Manufacturing process of a cask for spent nuclear fuel and a cask manufactured using the process

Abstract

Manufacturing process of a cask for a storage of spent nuclear fuel and / or other nuclear waste, during which at least two parts of the cask body comprising at least a cylindrical shell part (2) of the cask body and a bottom part (3) of the cask body with a round bottom are created separately and, after that, the cask is created by welding these parts of the body together. The cylindrical shell part (2) is placed in a positioning device to be rotated around its central axis and, after that cladding beads of a corrosion-resistant CrNi material with a highly basic agglomerated flux are laid onto its inner surface by means of an electroslag welding method. The bottom part (3) of the cask body comprising at least a round bottom (4) of the cask (1) is placed in the positioning device for laying cladding beads by means of the electroslag welding method onto this round bottom (4), the cladding beads are laid onto the round bottom (4) by an electroslag welding using a corrosion-resistant OrNi material with a highly basic agglomerated flux. Cask manufactured according to the process comprises a cylindrical shell part (2) and a bottom part (3) which define a storage space, wherein an inner surface (5) of the cylindrical shell part (2) and of the bottom part (3) is provided with an electroslag cladding from at least one strip-type corrosion-resistant CrNi material and highly basic agglomerated flux.

Description

[0001] Manufacturing process of a cask for spent nuclear fuel and a cask manufactured using the process

[0002] Field of the invention

[0003] A manufacturing process of a cask for spent nuclear fuel according to the invention is suitable for creation of a cask for storage of spent nuciear fuel from nuclear power plants, especially advantageously VVER 1000 and VVER 440 nuclear power plants, but also nuclear power plants of other types. The cask created according to this manufacturing process is provided with an inner anticorrosive layer protecting an inner surface of the cask for spent nuclear fuel, which process, when being carried out, does not pose any threat to the environment. Further, the cask manufactured using this process is presented herein.

[0004] Background of the invention

[0005] There are various casks for spent nuclear fuel and various processes of their manufacture. Most frequently, casks are manufactured as forgings with walls from ca. 300 mm to ca. 500 mm in thickness, which are provided on the inner side with an anticorrosive layer protecting the cask material against corrosion. The cask wall thickness is in close relation to the parameters of the fuel being stored, which govern the requirements for shielding and required wall thickness.

[0006] The most commonly used cask manufacturing process is to provide a forging that is nickel- plated on its inner side to create corrosion protection for the inner surface. Nickel-plating creates a sufficiently rugged anticorrosive layer on the inner surface of the cask. An example of the manufacturing process of such cask is described in document CZ291901. Such a cask manufacturing process , however, is disadvantageous for the following reasons:

[0007] - Nickel-plating of the inner surface of the cask for nuclear fuel is a highly challenging operation in the technological point of view as it is necessary to nickel-plate the inner surface of the whole body of the cask, the weight of which exceeds frequently 100 t, with the nickel-plated area of the inner surface exceeding 20 m2, which means that this manufacturing process requires a workplace capable of handling such a big and heavy object. - Given the above-mentioned requirements for the workplace which makes it possible to nickel-plate the cask inner surface, the number of thus specialized workplaces is very low, which fact often brings the necessity to transport forgings to be nickel-plated over long distances; moreover, after the nickel-plating process, the forgings must be transported back to be finished.

[0008] - Nickel-plating produces harmful gases which threaten occupational safety and cause difficulties in environmental protection.

[0009] - For the economic point of view, therefore, it concerns a highly costly cask manufacturing process .

[0010] There is also a cask manufacturing process according to which at least two protective anticorrosive layers are welded upon each other on the inner surface to provide its resistance against corrosion, whereby achieving the required corrosion resistance. The term “layer” means a system of cladding beads laid appropriately next to each other to cover the surface that is to be resistant to corrosion. As the cask forging is mostly made of carbon steel, a first layer is welded on the cask inner surface at first while the cask is being preheated, with the austenitic welding filler material being alloyed with Cr and Ni in low percentages only in order to achieve connection with the cask body material of low anticorrosive properties. Subsequently, a second and possibly further layers are welded onto the first layer, which is now done without preheating, while observing the prescribed maximum inteipass temperature, and using a common austenitic anticorrosive material chosen according to the purpose for which the welded product is used. The disadvantages of this manufacturing manufacturing process are, on the one hand, the necessity' to preheat the forging during the welding of the first layer, which is a very' costly operation for the dimensions of the cask and, on the other hand, the high labour intensity due to the necessity to weld two or more layers, as well as the consumption of welding filler materials due to the need to create more layers. The individual anticorrosi ve layers are made up of beads of the welded material laid side by side to cover gradually the entire inner surface of the cask.

[0011] Document EP3995251B1 describes a process of application of a covering material onto a metal surface by means of electroslag strip cladding usable in nuclear power engineering, which includes the use of a strip welding electrode made of austenitic steel, namely 308L or 309L alloy, and a flux containing 3-6 wt% of chromium, 1,2 to 2.5 wt% of niobium, 0.3 to 3.0 wt% of molybdenum, and other substances. Further, there is document EP2881211B1 which describes the process of laying the cladding material onto the metal surface by means of electroslag strip cladding with a flux containing more than 55 wt% of CaF2, the appropriate flux and the object with such a cladding, while using a strip electrode made of material 625 with trade name INCONEL. However, the strip electrode used in the second document in particular is a very expensive material and thus not cost-effective for use in spent fuel assemblies, which are very large.

