Indian engineering company Larsen & Toubro has handed over a 240 ton, 13.5 m diameter roof slab for the 500 MWe prototype fast breeder reactor currently under construction in India. The reactor is intended to be a template for a fleet of six similar commercial fast breeder reactors fuelled with uranium-plutonium oxide and equipped with a thorium blanket to breed fissile uranium-233.

The pool-type reactor vessel contains all of the primary cooling circuit, including 820 K hot pool and 670 K cold pool. The primary circuit is filled with sodium and blanketed by argon (since sodium explodes on contact with water, and oxidizes in air).

The roof slab above the reactor pool serves to structurally support elements of the reactor and also provide a radiation barrier to reduce dose within the reactor building to 25 µSv/hr (high-density concrete will be used for shielding). The roof slab also supports the primary sodium pumps, in-vessel fuel handling machine, intermediate heat exchangers, decay heat removal system heat exchangers, the control plug, and support rotatable plugs that provide external access to the core sub-assemblies requiring handling.

Air is used to cool the slab to its operating design temperature (T>383 K), which is intentionally warm to prevent deposition of sodium in annular gaps.

Initial carbon and stainless steel fabrication was carried out in L&T shops and dispatched in pieces to the site. Final assembly was performed under the RCC-MR construction code in a special fabrication shop adjacent to the Kalpakkam reactor site. Ten cooling boxes were inserted in the assembly. The main vessel and exterior safety vessel were also built by L&T.

A total of 1800 kg of metal was used to create 467 welds. Included in that sum were 301 critical thermocouple welds with stringent 100% pass requirements for liquid penetrant testing, radiographic testing and ultrasonic testing. All were successfully helium leak-tested in pressure mode at 0.5 bar (maximum permissible leak rate: 1×10-7 Pa cm3/sec). The bottom surface of the roof slab is insulated with reflective SS metal thermal insulation panels.

Final assembly involved machining of 26 flanges and 28 support pads of various sizes ranging from 350mm to 7000mm. Six special in-situ machines were deployed for machining, including an eight-arm, 45 ton rig for overhead machining of the 13.5 m diameter base support structure. They also supported work to drill and tap 1444 holes on the flanges and pads.

A team of 75 craftsmen and four machinists were mobilised to work on the project 24 hours a day. The last 12 months of the 30-month project (from order to delivery) were dedicated to on-site work. Final machining was completed in less than one month.

Initially, L&T staff made a series of mock-ups to verify manufacturing methods. Processes were qualified in presence of the customer’s quality control department. Work instruction sheets and checklists guided processes. Quality control measures during the process included operator qualification, machining set-up checks, quality control checkpoints, intermediate checks and tool-changing methodology. A thorough inspection was carried out and documented at an intermediate stage to ensure that the final tolerances would be met.

The final structure met its design requirements: perpendicularity 0.5 mm (maximum 1.4 mm), overall flatness 0.25 mm (maximum 0.3 mm), surface finish 0.6 µm–1.6 µm (maximum 3.2 µm), and profile within 0.2% of nominal dimensions (maximum 1%).

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This article was originally published in the October 2011 issue of Nuclear Engineering International (p27)

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