No other country that was denied nuclear technology after 1974 has done what India did at Kalpakkam.
Over 200 Indian companies, including MSMEs, built the now fabled FBR reactor from scratch.
IGCAR's design team and a sprawling network of Indian manufacturers turned drawings into nuclear-grade hardware.
Late & Over-budget. Yes. An achievement nonetheless.
Let's walk through what's inside the 'Main vessel' or the 'pot' submerged in sodium.
Start at the heart: the core subassemblies [1].
These are hexagonal units, 4.5 metres tall, packed with fuel pins.
Nuclear Fuel Complex (NFC) in Hyderabad fabricated every single one, supplying over 60,000 crimped tubes and 2.5 lakh precision fuel pin components.
Second, how do you control a reactor that breeds plutonium?
You need two completely independent shutdown systems.
MTAR Technologies built both the CSRDM and DSRDM [2], the control and safety rod drive mechanisms.
CSRDM stands for Control and Safety Rod Drive Mechanism.
During normal operation, CSRDM does two jobs: it controls the reactor's power level by raising or lowering the control rods (absorbing more or fewer neutrons), and it can "scram" the reactor, meaning slam all rods into the core to shut down the chain reaction instantly
The second (DSRDM) - Diverse Safety Rod Drive Mechanism is deliberately designed to be different.
It uses electromagnets.
Cut the power, and the safety rods fall into the core by gravity alone.
MTAR also built the grid plate [9], one of the most precision-critical components in the entire reactor.
Every core subassembly sits on it.
MTAR delivered the control plug [13] too, the structure that houses all those drive mechanisms from above.
Next, the vessels.
L&T built both the main vessel [4] and the safety vessel [3].
The main vessel is roughly 13 metres in diameter, and L&T achieved tolerances within ┬▒12mm across that entire span.
The safety vessel sits just 300mm outside the main vessel. That gap isn't wasted space. It's designed for robotic ultrasonic inspection.
If the main vessel ever leaks, the safety vessel keeps sodium levels high enough to keep cooling the core.
L&T supplied about 80% of all pool-type PFBR components, including the roof slab [11] from their Hazira facility and primary piping [10] from Powai.
BHEL Trichy handled the inner vessel [6] and thermal baffles with cooling pipes [5].
The inner vessel separates hot and cold sodium pools inside the reactor. The thermal baffles protect the main vessel wall from the 550┬░C sodium on the hot side.
Without these, the vessel's structural integrity would degrade within years.
Below the core sits WIL Walchandnagar's work: the core support structure [8] and the core catcher [7].
The core catcher is a circular pan covering the entire plenum area below the grid plate. Its job? Catch molten fuel debris if the worst happens.
It's designed to handle the meltdown of seven subassemblies simultaneously, preventing debris from reaching the main vessel while allowing natural convection cooling.
The core catcher is submerged in liquid sodium. Sodium touching the hot debris (in the event of fuel meltdown) on the pan heats up.
Hot sodium is less dense than cold sodium, so it rises. Cooler sodium from elsewhere in the pool flows in to replace it. That cooler sodium absorbs heat from the debris, warms up, rises, and the cycle repeats.
This continuous circulation pulls heat away from the molten fuel without any pumps running, without any electricity, without any operator pressing a button.
India is one of the few countries to incorporate an in-vessel core catcher in a fast reactor design.
Then there's Godrej.
They built the Large Rotating Plug (LRP) and Small Rotating Plug (SRP) [12].
These are massive, up to 8 metres in diameter, weighing nearly 120 tonnes combined.
They rotate 360 degrees with extreme precision to align fuel handling equipment with individual core positions.
This is how you refuel a sodium-cooled reactor without opening it up.
We talked about this fuel removal machine IVTM in our last post if you remember.
Lastly, don't forget to join us on 18th April 2026 @ 11:00 am for the Nuclear Sector webinar. Link in profile ЁЯН║
Sources: PIB India, https://t.co/aVX7KlS5xt, MTAR Technologies, Godrej Power & Energy, L&T Heavy Engineering, Nuclear Engineering International, World Nuclear News.