[0012] According to yet another known cask manufacturing cask manufacturing process , the cask inner surface is provided with a metal cladding using the process of hot application of suitable metals. This process makes it possible to create, for instance, zinc, aluminium, copper and other metal claddings on the surface required to be corrosion-protected. The disadvantage of this process is lower adhesiveness of the layer to the base material and its high porosity. Thus, the layer is not suitable for those applications in which fuel is loaded into the cask under water, for the subsequent process of drying the cask due to water penetrated into the pores of the inner surface would be unacceptably long.

[0013] Although it is possible, instead of forging from suitable steels, to manufacture the cask body also by casting to form a cast-iron body whose inner walls are then provided with a cladding made using one of the above-described processes, many of the above-mentioned issues remain in the cast-iron bodies of casks, with additional lower mechanical resistance of the cask caused by the lower density of the casting and, further, lower thermal conductivity of the cast material, which brings problems when storing spent fuel assemblies because such fuel assemblies still produce heat to be removed as effectively as possible to avoid cask overheating.

[0014] Subject of the invention

[0015] According to the first aspect of this invention, a manufacturing process of a cask for spent nuclear fuel from nuclear reactors, such as VVER 440, VVER 1000, but also PWR, BWR and others is presented, in which at least a cylindrical part of the cask body shell and a bottom part of the cask body are created first, which are then welded together, and, after producing these parts of the cask, an anticorrosive protective layer is created on their inner surface for a protection of the said inner surface of the cask against corrosion with one layer of an electroslag cladding, which anticorrosive protective layer is created using a strip welding filler material containing mainly chromium and nickel, hereinafter commonly referred to as “CrNi material” only, and a suitable highly basic agglomerated welding flux. The advantage of electroslag welding is that it causes minimum mixing of the layer being welded on with the base metal so that there is no danger of a potential degradation of the surface of the anticorrosive layer by the cask body material and a quality of the anticorrosive layer is secured in a depth required to avoid its degradation. An example of a suitable CrNi material used to create the anticorrosive layer of required properties for the protection of the said cask inner surface against corrosion is a material containing at least 18 wt% of chromium (Cr) and at least 8 wt% of nickel (Ni) or, more advantageously, one containing at least 18 wt% of chromium and 9 wt% of nickel or, even more advantageously, one containing 21 wt% of chromium and 11 wt% of nickel. An expert will understand that the welding filler material is expected to contain other elements, as usual in this area. Especially advantageously, the CrNi material then contains 0.3 to 0.9 wt% of niobium (Nb), which allows to achieve temperature stabilization of the resulting anticorrosive layer and to keep its anticorrosive properties even at the high temperatures reached in the cask when loaded with spent fuel. In its especially advantageous embodiment according to the invention, a CrNi material further containing 0.6 wt% of niobium (Nb) is used.

[0016] According to the present cask manufacturing process, an anticorrosive protective layer is created on an inner surface of the cask by electroslag welding of cladding beads of the above- mentioned CrNi material, wherein to lay the cladding, the appropriate part of the cask is placed in a positioning device, in which it is moved so as to provide an optimum position of the inner area of this part of the cask to apply an anticorrosive layer onto it by electroslag welding. An example of the positioning device used for electroslag welding of an anticorrosive layer on the inner surface of the cylindrical shell part is a pulley-type positioning device containing horizontal pulleys onto which the cylindrical shell part of the cask is placed and which then rotate this part by rolling it around its longitudinal axis, or a horizontal bench-type positioning device onto which the bottom part of the cask is placed and which makes it possible to move it. in a horizontal direction both around its vertical axis and in a rectilinear direction with appropriate transfer to create a rectangular area of weld cladding beads positioned next to each other in the centre of the round bottom of the bottom part. When laying the cladding beads, a position of the welding head must be kept so that the welding head remains in each place of the cladding at the same distance from the round bottom. An electroslag welding of cladding beads onto the inner surface of the parts of the cask body is always performed in a horizontal position from the top (perpendicular to the ground), with the welding head being in a fixed position when applying the anticorrosive layer and while moving the appropriate cask part being provided with cladding. The cylindrical shell part of the cask is rotated around its longitudinal axis, for instance, by laying the cylindrical shell part onto pulleys with the horizontal axis being in parallel with the mentioned longitudinal axis of the cylindrical shell part, so that by turning the pulleys the cylindrical shell part rotates on the positioning device on its outer circumference around its longitudinal axis, while the individual cladding beads are applied one by one with the welding head of the electroslag welding machine being placed in the appropriate position in the cylindrical shell part making it possible to apply the appropriate cladding bead onto its inner surface. When laying the cladding beads onto the round bottom of the bottom part of the cask, the round bottom rotates in the positioning device around the vertical axis, respectively it is moved in a plane for creating a central cladding. If the bottom part includes in addition to the round bottom also a wall segment of the cylindrical shell, the wall segment and the round bottom being connected, the cladding beads are applied to this part of the shell similarly as to the cylindrical shell part. The gap created between the cladding on the shell part of the bottom part and the cladding on the round bottom, which is not covered with any cladding bead, will be covered with a suitable cladding providing anticorrosive properties, as it is described in more detail below.

