Patent Number: 053389417
Section: description

SPECIFIC DESCRIPTION Referring first to FIG. 3, it can be seen that a radiation shielding container for receiving spent fuel elements of a nuclear reactor, especially for transport, but also for storage and adapted to be filled in a water basin as previously described, is represented at 10 and comprises a spherulitic cast iron body 11 whose outer surface is provided with a sealing coating 18 of a powder melt and consisting of nickel or a nickel-based alloy, including chromium/nickel 18-8 austenitic alloy. A similar coating, referred to as a powder melt coating can be provided at 19 at the interior of the container defining the space 20 receiving the irradiated fuel elements. The coating can be applied to the seats 13 in which the stepped shoulders 14 of the spherulitic cast iron cover 12 can be received. The entire container can be closed by a lid 15 which is attached by bolts 16 to the cover 12 and bolts 17 to the container body. FIG. 1 shows a cast body 1 having a surface 2 provided with open pores 3. On this surface a sealing layer 4 is applied to nickel or nickel-based alloy. As can be seen in FIG. 1, the coating 4 is applied galvanically, i.e. by electroplating techniques. It must be applied in a multiplicity of layers a, b, c, d and e in order to bridge the open pores 3. Not only is the open pore 3 not filled, but because of the potential characteristics in the region of the pore during the electrodeposition process, the coating 4 contains a cavity in the region of the pore which is only closed by a thin layer of the coating although the overall thickness of the layer is considerable. As a consequence, the coating is sensitive to mechanical disruption which can cause breakthrough to that cavity and expose the open pore to the action of water. The effectiveness of this type of coating in preventing the container from forming a galvanic element in a water basin during filling with the irradiated fuel elements, therefore, is limited. By contrast, as can be seen in FIG. 2, when the sealing layer 4 has a texture 5 of a layer solidified from a particle melt in which the particles have diameters substantially smaller than the diameters of the pores, the layer fills the pore 3 and the overall layer thickness can be substantially smaller. The layer thickness for example, may be of the order of 100 micrometers and the particle size can be of the order of 1 to 10 micrometers. The system has been found to be highly advantageous for spherulitic cast irons having the following compositions: 3.2 to 3.8% by weight carbon, 1.6 to 2.6% by weight silicon, 0.1 to 0.3% manganese, 0.025 to 0.06% by weight magnesium, the balance being iron and the usual elements unavoidably present in spherulitic cast irons. The preferred composition of the powder is 99 or more percent by weight nickel and phosphorous and other elements commonly present with nickel, and high purity nickel can be present here as well. As can be seen from FIG. 4, the spherulitic cast iron substrate 21 can be coated with powder 22 from a powder spray nozzle and a laser beam played back and forth across the powder coated surface as represented by the laser 24 to fuse the particles together and to the substrate. In principle, therefore, following casting of the body in an initial step at 30, represented in FIG. 5, the body of the container can be subjected to an abrasive surface treatment at 31 by shot peening or the like and can then be coated with the powder or droplets by plasma spray 32 or coated with the powder as described in connection with FIG. 4 in a separate application 33 followed by a laser fusion in a successive step 34. FIG. 6 shows the path of the laser beam 36 on the powder coated surface 35 of the substrate 37, i.e. the back and forth or reciprocating path previously mentioned.