Nuclear Energy

Safety first

Print edition : December 26, 2014

Construction of RAPP 7 and 8 in progress at Rawatbhatta in Rajasthan. Behind the two units under construction are the Rana Pratap Sagar Lake and the Aravalli hills. Photo: S. Subramanium

The construction of the inner containment wall and the reactor vault of RAPP 7 in progress. Photo: S. Subramanium

The structure where the end-shields of the calandria will be positioned in RAPP 7. Photo: S. Subramanium

A view of the Rajasthan Atomic Power Stations 5, 6 and 3 and 4 at Rawatbhatta. Photo: S. Subramanium

B.C. Pathak, Project Director, RAPP-7 and 8, Rawatbhatta.. Photo: S. Subramanium

At Rawatbhatta in Rajasthan, work on two 700 MWe indigenous reactors is in full swing, with safety as an important consideration.

THEIR sheer size overwhelms the lay visitor. But that does not mean that attention is not paid to detail. The twin reactor buildings that will house India’s indigenous 700 megawatt reactors are coming up at Rawatbhatta, Rajasthan. Named Rajasthan Atomic Power Project (RAPP) 7 and 8, the reactors are the biggest to be built by the Nuclear Power Corporation of India Limited (NPCIL), a public sector undertaking of the Department of Atomic Energy (DAE).

“Safety first, generation next” is the motto here, as the boards scream and the dedication and concentration of the engineers and workers show. We are standing on a floor 20 metres above the ground level, and a massive circular wall, 80 centimetres thick, is coming up around the floor. It is called the inner containment wall. Another wall called the outer containment wall, 60 cm thick, is coming up, outside of this. The inner containment wall has inner linings of carbon-steel sheets. A massive vault, made of cement concrete, has been built in the middle of the floor and there are two tall, gigantic pillars erected at the entrance to the vault.

Two more 700 MWe indigenous reactors are under construction—at Kakrapara in Gujarat. The reactors at Rawatbhatta and Kakrapara are pressurised heavy water reactors (PHWRs), which use natural uranium as fuel and heavy water as both coolant and moderator.

B.C. Pathak, Project Director, RAPP 7 and 8, said: “Work is progressing fast. This is an enormous job. We need to do a lot of concreting, piping, cabling, and so on. We have to proceed in a sequential manner.” He is a veteran in building nuclear power reactors, having made important contributions to building reactors at Tarapur in Maharashtra and Kudankulam in Tamil Nadu. He asserted that India’s 700 MWe PHWRs “are the latest in the PHWRs”, although among the CANDU (Canada Deuterium Uranium) reactors, which are also PHWRs, “there are higher-sized reactors” in other countries.

Pathak called the 700 MWe PHWRs at Rawatbhatta and Kakrapara “upgraded versions” of the two 540 MWe PHWRs at Tarapur (units 3 and 4). “We are continuously upgrading our technology [in the PHWRs],” Pathak said. After building a series of 220 MWe PHWRs across the country and then scaling them up to the 540 MWe PHWRs at Tarapur, the NPCIL has now turned its attention to building a series of 700 MWe PHWRs, and sites have been chosen for them.

“The size of RAPP 7 and 8 and that of Tarapur 3 and 4 are the same. They have the same number of calandria channels, that is, 396,” said Vivek Jain, Chief Construction Engineer, RAPP 7 and 8. But the coolant temperature parameters of 700 MWe reactors at Rawatbhatta and Kakrapara differed from those of the 540 MWe reactors. “So we are able to extract 700 MWe,” Vivek Jain explained.

The Rawatbhatta reactor building is 55 metres tall (as tall as a 20-storeyed building) and its foundation is 20 m deep. The cooling towers are 180 m tall from the ground level and the stack’s height is 120 m.

About 22 lakh cubic metres of sandstone rock was excavated to build the foundation for the reactor and turbine buildings for RAPP 7 and 8, said N.K. Mittal, Additional Chief Engineer (Control and Instrumentation), RAPP 7 and 8. “Excavating such a huge amount of rock is a big job,” he added. Eight lakh cubic metres of concreting is needed to build both the reactor and turbine buildings of the two units. About 90,000 tonnes of reinforced steel will go into the making of the two units. Both the units require 231 kilometres of tubing and 3,300 km of cabling.