[0017] When carrying out the cask manufacturing process according to the invention, an anticorrosive layer of a welded-on CrNi material of 2.5 to 8 mm or, more advantageously, at least 3 to 8 mm in thickness is applied onto the cask inner surface by electroslag welding. According to an advantageous example of embodiment, the applied anticorrosive layer created from the welded-on CrNi material has also other thicknesses within the above-mentioned range, such as 3.5 mm, 4 mm, 4.5 mm, but it can be of any other thickness within the above-mentioned range 2.5 to 8 mm of the welded-on CrNi material. It was found out that any thickness of the applied anticorrosive layer greater than 8 mm for the purposes of cask manufacture increases the manufacturing costs disproportionately without a substantial added value. The reasons for limitation of the welded-on layer thickness to at most 8 mm, therefore, are mostly economic, not that such a thickness could not be produced. Moreover, one takes into account the possibility to machine the applied anticorrosive layer to a lower the thickness to achieve a suitable surface roughness. In a majority of applications of the process according to the invention, it is more advantageous to remove as low quantity of material as possible so as to avoid unnecessary cost increase by welding on an excessively thick anticorrosive layer of cladding beads, which would then be machined to a much lower thickness. Moreover, one has to realize that increasing the thickness of the anticorrosive layer will aggravate heat removal out from the inner space of the cask, which Ills undesirable. The especially advantageous scope of thickness of the welded-on anticorrosive layer is 3 to 5 mm, or more advantageously 4 to 5 mm. These scopes of thicknesses of the welded-on anticorrosive layer will provide the required thickness of the anticorrosive material on the inner wall even after being possibly machined, and are also advantageous in the economic point of view. To achieve a suitable surface roughness, the applied anticorrosive layer is machined advantageously to a resulting thickness of 3 to 4 mm, or more advantageously to a thickness of 3 to 3.5 mm.

[0018] A newly applied cladding bead has to be laid after reaching the interpass temperature in each adjacent, already laid cladding bead, which is 180 °C or lower. For the purposes of this description, the term “interpass temperature” means a temperature of individual cladding beads directly neighbouring with the cladding bead being laid, i.e., the cladding beads over which the new cladd ing bead is being laid, immediately before starting the application of the new c ladding bead. Exceeding the interpass temperature may affect adversely the required properties of the welded-on anticorrosive layer.

[0019] The advantage of applying the inner anticorrosive layer by electroslag cladding of the CrNi material is that it is neither necessary to preheat the given part of the casks, which is very energy intensive in casks for spent nuclear fuel, nor is it necessary to do it in several layers.

[0020] The cask manufacturing process according to the invention uses especially advantageously the method of electroslag welding (ESW) No. 72 or, especially advantageously, No. 721 per CSN EN ISO 4063 to create an anticorrosive layer of cladding on the cask inner surface. A CrNi strip electrode of suitable width and a suitable flux are an example of especially advantageous materials used to create an anticorrosive layer on the cask inner surface or its part. The welding filler materials used to weld on a cask anticorrosive layer are, for instance, ESAB OK BAND 309LNb ESW strip together with ESAB OK FLUX 10.10 flux, the suitability of this combination being documented by qualification of tills method according to WPQR and with a level 3.2 inspection certificate according to EN 10204. In this application, the composition of these welding filler materials is provided by reference. But the invention is definitely not restricted to these welding filler materials, because they can be any CrNi materials and fluxes making it possible to apply an anticorrosive layer containing chromium and nickel using the electroslag welding method.

[0021] Advantages of the proposed solution as against the state of the art:

[0022] 1 ) The cladding laid using the electroslag submerged-arc welding method is qualified and manufactured as one of a single layer, with its chemical composition being satisfactory due to the minimum mixing with the base material in view of resistance to corrosion on the whole clad surface of the cask inner area.

[0023] 2) The uniqueness of the proposed technical solution lies in adding a protective layer without pre-heating the cask body, the wall thickness of which is 300 mm as a minimum. An example of the cask body according to the invention is a forging made of a material falling within the group of ASME SA-350M materials with wall thickness of approximately 470 mm.

[0024] The advantages of the manufacturing process of the cask and of the cask according to the invention are as follows:

[0025] 1. Reduction of manufacturing costs as a result of eliminating the necessity to transport the cask for galvanization to a specialized workplace and then back to the manufacturer to finish the cask.

[0026] 2. Worker protection by eliminating unnecessary' handling of the whole cask body in connection with transport to the galvanizing plant and back to the manufacturer and also by not having to use a galvanization process to protect the inner surface, which are all highly challenging and dangerous tasks.

[0027] 3. Protection of the environment by reducing cask transport over great distances, by avoiding the potential consequences of the galvanization processes or lower energy intensity owing to eliminating the need to pre-heat the cask needed to apply two protective anticorrosive layers. 4. Reducing the waste rate occurring when using the current processes of cladding anticorrosive inner surfaces, because the cask manufacturing process according to the invention allows to use fully mechanized processes eliminating the effects of human mistakes.

[0028] 5. Better repairability of any defects of the added anticorrosive layer. In the case of a nickel-plated surface, any repair of potential defects is highly complicated, where tampon galvanizing is used for repairs, which is a lengthy and difficult process. The difficulty of such repairs is directly proportionate to the size and depth of potential defects.

