The machining process of Zircaloy-4 nuclear fuel rod end-caps

Abstract

Zirconium alloy 4 (Zry-4) is one of the most used materials inside a water-cooled reactor’s fuel assembly (Schemel, 1977). The reasons for the extensive use of this material inside the fuel assembly, are its high corrosion resistance at elevated temperatures of 340 °C, and its low thermal neutron absorption cross-section (Schemel, 1977). One of the largest numbers of components manufactured from this material is endcaps, which are welded to the fuel rods to secure the UO2 inside the Zry-4 tubes. Zry-4 is a very ductile material which makes it great for forming using manufacturing procedures such as extrusion and rolling, but the challenge comes in when the material is machined using methods such as single point cutting procedures (turning) at high speeds. A side-effect of high speed-machining of ductile materials is galling. If tight dimensional tolerances are required, this issue can result in parts not meeting specifications. Furthermore, an added challenge of Zry-4 is that it has a high probability to ignite during machining if special care is not taken with the chips. Thus, it is especially important to have a predetermined and controlled machining setup combined with the correct inline measurement procedures, as well as a good final batch release procedure to ensure that the components satisfy the strict nuclear plant specifications for nuclear fuel assemblies. The aim of this study was • to review applicable cutting-tool principles and rotational machinability of ductile materials, with special reference to Zry-4, with the view • to find and apply the best possible cutting parameters to machine Zry-4 nuclear fuel rod endcaps in high volume production to circumvent galling, and • to measure the resulting dimensions and calculate the ensuing tolerances verified by using standard and numerical methods. Machining parameters that were investigated in this study, were the rotational speed, the cutting depth, the feed rate, and the cutting insert design. The machining was done, making use of different shaped ceramic tool inserts, such as the ISCAR PENTA TiAlN+TiN coated inserts, VCMT shaped TiCN+Al2O3+TiN coated inserts and VCGT shaped uncoated inserts. During consecutive machining procedures, using the VCGT inserts, the rotational speed, and feed rate was set at 1000 rpm and 0,25 mm/rev, respectively, with a cutting depth of 0,80 mm for the roughing cut. For the finishing cut, the cutting depth was set at 0,03 mm with a rotational speed of 2500 rpm and a feed rate of 0,09 mm/rev. Dimensional tolerances of machined endcaps were evaluated using conventional measuring techniques, such as micrometre dimensional measuring and profiling. The results obtained from this measurement procedure were compared with the measurements obtained from using the state-of-the-art DEA GLOBAL CMM, Zeiss Duramax, and the Zeiss O-Inspect coordinate measuring systems. These machines use a probe to measure the components either by line scanning or point scanning. After measuring all the components using the equipment available for this study it was found that the components could be machined with a 35% success rate. However, it is possible to increase this capability when measuring machines such as the multi-sensor CMM is being used, as these machines do have a higher repeatability rate than that of the conventional methods used in this study, with an added benefit of reduced cycle time.