EP1087408B1 - Process for producing a neutron-absorbing coating - Google Patents

Abstract

A process for producing a coating for absorption of neutrons produced in a nuclear reaction, comprises dipping a screening material in a dispersion bath, where it is coated with an element with a high neutron capture cross section and an electrolytic or autocatalytic metallic element. During coating, there is movement between the surface and the bath. The element with the high capture cross section is in the form of an electrically conducting cpd.

Description

The invention relates to a method for producing a coating for absorbing the neutrons produced in the nuclear reaction of radioactive materials. The invention also relates to an absorber element produced by the process.

For the treatment of radioactive materials originating, in particular, in the field of nuclear reactor technology, these are shielded from each other by the inevitably radiated neutrons, depending on the task, material and condition, for example for changing and / or checking and for transport and / or storage. To achieve a desired neutron absorption usually absorber elements in the form of various manholes, canisters, tubes or similar configuration are made, which surround a neutron-emitting object and shield him thereby. The use of such absorber elements enables, for example, the compact storage of neutron-emitting elements, in particular fuel elements from nuclear power plants.

EP 0 385 187 A1 discloses a fuel element storage rack in which absorber sheets form a number of ducts which surround the fuel elements over their entire length. These absorber elements are manholes or tubes made of a neutron-absorbing material, for example boron steel, a stainless steel with a boron content of 1 to 2%. Apart from the required production costs, these absorber elements are extremely expensive and the efficiency is limited because of the limited - boron content. In an attempt to increase the boron content, the deposition of a boron-nickel alloy was checked. Although the proportion of boron can be increased to up to 8%, the costs increase by a factor of about 10, so that an economical use of such pipes can not be considered.

For other tasks, such as the transport and / or storage of radioactive materials, methods are used in which deposited on the metallic surfaces of containers nickel layers.

US Pat. No. 4,218,622 describes a composite absorber element which has a thin carrier foil or a thin carrier plate onto which a polymer matrix in which boron carbide particles have been incorporated is applied. The material of the carrier film or of the carrier sheet is preferably glass fiber-reinforced polymer. The boron carbide particles are uniformly distributed on the surface of the polymer matrix, with a boron concentration of up to 0.1 g / cm ² . When using the composite absorber part in a fuel storage rack, this absorber element has a thickness of up to 7 mm, is configured in the form of a foil or sheet, and suspended between an inner wall and an outer wall. Whether a homogeneous distribution of the arranged on the surface of the polymer matrix boron carbide particles is ensured over a longer time, in particular with regard to a possible abrasion on the surface, can not be found in the US-PS 4,218,622.

EP 0 016 252 A1 describes a method for producing a neutron-absorbing absorber element. In the process, boron carbide is applied to a substrate together with a metallic substance by means of plasma spraying, the boron carbide being incorporated into a matrix of a metallic substance. The process also takes place in such a way that oxidation of the boron is avoided. The absorber element thus produced should be stable to a liquid medium, as it is present for example in a fuel storage pool. The thickness of the plasma sprayed metal and boron carbide layer is at least 500 μm. The proportion of boron carbide is about 50% by volume. As the metallic substance, aluminum, copper and stainless steel are considered, the substrate containing the same metallic substance as the sprayed layer. To achieve effective neutron absorption, a relatively thick layer of boron carbide is required, in particular, the thickness of the layer is 3 to 6 mm.

From DE-AS 1 037 302 and DE 2 361 363 it is known to provide pipes, in particular cans, on their outer surface by electrolytic means with absorber material for protection against radioactive radiation. With regard to the process engineering processes and devices for the technical implementation of the physico-chemical state changes and material conversions for applying the absorber materials, no information can be taken from DE-AS-1 037 302 and DE 2 361 363.

EP 0 055 679 A2 discloses processes for the production of shielding elements, boron carbide being applied to the surface of the shielding element either in a plasma coating process, or according to a electrolytic or chemical Vorvernickelung of the shielding element boron carbide scattered as a powder on the surface and the shielding element is then electrolyzed or chemically nachvernickelt. According to these methods, only small quantities of boron carbide in the order of 20 wt.% With respect to nickel are applied to the surface. It therefore requires very strong layers, so that these prior art methods are uneconomical. In practice, these methods were not used, since they are not technically feasible in terms of process engineering. The application of a powder to a surface in the sense of spreading is not a measure that ensures a secure industrial production.

All previously known methods and shielding elements produced therefrom can be regarded as uneconomical in terms of large production costs and a large cost of materials. In addition, the variability of the shape of the shielding and the extension of the application options is limited.

