ABSTRACT
The reactor vessel of Monju is a circular cylindrical shell, made of austenitic stainless steel about 7 m in diameter, 18 m high, and 50 mm thick. Figure 24.1 shows a schematic sketch of a reactor assembly (RA). To improve the structural reliability by reducing the number of welded parts, the vessel is composed of 12 pieces of forged rings and has only circumferential welds. The shield plug had to be firm enough to support the components installed on it and to keep them properly aligned with the core internals. For this reason, the plug was manufactured with a proven thick rib structure. Since the main circulation pump in the primary system is vertical with a thin shaft about 6 m long, much consideration was given to the fabrication of rotating parts to ensure a highly controlled and balanced rotation. The balance of the shaft was tested under the same high-temperature conditions as in the actual operation. In the helical coil-type steam generator (SG), butt welds were used for both the tube-to-tube and the tube-to-tubesheet joints of the SG. An automated welding machine was developed, principally for the tubesheet welds. The shield plug, fuel handling machine, exvessel storage tank, and other major components were assembled in advance at the factory, and assembly tests were conducted to confirm the performance of the system prior to reassembly on site.
Construction of Monju began in October 1985 with the foundation work for the reactor building and reactor auxiliary building, which are located at the center of the plant. The construction of the reactor containment building (RCB) was completed in April 1987. The reactor vessel was installed in October 1989 (Figure 24.2). Figure 24.3 shows the core internals after installation. Construction was completed in April 1991. Functional tests were carried out between May 1991 and December 1992. Reactor criticality was achieved in April 1994. The entire construction process took ~6 years and criticality was achieved in ~3 years after carrying out various functional and start-up tests.
