Reviving reactors

Using indigenous technology, the Rajasthan Atomic Power Station has revived two reactors shut down in 1994 by plugging a leak in one and replacing the coolant channels in the other.

THE Rajasthan Atomic Power Station (RAPS), situated at Rawatbhatta, 65 km from the industrial town of Kota on the edge of the Rana Pratap Sagar lake, has made major technological breakthroughs in recent times. Its engineers have put the RAPS-1 reactor, which had been virtually given up as beyond repair, back in action. They have also performed a virtual "transplant" in the RAPS-2 unit. In only the second instance of such an operation in the world, they removed 306 coolant channels in RAPS-2 and were expected to replace them with new coolant channels by December 20. RAPS-1 generates 160 MW of electricity now, although it had earlier been de-rated to 100 MW. The second unit, which is as good as a new reactor, will attain criticality towards the end of February 1998 and generate its full capacity of 220 MW by end-March.

In terms of cost, RAPS spent a mere Rs.1 lakh in reviving the first unit. This amount was spent on developing tools, tackles and manipulators to repair a leak from a contraption called Over Pressure Relief Device (OPRD). It took RAPS only six months to develop the tools and just 24 hours to plug the leak. When there was a similar leak in a reactor at Douglas Point in Canada, the authorities there reportedly took three years to finalise preparations and a further 10 months to stop the leak. It cost them a few million dollars.

The credit for the achievements made at RAPS-1 and RAPS-2 units should go to RAPS Project Director V.K. Chaturvedi and his team. Chaturvedi said that the OPRD experience had set a precedent for similar high-technology jobs in other nuclear areas. He attributed the success of the OPRD repair work and coolant channel replacement programme to two senior maintenance engineers, R.K. Gargye and D.K. Sisodia. Chaturvedi described Gargye as the brain and said Sisodia as another central figure behind both operations.

In 1962, the Government of India decided to set up two reactors of CANDU (Canada-deuterium-uranium) design at Rawatbhatta on the model of the Douglas Point Generating Station with the help of Atomic Energy of Canada Limited (AECL). AECL provided the design and also most of the equipment for the first unit of 220 MW capacity. It went critical on August 11, 1972.

Work on the second unit had just begun with Canadian assistance when India conducted its peaceful nuclear explosion on May 18, 1974 at Pokhran, Rajasthan. Four days later, Canada pulled out of the construction of the second unit. Indian engineers took up the challenge and built the unit.

The RAPS units are prototypes of India's Pressurised Heavy Water Reactor (PHWR) programme, which uses uranium as fuel and heavy water as coolant and moderator. Atomic power stations built subsequently at Kalpakkam, Narora and Kakrapara are all PHWRs. All nuclear power stations in the country come under the Nuclear Power Corporation of India Limited (NPC).

RAPS-1 was shut down on February 2, 1994 after helium gas and heavy water leaked from the reactor's OPRD. The OPRD is placed at the top of the calandria, which is a large cylindrical reactor vessel with heavy water (moderator). The OPRD's purpose is to check the rise of pressure inside the calandria in case of an accident. The problem arose after a nickel gasket seal in the OPRD gave way. Chaturvedi said that nitrous oxide, which formed in the calandria vault due to radiation, began to eat into the nickel seal and once the nickel seal gave way, heavy water and helium began to leak. Heavy water started collecting in the calandria vault when the unit was in operation. The Atomic Energy Regulatory Board (AERB), responsible for maintaining safety standards in nuclear power plants in the country, insisted that the reactor could not be run if the leak was not plugged. And the Canadians suggested that the reactor be "boxed up".

Three groups - two from the Bhabha Atomic Research Centre (BARC), Trombay, and another from the NPC - set to work. The NPC group suggested that an additional metallic seal resistant to radiation be placed as a back-up inside the existing nickel seal. A special metal, indium, which is produced by the Nuclear Fuel Complex in Hyderabad, was selected for the purpose. An indium seal would be resistant to radiation and the corrosive nitrous oxide atmosphere. Indium's melting point is low, it is flowable and it could occupy all cavities.

But the problems of placing the indium seal were daunting. It had to be a "blind operation" in a high-radiation zone. The seal had to be taken through a tiny circular opening 9.52 centimetres wide. RAPS had to develop remotely operated manipulators, tools and tackles that could reach a location difficult to access and place the indium seal accurately from a point about five metres from the reactor-top.

Chaturvedi said that finally they devised a flexible manipulator in the shape of the human hand which could pass through the hole and reach the spot. "Our strategy was to place the indium seal inside the nickel seal."

However, in the mock-up trials held in a simulated calandria, RAPS technicians ran into problems. Although indium melted and flowed inside the tube, it began to solidify and choke the tube. It was like wax from a burning candle solidifying the moment it fell on the ground.

The strategy now was to melt the indium inside the reactor. But this also posed a problem. The reactor had fuel and any heating inside the reactor had to be done without starting a fire. The indium liquid had to be flowed accurately in the cavities, ensuring that it did not spill on fuel bundles (natural uranium). It was, therefore, decided to fabricate a free-floating manipulator that would reach the cavities accurately. It was also decided to melt the indium inside the calandria using a heating device whose design looked like an umbrella. A Mumbai-based firm offered to develop the devices, called heating candles, in six months at a cost of Rs.28 lakhs. Chaturvedi said that RAPS itself developed the heater at its Reactor Component Workshop at a cost of Rs.2,000. The heater worked and RAPS made another manipulator for placing the indium.

Since placing the indium seal would be "a totally blind operation", repeated trials were held. Gargye said, "We did it ten times and we succeeded all ten times. This is a regulatory requirement." What if any component failed? "We had back-up systems and conducted failure analyses on each component."

