NPCIL Begins Geotechnical Site Studies for Kakrapar Units 5 and 6 Nuclear Project

The Nuclear Power Corporation of India Limited (NPCIL) has formally commenced pre-project geotechnical site investigations for Units 5 and 6 at the Kakrapar Atomic Power Station (KAPP) in Surat district, Gujarat. The undertaking represents a key preliminary milestone in the planned installation of two indigenously designed 700 MWe Pressurised Heavy Water Reactors (PHWRs). Once completed, the twin reactors will add 1,400 MW of zero-carbon baseload electricity generation, expanding the total installed capacity at the Kakrapar complex from its current 1,840 MW to 3,240 MW.

The initiation of physical field investigations at Kakrapar-5 and Kakrapar-6 marks the continued expansion of India’s standardized 700 MWe heavy-water reactor platform. By leveraging established regional infrastructure, civil engineering supply chains, and technical expertise cultivated during the construction and commissioning of Kakrapar Units 3 and 4, NPCIL aims to streamline site integration and project execution for the new units.

Technical Architecture of the 700 MWe PHWR Generation

India’s 700 MWe Pressurised Heavy Water Reactor platform represents a major technological achievement in the country’s nuclear energy program. Developed indigenously by NPCIL engineers in collaboration with domestic research institutions and industrial manufacturing partners, the design evolved from the earlier 220 MWe and 540 MWe reactor generations. The 700 MWe PHWR utilizes natural uranium dioxide fuel bundles and heavy water (deuterium oxide) as both moderator and primary coolant.

The nuclear steam supply system centers around a horizontal cylindrical stainless-steel reactor vessel known as the calandria, which houses several hundred zircaloy pressure tubes containing the fuel assemblies. Heavy water flows through these pressure tubes under high pressure to absorb heat generated by nuclear fission, transferring thermal energy via closed piping loops to heavy-duty steam generators. These steam generators produce dry saturated light-water steam to drive a high-efficiency steam turbine generator set.

A key engineering innovation of the 700 MWe PHWR design is its passive decay heat removal system (PDHRS). In the event of a total station blackout or loss of power to primary coolant circulation pumps, the PDHRS operates through natural convection to cool the reactor core without requiring external electrical power or manual intervention. Furthermore, the containment architecture features a double-containment structure comprising an inner prestressed concrete containment envelope surrounded by an outer reinforced concrete building, providing robust defense against environmental extreme events and internal pressure surges.

Geomechanical Engineering and Nuclear Safety Baselines

The geotechnical investigations initiated at the Kakrapar-5 and Kakrapar-6 site constitute a mandatory engineering requirement prior to finalizing structural foundation design for heavy civil construction. Specialized engineering teams are carrying out deep core borehole drilling, geophysical dynamic seismic testing, and sub-surface rock mechanics evaluations across the proposed reactor building footprints and auxiliary facility zones.

These field surveys are combined with detailed laboratory testing of soil and rock core samples to establish comprehensive baseline parameters for shear wave velocity, ultimate bearing capacity, rock mass rating (RMR), and local hydrogeological conditions. Subsurface characterization is vital because nuclear power plant structures impose extraordinary static and dynamic loading demands. A typical 700 MWe reactor building exerts massive structural loads upon its founding bedrock through a heavy reinforced concrete raft foundation, while also requiring full resilience against dynamic forces from potential seismic ground motions.

Precise geomechanical data allows civil design engineers to optimize foundation depths, specify ground-improvement protocols, and analyze complex soil-structure interaction (SSI) under extreme transient conditions. Regulatory frameworks enforced by the Atomic Energy Regulatory Board (AERB) mandate strict adherence to site characterization standards to guarantee that containment structures, turbine pedestals, and safety-critical fluid piping maintain absolute structural integrity throughout decades of operational service.

Strategic Scale-Up of India’s Domestic Nuclear Fleet

The progression of Kakrapar Units 5 and 6 aligns with India’s broader national strategy to expand domestic nuclear power generation capacity. NPCIL currently operates 24 commercial nuclear reactors with a total installed capacity of 8,780 MW. To address rising industrial electricity demand while advancing long-term decarbonization targets, the Indian government has established a structured roadmap to increase nuclear capacity to approximately 22 GW by 2031–32, serving as an operational milestone toward achieving 100 GW of nuclear power capacity by 2047.

To accelerate construction schedules and optimize capital outlays, NPCIL is executing a fleet-mode procurement model for its standardized 700 MWe PHWR design. Fleet procurement enables domestic manufacturing companies to establish dedicated manufacturing lines for long-lead nuclear components, including forged calandria end-shields, steam generators, main coolant pump assemblies, remote fuelling machines, and heavy-wall containment isolation valves. This industrial scale-up has significantly increased domestic content, with local engineering enterprises supplying over 80 percent of major nuclear steam supply system components.

Expanding existing multi-unit nuclear sites like Kakrapar offers substantial operational and logistical advantages. Shared cooling water intake structures, grid interconnection infrastructure, established environmental surveillance programs, and trained maintenance workforces lower capital expenditure and reduce site preparation timelines compared to greenfield developments. Additionally, the operational feedback from Kakrapar Unit 3 (commercial since mid-2023) and Kakrapar Unit 4 (grid-connected in early 2024) provides empirical baseline data that helps engineers refine construction methodologies for Units 5 and 6.

Quality Assurance and High-Pressure Fluid Circuit Integrity

The operational safety and reliability of pressurised heavy water reactors depend fundamentally on the leak-tight integrity and performance of their high-pressure fluid circuits, hydraulic actuators, and control systems. Operating under elevated temperature and pressure regimes, primary heat transport piping and secondary steam circuits demand rigorous non-destructive testing, pressure validation, and dynamic testing prior to plant commissioning and during periodic refueling outages.

Every valve system, hydraulic snubber, dynamic pipe restraint, and high-pressure gas purging circuit integrated into a nuclear facility must adhere to strict quality assurance specifications. Hydrostatic test rigs, specialized pneumatic test benches, and automated performance qualification setups are essential for verifying that key components maintain full pressure retention under maximum working limits and operational transients.

As India accelerates the expansion of its civil nuclear energy infrastructure, the requirement for custom, highly reliable test equipment and fluid engineering systems continues to grow. Neometrix Defence Limited contributes to this advanced engineering ecosystem by designing and manufacturing specialized hydraulic test benches, high-pressure gas systems, and turnkey test rigs engineered to satisfy the strict performance standards of critical energy and industrial sectors.