[0029] Description of drawings

[0030] The examples of the embodiment of the equipment will be described below with references to the drawings, in which:

[0031] Fig. 1 presents a sectional view of the cask according to the first example;

[0032] Fig. 2 shows a top view of the bottom part;

[0033] Fig. 3 shows a top view of the botom part with the annular area of the anticorrosive layer being highlighted;

[0034] Fig. 4 presents a sectional view of the cylindrical shell part;

[0035] Fig. 5 presents a view of the bottom part with a wall segment;

[0036] Fig. 6 presents a sectional view of the cask according to the second example.

[0037] Invention embodiment examples

[0038] To facilitate the understanding of the cask manufacturing process and of the cask manufactured using the process according to the in vention, examples of the possible embodiment of this invention are described below with references to the enclosed drawings. These examples only serve to facilitate the understanding of and to illustrate the invention and should not be understood as restricting the invention to the described examples only. The drawings atached hereto are schematic only and are not intended to restrict the extent of the protection of the invention to the features depicted in them. Some elements may be exaggerated in the drawings for illustrative purposes, so that such elements are not drawn to scale. The dimensions of the individual elements provided on the drawings may not correspond to their actual dimensions in the application of the invention. Furthermore, expressions like “fest”, “second” and similar as used in the description and in the claims are intended to distinguish among similar elements, and it does not need to be to describe succession, temporality, space, superiority or any other way. One has to understand that the expressions thus used here are interchangeable under certain circumstances and that the embodiment of the invention as described here is able to perform in other sequences than as described or depicted here. Similarly, expressions like “on the top”, “below” and similar as used in the description and in the claims are intended for descriptive purposes, they relate to the orientation shown on the drawing and do not necessarily need to correspond to the relative situation. One has to understand that the expressions thus used here are interchangeable under certain circumstances and that the embodiment of the invention as described here is able to operate also in other orientations than as described or depicted here.

[0039] In the first example of the manufacturing process of a cask for nuclear fuel or nuclear waste storage according to the invention, body 1 of the cask as shown in Fig. 1 to 3 was manufactured, consisting of two parts, namely a cylindrical shell part 2 and a bottom part 3. The cylindrical shell part 2 was a cylindrical forging of 2655 mm in outer diameter, approx. 470 mm in wall thickness and 4220 mm in length, and the bottom part. 3_was a forging consisting only of a round bottom 4 of 2655 mm in diameter and 470 mm in thickness. All these forgings of the cask’s body according to this example of embodiment were made of a material falling within the group of ASME SA-350M materials.

[0040] An expert will understand that to manufacture parts of the cask by forging, their thickness may not be identical everywhere, which means, with respect to possible tolerances, that the wall thickness shown in both examples is the Intended one and, therefore, approximate only.

[0041] Fig. 1 shows a cross section of the first example of the cask 1 manufactured using the manufacturing process of a cask for nuclear fuel according to the invention. This cask 1 comprises the cylindrical shell part 2 of approx. 2655 mm in outer diameter and approx. 1715 mm in an inner diameter, and the bottom part 3 of the same outer diameter. The wall thickness is 470 nun. When using the cask manufacturing process according to the invention, the cylindrical shell part 2 was clamped first in a positioning device, which allows to rotate this cylindrical shell part 2 around its longitudinal axis to facilitate the laying of an electroslag cladding onto its inner surface 5 around the whole inner circumference with the welding head in a horizontal position from above according to CSN EN ISO 6947. Tire w elding filler material was clamped in the electroslag welding equipment in the form of a CrNi material strip 60 mm wide and 0.5 mm thick marketed as ESAB OK BAND 309LNb ESW, while a ESAB OK FLUX 10.10 flux was used as a highly basic agglomerated flux. The ESAB OK BAND 309LNb ESW welding filler material contains 21 .14 wt% of chromium (Cr) and 11 wt% of nickel (Ni). In addition to other elements, this material also contains approx. 0.6 wt% of niobium (Nb), the presence of which in the resulting anticorrosive layer stabilizes its anticorrosive properties even at the temperatures reached when storing nuclear fuel in the cask. The anticorrosive protective layer was laid onto the inner surface 5 of the cylindrical shell part 2 of the cask body using the electroslag welding method ESW type 721 in the horizonal position from above PA according to standard tSN EN ISO 6947 as already mentioned above. The first cladding bead of 5 mm in thickness was laid on the inner surface 5 of the cylindrical shell part 2 of the cask body being rotated gradually, whereby creating a strip of a welded-on anticorrosive material of 60 mm in width around the inner circumference of the cylindrical shell part 2. After finishing the first cladding bead around the whole inner circumference of the cylindrical shell part 2 (i.e., 360° of its inner circumference), which created a ring of the anticorrosive material on this part 2 of the cask body, the same way was used to lay another cladding bead after moving the welding head to the required position. The newly welded cladding bead was laid so as to overlap the previously laid cladding bead in approx. 4-6 mm. The size of the overlap of the neighbouring cladding beads was chosen advantageously so as to provide continuous coverage of the inner surface of the part of the cask in question and advantageously also the quantity of claddi ng beads. Laying a new cladding bead was started after the temperature of the adjacent, previously laid, cladding bead dropped to 170 °C in order to keep the rule that a new cladding bead is to be laid no sooner than when the adjacent, previously laid, cladding bead reached an mterpass temperature of 180 °C or lower. A similar procedure was used to lay other cladding beads until covering the inner surface of cylindrical shell part 2 with an integral anticorrosive layer of 5 mm in thickness. Subsequently, this anticorrosive layer was chip-machined to a resulting thickness of 3 mm, while achieving surface roughness Ra 12,5. In addition to the gradual laying of the individual cladding beads onto the cylindrical shell part 2 “next to each other”, other examples of embodiment of the cask manufacturing process also include other procedures of laying the cladding beads onto the cylindrical shell part 2, for instance, laying the 1st, 3rd, 5th, 7th, 9th, etc., i.e., every other cladding bead. In such a procedure, it is advantageous to lay the next cladding bead at a distance from the already created one, which is lower than the width of the cladding bead which will cover the distance. This will ensure continuous connection of the individual cladding beads to form the anticorrosive layer without any defects. Also, when laying the cladding bead overlapping at least one cladding bead created in its neighbourhood, or two neigbouring cladding beads, for instance, when laying cladding beads 2, 4, 6, 8 onto already laid 1 ,3,5,7 and 9 in the example above, it is important to observe the rule that a new cladding bead may only be laid after reaching the interpass temperature of each neighbouring, already laid, cladding bead, which is 180 °C or lower. According to yet another process, which is not depicted here, it is possible to lay individual cladding beads so that they do not overlap each other, and to connect them with a suitable cladding providing the anticorrosive properties while keeping the above-mentioned interpass temperatures. This process, however, is substantially more challenging both in terms of time and costs than creating the anticorrosive layer from overlapping cladding beads. In some of these examples of embodiment, the inner surface of the anticorrosive layer was machined, in others it was chip-machined, in one of the examples to surface roughness Ra 25, and to Ra 12.5 in another.

