Thursday 26 July 2007

Nuclear Engineering--PHWR

BUSINESS INDIA, October 7-20. 1996

Nuclear Heart Transplant

The heart of the 16-year-old Rajasthan-II reactor is being removed and replaced by a more robust brand new one, in a marvel of nuclear engineering

Shivanand Kanavi

In 1967, when Dr Christiaan Barnard conducted the world's first successful human heart transplant in South Africa, he created history. The engineers of Nuclear Power Corporation (NPC) are carrying out another type of heart trans­plant at Rawatbhatta, by changing the coolant channels of the nuclear power reactor. Thereby, they hope to extend the life of the Rajasthan Atomic Power Sta­tion-II (RAPS-II) by roughly 30 years.

On successful completion of the job to take roughly three years, NPC would have also mastered a new proprietory technol­ogy, developed indigenously at a highly competitive price. Thereby posing seri­ous competition to the Canadians in the international reactor services market for Pressurised Heavy Water Reactors (PHWR).

Nuclear heart surgery involves a great deal of analysis, design, precise planning, skill as in human heart surgery and in addition great radiation risk too, if mis­handled. Hence, to appreciate the com­plexity of the operation, it will help to know how the workhorse of the Indian nuclear programme - the 210 MW PHWR works.

PHWR produces power by bombarding neutrons on natural uranium (99.3 per cent U238 and 0.7 per cent U235). The right neutron speed can split the uranium nucleus into two nearly equal halves, releasing energy and more neutrons than consumed in fission. The released neutrons are slowed down through a series of collisions with deuterium in heavy water without being absorbed, much like a sprinter is slowed down while passing through a crowd.

Ordinary water is a compound of hydrogen and oxygen, whereas heavy water is made up of deu­terium - a heavier isotope of hydrogen - and oxygen. The resulting compound is about 10 per cent heavier than ordinary water and hence the name. While ordinary water absorbs neu­trons thereby stopping the reaction, its heavier cousin does not do so. This prop­erty of heavy water makes it a good 'moderator'.

With more neutrons released than consumed by fission, a chain reaction sets in. Neutron absorbers like cadmium are used to strike the right balance between neutron release and absorp­tion rates, thereby prevent­ing a run away reaction leading to a nuclear explo­sion, while still sustaining enough of the reaction for power production.

Uranium mined by the Uranium Corporation at Jaduguda, Bihar and converted into 'yellow cake' is refined and converted into fuel bundles by the Nuclear Fuel Complex at Hyderabad. These fuel bundles are placed in coolant channels made of zirconium alloy which is almost transparent to neutrons. Pres­surised heavy water flows through the coolant channels and carries away the heat produced during nuclear fission. The hot heavy water at 270 degree celsius then transfers the heat to ordinary water in the steam generator. The steam thus produced then turns a conventional tur­bine-generator producing electricity.

The coolant channels are housed in a cylindrical steel vessel called the calan­dria. The calandria contains heavy water which acts as a moderator. The two ends of calandria are cov­ered by nearly a metre thick steel end shields housing a lattice of 306 coolant channels. The entire reactor is inaccessible and is in a metre thick con­crete vault, once the reactor starts up. All defuelling and fuelling has to be done through remote control. Thus unlike a conventional power station, any minor repair later, is a herculean task and needs careful planning and execution.
The boiling water reactor technology developed in the US by General Electric, Westinghouse, etc, needs enriched ura­nium requiring expensive enrichment processes. The PHWRS developed by Canada as pointed out earlier, use natural uranium. Moreover, Canada offered the technology at very attractive terms and even showed willingness to involve Indi­ans to some degree in developing and sta­bilising the design.

However, Pokharan in 1974, exploded all international nuclear co-operation with India. Canadians even abandoned RAPS-IT halfway. There was an embargo placed on all nuclear-related sales to India and every wheel had to be painstak­ingly reinvented by the Department of Atomic Energy and then taught to the Indian industry.

Today a veritable nuclear industrial infrastructure has been built. Industries like L&T, Walchandnagar, Bhel, Machine Tool Aids & Reconditioning, KSB Pumps, etc, are doing high precision fab­rication of end shields, calandrias, coolant channels, fuelling machines, steam generators, pumps and other sub­systems for the PHWRS.

The technological embargo, however, led to another serious problem. There was a fracture in a coolant channel in the reac­tor at Pickering Unit-II, Canada, in August 1983. Such a fracture leading to what nuclear engineers call a 'loss of coolant accident' is every reactor opera­tor's worst nightmare, as it might lead to 'core melt down' and a serious nuclear accident as in the Three Mile Island in the US or even worse. Some readers might also remember the Hollywood version of loss of coolant accident in Jane Fonda & Jack Lemon’s China Syndrome.
Fortunately, the loss of coolant in a PHWR does not lead to a core melt down. It is one of the inherent design superiori­ties of PHWR. But due to the embargo, Indians, who were using the same design in Rajasthan, were denied detailed knowledge of the accident, its cause and the remedial actions taken. They had access only to some general discussion in international conferences.

