As described in Section 2.15.3.2.7, two types of pin spacing for fuel assemblies, the grid type and the wire type, have been adopted for all FBRs. The wire type is more widely used except those for the Dounreay Fast Reactor in UK.34 Here, the rod fabrication and assembly are described taking a wire spacer type fuel assembly from the MONJU as an example.

The lower end plug is TIG-welded (tungsten inert gas-welded) to a cladding tube made of SUS 316 based alloy; this is done outside the PFPF. Clad­ding tubes with lower end plugs are then transferred to PFPF along with blanket pellets of depleted UO2 and the other cold components such as plenum sleeves and plenum springs. After adjusting the col­umn length of MOX pellets and measuring their weight, they are loaded into each cladding tube
with the other components; this is done in a glove box under a helium gas atmosphere. Then, an upper end plug is TIG-welded to the cladding tube. In this welding, the position ofthe weld electrode is adjusted automatically using image analysis. Figure 27 shows photographs of a welding torch installed in the glove box and an image display showing the position of the weld electrode.

Decontamination of the fuel rod surface is carried out prior to a contamination check. The fuel rods which pass the contamination check are brought from the glove box and are sent to the helium leak test to certify tightness of the welded part. An X-ray check of the welded part to confirm its soundness is also carried out prior to wrapping a spacer wire around the fuel rod. Finally, each fuel rod is checked for its weight, straightness, gap between spacer wire and fuel rod, g-ray spectrum from Am in the MOX pel­lets, and general surface appearance. Next, 169 fuel rods are transferred to the automated assembly sta­tion where 15 layers of fuel rods, consisting of 8-15 rods in each layer, are prepared at first. The layers of fuel rods are fixed to the entrance nozzle one by one to get a hexagonal cross-section. This bundle of 169

fuel rods is inserted into a wrapper tube, and then this wrapper tube is TIG-welded to the entrance nozzle.

Figure 28 shows a photograph of a bundle being inserted into a wrapper tube at the assembly station. The completed fuel assembly is then moved to an inspection station to confirm its straightness, twist, distance between opposite outer surfaces and appear­ance through automatic and remote operations.