[0042] Fig. 2 shows the bottom part 3 of the cask body, which, in this example of embodiment, formed by a round bottom 4 of the cask X. The bottom part 3 was placed in a positioning device for laying the cladding beads onto the round bottom 4 using the electroslag welding method, which the cladding beads were created in 5 mm thickness by an electroslag welding from the above-mentioned CrNi corrosion-resistant material ESAB OK BAND 309LNb ESW, while ESAB OK FLUX 10.10 flux was used as a highly basic agglomerated flux. To cover the whole surface of the round bottom 4 with the anticorrosive layer, the cladding beads in the central area 2 of the anticorrosive layer of the round bottom 4 and the cladding beads in an annular area 6 arranged around the circumference of the round bottom 4 were laid separately. To the cask manufacturing process according to this invention, it applies in general that the reason for laying the cladding beads differently in the central area 7 and in the annular area 6 around the central area 7 is the fact that the central area 7 of the anticorrosive layer cannot be simply created by laying annular cladding beads because, in this area, it is complicated and even impossible to reach the required angular velocity and it is impossible to meet the requirement for a constant welding rate. According to this invention, therefore, this problem has been eliminated by dividing the area of round bottom 4 into the central area 7 and the annular area 6 around its edge.

[0043] In the central area 7, the cladding beads are laid in rows next to each other, i.e., similarly as when covering the inner surface of the cylindrical shell part 2. Covering the central area of the round bottom 4 with rows of adjacently laid cladding beads will ensure creation of an anticorrosive layer in this area while keeping the required welding parameters. Fig. 2 shows an example of embodiment of the bottom part 3 of the cask according to this invention with an anticorrosive layer in central part 7. When creating the central part £> the cladding beads were laid gradually next to each other, whereby creating an integral area of an anticorrosive layer in the centre of round bottom 4. These cladding beads were laid by a stationary welding head onto the bottom part 3 while performing rectilinear movements to create the welding beads next to each other in rows of identical and / or different lengths. In this example of embodiment, the central area 7 of the anticorrosive layer of the bottom part 4 was made of twelve cladding beads laid next to each other, which, in the places of their connection, were overlapped by approx. 4- 6 mm, while the length of the longest cladding beads of the cen tral area 7 was approx. 600 mm. The individual cladding beads were laid by rectilinear movement of bottom part 3 in one direction, and after finishing each cladding bead, the botom part 3 was moved laterally byan appropriate distance and to the level of the start of the new cladding bead neigbouring with the just finished one, after which the new cladding bead was laid. The whole cycle of laying the cladding beads in the central area 7 was performed similarly to that used when creating the anticorrosive layer on the cylindrical shell part 2, including the necessity to observe the interpass temperatures, which is why it will not be described in more detail below, as it is possible to use by analogy the procedures already described in connection with the creation of the anticorrosive layer on the surface of the cylindrical shell part 2 to create the rows in the central area 7. The created central area 7 of the anticorrosive layer was subsequently machined around its outer circumference to form a circular shape of 500 mm in outer diameter as shown in Fig. 2 by a circle having its centre in the centre of the round bottom. 'fhus the created central area 7 of the anticorrosive layer of a circular shape of 500 mm in diameter is shown in Fig. 3, with an uncovered area of an annular shape created between its outer circumference and the outer circumference of round bottom 4. Subsequently, the annular area 6 of the anticorrosive layer was created around the circumference of the central area 7 with overlapping by 20 mm, by laying eleven concentrically arranged the cladding beads 60 mm wide each, where the adjacent beads overlapped each other by approx. 4 to 5 mm. An uncovered area 13 490 mm wide was left on the outer edge of this annular part 6 for connection of the bottom part 3 with the cylindrical shell part 2. The cladding beads of the annular part 6 were laid so that the bottom part 3 was rotated around its central axis, i.e., the axis of symmetry going through the centre of the round bottom 4, while the welding head was stationary. Thus, the annular area 6 of the anticorrosive layer of the round bottom 4 was created without the anticorrosive layer being laid on the surface of round bottom 4 which forms an area 13 for a connection, to which the cylindrical shell part 2 will be placed subsequently. The cladding beads of the annular part 6 were laid gradually concentrically next to each other during the rotating movement of the bottom part 3 around the central axis, which, at this moment, forming a vertical central axis, thus creating an annular area of the anticorrosive layer of the round bottom 4. The cladding beads of both the central area 7 and the annular area 6 were laid 5 mm in thickness. When laying the neighbouring cladding beads, the rule was observed according to which the new cladding bead could not be laid before the interpass temperature of the adjacent, already laid, cladding bead dropped to at least 180 °C or lower. After finishing the laying of the anticorrosive layer in the central area 7 and in the annular area 6, the anticorrosive layer of the round bottom 4 was chip-machined to a resulting thickness of 3 mm and a surface roughness Ra 12.5.

