Fuelling power

Print edition : March 16, 2002

The Nuclear Fuel Complex in Hyderabad is all set to expand its operations to cater to the increasing needs of the country's nuclear electricity programme.

THE Nuclear Fuel Complex (NFC), Hyderabad, which forms the heart of the country's nuclear electricity programme, is all set to expand in a big way and further the aims of the Department of Atomic Energy (DAE) as outlined in the Vision 2020 document. The document envisions the generation of 20,000 MWe of nuclear electricity by 2020.

Seamless calandria tubes manufactured at the NFC for the 540 MWe PHWRs coming up at Tarapur.-H. SATISH

The NFC is a massive and versatile industrial facility that produces natural uranium fuel pellets for the 12 Pressurised Heavy Water Reactors (PHWRs) operating in various parts of the country and enriched uranium fuel pellets for the two Boiling Water Reactors (BWRs) at Tarapur, Maharashtra. Besides, it manufactures core components such as calandria tubes, coolant tubes, cladding for the fuel pellets, end-caps, end-plates, garter springs, spacer pads and bearing pads not only for the 14 reactors but the Fast Breeder Test Reactor (FBTR) and the proposed Prototype Fast Breeder Reactor (PFBR), both at Kalpakkam, Tamil Nadu. Plans are under way to build two more NFCs to meet the requirements of a string of PHWRs that will attain criticality in the coming years.

Dr. Chaitanyamoy Ganguly, Chairman, NFC Board and Chief Executive, said: "In another 10 years, we will have 16 PHWRs of 220 MWe each and 12 PHWRs of 500 MWe each. Our strategy is to have a centre exclusively for manufacturing natural uranium fuel pellets for 220 MWe PHWRs." The NFC, with its present capacity of manufacturing 600 tonnes of natural uranium fuel pellets, can take care of the fuel needs of 16 PHWRs of 220 MWe each, which works out to 560 tonnes a year.

Dr. Ganguly said: "So we will have NFC-II in the 10th or 11th Plan to meet the requirements of all PHWRs of 500 MWe capacity." There are plans for an NFC-III which will take care of Fast Breeder Reactors that use plutonium-based fuel. While Russia will supply the enriched uranium fuel for the VVER-1000 reactors of 1000 MWe capacity each to be built at Koodankulam in Tamil Nadu, the NFC will convert that fuel into pellets.

The NFC is geared to meet the requirements of natural uranium fuel pellets and core components of the two huge PHWRs of 540 MWe each coming up at Tarapur; the third and fourth reactors at Kaiga in Karnataka; and the fifth and sixth units at Rawatbhatta, Rajasthan. The Kaiga and Rawatbhatta units will have a capacity of 220 MWe each.

Newly fabricated fuel bundles.-H. SATISH

The NFC is a world pioneer in making seamless tubes for use in reactors and other industries. The father of the technology is R. Kalidas, Deputy Chief Executive, NFC. Kalidas said: "We are the only people in the world to manufacture thin-walled large diameter seamless tubes such as calandria tubes for the PHWRs. The usual method of making these tubes is to form sheets and weld them. To get uniform thickness through the seamless route, that too in six-metre long tubes, is very difficult and we have achieved it."

The coolant channels and tubes are made of welded tubes everywhere. However, Kalidas said that the NFC was making seamless tubes in a wide variety of materials such as zircaloy, stainless steel, inconel, titanium alloy and so on in sizes from 4 mm to 250 mm in diameter.

The NFC also manufactures a variety of exotic, strategic alloys and components for use in missiles, the launch vehicles of the Indian Space Research Organisation (ISRO), combat aircraft and for the Navy and the electronics industry. The high purity (99.999 per cent pure) metals and alloys include tantulum, selenium, gallium, cadmium, antimony and bismuth. Another product is magnesium powder used in the manufacture of explosives. The NFC makes ferritic steel foil of 0.05 mm thickness, besides titanium foil.

IT was the vision of Dr. Homi J. Bhabha, the father of India's nuclear programme, that helped the NFC to grow into a versatile industrial complex. In 1965, he decided to establish the NFC in Hyderabad. Construction began in 1969 and various units went into operation in 1971. The NFC today is spread on 1,200 acres (486 hectares) of afforested land in the once desolate, rocky Moula Ali area and has a workforce of 4,000. The late Dr. Brahm Prakash was primarily responsible, as project director, for the structure and substance of the NFC programme as one sees it today. Electronics Corporation of India Limited (ECIL), another undertaking of the DAE, has its facility nearby.

