IN the various facilities at the Kudankulam Nuclear Power Project (KKNPP) the engineers of Nuclear Power Corporation Limited (NPCIL) are proud of the great lengths to which they have gone to ensure safety. Multiple layers of safety have been built into the two Russian VVER-1000 reactors to prevent radioactivity from escaping into the atmosphere in the case of an accident. The reactor and turbine buildings can also withstand natural disasters such as earthquakes, cyclones and tsunamis, NPCIL engineers said.
In the reactor building, the fuel, enriched uranium, is encased in tubes housed in the Reactor Pressure Vessel (RPV). The RPV has a 22-cm-thick stainless steel wall and weighs 350 tonnes. It is filled with water, which acts as a shield. The vessel itself is kept inside a concrete vault whose wall is 1 metre thick. The vault is topped by two domes for double containment. The inner dome has a 1.2-m-thick concrete wall lined on the inside with a 6 mm layer of steel. The outer dome has a 60-cm-thick wall. All this to prevent any radioactivity leaking into the atmosphere in the case of an accident.
The newest safety measure is the Passive Heat Removal System (PHRS), which has been built on top of the containment dome of the reactors. On top of the outer dome, at a height of 80 metres, sits a box-like structure. This is the tertiary dome of the PHRS. The entire PHRS comprises several heat exchangers and steel pipes of 3-m diameter. The PHRS is a new design conceived by us. The Russians did the detailing, said A.K. Pal, Chief Construction Engineer, KKNPP. True to its name, it has no active components and does not need electricity. It works automatically on the thermo-syphon principle.
M.K. Balaji, Chief Superintendent, KKNPP, said that in the event of blackout or if the circulation of reactor coolant failed, power supply was available from the grid to remove the decay heat. In addition, four diesel generators are available, which will generate electricity to keep the reactor in a safe shut-down and cool mode. While power from one diesel generator is enough to keep the reactor cool, three more are available. Even in the extreme condition of all these four generators failing, we can still keep the reactor cool with the ambient natural air by means of the PHRS, said Balaji.
The PHRS allows the hot air (which tends to move up) inside the reactor building to escape into the atmosphere and simultaneously allows cool air from the atmosphere to enter inside and cool the reactor.
Another unique feature is the fish protection facility. S. Venkatesh, Senior Commissioning Engineer, KKNPP, called it an engineering marvel. The plant uses sea water for condenser cooling. Concrete pipelines ferry sea water to the pump house on the shore by gravity. Small fish get trapped in the water and enter the cooling facility. At the place where the pipelines are laid on the seabed, compressors blow air into the tunnels, creating an air-bubble curtain and waves on the ocean surface. This buoyancy enables the fish to move up, float on the surface and not go down and enter the pipelines. This fish protection facility was qualified by an actual test in Russia.
The reactor has another important safety feature called the core-catcher, a steel vessel weighing 101 tonnes. In the case of a severe accident, the molten radioactive fuel from the RPV will fall into the core-catcher through a funnel and be cooled.
It will not fall on the floor of the reactor building and contaminate it. Once the emergency core cooling system is activated, the area around the core-catcher will be flooded with water.
When the tsunami devastated the nearby Kanyakumari coast in December 2004, no water entered the KKNPP site because it is 7.5 m above the mean sea level and the tide level during the tsunami was only 5 m.
Engineers who will commission and operate the two reactors at Kudankulam have been given extensive training because the technology of the VVERs is different from that of the PHWRs that NPCIL has built so far. A Nuclear Training Centre has been set up at Kudankulam.
Said Balaji: A nuclear power plant is multidisciplinary. So we have a custom-made training programme for graduate engineers from different disciplines to make them full-fledged nuclear engineers in the operation and maintenance [O and M] of the VVERs. This is a one-year programme. We train them for techno-managerial positions. We are taking a lot of care because there should be no failures during the commissioning of the reactors.
The KKNPP has a simulator, bought from Russia, which is a replica of the control room of the reactors. In it, we can simulate the start-up of the reactor, its operation and a spectrum of abnormal conditions, said Suresh Kumar Pillai, training superintendent, KKNPP.
The instructor can create a malfunction and watch how trainees respond. Even the tripping of a reactor can be simulated. A printout will show how the trainees reacted, and they are evaluated. Two cameras produce pictures on how they responded to an abnormal situation. There are hidden microphones to record the trainees conversation.
