Critical feat

INS Arihant’s reactor, which has gone into operation, propels India into the exclusive club of countries that are able to build nuclear-powered submarines.

The 80 MWe Pressurised Water Reactor at Kalpakkam, an August 2009 photograph. In the foreground is the pressure hull and behind it is the shield tank containing water. An identical reactor powers INS Arihant.

IT was October 23, 2004, and Prime Minister Manmohan Singh was to inaugurate the construction of the Prototype Fast Breeder Reactor (PFBR) at Kalpakkam, about 60 kilometres from Chennai, in Tamil Nadu. On the way to the auditorium where the function was to take place, his car took a detour and stopped in front of a nondescript building called Plutonium Reprocessing Plant (PRP). The PRP was a facade behind which a project of great secrecy was under way. As Manmohan Singh stepped inside the building and entered a big hall, he was amazed to see a massive device with a pressure hull and a shielding tank with water inside. It was India’s first Pressurised Water Reactor (PWR). It would generate 80 MWt using enriched uranium as fuel and light water as both coolant and moderator. The then PWR Project Director S. Basu, Chairman of the Atomic Energy Commission (AEC) Anil Kakokdar, and Director of the Bhabha Atomic Research Centre (BARC) Srikumar Banerjee, explained the features and importance of the PWR to Manmohan Singh. The PWR that Manmohan Singh saw was a “half boat”. His visit to the plant was not revealed to the press.

Two years later, on September 22, 2006, when the PWR reached its first criticality, that is, when it went into operation, an “invisible team” was thrilled. BARC scientists spent the next few weeks performing experiments on the physics of the reactor before the PWR started generating a stable supply of power. The event was again kept a secret.

On July 26, 2009, when Gursharan Kaur, the Prime Minister’s wife, broke a coconut on the hull of a submarine in the Ship Building Centre at Visakhapatnam harbour and named the boat “Arihant”, hundreds of engineers and scientists of the Department of Atomic Energy (DAE), the Defence Research and Development Organisation (DRDO) and the Indian Navy were overjoyed. The gathering that was present erupted in applause as the sluice gates of the dry dock opened, seawater gushed in and INS Arihant started floating.

Arihant is 111 metres long, 11 m wide and about 15 m tall. It has a surface displacement of about 6,000 tonnes. The launch marked the culmination of a saga of 25 years of selfless intellectual and physical work on the part of DAE, DRDO and navy personnel, who were never in the limelight. For Arihant is no ordinary submarine powered by diesel. It is a nuclear-powered submarine and the reactor that will propel it is identical to what Manmohan Singh saw at Kalpakkam on October 23, 2004.

Manmohan Singh made clear India’s policy on that day. “We do not have any aggressive designs, nor do we seek to threaten anyone…. Nevertheless, it is incumbent upon us to take all measures necessary to safeguard our country and keep pace with technological developments worldwide. It has been rightly said that eternal vigilance is the price of liberty.” Both the Prime Minister and Defence Minister A.K. Antony remembered the contribution of the Russians who helped India achieve “a historical milestone” in the Advanced Technology Vessel (ATV) programme. Manmohan Singh thanked “our Russian friends for their consistent and invaluable cooperation, which symbolises the close strategic partnership that we enjoy with Russia”.

August 10, 2013, was yet another significant day. At 1-20 a.m. on that day Arihant’s PWR reached its first criticality, and that truly propelled India into an exclusive club of countries that can build nuclear-powered submarines of their own: Russia, the United States, the United Kingdom, France and China. Arihant’s nuclear propulsion will enable it to stay under water for months on end, obviating the need to come to the surface periodically to recharge batteries, unlike diesel-powered boats.

Besides its torpedoes, what makes Arihant a lethal platform are a dozen K-15 underwater-launched missiles armed with nuclear warheads. These missiles can take out targets situated 700–750 km away. A booster will erupt into life underwater and drive the missile to the surface. It will then dart 20 km into the air, trace a parabolic path and hit targets on land. The K-15 missiles, which are under serial production, will be the last to be integrated with Arihant before it becomes operational in about 18 months from now. Arihant has an option. In lieu of a dozen K-15 missiles, it can carry four heavier and more lethal K-4 missiles, which can make scorched earth of places as far as 3,000 km away. A.K. Chakrabarti, who was Director, Defence Research and Development Laboratory, Hyderabad, is the architect of the K-4 and K-15 missiles, developed by the DRDO.

The significance of the PWR on board Arihant reaching criticality is multidimensional. It announced that India had mastered the technology of building civilian PWRs on shore for electricity generation and for using the PWRs to propel submarines. The identical PWRs at Kalpakkam and on board Arihant are forerunners of large-sized PWRs, called the Indian PWRs, of 900 MWe, whose design is almost complete.