From a height of 20 m, we see two heavy-duty crawlers at work at the ground level. While one crane can lift and swing around a weight of 1,350 tonnes, another can lift 650 tonnes. These cranes are needed for lifting and installing the calandria (reactor vessel), which weighs 42 tonnes, the end-shields (which will cap the calandria vessel) weighing 103 tonnes each, the 275-tonne generator transformer and the 316-tonne generator stator.

Rawatbhatta is a small town in Chittorgarh district, 65 km from the industrial town of Kota. It is an inland site. Its nuclear power reactors are situated on the banks of the expansive Rana Pratap Sagar Lake, fed by the Chambal river. The Aravalli hills ring the lake on one side. Townships, housing the quarters for the NPCIL staff working in the units, are situated 12 to 15 km away from the nuclear power station. At the entrance of one of the well-kept townships is a towering statue of the Rajput warrior Rana Pratap Singh astride his horse Chetak.

Rawatbhatta is host to India’s first and biggest nuclear park. It already has six PHWRs of various installed capacities while unit 7 and 8, of 700 MWe each, are under construction. Rawatbhatta exemplifies the journey of the Indian PHWRs. It has the distinction of hosting India’s very first as well as the latest PHWRs.

The first PHWR in India, built by the Canadians, went critical on August 11, 1972. It has remained permanently shut down for the past few years. The DAE has not decided so far whether to decommission the reactor or rehabilitate it.

India detonated its peaceful nuclear device in May 1974 when Canada was building the second unit at Rawatbhatta. Canada walked out of the project in protest. India started building the second reactor, undaunted by punishing technology denial regimes put in place against it by the West in the wake of the explosion. The second unit, which has a capacity of 200 MWe, reached criticality on October 8, 1980. Units 3 to 6 are 220 MWe each. Besides these reactors, Rawatbhatta is home to a DAE-owned plant that manufactures heavy water. A Nuclear Fuel Complex (NFC) is coming up to fabricate nuclear fuel bundles for India’s 700 MWe PHWRs. This is the second NFC of the DAE. An NFC is already functioning in Hyderabad, where nuclear fuel bundles for the 220 MWe PHWRs are made.

On September 6 this year, the Rajasthan Atomic Power Station (RAPS) 5 achieved a rare distinction. The reactor operated continuously for 765 days, generating power to its full capacity of 220 MWe almost every day. This is the second longest continuous run in the world for a PHWR and it was the first reactor in Asia to reach this milestone. The longest operating record belongs to a PHWR of the Pickering Nuclear Generating Station in Canada for a continuous run of 894 days, which ended in 1994.

R.K. Sinha, Chairman, Atomic Energy Commission, who unveiled a plaque at RAPS to celebrate RAPS 5’s feat, said the event conveyed a powerful message to the world about the robustness of India’s indigenous nuclear power programme. “We were not hankering for a record. Our only aim was to demonstrate that we could generate nuclear power in a safe and reliable manner.”

The NPCIL never attempted this record “although our focus was on safe and reliable generation of power” all through the continuous run, said K.C. Purohit, Chairman and Managing Director, NPCIL.

S.K. Sharma, Site Director, Rawatbhatta, was proud that the RAPS 3 to 8 units exemplified the concept of “poorna swadeshi” and RAPS had received the award for outstanding performance among the NPCIL stations.

Vinod Kumar, Station Director, RAPS 5 and 6, attributed the factors behind the record to the micro and meticulous planning done by the reactor engineers during the unit’s biannual shutdown, improvement in the ventilation systems, dust control, and assessment of the health of the critical equipment.

Ambitious plans

The NPCIL has ambitious plans to build more indigenous 700 MWe PHWRs besides those coming up at Rawatbhatta and Kakrapara. Twelve 700 MWe PHWRs will come up, four each at Mahi-Banswara in Rajasthan, Gorakhpur in Haryana and Bhimpur in Madhya Pradesh. Besides, two more will come up at Chutka, also in Madhya Pradesh.

That the pace of work at Rawatbhatta is in full swing for building RAPP 7 and 8 can be seen from the fact that equipment and components have started arriving at the site. The 42-tonne calandria, that is, the reactor vessel, for unit 7 is already at the site. Godrej and Boyce Manufacturing Company manufactured it.