From US-A-3,625,821 a fuel assembly for nuclear reactors is known which comprises a fuel tube having an inner surface coated with a support metal having a low neutron capture section in which layer combustible reactant cages are incorporated. Boron compounds can serve as reactant compounds.

The production of boron steel is very expensive. The steel is melted and boron is enriched by complex processes up to the 10 valence and mixed with the molten steel. This results in a boron steel with 1.1 to 1.4 wt .-% boron. This steel can be very bad work, is extremely brittle and can be poorly welded. Shielding elements made of this have an extremely high weight with average absorption properties. For example, are made of boron steel produced inner storage containers, so-called baskets, known for the intermediate storage of fuel elements, which have a weight of about 10 tons.

From WO 98/59344 a method for producing a coating for neutron absorption is known, wherein corresponding surfaces of a shielding element is provided with a boron / nickel layer, wherein in the Dispersion bath boron in elemental form or boron carbide present. Although high Boreinbauraten can be achieved, but the incorporation rate is limited when using boron in elemental form and the coating has a high hardness and thus a high brittleness. Boron carbide has only poorly conductive properties, at best semiconductor properties, and is therefore difficult to electrolytically or not at all controllable. This results in only slow layer structures and poor layer formations. Due to the relative movement generated results in a certain randomness in the layer structure. As a result, the process is very complicated as a whole, because it is very demanding in terms of the materials used, the process management and the like.

Starting from the prior art, the present invention has the object to improve a method for producing a coating or by shielding the absorption of the produced during the nuclear reaction of radioactive materials neutron further, which is economical and easy to use, the effectiveness of the absorption increases, with respect to the base materials and shape of the shielding allows greater variability, is technically well controlled and in particular allows the production of lighter absorber elements with at least the same absorption qualities.

For the technical solution of this object, a method for producing a coating for absorbing the resulting in the nuclear reaction of radioactive materials neutrons according to claim 1 is proposed.

It has been shown that the training z. B. a Bornickelschicht in a dispersion bath with temporary relative movement between the surface to be coated and the dispersion bath brings very good results. The use of conductive compounds of elements with high neutron capture section results in a good electrolytic controllability and it has surprisingly been found that the incorporation rates can be significantly increased. This results in the possibility of forming much smaller layer thicknesses.

Elements with a high neutron capture range include elements from the group boron, also in elemental form or boron carbide, gadolinium, cadmium, samarium, europium or dysprosium. The high neutron capture section represents the size of the capture cross section for neutrons of the respective element. As conductive compounds in particular metallic compounds have proven to be particularly useful. These include metal borides such as iron boride, nickel boride and the like. The list is exemplary and extensible in relation to the named elements. The conductivity stands for the good electrolytic controllability, so that the process can be performed under less demanding conditions with high reliability and reproducibility.

As electrolytically or autocatalytically depositable metallic element are in particular nickel, cadmium or copper. The high neutron capture section element or its compounds are incorporated in this metal matrix with the corresponding effect.

It is particularly advantageous to use isotopes of the respective elements which have an enlarged neutron capture section. For example, it is known that the use of ¹¹ B signifies a neutron capture section of 0.005 barn, while the use of the isotope ¹⁰ B signifies 3837 barn. This results in the possible lower layer thicknesses.

This results in a much greater effectiveness due to the high storage rates. The absorption layers are in the order of magnitude of up to 800 μm. In addition, a special advantage is the independence of the process from the base material. Advantageously, inorganic base material is to be used, for example steel, stainless steel, boron steel, titanium, aluminum, copper, nickel and the like, including corresponding alloys. Despite its organic character, carbon fiber material may be considered as the base material. Carbon fiber material has the particular advantage of galvanotechnical manufacturability of the absorption element.

Also according to the invention it is possible to manufacture the absorber element in the finished state or in individual parts. Due to the independence of the base material very easy machinable materials can be used. On the other hand, even very complicated forms of absorber elements, containers, baskets and the like, can be completely prefabricated and then coated according to the invention.

Because of the high rate of incorporation, the shield is extremely effective so that the layers can be extremely thin. Thus, weight savings of up to 50% are possible with respect to shielding elements which can be produced by conventional methods. The currently used in the container program for fuel storage storage containers (baskets) of previously about 10 t can be after the process according to the invention now produce in the order of 4 to 6 tons.

The base material can be prefabricated as a finished part or item, so that can be formed from the items finished absorber elements. The assembly of the absorber elements or the parts of absorber elements to complete bearings or baskets can be made by positive and / or positive connections. The invention also enables the coating of complete storage racks and baskets. The coating in the dispersion bath is carried out either chemically or electrolytically.

