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THE 23-year-old first unit of the Madras Atomic Power Station (MAPS-1) at Kalpakkam, near Chennai, has virtually had a rebirth. Its lifespan has been extended by another 30 years and capacity stepped up to 220 MWe from 170 MWe. (The lifespan of a nuclear power reactor is around 30 years.) Engineers of Nuclear Power Corporation of India Limited (NPCIL) achieved this brilliant feat of engineering in about 14 months at a fraction of the time and cost it takes to build a new reactor.
"This reactor is as good as new. It is good enough for the next 30 to 40 years," said S.K. Jain, Chairman and Managing Director of NPCIL, on January 21 at Kalpakkam. The task was achieved at a cost of Rs.222 crores; a new reactor would have cost Rs.1,100 crores. "This job amounted to working on the heart of the reactor. We had to handle highly radioactive components," he said. NPCIL developed advanced technology in automation, robotics, remotely handled tubes and so on for the purpose. The Bhabha Atomic Research Centre (BARC), Trombay, helped NPCIL in all these, said Jain.
According to S. Krishnamurthy, Station Director, MAPS, the engineering feat involved four major jobs. They were: en masse coolant channel replacement (EMCCR), which involved cutting and replacing 306 coolant channels, each 80 mm wide and six metres long; replacement of eight steam generators and 88 heat exchangers; replacement of all 606 feeder pipes; and installation of sparger channels in the calandria or reactor vessel, which forms the heart of the reactor.
Work on the reactor began in October 2004 and went on for 14 months and was smooth and incident-free. On January 4, MAPS-1 attained re-criticality and it was connected to the southern grid on January 18.
"In this campaign, the main components in the heart of the reactor, which is the primary coolant system consisting of coolant channels, feeders, steam generators and spargers, have been changed. All life-limiting components of the primary systems have been replaced. Hence it is a rebirth for the reactor, which can go on for an extended period of 30 years. The reactor will be younger and stronger," said Krishnamurthy.
The engineers took the opportunity of the life-extension work to upgrade the reactor's safety and tackle obsolescence by replacing electrical switchgear, instrumentation components and piping material in different systems. The common control room for the two reactors in MAPS has been given a face-lift and miniaturisation has been introduced in its controls, systems and computer consoles.
"The experience has given a lot of confidence to NPCIL engineers in planning and executing a mammoth job of this kind, which requires handling an enormous amount of heavy equipment and taking care of radioactive components which are removed from the reactor," said Krishnamurthy.
What has attracted the attention of the nuclear industry all over the world is the replacement of the feeder pipes, which are made of carbon steel and are among the most critical components. The work required skilful fabrication of piping to suit the intricate routing and matching the dimensions of the earlier piping. "This is the first time that feeder replacement has been done in a reactor anywhere in the world," said Jain.
The reactors in MAPS belong to the first generation of Pressurised Heavy Water Reactors (PHWRs) built indigenously. The PHWRs use natural uranium as fuel and heavy water as both coolant and moderator. The PHWRs are the mainstay of India's nuclear electricity programme. The EMCCR was first attempted in the second unit of the Rajasthan Atomic Power Station (RAPS) at Rawatbhatta, near Kota, and the reactor attained re-criticality on May 27, 1998. The same was then done in the second unit at MAPS. The campaign began in January 2002 and ended in June 2003, and the capacity of the reactor was stepped up to 220 MWe from 170 MWe.
However, in the refurbishment of MAPS-1, the technology used in the EMCCR in RAPS-2 and MAPS-2 was fine-tuned and several innovations were used.
In a reactor, the coolant tubes, also called pressure tubes, are inside the calandria tubes. The coolant tubes house the nuclear fuel and also convey heavy water for the transfer of heat from the fuel to the steam generators. The steam thus produced drives the turbine to generate electricity. Both the coolant tubes and the calandria tubes carry heavy water and between them are two circular garter springs. The coolant tubes with end-fittings on either side are called coolant channels and there are 306 coolant channels arranged in a circular lattice that has a diameter of 4.8 metres.