N. Nagaich, Executive Director, Corporate Planning and Corporate Communications, NPCIL, said the calandria was a high-integrity, special-grade stainless steel vessel. It is at the heart of the reactor core (which houses the natural uranium fuel assemblies), where the fission process takes place in the presence of circulating heavy water, which acts as moderator and coolant. The manufacturing of calandria requires high levels of skill in heavy fabrication, distortion control and precision machining, said Nagaich. It requires skilful handling of the equipment during the various stages of its manufacturing and transportation to the reactor site.

Besides the calandria for unit 7, the two end-shields needed for reactor 7 have reached the site from Hazira, Gujarat. The end-shields for the eighth reactor will arrive at the site in a few months. They were fabricated by Larson & Toubro. End-shields are huge circular plates, made of steel, that are used to cap the reactor vessel on either side. They are filled with 10-mm-diameter balls made of carbon-steel, and offer heavy shielding to prevent radiation from escaping from the reactors, said Mittal. By November 15, one of the two end-shields for the seventh unit had been filled with steel balls and was ready for installation on the calandria. The same day, the NPCIL was awaiting permission from the Atomic Energy Regulatory Board, the watchdog on safety at the nuclear facilities in India, to install that end-shield in the calandria.

Pathak, who played key roles in building the 540 MWe reactors at Tarapur and the Russian 1,000-MWe reactors at Kudankulam, said super-heavy concrete had been used for the first time at the Rawatbhatta site for building the seventh and eighth units instead of the normal and heavy concrete which were used in such construction. Super-heavy concrete was used for building the calandria vault which houses the reactor. The wall of the calandria vault is 2.5 m thick. Super-heavy concrete is a mixture of concrete, hematite and steel balls. Its density is high. Super-heavy concrete can provide powerful shielding against radiation.

Another major feature of units 7 and 8 is that their inner wall is lined (embedded) with carbon-steel plates of 6 mm thickness, just as at the Kudankulam plant. The inner and outer containment walls surround the reactor, and a dome sits on top of these walls. The concrete dome is to prevent radiation from escaping into the atmosphere in case of an incident or accident. The inner containment wall is cast in 10 rings; seven rings are already in place for RAPP 7.

Each carbon-steel ring consists of 16 segments and they have been erected and welded together. Then concreting is done on either side of the rings to make the inner containment wall. As a safety measure, even the joints in the wall underwent non-destructive testing. While the inner containment wall stands at a height of 50 m from the ground level, the outer containment wall is at a height of 55 m from the ground level. According to Mittal, steel rods with a diameter of 36 mm went into the construction of various facilities in the reactor and turbine buildings. “The structures have to sustain heavy loads, including equipment and seismic loads. Hence such thick steel rods,” he said.

Sanjeev Kumar Singla, Additional Chief Engineer (Planning and Coordination), RAPP 7 and 8, said the steel liners of the inner containment wall would not allow any radiation from the reactor to escape into the atmosphere. For the space between the inner and outer containment walls is a vacuum and the pressure is less than in the atmosphere. So any radiation leak coming from the reactor would get absorbed in this space and it would not escape into the atmosphere, Singla said.

When Frontline visited RAPP 7 and 8 recently, the building of the inner and containment walls and the calandria vault for the seventh unit was in progress. “Everything has to be done as per the drawing,” said Pathak. Workers had erected a component that looked like a giant nail and engineers were checking its coordinates. Concrete floors were being cured with water, and the wetting lasted several days.

Compact layout

An important feature of the entire plant (all eight reactors) is that its layout is compact. The buildings are closely placed. “This reduces the length of the piping and cabling to be done,” said the Project Director. What has added to the compactness of the layout of units 7 and 8 is that the huge storage bay which houses the spent fuel is located inside the nuclear island, which comprises the reactor building, the spent fuel storage bay and other structures. So the travel of the spent fuel assemblies from the reactor core to the spent fuel storage bay does not take time.

The estimated cost of the two units is Rs.12,300 crore. While RAPP 7 is expected to be commissioned in early 2018, the eighth unit will reach criticality a few months later.