RPV fabrication processes

Fabrication of an RPV requires many years to complete. The fabrication process is a series of carefully controlled operations with quality control checks after each step. The first of these controls starts with the steel supplier. The steel supplier must perform chemical analyses and destructive testing of the ingot material immediately after pouring the base metal. Each step in the procedure, from acceptance of raw materials at the shop to final shipment, is documented so that the complete history of the RPV is known. 

Irradiation Embrittiement of Reactor Pressure Vesseis (RPVs) 

T vo basic methods of RPV fabrication have been used. The first method used rolled and welded plates to form separate shell courses. Thus, an RPV fabricated in this manner has both longitudinal (axial) and circumferential (girth) weld seams. In some of the older (pre-1972) PWR RPVs, these longitudinal seams are of particular concern for RPV integrity due to high levels of Cu in the welds. All BWR and most of the PWR RPVs used this rolled and welded plate construction. 

The RPV assembly consists of a cylindrical shell, a spherically dished bottom head and a ring flange to which the upper closure head is bolted. The RPV and closure head are made as subassemblies from a combination of formed plates and forgings. 

1 Cylindrical shell fabrication method used by Combustion Engineering (C-E) 

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RPV shell plate material specifications have evolved since the beginning of the commercial nuclear power industry in the mid-1950s. Due to safety concerns for nuclear RPVs, cautious steel-making was paramount, as well as careful RPV fabrication and welding practices. Nevertheless, the need for familiarity in steel-making and fabrication processes initially led to the selection and use of old steels typical of prior (non-nuclear) pressure vessel service. Table 1.1 lists the principal steel plate materials used in construction of nuclear RPV components. All of the plate materials used... 

At this point in the fabrication process for the later RPVs, the welds were given a hot UT inspection to assure compliance with a pre-service examination. The shell course was then placed in a furnace and given a post-weld heat treatment (PWHT). The welds were then ground smooth on the external and internal surfaces. The ASME Code-required radiographic (RT) and MT inspections were then performed. Any required repairs of unacceptable indications were made at this point using manual metal arc (MMA) welding and a repeat of the preheat-post-weld heat treatment. 

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