RAPS technicians also made a closed circuit television camera and a periscope to look inside the calandria. This camera, resistant to radiation, worked admirably well and produced good pictures. (An imported camera costs between Rs.50 lakhs and Rs.1 crore).

The trials were completed by January 25, 1997, and on February 6, 1997 the AERB allowed RAPS to place the indium seal in the reactor. The actual job was taken up on February 8. The work of in situ seal formation inside the calandria was taken up on February 10. Chaturvedi said that the most crucial operation lasted about eight hours and a perfect placement of the liquid metal seal was achieved inside the reactor. After the seal cooled down, the closed circuit camera was lowered. "We completed a difficult job perfectly."

Subsequent operations were relatively simple. It took a few hours to close the opening on top of the reactor through which all the manipulators were lowered into the calandria. Helium leak detection tests proved that the new indium seal was in sound health. The leak was stopped completely. With this, RAPS-1 received a new life. The reactor was started up on March 31. It ran continuously for 64 days. Impressed, the AERB permitted RAPS to step up generation to 160 MW. (In the mid-1980s, the unit was de-rated to 100 MW because of a crack in the calandria side of the end-shield.)

RAPS Deputy General Manager S.K. Agrawal described Operation OPRD "a major technological achievement". He said, "We have given a new lease of life not only to the reactor but to neighbouring villages; their prosperity depended on it." More than 40 persons involved in the operation were given medals and citations. A plaque unveiled by NPC Chairman and Managing Director Y.S.R. Prasad on September 12, 1997 says: "RAPS-1 Revival. In pursuit of excellence accomplished by RAPS employees."

EARLIER, RAPS engineers were engaged in another ambitious project, the en masse removal and replacement of the coolant channels in RAPS-2. It is a transplant operation known as re-tubing in nuclear terminology. The reactor had to be de-fuelled and decontaminated before the 306 coolant tubes made of zircalloy and which carry heavy water were removed. The target date for the removal was April 1997; the work was, however, completed in September 1996. New coolant tubes made of an improved metal called zirconium niobium (Zirc-Niobium) have been put in their place.

The coolant tubes, also called pressure tubes, are inside the calandria tubes. What separates these tubes are two circular garter springs. Both the coolant tubes and the calandria tubes carry heavy water. The coolant tubes with end-fittings on either side are called coolant channels. The 306 coolant channels in the calandria tubes appear in the form of a circle with a diameter of 4.8 m.

How did micro-cracks occur in the coolant tubes in RAPS-2? When heavy water flowed in the coolant tubes, vibrations occurred and the garter springs moved from their positions. This resulted in the coolant tubes sagging and coming into contact with the calandria tubes. While the temperature inside the coolant tubes was 295 Celsius, the temperature in the calandria tubes was about 70 Celsius. The temperature of the coolant tubes came down at the points of contact, resulting in the formation of zirconium hydride and the micro-cracks. If these cracks had widened, heavy water would have leaked from the coolant tubes into uranium fuel bundles and this may have led to a dangerous situation.

When RAPS investigated the second unit, it found that the garter springs had moved in about 10 to 12 coolant channels. These channels had sagged and "contacted" the calandria tubes. Facing a similar situation, the Canadians had earlier shut down their Pickering and Bruce stations.

Chaturvedi said that the five nuclear power units at Kalpakkam, Narora and Kakrapara were causing more worry than RAPS; all these had coolant tubes made of zircalloy. So it was decided to de-fuel RAPS-2, take out all the 306 coolant tubes and examine them metallurgically, he said.

Investigations took a year, from September 1994 to September 1995. It was decided to go ahead with the coolant channel removal and replacement project. This included devising additional safety systems and rehabilitating the unit with repair and replacement of other equipment.

By January 1996, technicians had de-fuelled the reactor. The systems were then de-contaminated by circulating chemicals, which dissolved radioactive substances. Hot-conditioning came next: the systems were heated to 220 Celsius and water circulated so that they became resistant to corrosion. This was over in April 1996. The systems were later drained of heavy water and dried.

After trials, the removal of the coolant tubes began in May 1996 and was completed by September 1996, although it was scheduled to be completed only by April 1997. Simultaneously, said Chaturvedi, the task of disposing of the radioactive tubes was also undertaken. The 306 tubes and their 612 end-fittings were cut, removed and buried in 4.5-metre-deep tile-holes. The holes were concreted on all sides to prevent radiation from escaping.

The replacement of coolant tubes made of zirconium niobium began on May 24, 1997. When this correspondent visited the unit on November 27, work was on. By December 12, 1997, about 290 new tubes had been placed in position and all 306 tubes were expected to be in place by December 20. The reactor will then be fuelled and a range of tests, including helium leak testing, air testing and hydro testing, will be carried out. Various systems will then be run one by one before all plant operations are integrated.

Canada demanded $350 million (Rs.1,200-1,300 crores) to remove and replace the channels in RAPS-2. Chaturvedi said that RAPS needed only Rs.84 crores to do the job. He said that RAPS spent Rs.4 crores at the site and the components cost was Rs.80 crores. The components included new end-fittings and new coolant channels made of zirconium niobium.

Chaturvedi said that the reactor would attain criticality by the end of February 1998. It was called criticality because it has a new core, he said. This would be followed by a lot of reactor physics experiments. The reactor would be connected to the Rajasthan grid by March-end. The life of the reactor, according to Chaturvedi, will be 50 years and its full-power life will be 30 years. Chaturvedi added, "This is the first time in an operating station that a whole core has been taken out and replaced with a new core."