On the strategic side, Arihant completes India’s nuclear triad. In other words, India can launch nuclear warheads from land, air and sea. India already has a fleet of Agni and Prithvi missiles, which can carry nuclear warheads from surface to surface. It has aircraft such as Mirage-2000 and Sukhoi-Mark I, which can deliver nuclear weapons. And now it has Arihant, which can fire missiles armed with nuclear warheads from underwater.

Since India has declared a policy of “no first strike”, that is, it will not use nuclear weapons first in case of a confrontation with another country, Arihant, with its nuclear weapons, offers a second-strike capability.

Srikumar Banerjee, then BARC Director, told Frontline on August 10, 2009, in an extended interview at Trombay: “On the strategic side, today, Arihant gives teeth to our no-first-use doctrine. For your system should not be vulnerable [to attacks from the enemy]. So the safest way to keep your nuclear weapons is to keep them underwater.” Banerjee called Arihant “a major technology in itself” and said “the whole platform is a very complex combination of various technologies”.

In the assessment of Basu, who is now BARC Director, Arihant’s reactor going into operation was “a big achievement in terms of our defence preparedness”. In his estimate, making the reactor compact and installing it in the cramped space of the hull of the submarine and building the boat itself “is one of the biggest technological achievements of the country in terms of precision and the sheer volume of work and reliability”. What is at premium in a submarine is space. The reactor’s compactness and the adversarial conditions, such as the rolling and pitching it will encounter in the depths of the sea, made this technology “very complex”. That is why not many countries had built nuclear-powered submarines, he pointed out.

R.K. Sinha, AEC Chairman, said: “It is a moment of jubilation for the entire nuclear energy section in this country, which has worked to make a reactor of this type. It is really a demonstration of a very advanced technological capability in challenging areas of nuclear reactor design, manufacture and commissioning. I am proud that this effort has resulted in the reactor reaching criticality.” He asserted that Indian industry had come of age in the manufacture of Arihant’s reactor pressure vessel. There were many other important components and systems in the reactor, Sinha said, which were completely indigenous.

Avinash Chander, Scientific Adviser to the Defence Minister and DRDO Director-General, was upbeat. “Arihant completes the nuclear triad. It is the most survivable element in the nuclear triad [because it is underwater]. When the submarine becomes operational, it will add a lot of strength to the credible deterrence of the country,” he said. The DRDO had been actively working for the last several months to make this activity [criticality] happen and “we are happy that it has happened,” added Avinash Chander, who was Programme Director, Agni-III, IV and V missiles. “We are getting ready for the follow-up action of going for sea trials,” he said.

Multidisciplinary effort Building Arihant’s reactor was essentially a multidisciplinary effort that involved fuel development, thermal and mechanical engineering to manufacture the reactor pressure vessel, steam generators and high pressure components, control rod mechanism, control and instrumentation, electromechanical systems, drive mechanisms, and so on. “It is a marriage of all these systems to make the reactor work efficiently,” Banerjee said in August 2009. “It is not desktop research at all,” he emphasised.

BARC’s engineers and scientists were involved in all this, from engineering the concept to the final product development. For everything had to be developed from scratch and there was absolutely no technology available to India on the PWR.

While V.K. Mehra gave leadership to the reactor development programme and H.S. Kamat was in charge of fuel development, B.K. Bera, A.K. Suri and R.P. Singh played important roles on the fuel side. The contribution of G.P. Srivastava, M. Mahapatra and R.K. Patil was formidable in control and instrumentation. R.S. Yadav dealt with the design and manufacture of one of the most important components, the reactor pressure vessel. C.G. Utge was responsible for the development of high-pressure, high-temperature equipment.

Why was PWR, not Pressurised Heavy Water Reactor (PHWR) technology which India had mastered and used to build several commercial reactors, chosen to propel the submarine? PWRs use enriched uranium as fuel and light water as coolant and moderator. In contrast, PHWRs use natural uranium as fuel and heavy water as both coolant and moderator. “PHWR is not something which you can make into a compact form,” said Banerjee, who later became AEC Chairman. Besides, nuclear energy generation depends on the quantity of fissile material available in the reactor and the PWR lent itself admirably for this with a high availability of fissile material (uranium-235) in enriched uranium. While plutonium also can be used as fuel, enriched uranium-driven fuel is generally adopted for reactors that propel submarines.

The question now arose whether India had the capability to enrich uranium. (If the non-fissile U-238 is removed from natural uranium, then the U-235 concentration will go up. This is called enrichment of uranium. This is done by a series of chemical and physical processes. If one uses enriched uranium as fuel, the availability of neutrons is high enough to generate electricity and one can use light water as coolant and moderator.)