The relative movement between the surface to be coated and the dispersion bath can be effected, for example, by a movement of the element to be coated in the dispersion bath. As is known, elements such as boron and the like are such that circulating or circulating the dispersion is practically not economically possible. Any circulating or pumping unit would be worn out in no time. Nevertheless, on the one hand a further good mixing or repeated mixing of the dispersion is to be achieved by the relative movement, on the other hand a directional supply of the dispersion to the surface to be coated. In addition to the movement of the element itself and the entire coating system for the purpose of generating the relative movement can be moved. For example, the implementation of the coating in a kind of drum is conceivable. The relative movement can also be effected by mechanical movement of the bath, blowing gas, in particular air, ultrasound support and combinations thereof.

With particular advantage, the invention proposes that the surface to be coated is arranged in the dispersion bath pointing upwards. This means that the surface to be coated in such Disperse bath is arranged that due to gravity, the particles in the dispersion fall to the surface. This arrangement according to the invention, in particular in combination with the temporary generation of a relative movement between the surface and the dispersion bath, favors excellent coating results.

With particular advantage is proposed by the invention that the coating process is carried out in a ceramic or glass pan. This ensures a special purity of the dispersion bath.

With the invention, an easily feasible, economical and very effective method for the production of absorber elements for neutron absorption is specified, which makes particular basematerial-independent absorber elements that are considerably easier with comparable absorption effects than known shielding.

The invention also relates to absorber elements produced by the process described in claims 1-13. These are characterized by having a coating formed of a high neutron capture section element and nickel having a proportion of the element or its high neutron capture section compound of up to 60% by volume and 40% by volume, respectively. The layer thickness is 350 to 500 microns to 800 microns, wherein the layer is formed on an inorganic base material such as steel, titanium, copper or the like. Layer thicknesses up to 2000 μm can be realized. The training takes place chemically or electrolytically. The shielding element may have been coated in finished form or composed of individual coated individual parts. As the electrolyte come, for example, in question without electroless nickel-phosphorus or electrolytic nickel.

In one experiment, conventional steel plates were electrolytically coated in a nickel / boron carbide dispersion bath. The plates were turned every half hour in the bath and temporarily moved up and down to create a relative movement between the surfaces and the dispersion bath on the one hand, on the other hand to arrange each surface to be coated facing up in the bathroom. Boron carbide in the range of 40% by volume could be incorporated into the nickel matrix as subsequent analyzes showed.

Claims (14)

1. A method of producing a coating for the absorption of neutrons produced in the nuclear reaction of radioactive materials, wherein a portion of an element consisting of a base material is provided on its surfaces predetermined therefor in a dispersion bath with a layer formed from an element with a high neutron capture section and a metallic element which is electrolytically or autocatalytically precipitable, whereby relative motion is produced during the coating process between the surface to be coated and the dispersion bath, at least periodically, wherein at least one element from the group consisting of boron, gadolinium, cadmium, samarium, europium or dysprosium is used as the element with a high neutron capture section, characterised in that the element with a high neutron capture section is present in the dispersion bath in an electrically conductive compound and is deposited with a proportion of more than 20 volume % in the coating.
2. A method as claimed in Claim 1, characterised in that one of the elements from the group nickel, cadmium or copper is used as the electrolytically or autocatalytically precipitable metallic element.
3. A method as claimed in one of the preceding claims, characterised in that a metallic compound is used as the conductive compound of the element with a high neutron capture section.
4. A method as claimed in one of the preceding claims, characterised in that metal boride is used as the conductive compound of the element with a high neutron capture section.
5. A method as claimed in one of the preceding claims, characterised in that the element with a high neutron capture section is used in the form of an isotope with an increased neutron capture section.
6. A method as claimed in one of the preceding claims, characterised in that the relative movement is produced by movement of the element to be coated.
7. A method as claimed in one of Claims 1 to 5, characterised in that the relative movement is produced by the injection of gas and/or ultrasonic loading.
8. A method as claimed in one of the preceding claims, characterised in that the formation of the layer occurs chemically.
9. A method as claimed in one of Claims 1 to 7, characterised in that the formation of the layer occurs electrolytically.
10. A method as claimed in one of the preceding claims, characterised in that a layer of thickness of up to 800 µm is produced.
11. A method as claimed in one of the preceding claims, characterised in that the element with a high neutron capture section or its compounds is incorporated in the metal matrix in an amount of up to 60 Vol.%.
12. A method as claimed in one of the preceding claims, characterised in that the dispersion bath is thoroughly mixed during the coating process, at least periodically.
13. A method as claimed in one of the preceding claims, characterised in that the method is performed in a ceramic or glass bath.
14. An absorber element produced by the method as claimed in at least one of the preceding Claims 1-13.