried out on a stainless steel casing of parallelepiped shape and using a chemical bath instead of an electrolytic bath. The operations carried out are identical, namely a pre-nickel plating of the substrate, then a gravitational deposition of a first layer of boron carbide particles, then a chemical nickel plating allowing the particles to be bonded, followed by a new deposition of a monolayer. boron carbide particles which are then bonded together by chemical deposition of a nickel layer, these successive operations continuing until the coating comprises a sufficient quantity of boron carbide per cm2.

The invention is not limited to the embodiments which have just been described, on the contrary it includes all variants thereof.

Thus, the neutron absorbing coating can be produced on the housing by a method different from those which have been described above, that this deposition can be carried out both on the housing having its final shape and on a blank which is then shaped and that the metal envelope may be made of a metallic material other than stainless steel, for example aluminum.

The thickness of the metal casing can be different from 2 mm, but however in the case of boxes for the storage of fuel assemblies under water, it is necessary that this thickness is between 1.5 and 2.5 mm, for reconcile both the requirements concerning the mechanical strength of the box and the heat exchanges between the fuel assembly contained in the box and the pool water.

The invention applies not only to the storage of fuel assemblies for a water nuclear reactor but it also applies to the transport of irradiated materials using boxes according to the invention as transport containers.

Claims (12)

1.- Case for the storage under water of irradiated fuel assemblies with square section constituted by a metallic parallelepipedic envelope with square section of sufficient dimensions to receive a fuel assembly, open at at least one of its two ends and coated externally with a neutron absorbing material consisting of particles of boron carbide coated in a metallic binder constituted by nickel, characterized in that the thickness of the metallic envelope is between 1.5 and 2.5 mm and that l the thickness of the neutron absorbing coating is at most equal to 2 mm, the mass of boron carbide per cm 2 of surface of the housing being greater than 0.146 g over the entire external surface thereof, with the exception of the areas adjacent to the edges. 2. Housing according to claim 1, characterized in that a continuous layer of nickel covers the absorbent coating. 3.- Housing according to any one of claims 1 and 2, characterized in that the thickness of the neutron absorbing coating is close to 1 mm. 4.- A method of producing a housing according to any one of claims 1, 2 and 3, characterized in that one realizes a tubular envelope with a thickness between 1.5 and 2.5 mm, qu 'there are along four generators arranged in two perpendicular axial planes, covers over the entire length of the tubular casing,
that a coating consisting of boron carbide coated with nickel is produced on the external surface of the tubular casing, the covers preventing the deposition of coating in the region near the generators along which they are arranged, by high projection temperature of a powder constituted by grains of boron carbide B4C whose external surface is entirely coated with a layer of nickel which is deformed cold the tubular envelope, after coating, so as to give it the form of a parallelepipedic box, the edges of which correspond to the generators along which the covers preventing the deposit of coating have been placed. 5.- Method according to claim 4, characterized in that the powder is sprayed onto the metal surface using a plasma torch. 6.- Method according to claim 4, characterized in that the grains of boron carbide comprise a very thin palladium initiation layer which is itself covered by the nickel layer. 7.- Method according to claim 4, characterized in that the nickel layer on the boron carbide grains has a thickness between 2 and 10 microns. 8.- Method according to claim 4, characterized in that the boron carbide represents a propbrtion of 20 to 50% by mass relative to nickel. 9.- Method according to claim 4, characterized in that the nickel layer on the boron carbide grains is obtained chemically. 10.- A method of producing a housing according to any one of claims 1, 2 and 3, characterized in that a rectangular metal envelope is produced from a sheet, which is produced on the lateral surface external of this case, a prior deposition of nickel, then a gravity deposition on this first layer of nickel, the surface to be coated being substantially horizontal, of particles of a certain particle size in boron carbide, so as to constitute a layer of which the thickness corresponds substantially to the size of the particles, then a deposit of nickel ensuring the bonding of the particles with the first layer of nickel and the bonding of the particles together, a deposition of a new layer of particles then a deposition of nickel at through this new layer of particles, if the quantity of boron carbide is not sufficient to obtain the desired effectiveness of the coating absorbing the neutrons, then possibly a new deposit of particles of boron carbide and a new deposit of nickel, these operations being repeated a sufficient number of times to obtain a quantity of boron carbide greater than 0.146 g iar çm2 of the lateral surface of the case by rotating the metal casing of a quarter turn after each boron carbide deposition operation followed by the fixation of these particles by nickel. 11.- Method according to claim 10, characterized in that the nickel coatings are obtained by electrolytic means. 12.- Method according to claim 10, characterized in that the nickel coatings are obtained chemically.