The first unit of the Madras Atomic Power Station (MAPS) at Kalpakkam went critical on January 4, 2006. Unit 1 of the MAPS, which is a 22-year-old nuclear plant, first went critical in July 1983. Going critical for the second time was the result of the replacement of some critical primary components of the aging nuclear plant with new and improved material. The life of the 22-year-old 220 MW nuclear power plant has been extended by 25-30 years with the replacement of the primary components. Changing the primary components of unit 1 was undertaken in late 2004 after it was shut down in 2003; the job was completed in a year’s time. A similar exercise undertaken in unit 2 of MAPS was completed in July 2003 in one and a half years’ time.
The life extension programme of a nuclear power plant gains significance on two counts – savings in terms of time and money.”We were able to complete the replacement of critical components at one tenth the cost compared with such work undertaken globally,” said S. Krishnamurthy, Station Director of MAPS. “And compared with the cost of constructing a new nuclear power plant of similar capacity, we were able to extend the life of the plant at nearly one fifth the cost.”As a rule of thumb, constructing a new nuclear power plant would work out to Rs.5.5 crores per MW if completed in five years’ time. And while the time taken to construct a new power plant would have taken five years, refurbishment or replacement of many primary components of unit 1 was done in nearly a year’s time – 14 months to be precise. It took 19 months for unit 2.
Life extension programme of the two units was done using indigenous technology. It may be recalled that a similar exercise was first undertaken with the two units of the Rajasthan Atomic Power Station (RAPS) in 1998. MAPS and RAPS are of the Pressurised Heavy Water Reactor (PHWR) type that uses natural uranium as fuel and heavy water as coolant and moderator. While only the 306 coolant channels were replaced in the two units of RAPS, steam generators and 306 coolant channels were replaced in both the units at MAPS. In the case of MAPS unit 1, the feeder pipes was also replaced. “Feeder pipe replacement was done for the first time in the world,” said Mr. Krishnamurthy with pride. “Though we had not seen wall thinning of the feeder pipes, we had seen this happening in other reactors. So we replaced the pipes as a pro-active measure.” About 0.25 per cent of chromium was added to the feeder pipe carbon steel to improve erosion resistance and increase its thickness. In the case of steam generators, the 88 old ones were replaced with the same material. “But we improved the blow-down conditions. With this the life [of the steam generators] is expected to be more,” he noted. The improvement in blow-down condition makes sure that impurities do not build up. In the case of the coolant channels, the zircaloy material that was used previously was replaced with zirconium-niobium. Niobium content is 2.5 per cent. There was a compelling need to change the composition of the material. Zircaloy material has a tendency to pick up hydrogen from the coolant to form hydrides. “Hydride formation leads to brittleness of the material and in turn leads to loss of integrity,” Mr. Krishnamurthy explained. Loss of integrity, in simple terms, means the formation of cracks.
While the zircaloy material has a natural tendency to pick up hydrogen from the coolant, the garter springs that separates the coolant tubes from the calandria tubes had also shifted in some cases due to flow induced vibrations in the coolant tube. The garter spring displacement led to sagging of the coolant tubes. This resulted in the coolant tubes coming in contact with the calandria tubes resulting in the reduction of temperature of the coolant tubes and formation of cold spots. “This [cold spot] acted as an accelerating mechanism for hydride formation,” he explained. “But we did not see any cracks in any calandria tubes.”
The use of zirconium-niobium material imparts greater strength and higher resistance to hydride formation. This makes sure that the coolant channels have a longer life compared with channels made of zircaloy.
Another step in preventing or delaying the displacement of garter springs was undertaken at MAPS. “We have put four garter springs instead of two. And these springs are tight fit ones and would not get displaced as easily as the earlier ones,” said Mr. Krishnamurthy. Apart from replacing the primary components, an innovative method of introducing the moderator heavy water into the reactor vessel (calandria) was designed and implemented. It may be recalled that the moderator flow into the reactor vessel had to be rearranged in early 1990 as one of the flow distributor plates had given way. This led to forced reduction in power production (derating) – from 220 to 175 MW.
Full capacity restored
“With the new design now implemented, the unit 2 has been restored to its full capacity of 220 MW and the unit 1 would also be restored similarly when it is connected to the grid,” he highlighted.”The MAPS engineers have done an excellent job in increasing the automation levels,” commented S.K. Jain, Chairman and Managing Director of the Nuclear Power Corporation of India Limited (NPCIL). “Increasing the automation level results in lowering the radiation exposure [to workers] considerably. Also, the job was done faster.”The life extension programme undertaken at RAPS and MAPS have helped build the confidence level of NPCIL in a different way.
“It has given us a great amount of confidence in decommissioning the [PHWR] nuclear plants some 20-25 years later,” he noted, “as we have already replaced half the critical components at a reasonable cost and time.”Having successfully completed the replacement work at MAPS, the NPCIL has undertaken a similar job at Narora unit 1. Narora unit 1 attained criticality in March 1989. “We have already shut down the unit 4-5 weeks ago for replacement as the plant had already completed the stipulated period for replacement of coolant channels,” Mr. Jain said. As the design of the steam generator is different, the replacement would be restricted to the 306 coolant channels and 612 feeder pipes.”Our ultimate aim is to achieve 100 per cent automation and to complete the job in less than a year’s time, say about ten months,” he said. An ambitious target indeed; the NPCIL’s track record, however, shows that it is quite capable of achieving it.