The coolant tubes, made of zircaloy, were found to be one of the life-limiting components of the reactor. Zircaloy is brittle and susceptible to hydriding, that is, formation of metal hydrides when it picks up hydrogen. If hydriding continues beyond a limit, blisters would form on the coolant tubes and affect the integrity of the tubes. This happened in RAPS-2 and MAPS-2, and the coolant tubes developed micro-cracks. Hydriding accelerates if the coolant tube comes in contact with the relatively cold calandria tube. This happens when the garter springs move away from their positions because of vibrations from the flow of heavy water.
In MAPS-1 it was decided to replace the zircaloy coolant tubes with those of zirconium-niobium, which has high resistance to hydriding. It was also decided to use four garter springs instead of two and ensure that they did not move from their positions so that the spacing between the coolant tubes and the calandria tubes was maintained.
The massive job of replacement of the 306 coolant channels was divided into two main constituents: the removal and disposal phase, and the reinstallation phase. The removal phase involved cutting the coolant channels from both the ends and removing 612 end-fittings made of stainless steel. This was precision work that required detailed planning and defuelling of the reactor.
Since the EMCCR involved working on the radioactive innards of the reactor, quality assurance methods were adopted at every stage. To handle the highly radioactive coolant tubes and end-fittings that were cut and removed, heavily shielded working platforms and shielding flasks were used. Many of these operations were done remotely because radioactivity was involved. "The magnitude and complexity of the task required extensive mock-ups at different stages of work," said V.A. Subramani, Chief Superintendent, MAPS.
Each shielding flask weighed eight tonnes. The cut coolant tubes and their end-fittings were slid into the shielding flasks, transported outside the reactor building and disposed of in underground silos in a waste management facility which is isolated from the environment. Special methods were devised to handle safely the flasks and minimise radiation exposure to the personnel handling them.
Another gigantic task was the replacement of all the 606 carbon steel coolant feeder pipes. They are of different diameters and complex shapes and convey heavy water to the coolant channels and the steam generator. In the other PHWRs it was found that the wall of the feeder pipes became thin owing to corrosion. As a pro-active measure, NPCIL replaced all the feeder pipes in MAPS-1 to take care of the possible wall-thinning, thus matching the life of the feeder pipes with the extended life of the coolant channels. Replacement of the feeder pipes involved precision-fabrication of pipes, matching the connections with the end-fittings and the reactor headers. Besides, given the narrow spacing between the feeders, detailed procedures were worked out for precision welding and rehearsals were done to confirm the routes of the pipes.
Another demanding job was the replacement of heat exchangers in all the eight steam generators. Each steam generator has 11 heat exchangers, and each heat exchanger has 195 half-an-inch-diameter tubes made of a material called monel-40. Each heat exchanger weighs about three tonnes. Wall-thinning as a result of corrosion was noticed in a few tubes during in-service inspection. Utilising the long outage during the EMCCR, all the 88 heat exchangers in the first unit were replaced. The heat exchangers were cut at the connecting places, rigged out, and taken out of the reactor building. The new heat exchangers were trimmed to size and matched precisely in their original locations. Replicas were first used in this.
Said Krishnamurthy: "The removal and replacement of the heat exchangers required a high degree of skill in handling heavy equipment in difficult locations. The total weight handled was of the order of 300 tonnes for the old heat exchangers and another 300 tonnes for the new ones."
Another achievement was the re-installation of sparger channels in the calandria. The technology for this was developed indigenously by the engineers. The job required precision machining on the reactor vessel, which had to be done remotely, and special tooling was developed for this.
Said Jain: "From RAPS-2 to MAPS-1, we have come a long way." From 1974, when India conducted its first nuclear explosion at Pokhran, it suffered from a technology-denial regime. This forced India to develop the technology and the tools to find out the health of its nuclear reactors. "We have not only enhanced MAPS-1's life but made it more safe. They have been brought to the international level," said Jain.
The next reactor being readied for EMCCR is the first unit at the Narora Atomic Power Station in Uttar Pradesh.
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