[0044] The cylindrical shell pan 2 with the anticorrosive layer and the bottom part 3 with the anticorrosive layer were subsequently assembled to form the cask L This assembly was performed by placing the cylindrical shell part 2 onto the bottom part 3 in the area 13 for a connection on the round bottom 4, which was not covered with the anticorrosive cladding, and joining them together by means of a narrow gap weld 9 in the whole width of the wall of the cask 1 with the weld root located inside the cask. After making the narrow gap weld 9, its root was chip-machined and, after that, the resulting body of the cask 1 was placed again in the positioning device which allowed to rotate the cask around its longitudinal axis to facilitate the cladding of the inner surface of cask 1 to cover the vicinity of the root of the narrow gap weld 9 on the cylindrical shell part. This coverage was performed with an anticorrosive cladding 10 by laying the cladding beads next to each other by means of a gas-filled tube electrode, which were joined by overlapping the above-described, already laid anticorrosive layer. Finally, an anticorrosive layer was applied onto the seating surfaces of the cask lids and related areas which could not be protected by means of the electroslag cladding, using a welding method No. 121 — submerged-arc wire welding. Any other places which were not covered with an anticorrosive layer, such as those between the two cladding beads not overlapping each other, or other not treated places inside the individual parts of the cask body or already the whole cask body, were provided with an anticorrosive layer by submerged-arc or gas-filled tube electrode welding. However, it is also possible to use other processes of creation of an anticorrosive layer, such as a multi-layer cladding, as they are small areas where there is no need to pre-heat the material.

[0045] In the second example of the manufacturing process of a cask for nuclear fuel or nuclear waste storage according to the invention, the identical parts are identified with identical reference numerals. According to this example, the cask body I shown in Fig. 4 to 6 was manufactured, formed by two parts, namely a cylindrical shell part 2 and a bottom part 3. In this case, the cylindrical shell part 2 was a cylindrical forging of 2655 mm in outer diameter with a wall thickness 470 mm and 3240 mm in length, while the bottom part 3 was a forging of total height 980 mm, consisting of a round bottom 4 of 2655 mm in diameter and 470 mm in thickness and of a wall segment of 2655 mm in outer diameter, a wall thickness 470 mm and 510 mm in height measured from the round bottom 4, with the wall segment being set perpendicularly onto the round bottom 4 to form a part of the cylindrical shell of the cask, which will follow the cylindrical shell part 2 in the cask 1. All these forgings of the cask body according to this example of embodiment were made of a material falling within the group of ASME SA-350M materials. The advantage of this arrangement Is the easier performance of the weld of the cylindrical shell part 2 and the bottom part 3. In this example of embodiment, the laying of an anticorrosive layer of individual cladding beads onto the cylindrical shell part_2 will not be described again as the activity is identical with the above-described application of the anticorrosive layer onto the cylindrical shell part ? in the first example. Similarly, an application of the anticorrosive layer of the cladding beads onto the round bottom 4 of the botom part 3 is identical. According to this example of embodiment of the cask manufacturing process , the anticorrosive layer was applied onto the round bottom 4 using the same procedure as described in detail in the first example of the cask manufacturing process and, subsequently, the whole bottom part 3 was tilted by 90° so that its central axis (i.e., the axis of symmetry' going through the centre of the round bottom) changed from vertical to horizontal, after which, whilerotating the bottom part around this horizontal axis, circumferential cladding beads were laid one by one onto the inner surface of the wall segment in the thickness of 4 mm by means of the electroslag welding method using a corrosion-resistant CrNi material with a highly basic agglomerated flux similarly as when being laid onto the cylindrical shell part 2, where every two adjacent circumferential cladding beads of the botom part were laid while overlapping each other by approx. 4-5 mm to be joined solid. New cladding beads were laid no sooner than before the interpass temperature of the adjacent, already laid, cladding bead reached at most 180 °C. Subsequently, the space between the cladding on the wall segment and the cladding on the round bottom of the bottom part was enclosed with a cladding bead of an anticorrosi ve CrNi material.