The Indian nuclear power programme is based on a three-stage strategy. In the first stage, the PHWRs, currently operating, form the backbone. They use natural uranium as fuel and heavy water as both moderator and coolant. The Fast Breeder Reactors, which will use plutonium-uranium oxide as fuel and liquid sodium as moderator, will comprise the second stage. A beginning was made in the second stage with the FBTR attaining criticality in 1985. The Advanced Heavy Water Reactors, which will use thorium as fuel, form the third stage.

At the uranium oxide plant.-H. SATISH

The range of chemical and metallurgical processes involved in the manufacture of fuel pellets and components of reactors is astounding. Hundreds of machines hum in scores of huge shops. These include the Natural Uranium Fuel Plant, Zircaloy Fabrication Plant, Special Tubes Plant, New Uranium Oxide Fuel Project, New Zircaloy Fabrication Project, Extrusion and Piercion Plant, Melt Shop and so on. Dr. Ganguly said: "The work done at the NFC is far more complicated than that at an integrated steel plant. Safety has been given paramount importance. Our safety records are outstanding."

The Uranium Oxide Plant has two wings where natural uranium dioxide fuel pellets for PHWRs are made. Uranium is mined at Jaduguda, Bhatin and Narwapahar in Bihar by Uranium Corporation of India Limited. Through an elaborate process based on indigenous know-how and equipment at the mill complex at Jaduguda, UCIL converts the uranium ore into magnesium diuranate, known as yellow-cake (Frontline, September 10, 1999). The yellow cake is the starting material for making natural uranium pellets.

In the first wing, the yellow cake is converted into nuclear grade uranium dioxide powder. In the second wing, this is made into pellets. P.S.A. Narayanan, General Manager, Uranium Oxide Plant, says that the yellow cake is first dissolved in nitric acid and by solvent extraction, 99 per cent pure uranium is got as uranyl nitrate solution. This is precipitated into ammonium diuranate, which undergoes heat treatment to become nuclear grade uranium dioxide powder.

In the other wing, the powder is converted into cylindrical uranium oxide pellets. The process, Narayanan said, called for extreme care. It is done in an enclosed atmosphere. The operators wear gloves, safety shoes and respirators. The floor is kept clean. The air is sucked out by exhausts so that no uranium particulate stays inside the room. At the New Uranium Oxide Fuel Plant, no operator is required as the powder is automatically compacted into pellets.

These green pellets with a density of 5.6 gm/cc are densified to 10.5 gm/cc to attain reactor requirements. To achieve this, the pellets undergo sintering at 1,700 C in hydrogen atmosphere. R.N. Jayaraj, Deputy General Manager, said: "We burn the pellets so much in the furnace that they become hard as bricks." The pellets are densified by reducing their diameter and height. The mass remains the same. The pellets then undergo centreless grinding. "All this technology is our own," Jayaraj said. Three more furnaces will soon be added to the existing four to cater for the fuel requirements of TAPS 3 and 4.

The pellets are stacked in tubes made of zircaloy, an alloy consisting of zirconium, tin, iron and nickel. The two ends of the tube are closed by end-caps, made again by zircaloy. Nineteen such fuel rods are arranged in circles to form a a fuel bundle, weighing 15 kg. The tube that holds these 19 rods is also made of zircaloy. Dr. Ganguly said: "A single fuel bundle can generate 6.5 lakh units of electricity. When we sell it to the NPC, we charge about Rs.25 lakhs."

The welding of end-caps, according to Narayanan and Jayaraj, is the most critical operation because the integrity of the welds should confirm strictly to specifications. The 19 rods are welded together at both ends with an end-plate, also made of zircaloy. These 19 rods should neither touch one another nor be loose.

The raw material imported for the two BWRs at Tarapur is uranium hexafluoride, which is converted into fuel pellets at the NFC. This is done at the Enriched Uranium Oxide Plant through the ammonium diuranate route to make enriched uranium oxide powder. The sintered fuel pellets are then produced by a process similar to that of the PHWR fuel. Russia now supplies enriched fuel for the BWRs. There are 36 rods in a fuel bundle for TAPS 1 and 2. The sheathing, fuel tubes, end-caps and end-plates for TAPS 1 and 2 are made of zircaloy.