While some problems are expected due to ageing, after 30 years of run­ning the reactor, it was significant that the accident at Pickering occurred after only ten full operating years. The accident at Pickering, however, alerted Indians and some design modifications were made in all reactors after Rajasthan I & II and Madras I &II. In Kakrapara II, Kaiga-I &II and RAPS-III & IV a new zirconium alloy with an addition of 2.5 per cent niobium was used for coolant channels. The new alloy has vastly better characteristics than the earlier zircolloy-2 and should give no problem for 30 years.

However, such a coolant channel frac­ture could still occur at RAPS-I&II and MAPS-I&II, which use the old design. Hence they were closely monitored. To take remedial action, NPC set up a core group of engineers called Coolant Chan­nel Replacement Group to work out the entire details of an exercise to replace all the 306 coolant channels in the older designs starting with RAPS-II.

NPC engineers in Bombay and on site at Rawatbhatta have risen up to the task admirably and are today probably the most excited group in the entire DAE. In fact, V.K. Chaturvedi, the project director at Rawatbhatta has become a legend of sorts with his hands-on leadership. The best place to meet him is not his residence or office but the reactor site itself where he is found at all odd times.

In a record time of four months they have already cut and removed all the 306 coolant channels at RAPS-II and sealed the highly radioactive channels in a spe­cially constructed underground concrete mausoleum. According G.R. Srinivasan, director environment and public aware­ness at NPC, "The task has been carried out with a surprisingly low radiation exposure to personnel, well below safe levels, a fact which has amazed many."

Canadians had done the same, taking longer time and using advanced remote controlled equipment. It was rumoured internationally that either Indians cannot develop the technology or they will use crude and callous methods and expose their personnel to heavy doses of radiation. The achievement of the coolant channel replacement group led by R.C. Arya, director, reactor services and the on-site team led by Chaturvedi, increases in significance in this background.

If everything goes well then even fit­ting the new channels will be finished between December 1996 and September 1997. They would then have completed the entire project, from defuelling to handing over for start-up within 36 months, as opposed to 44 months taken by the Canadians.

This has commercial implications. The Canadians spent nearly $300 million whereas the Indians would spend $72 million to do the same. With PHWRS operating in South Korea and Argentina there is a good opportunity for the Indians to offer coolant channel inspection and replacement services at highly competi­tive rates.

The genesis of the coolant channel problem, lies in a confluence of factors. These pressurised tubes are separated from the calandria tubes by a concentric gap of 8 mm. The separation is main­tained using two garter springs kept at certain intervals. While the heavy water in coolant channels is at about 270 degree celsius, the calandria tubes are sur­rounded by the moderating heavy water at 70 degree celsius. Due to vibration within the tube the springs in the old design, tend to move from their positions leading to lack of support at certain portions of the coolant channel. The weight of the fuel bundles (uranium is heavier than gold!), thermal stresses and irradiation, lead to sagging of the coolant channels.

In the extreme conditions existing inside the coolant channel, minute amounts of heavy water break into deu­terium and oxygen. Normally zirconium forms an oxide layer by combining with oxygen while deuterium is released as gas. But tiny amounts of deuterium are also absorbed by zirconium forming a brittle 'hydride'. This deuterium pie is so slow that one need not worry about it for 30 years.

However, if the sag in the coolant channel leads to contact with the colder calandria tube then the cold spot devel­oped at the point of contact leads to accu­mulation of deuterium in the zircolloy at that point. This can lead to blistering and even a possible fracture, as it happened in Pickering Unit-II. Niobium-stabilised zirconium however has much less deu­terium pick up and better thermal creep characteristics. Thus the replacement of old channels by the new niobium-sta­bilised zirconium alloy channels with four tight fitting garter springs, which will not move easily, will prevent sagging and add another 30 years to reactor life provided all other systems continue to work well.

Nuclear Power Corporation today has under 2,000 MW of gen­erating capacity. For NPC to gener­ate funds through internal accruals for further expansion it needs a minimum of 5,000 MW of generat­ing base. At a crucial phase in NPC'S evolution, funds from the Central government have slowed down to a trickle with extreme short sightedness. With no interna­tional institution like World Bank ready to fund nuclear power, the NPC has been left high and dry. Since building a new power plant is always very expensive, every megawatt squeezed out of existing old plants at a marginal cost, is heavenly light for NPC and a power-starved India.

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