A feat at Tarapur

CHECKING A NUCLEAR fuel bundle made of natural uranium for integrity before it is loaded into the 540 MWe TAPP-4. - VIVEK BENDRE

The twin reactors of the Tarapur Atomic Power Station undergo modernisation, safety upgradation and life extension.

THESE are exciting times at Tarapur, Maharashtra, where India's first commercial nuclear power station was built. The two imported reactors of the Tarapur Atomic Power Station (TAPS-1 and 2) have been modernised, their safety upgraded and life extended, and they have enough fuel - low enriched uranium (LEU) - to see them through for several years. Nearby, the third indigenously built Pressurised Heavy Water Reactor (PHWR) of 540 MWe capacity is getting ready to reach criticality by April-end or the first week of May. The loading of natural uranium fuel bundles into the reactor core has already begun. The fourth unit, with a similar capacity, which reached criticality in March 2005, is now operating smoothly and generating about 500 MWe.

TAPS-1 and 2 are light water reactors (LWRs), built by General Electric of the United States and designed by Bechtel. Both units, which started generating electricity in 1969, use LEU as fuel and light water as both moderator and coolant. TAPS-1 and 2 were built as a turnkey project, to demonstrate that commercial nuclear electricity was viable in India. Their original capacity was 210 MWe each but they were derated to 160 MWe each in 1985.

Under an agreement signed in 1963, the U.S. agreed to supply LEU to TAPS-1 and 2 for 30 years. However, after India conducted its peaceful nuclear experiment in May 1974, the U.S. reneged on its commitment to supply the fuel. It cited domestic legislation, particularly the Nuclear Non-Proliferation Act of 1978, to deny LEU to the two units. In 1983, France agreed to supply enriched uranium for 10 years up to 1993 when the agreement between India and the U.S. was to expire.

China supplied about 30 tonnes of enriched uranium in 1995, and Russia 51 tonnes in 2001. Again, in March 2006, Russia agreed to supply 60 tonnes of LEU, and a part of the consignment reached India in the week beginning March 21. About 30 tonnes of LEU, in the form of pellets, reached Hyderabad in chartered flights. The pellets were taken in trucks to the Nuclear Fuel Complex there where they would be clad in zircaloy tubes and despatched to TAPS-1 and 2 to be fed into the reactor core. According to Department of Atomic Energy (DAE) officials, TAPS-1 and 2 will face no shortage of fuel until 2012.

In four and a half months, the Nuclear Power Corporation of India (NPCIL) engineers refurbished, modernised and extended the life of these ageing reactors at a cost of Rs.20 crores, when similar reactors in other parts of the world have been shut down. The two units were reconnected to the grid on February 16 and are now generating 160 MWe each. While modernising the two units, the NPCIL engineers upgraded their safety to international standards. The Atomic Energy Regulatory Board (AERB), which is entrusted with maintaining safety in nuclear power stations in India, has permitted the NPCIL to operate the two units for another five years. But TAPS engineers are confident that they will serve for another 15 years.

S.K. Jain, Chairman and Managing Director, NPCIL, said what stood the TAPS- 1 and 2 engineers in good stead was that they had systematically maintained records and data on the performance of the two units for the past 37 years. Besides, GE and Bechtel had handed over to them all the documents relating to the plant, including the routes of the cables that led into the two units.The records and data helped the engineers in assessing the life of the station and doing the engineering for the upgrade.

"We did reverse engineering. This plant is worth Rs.1,800 crores. We have got back Rs.1,800 crores by spending just Rs.20 crores. This makes economic sense," U. Ramamurty, Station Director, TAPS-1 and 2, said. He pointed out that retro-fitting was difficult as everything had to be done within the limited space that was available.

"It is as good as a new plant," said M.S.S. Raghavan, who belonged to the first batch of engineers to work in TAPS and was its shift-charge engineer. Raghavan, who retired as Director (Projects), NPCIL, in 1994, visited TAPS-1 and 2 in March after the two units were reconnected to the grid. "I am happy to see that all safety requirements have been incorporated," he said. For instance, the emergency diesel generators' capacity had been stepped up. The electrical cables had been neatly laid and taken to the containment building, incorporating international safety standards. The safety systems' cabling and relay equipment had been completely isolated, separated and duplicated, Raghavan said.

Although the two units had a chequered career, including the shortage of fuel, their fortunes have looked up since 1995 when their capacity factor went up steadily. Long outages were cut short. The time taken to refuel the reactors was reduced. "We reached 90 per cent capacity factor in 2003-04. We generated 2,626.6 million units in 2004 - the highest generation in a calendar year in the station's history," said Ramamurty.

Since the two units were having a smooth run, TAPS' top officials wanted to receive long-term clearance from the AERB to operate them. The AERB suggested that TAPS engineers do an exhaustive review of the two units, including an assessment of their life and the steps needed to manage their ageing and upgrade their safety, and submit a report.

Engineers of TAPS began the work, concentrating on three aspects: reviewing the operating safety of the two units, reviewing their ageing management and conducting a safety analysis based on the station's design. The rationale behind doing these three reviews was that during its long innings, the station had undergone many ups and downs and its material condition would have possibly changed. The design of the plant was based on the codes and guides of the 1960s. Safety standards in vogue today are far more stringent than they were at that time. The AERB made it plain to TAPS' officials that the two units should meet international safety standards if they wanted long-term authorisation to operate them. "So we identified the areas that required refurbishment. We did a probable safety analysis, which gave a good insight into the safety of the plant. We identified the equipment that had to be overhauled or replaced," said Ramamurty.

Civil structures, including the containment building, were studied and the strength of their cement concrete was assessed. An assessment of the life of the cables was done. The suppression pool, which holds 1.25 lakh cubic feet of water, comes into play during conditions of loss of coolant accident (LOCA) when the fuel core would melt.

The water would be injected with the reactor core in case of LOCA. After the modernisation campaign began, all the water was drained and the flooring of the suppression pool chamber cleaned and its steel lining checked for integrity.

Ageing management included replacement of emergency diesel generators and battery chargers, routing of cables based on the residual life assessment studies and so on.

Safety upgradation work included the retrofitting of emergency diesel generators, segregating distribution of electrical cables, replacing cables, reconfiguring load centres, modernising the reactor shutdown control system and control rod drive mechanism, and so on. Indeed, the trenches through which the electrical cables were laid were strengthened by rebuilding their walls and floors and buttressing them with steel bolts so that they would not cave in over the cables in case of an earthquake. Tall segregation walls were built in rooms that had a tangle of cables so that a fire could be localised.

According to S. Bhattacharjee, Chief Superintendent, TAPS-1 and 2, a supplementary control room for both the units was built lest a fire in the main control room the operators unable to occupy it. "We also upgraded the system for cooling the spent fuel," he said. A lot of seismic revaluation of equipment was done to check whether they would remain in place when an earthquake occurred. This was done by placing the equipment on shake-tables and subjecting them to jolts. Experts did a "seismic walk down the plant" and prepared a checklist of the areas that should be made more secure, Bhattacharjee said. The building, which housed the emergency diesel generators, underwent tests to check whether it could withstand earthquakes. Fire protection systems also underwent seismic reviews.

About 50 people worked continuously to enhance safety. The Design Group from Bhabha Atomic Research Centre, Trombay, helped in this. Operators and maintenance personnel wrote examinations and underwent viva voce so that their competency in operating the modified systems could be assessed.