So a small plant was set up at Ratnahalli near Mysore in 1990 for enriching uranium, and work on designing the enriched uranium fuel for the submarine’s nuclear power pack also began. BARC made a technological breakthrough in developing all the centrifuges needed for enriching uranium without any external help. The centrifuges were needed to separate U-238 from U-235 so that the concentration of U-235 went up, but the separation technology itself was very complex. To sustain the centrifugal forces, centrifuges were to have a high strength-to-weight ratio. Yet, they had to be thin. So maraging steel was used in the manufacture of centrifuges.

The next step was to process the enriched uranium into fuel. Banerjee said: “This is also crucial because unlike in the case of fuel for the land-based reactor, here the fuel had to be monolithic. This required special fabrication techniques that allow you to make the fuel withstand the rolling, pitching and other movements of the submarine…. In Trombay, we developed the right kind of fuel.”

Reactor development The reactor development itself was a big and tough task. At the heart of the reactor is its pressure vessel, which houses the fuel. Developing the pressure vessel entailed the use of a special technology and a special steel. The material had to have high fracture toughness and the toughness had to be retained even if the steel got exposed to radiation. So a special type of steel was developed to withstand the radiation environment.

The design of the vessel was another major challenge. The issue of the reactor’s compactness came in. The entire PWR had to fit into the cramped space of the submarine’s hull. Steam generators, tall structures consisting of a maze of pipes, posed another big problem. They produced steam to drive the turbine which generated electricity. So the steam generator and the pressure vessel were designed in such a way that every small space in the hull was made use of. This was a very important mechanical engineering design, which BARC engineers, after many trials and efforts, evolved.

Development of hundreds of subsystems and high-pressure valves and pumps posed various challenges, which were met by BARC engineers. Indian industry rose to the occasion by manufacturing them. The entire reactor structure had to be designed in such a way that it is stable when the submarine accelerates. What had to be taken into account here was that the reactor was housed in a submarine that sped under water. The thrust generated by the submarine’s propulsion required a design for the reactor that was different from that of a nuclear power reactor on terra firma.

“In designing the propulsion of the submarine, we had to take into account the various sea conditions, including rough sea, the submarine’s pitching and rolling, the effect of saline water, enemy action which includes underwater explosions/depth charges and internal conditions,” explained Basu. “Yet another factor is that the propulsion plant had to be compact and so weight and volume had to be minimised. Thirdly, the plant had to be very reliable. It is moving under water, hundreds of kilometres away from the shore. In case of an accident, no help will be available from outside. So back-up safety systems should function perfectly.”

So, the design of the safety system was crucial. BARC went for passive safety systems, which would not need an external source of electricity, to come into action. The passive thermo-siphoning system will come into play in abnormal conditions. Since a submarine’s reactor has no exclusion zone, unlike its counterpart on land where no human settlement is allowed a few kilometres around it, gamma shielding, and partly neutron shielding, by water was done.

In land-based reactors, control rods fall by gravity and bring the reactors to a halt in case of an accident. But the rolling and pitching of the boat demands that the control-rod mechanism is designed suitably to take care of the submarine’s various movements. “Since power has to be generated in a regulated manner, it puts a lot of restrictions on the design of the control mechanisms. Diverse techniques were used to design them. We had to take into consideration the possibility of the boat going upside down. So special sensors and drives were made for ensuring a safe and reliable operation of the control-rod mechanisms,” said Srivastava in August 2009. Indeed, 13 control mechanisms were accommodated within a diameter of 0.8 metre.

BARC also built a simulator at Visakhapatnam to train navy personnel in operating the reactor. When the Russians were shown this simulator, they were amazed at its sophistication.

In the Arihant project, which went under the name of ATV programme, DRDO laboratories contributed sonars, sensors, sound absorption materials, communication equipment, electronics and weapons. While the Naval Physical and Oceanographic Laboratory (NPOL), Kochi, contributed sensors to Arihant, special acoustics were done by the Naval Science and Technology Laboratory (NSTL), Visakhapatnam.

In the end, as Banerjee emphasised, it boiled down to teamwork in a multidisciplinary project and he called the platform “a very complex combination of various technologies”. As Kakodkar said, “This PWR technology is very complex. You have to make it extremely compact and pack it in the cramped space of the submarine’s hull. It was a big challenge.”

Today, India can assert that it has mastered the technology of developing and manufacturing nuclear propulsion for driving submarines. The proof of it lies in three more nuclear-powered submarines being built at Visakhapatnam. When the four submarines, including Arihant, patrol the seas, India will have achieved the status of a blue-water navy.