[0046] Fig. 4 shows a cross-section of the cylindrical shell part 2 with cladding beads laid onto inner area 5, where the beads were laid one by one overlapping each other, while in the bottom part where the narrow gap weld 9 will be made to connect the cylindrical shell part with a wall segment 8 of the bottom part 3 as depicted in Fig. 5, an area was left with no anticorrosive layer welded on. Fig. 6 shows a cross-section of the assembled cask 1 according to the second example of embodiment.

[0047] The cylindrical shell part 2 with the anticorrosive layer and the bottom part 3 with the anticorrosive layer were subsequently assembled to form the cask L This assembly was performed by joining cylindrical shell part 2 with the wall segment 8 of the bottom part 3 by means of the narrow gap weld 9 in the whole w'idth of the wall of the cask 1 with the weld root located inside the cask J.. After making the narrow gap weld 9, its root was chip-machined and, after that, the resulting body of the cask 1 was placed again in the positioning device which allowed to rotate the cask around its longitudinal axis to facilitate the laying of an electroslag cladding onto the inner surface of cask 1 with the welding head in a horizontal position to cover the area of the root of the narrow gap weld 9. This coverage was performed with an anticorrosive cladding 10 of five cladding beads laid next to each other and joined by overlapping the abovedescribed, already laid anticorrosive layer of the cylindrical shell part 2 and of the wall segment 8 of the bottom part. As any of the procedures described for application of an anticorrosive layer onto the inner surface 5 of the cylindrical shell part 2 can be used to lay the cladding beads, these procedures are referred to without describing again the precise procedure of laying the cladding beads. Finally, thus applied anticorrosive layer covering the joint of both parts of the cask 1 was chip-machined to a Ra 12.5 roughness. An anticorrosive cladding using method No. 121 (submerged arc welding) or gas -filled tube electrode was laid onto the seating surfaces of the cask lids and the related areas which could not be protected with electroslag cladding.

[0048] The above-described embodiments of the cask manufacturing process with the described procedures of application of an anticorrosive layer onto the individual parts of the cask are not intended to be defined strictly by them, as this process can be modified and, virtually, it is possible to start with different areas of the individual parts of the cask in different order. Also, it is possible to combine the features of the described examples of embodiment, whereby creating other examples of embodiment. The presented examples of embodiment, however, represent an optimized process.

[0049] The CrNi strips used to perform the process according to this invention will be chosen advantageously from the following sizes: 30 x 0.5 mm; 60 x 0.5 mm; 90 x 0.5 mm; and 120 x 0.5 mm. Although other dimensions of CrNi strips are also possible, each of the strip size requires using different welding parameters.

[0050] Possible rotating and movement velocities of the individual parts of the cask are not subject of this invention as they will be determined on the basis of the results of the required welding qualification parameters and may differ for the different sizes of strips and welding filler materials in use.

[0051] In the examples of embodimen t described here, both the cylindrical shell part and the bottom part are manufactured advantageously from one piece of steel forged to the respective shapes. According to other examples of embodiment covered in this application, however, it is possible to create both the cylindrical shell part and the bottom part out of more pieces welded together. Other examples of embodiment of this invention are expected, according to which a cask can be manufactured, for instance, by applying an anticorrosive layer onto all individual parts first except the areas of their joints and, after welding these parts together to form a cask, the individual joints of these parts are covered with an anticorrosive cladding to finish coverage of the inner surface of the cask with the anticorrosive layer, or these parts can be joined in advance into units, onto which the anticorrosive layer is applied only after they have been welded together similarly as in the above-described examples where the individual cask body parts were made from one piece.

[0052] The above-provided description of the examples of invention embodiments was presented in order to illustrate the invention, not being intended as an exhaustive description of possible embodiments or as an invention being restricted to the form published here. The description has been designed to best explain the principles of the invention and the practical application of those principles when putting the invention into practice, so as to enable other persons skilled in the art to make the best use of the invention in various embodiments and various modifications suitable for the particular intended use. It is intended that the scope of the in vention not be limited to the description herein, but be defined by the patent claims set forth below.

[0053] Industrial applicability

[0054] The practical application of the proposed solution is expected mainly for protection of the inner surface of casks for spent nuclear fuel

[0055] Reference numerals

[0056] 1 Cask body

[0057] 2 Cylindrical shell part

[0058] 3 Bottom part

[0059] 4 Round bottom

[0060] 5 Anticorrosive layer of shell part

[0061] 6 Annular area of the anticorrosive layer (of round bottom) Central area of the anticorrosive layer (of round bottom) Wall segment of bottom part Narrow gap weld Anticorrosive cladding between wall segment of bottom part of cask body shell Anticorrosive cladding of sealing surfaces ofcask body shell Anticorrosive cladding in cask body weld area Connection area

