Challenges in PFBR civil construction

Civil works of PFBR has posed numerous design and construction challenges. Eight safety related nuclear buildings of PFBR are founded on a single raft, first of its kind in Indian nuclear Reactors. Construction of eight interconnected buildings on the common raft has been challenge for construction material transportation. The construction of Nuclear Island Connected Buildings (NICB) raft was completed in three and a half months. This massive 100mx100m raft for NICB, requiring 35000 cubic meter of concrete was undertaken in twelve pours. This Herculean task required storage of massive quantities of raw materials and ice. The construction of Turbo Generator (TG) building including TG mat and deck and interconnection with NICB posed enormous construction challenges that were well planned and executed. The intake and outfall structures too are exemplary construction handled effectively at PFBR. The excavation for the nuclear and power islands was large and deep. Rock out crops were found starting at various depths of excavation, requiring extensive planning controlled blasting and mechanization in excavation. This also required innovative dewatering procedures in sandy terrain often mixed with clay. Ground water table at PFBR site being high, water proofing for massive underground structure of PFBR was required to be carefully engineered and handled. This started with innovative grouting of excavated strata below the raft till required permeability was met and bone dry condition was achieved for laying of waterproofing membranes. Concurrent construction and regulatory reviews and stage wise clearance for civil construction required dynamic construction planning and sequencing. The scope of civil construction in PFBR expands to critical steel fabricated structures integrated with civil works. This includes huge embedment erected to very close tolerances. Large size panels for upper lateral, lower lateral and bottom shields were fabricated to intricate contours with tight tolerances and were transported to locations from far off distances and erected to tolerances. Massive fixturing had to be resorted to meet tolerances of reactor vault. The reinforcement and concreting placement required protection of the biological shield cooling pipes integrated with the vault liners. Novel construction practices and stringent quality control procedures were implemented to accomplish the critical erection requirement. Integration of cooling coils inside the safety vessel flanges required special contour bending for numerous reinforcement rods and development of special construction methodology. The neutron detector box integration with bottom shield was also a construction challenge well addressed by site. Many features of Nuclear Island Connected Buildings (NICB) required development of innovative construction techniques. This included construction of underground slabs at Steam Generator Building after erection of the massive sodium storage tanks. Maintaining the stringent requirements such as clean room conditions was a major challenge and was met effectively. Equipment erection in parallel to civil works required innovative solutions for clean condition maintenance while civil works are in progress. Heavy density hatches weighing approximately 150 MT is also a major challenge in the civil construction above argon buffer tanks. The construction of huge cells in reactor buildings require heavy density concrete to be poured in controlled manner to avoid segregation. Three densities of heavy concrete, 3.2, 3.8 and 4.2 gm/cc are used in Reactor Containment Building (RCB). Many mock ups were carried out for demonstrating the construction procedures and methodology for critical construction activities. The construction engineers ensured that constructability of all critical areas where concrete flow could be doubtful is established on mock ups prior to actual construction. In conclusion it can be said that the civil construction of PFBR is a marvel of technology handled professionally by competent engineers deploying scientific construction techniques and stringent quality control procedures