The Special Tube Plant is a central facility where the core components of a reactor such as calandria tubes, coolant tubes, end-caps, end-plates, bearing pads, spacer pads and garter springs are manufactured. All these are made of zircaloy. Increasingly, the coolant tubes are now made of zirconium-niobium which is resistant to cracks (Frontline, January 9 and July 17, 1998).

In a reactor, the calandria is the reactor vessel which contains the coolant tubes. In a PHWR, the calandria tubes and the coolant tubes carry heavy water and in between are garter springs. The coolant tubes with end-fittings are called coolant channels and there are 306 of them in the calandria. Inside the coolant tube is the fuel tube with the fuel pellets.

Three types of coolant channels are made at the Special Tubes Plant. While the coolant channels for the PHWRs are circular, they are square for the two BWRs. Both are made of zircaloy. For the FBTR and the PFBR, the coolant channels, made of stainless steel, are hexagonal in configuration.

The starting material for making zircaloy is zircon sand, found along the Tamil Nadu, Orissa and Kerala coasts, and supplied by Indian Rare Earths Limited. At the NFC, zircon is processed and purified to reactor grade zirconium oxide, which is further processed into hard zirconium crystals called zirconium sponge. This is briquetted with by adding tin, iron and nickel and made into ingots which are then converted to seamless tubes, sheets and bars.

The reactors' core components are made of zircaloy or stainless steel in the Hot Extrusion Plant. Extrusion is a tremendously hot process in which zircaloy or stainless steel billets are pushed through a die to make tubes, rods and so on. When the Frontline team visited the plant, a titanium billet with a bore of 30 mm was being expanded to a bore of 100 mm in a vertical hydraulic press. (The NFC supplies titanium tubes to the defence sector.) In an adjacent horizontal press, a zirconium alloy ingot with a diameter of 370 mm was being extruded to 230 mm diameter in one shot with a force of 3,780 tonnes.

The plant makes seamless calandria tubes of zircaloy with a diameter of 132.5 mm and a wall thickness of 1.40 mm. The Plant was racing against time to make calandria tubes for TAPS 3 and 4 (540 MWe each), which would become critical in 2005-2006. Each reactor needs 425 tubes. They have to be supplied by April 2002. The plant is also busy making zirconium-niobium coolant tubes for TAPS 3 and 4. The coolant tubes were 6.425 m long, had a diameter of 112.67 mm and a wall thickness of 4.3 mm. The NFC made tubes of special grade stainless steel - SS 410 - for reactors.

According to S. Chaudhary, Senior Manager, the largest tube made at the plant had a diameter of 255 mm and a wall thickness of 50 mm. The smallest it made had a diameter of 4.7 mm and 0.6 mm wall thickness.

The pilgering mill at the NFC is the biggest such facility in Asia. Cold pilgering is done in order to reduce wall thickness and diameter which impart improved mechanical and metallurgical properties to the tubes.

Indigenisation had been a motivating force all along. Jayaraj said that the NFC had the expertise to make machines in-house to fabricate the fuel. "We have achieved 100 per cent indigenisation in this," he said. When embargoes were imposed on India after the May 1974 Pokhran nuclear test, the NFC and industries in Bangalore and Kolkata fabricated the machinery to weld end-caps to the fuel rods. These machines required no manual intervention unlike the imported Canadian machines. The indigenous machines cost a fraction of the imported ones. Machines were made at the NFC to attach the bearing-pads to the fuel pins when other countries refused to sell such machines to India. The NFC made the hexagonal coolant channels for the FBTR after France refused to supply them from 1974.

NFC officials said that quality assurance, especially in the matter of integrity of the fuel bundles, was given paramount importance. Out of 42,000 fuel bundles fabricated at the NFC in the last 18 months, there was no leak of fuel from a single bundle. Narayanan said: "This speaks for the process control, testing systems, equipment capability and reliability." The fuel bundles were subjected to rigorous checks including leak testing using helium gas in two stages. The integrity of the welds were checked by non-destructive testing methods. The fuel bundles were dropped from a height of 30 feet to check whether they developed cracks. The NFC officials said that the uninterrupted supply of good quality fuel from the NFC was one of the factors in the nuclear power stations in the country achieving 85 per cent capacity factor in 2002.

The Health Physics Unit kept a tab on the health of employees and the immediate environment. The employees wear dossimeters to record any radiation doses they receive. They underwent tests in whole-body counters and lung-body machines. The HPU monitored the air quality in and around the NFC 24 hours a day. The Occupational Health Centre monitored the health of every employee from the day of recruitment until the day he or she retired.