Claims

AMENDED CLAIMS received by the International Bureau on 27 November 2025 (27.11.2025)Claims1. Manufacturing process of a cask for a storage of spent nuclear fuel and / or other nuclear waste, during which at least two parts of the cask body comprising at least a cylindrical shell part of the cask body and a bottom part of the cask body with a round bottom are created separately and, after that, the cask is created by welding these parts of the body together, characterized in that the cylindrical shell part (2) is placed in a positioning device to be rotated around its central axis and, after that, while gradually rotating the cylindrical shell part (2), circumferential cladding beads of a corrosion-resistant CrNi material with a highly basic agglomerated flux are laid onto its inner surface in thickness from 2.5 mm to 8 mm by means of an electroslag welding method gradually around its circumference and, after finishing the laying of the first cladding bead, while gradually rotating the cylindrical shell part (2), further cladding beads are laid gradually until an anticorrosive layer is created on the inner surface of the cylindrical shell part (2) by these cladding beads; the bottom part (3) of the cask body comprising at least a round bottom (4) of the cask (1) is placed in the positioning device for laying cladding beads by means of the electroslag welding method onto this round bottom (4), the cladding beads are laid onto the round bottom (4) in thickness from 2.5 mm to 8 mm by an electroslag welding using a corrosionresistant CrNi material with a highly basic agglomerated flux, and to cover the whole surface of the round bottom (4), the cladding beads of a central area (7) of the anticorrosive layer of round bottom (4) and the cladding beads of an annular area (6) of the anticorrosive layer of the round bottom (4), arranged around the circumference of the round bottom (4), are laid separately, with the cladding beads of the central area (7) being gradually laid onto the bottom part (3) next to each other during rectilinear movements of the bottom part and with the cladding beads of the annular area (6) being laid concentrically next to each other during the rotating movement of bottom part (4) around the vertical central axis to create an annular area around the circumference of the round bottom, after which the annular area and the central area will overlap each other to ensure the coverage of the whole surface of the round bottom.AMENDED SHEET (ARTICLE 19)2. Manufacturing process of a cask according to claim 1, characterized in that the central area (7) of the anticorrosive layer is created first on the bottom part (3) from cladding beads laid next to each other, after which this central area (7) is machined around its circumference to make a round shape with its centre in the centre of the round bottom (4), and then the cladding beads of the annular area (6) are laid.

3. Manufacturing process of a cask according to any of claims 1 and 2, characterized in that the bottom part (3) comprising in addition to the round bottom (4) also a wall segment (8), is placed in the positioning device and rotated for laying the cladding beads onto the inner surface of the wall segment (8) of bottom part (3) so that its central axis changes from vertical to horizontal, then circumferential cladding beads of thickness from 2.5 mm to 8 mm are gradually laid onto the inner surface of wall segment (8) by electroslag welding using a corrosion-resistant CrNi material with a highly basic agglomerated flux while rotating the bottom part (3) around this central axis, and then the area between the anticorrosive cladding of the wall segment (8) of the bottom part and the anticorrosive cladding of the round bottom (4) of the bottom part is provided with an anticorrosive layer from cladding beads of corrosion-resistant CrNi material.

4. Manufacturing process of a cask according to any of claims 1 to 3, characterized in that every two adjacent cladding beads are laid while overlapping each other, with the newly applied cladding bead being laid only after the interpass temperature of the adjacent, already laid, bead is at most 180 °C.

5. Manufacturing process of a cask according to any of claims 1 to 4, characterized in that the cladding beads are laid onto the inner surface of the cylindrical shell part (2) of the body of cask (1) gradually one by one, or every other cladding bead is laid with a strip being left between two neighbouring cladding beads of width smaller than that of the cladding bead to be laid between them, wherein the cladding bead overlapping at least one of the laid cladding beads being only laid after reaching the interpass temperature of at most 180°C or, more advantageously, at most 150°C, of such already laid adjacent cladding bead.AMENDED SHEET (ARTICLE 19)6. Manufacturing process of a cask according to any of claims 1 to 5, characterized in that the cladding beads of thickness from 3 mm to 5 mm are laid onto the inner surface of the cylindrical shell part (2) and of the bottom part (3).

7. Manufacturing process of a cask according to any of claims 1 to 6, characterized in that after laying the cladding beads onto the inner surface of the cask, they are machined to an identical width with a surface roughness Ra 25 or lower or, more advantageously, Ra 12.5 or lower.

8. Cask manufactured according to at least one of Claims 1 to 7, comprising a cylindrical shell part (2) and a bottom part (3) which define a storage space, characterized in that an inner surface (5) of the cylindrical shell part (2) and of the bottom part (3) is provided with an electroslag cladding from at least one strip-type corrosion-resistant CrNi material and highly basic agglomerated flux in thickness from 2.5 mm to 8 mm.

9. Cask manufactured according to claim 8, characterized in that the above-mentioned striptype corrosion-resistant CrNi material contains at least 9 wt% of nickel (Ni) or, more advantageously, 11 wt% of nickel (Ni), and at least 19 wt% of chromium (Cr) or, more advantageously, 21.00 wt% of chromium (Cr).

10. Cask manufactured according to Claim 8 or 9, characterized in that the above-mentioned strip-type corrosion-resistant CrNi material contains 0.3 to 0.9 wt% of niobium (Nb).

11. Cask manufactured according to any of Claims 8 to 11, characterized in that the degree of roughness of the inner surfaces is at least RA 25 or lower or, advantageously, the degree of roughness of the inner surfaces is at most Ra 12.5.AMENDED SHEET (ARTICLE 19)

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