Classifications

• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21F—PROTECTION AGAINST X-RADIATION, GAMMA RADIATION, CORPUSCULAR RADIATION OR PARTICLE BOMBARDMENT; TREATING RADIOACTIVELY CONTAMINATED MATERIAL; DECONTAMINATION ARRANGEMENTS THEREFOR
  • G21F9/00—Treating radioactively contaminated material; Decontamination arrangements therefor
  • G21F9/28—Treating solids
  • G21F9/34—Disposal of solid waste
  • G21F9/36—Disposal of solid waste by packaging; by baling

Definitions

• the invention relates to a case for the storage under water of irradiated fuel assemblies and to a method for producing such a case.
• 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.
• 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 .
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the powder of boron carbide is covered with a layer of nickel which thickens over time.
• 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%.
• 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.
• 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.
• 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.
• 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.
• the tubular envelope 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.
• 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.
• 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.
• tubular 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.
• the method described therefore made it possible to obtain a thin-walled housing comprising an effective absorbent layer but of small thickness.
• 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.
• 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.
• 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.
• 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.
• the coating operation is terminated by an electrolysis of 2 hours with a current density of 2 amperes per dm2 in order to
• the box is then removed from the rinsed and dried electrolysis tank.
• 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.
• 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 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.

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• Engineering & Computer Science (AREA)
• Environmental & Geological Engineering (AREA)
• Physics & Mathematics (AREA)
• General Engineering & Computer Science (AREA)
• High Energy & Nuclear Physics (AREA)
• Other Surface Treatments For Metallic Materials (AREA)
• Physical Or Chemical Processes And Apparatus (AREA)

Applications Claiming Priority (4)

Publications (2)

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ID=26222158

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• 1981
  • 1981-12-22 YU YU305181A patent/YU305181A/xx unknown
  • 1981-12-30 ES ES1981279675U patent/ES279675Y/es not_active Expired
  • 1981-12-31 WO PCT/FR1981/000175 patent/WO1982002453A1/fr not_active Ceased
  • 1981-12-31 BR BR8108942A patent/BR8108942A/pt unknown
  • 1981-12-31 EP EP81402101A patent/EP0055679A3/de not_active Withdrawn
• 1982
  • 1982-04-05 ES ES511154A patent/ES8506411A1/es not_active Expired
  • 1982-04-05 ES ES511155A patent/ES8506412A1/es not_active Expired

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