The Kudankulam Nuclear Power Plant (KKNPP) in Tamil Nadu has become part of the regular news in the recent past. Safety of the public has been given the utmost priority at all stages of the KKNNP construction, including from the selection of the site, designing the processes, and erection of the plant buildings and equipment. The Nuclear Power Corporation of India Limited, which has built the two Russian reactors at Kudankulam, has so far clocked a cumulative 350 years of safe reactor operation in the 20 reactors that are operating in India.
KKNPP has been built on the sea coast like any other electricity generating station including the coal-fired plants. Since the sea water level rises due to wave run-up, storm surge, tide variation and tsunami, the plant has to be protected from these natural events. KKNPP is well protected from a possible rise in sea level by locating the entire plant site at a higher elevation. The safe grade elevation of KKNPP site has been kept at 7.5 metres above the MSL (mean sea level) and a shore protection bund has been constructed all along the shore to a height of + 8.0 metres to the MSL. All the buildings along with their respective equipment are located at higher elevations as shown in Figure 1 (at left). In addition to having a higher grade elevation, all the safety-related buildings are closed with double gasket leak tight doors.
KKNPP is located in Indian Seismic Zone II, which is the least seismic potential region of our country. However, for designing of the plant, detailed studies were conducted to conservatively estimate the extent of ground motion applicable to the specific site with reference to seismotectonic and geological conditions around it so that the nuclear plant was designed for a level earthquake which has a very low probability of being exceeded. The plant's seismic sensors safely shut down the reactor in case the seismicity exceeds the preset value. Thus, despite KKNPP being located in a very low seismic zone, it is adequately designed to withstand the seismic events.
The two reactors that have been built at Kudankulam are advanced models of the Russian VVER-1000 MW Pressurised Water Reactor which is a leading type of reactor worldwide. VVER is a Russian nomenclature for water-cooled and water-moderated reactors. Each reactor at Kudankulam will generate 1000 MW. It uses low-enriched uranium fuel in oxide matrix, housed in sealed zirconium-niobium alloy tubes. KKNPP VVER 1000 adopts the basic Russian design with enhanced safety features to make it in line with IAEA GEN III reactors. Further, certain additional safety features were incorporated like passive heat removal system and core catcher, taking it to GEN III+ category.
The following safety functions are performed in any operational state of the reactor:
Control of the Reactivity (control of fission chain reaction), removal of heat from the fuel core and confinement of radioactivity.
For the control of reactivity, control rods are provided, which will ensure the shutdown of the reactor, thereby terminating the chain reaction, whenever the action is called for. The control rods are designed to fall by gravity to shut down the reactor.
The salient safety features of KKNPP
- Passive heat removal system to provide cooling for the removal of decay heat using atmospheric air.
- Higher redundancy for safety system.
- Double containment.
- Additional shut down systems like quick boron and emergency boron injection systems to ensure absolute safety for shut down of the reactor, when needed.
- Core catcher to provide safety in the unlikely event of fuel melt-down
- Passive hydrogen re-combiners which do not need any power supply to absorb any hydrogen liberated inside the containment.
The above systems have been developed based on extensive R & D and simulated testing by the Russian design institutes. The functional performance of these systems have been established during the commissioning stage.
A large number of process systems are provided for the purpose of heat removal from the reactor fuel core. In addition, to remove the decay heat after the shut down, a series of safety systems are provided which are backed by the diesel electricity generator sets. The safety systems are provided in four trains, each train containing a set of safety systems, both active and passive systems. Each set of safety trains is provided with a dedicated diesel generator set of 6 MW. The passive heat removal system provides the core cooling in case of rare occasion of non availability all the diesel generators. This system uses the simple atmospheric air to take away the heat from the reactor through steam generators by using the natural principle of convection. One safety train is sufficient to completely ensure heat removal from the fuel core. However, three additional safety trains, i.e., additional 300% systems are provided making the KKNPP reactors among the safest reactors.
The confinement of radioactivity is achieved by the principle of defence in depth. This concept provides a set of barriers, one after the other, so as to contain radioactivity within the reactor building. This concept is illustrated in Figure 2 (at right).
The reactor building has double containment structure. The primary or inner containment is a pre-stressed concrete structure, with the thickness of 1.2 metres. This inner containment is provided with leak-tight inner steel liner. The outer containment known as secondary containment is a reinforced concrete structure with thickness of 0.6 metres. The multiple barriers, as shown in Figure-2, including the containment structure, ensure that no radioactivity reaches the public domain. The double containment structures also protect the plant from external hazards like hurricane, shock waves, air attacks, seismic impact, floods, etc.
In addition, there are two important systems which provide safety function, viz., hydrogen re-combiners and a core catcher. The hydrogen re-combiners are passive devices. Hydrogen, if generated during any accident conditions, is recombined in the passive hydrogen re-combiners to convert it to water. This prevents any hydrogen explosion within the containment as happened at Fukushima in Japan in March 2011. There are 154 hydrogen re-combiners at various locations within the containment.
The core catcher is a special feature of KKNPP. It is a huge vessel weighing 101 tonnes. In case of an extreme hypothetical case, wherein an event causes damage to the fuel core resulting in partial core damage, the core catcher will collect the molten core, cool it and maintain it in sub-critical state.
At Kudankulam, a fish protection facility is provided in the intake of sea water. This facility assists juvenile fish, which drift along with the flow of cooling sea water, from not getting trapped in the machinery. The fish are helped in getting back to the sea and the fish population is thus conserved.
The product water and domestic water requirement of KKNPP are fully met by a desalination plant at the KKNPP site, based on Mechanical Vapour Compression technology. Thus it can be safely concluded that the reactors at KKNPP are the built with the state of the art technology, with the best safety features that will ensure safe operation of the reactors, without any impact to the environment and the public.
(M. Kasinath Balaji is Site Director, KKNPP and S.V. Jinna is Chief Engineer, Engineering & Utility Services)
