Design and development of thick plate concept for rotatable plugs and technology development for future Indian FBR

Prototype fast breeder reactor (PFBR) is at an advanced stage of construction in Kalpakkam, India. Top shield (consisting of roof slab and rotatable plugs) of PFBR is of box type construction. It is a Class-1 component being a part of the primary leak-tight boundary of the reactor assembly. In the future commercial fast breeder reactor (CFBR), it is planned to adopt thick plate concept for rotatable plugs. The thickness of the plates required for the rotatable plugs of CFBR was arrived at based on finite element analysis considering mechanical and thermal loads. The main advantage of using thick plate for the rotatable plugs is that it eliminates the possibility of lamellar tearing which exists in the box type construction. The successful realisation of this concept necessitates the indigenous development of thick plate narrow gap welding technology. Hence a technology development programme for realising 800 mm thick narrow gap welds using submerged arc welding process was undertaken. Three numbers of welded joints were made to demonstrate the successful development of thick plate narrow gap welding technology. Repair procedure for the weld was also established. Non-destructive examination and destructive testing were carried out and the results were analysed critically. The attenuation of ultrasonic wave in narrow gap weld as compared to that in the base metal was examined. This paper discusses the need and conceptual design of thick plate concept for rotatable plugs in CFBR, the approach adopted for technology development of thick plate narrow gap welding and the salient experiences gained during the developmental project. Recommendations are made regarding the RCC-MR code rules for the non-destructive volumetric examination of welds of thick carbon steel plates.