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Core Safety of Indian Nuclear Power Plants (Npps) Under Extreme Conditions

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Core Safety of Indian Nuclear Power Plants (Npps) Under Extreme Conditions Sadhan¯ a¯ Vol. 38, Part 5, October 2013, pp. 945–970. c Indian Academy of Sciences Core safety of Indian nuclear power plants (NPPs) under extreme conditions JBJOSHI1,∗, AKNAYAK2, M SINGHAL3 and D MUKHOPADHAYA4 1Homi Bhabha National Institute, Anushaktinagar, Mumbai 400 094, India 2Reactor Engineering Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India 3Nuclear Power Corporation of India Limited, Anushaktinagar, Mumbai 400 0094, India 4Reactor Safety Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India e-mail: [email protected] Abstract. Nuclear power is currently the fourth largest source of electricity produc- tion in India after thermal, hydro and renewable sources of electricity. Currently, India has 20 nuclear reactors in operation and seven other reactors are under construction. Most of these reactors are indigenously designed and built Heavy Water Reactors. In addition, a 300 MWe Advanced Heavy Water Reactor has already been designed and in the process of deployment in near future for demonstration of power production from Thorium apart from enhanced safety features by passive means. India has ambi- tious plans to enhance the share of electricity production from nuclear. The recent Fukushima accident has raised concerns of safety of Nuclear Power Plants world- wide. The Fukushima accident was caused by extreme events, i.e., large earthquake followed by gigantic Tsunami which are not expected to hit India’s coast considering the geography of India and historical records. Nevertheless, systematic investigations have been conducted by nuclear scientists in India to evaluate the safety of the current Nuclear Power Plants in case of occurrence of such extreme events in any nuclear site. This paper gives a brief outline of the safety features of Indian Heavy Water Reactors for prevention and mitigation of such extreme events. The probabilistic safety analysis revealed that the risk from Indian Heavy Water Reactors are negligibly small. Keywords. Indian NPPs; PHWR; AHWR; heavy water reactor; core safety; extreme events; thorium. 1. Introduction Sustainable, reliable and affordable energy supply is a foundation of economic stability and growth for any country. It is more or less well-known that the per capita energy consumption ∗For correspondence 945 946 JBJoshietal and quality of life are strongly related. In view of this, there is strong energy demand around the world, which is growing due to rise in population. From a sustainable energy perspective, there are limited options. Increase in fossil fuel based energy supply inevitably leads to environmen- tal issues. Further, the existing reserves of coal would be inadequate to meet an enhanced rate of energy consumption comparable to today’s world average per capita level. Renewable energy sources like solar, wind, etc. are fast emerging as promising alternatives to traditional energy. However the technology for large scale production of energy from renewable resources is in nascent stage and will take long time to make up for the exponential energy demand. Moreover, these energy sources must be deployed to the fullest extent possible to meet the large concen- trated energy needs for industries and urban centers. Thus, not only in India, but from a global perspective, nuclear energy has become a viable, sustainable and clean energy source of future. Today, the world has about 437 nuclear power reactors in operation around the world which generates about 371 GWe (http://www.iaea.org/pris). In the entire history of commercial nuclear power so far, only three major accidents leading to damage of reactor core, have taken place (at Three Mile Island (Tolman et al 1988), Chernobyl (The Chernobyl Accident 1992)and recent Fukushima (IAEA mission report 2011)). In the case of the Three Mile Island accident (Tolman et al 1988), even though the core had melted, there was no radiation exposure to any plant operator or any member of the public in spite of a series of human errors. This was all because of proper adherence to the defence-in-depth principles in the design of the plant con- structed in the early seventies. The Chernobyl nuclear power plant (Sehgal 2011) was one of the Soviet nuclear power stations. The reactor design did not fully conform to stipulated principles of defence-in-depth applied to the reactors of such vintage constructed in the Western world. More specifically, the accident was a consequence of several design deficiencies compounded by a series of human errors. The recent Fukushima accident (Srinivasan & Gopi Rethinaraj 2013) has been a concern for public since it happened especially with regard to safety of nuclear power plants. The accident in Fukushima occurred mainly due to extreme events such as earthquake of magnitude of 9.1 on Richter scale followed by gigantic tsunami of height 14–15 m. These resulted in prolonged station blackout conditions with unprecedented devastation of on- and off- site infrastructures such as destruction of fuel tanks of the diesel generators; flooding of the diesel generator building, etc. which serve as emergency power supply to nuclear reactors. Of course, the accidents led to release of radioactivity due to probable melt down of reactor core and hydro- gen explosion in the containment; however, there were no casualties in the accident since the post accident habitation programme was well-managed by Japanese Government. Out of these accidents, the Chernobyl accident was definitely more severe in all its dimensions resulting in causalities. These accidents have reinforced the necessity for further improvement of safety in the design of nuclear power plants. The lessons from the accidents have led to a higher attention to improve- ment in man–machine interfaces to minimize operator actions and errors in case of any extreme events and retrofitting additional safety systems to cater to such low probable extreme events for example that happened in Fukushima. Rigorous calculations have been performed for cur- rently operating plants in India to prove that there is no degradation of the health of reactor in case of any postulated extreme events. In addition to deterministic analyses, probabilistic safety assessments have been mandatory for identification of design weakness which is supplemented by providing redundancy and diversity in necessary systems such that coolability of reactor is maintained in low probable extreme events. In fact, the current design and safety standards are rigorous and adequate from the design, operation, accident prevention and mitigation perspec- tive. Regulatory infrastructures all over the world have been substantially augmented over the years with most of the key regulatory philosophies and practices based on internationally agreed Core safety of Indian NPPs 947 standards. The mechanisms of information exchange across the world have been substantially augmented to learn from each other’s experiences. The designs of next generation reactors have considered extremely low probability Beyond Design Basis Accidents as a part of the design. Considering the need for a much larger scale of deployment of nuclear power in the future, the safety goals for future nuclear power plants are sought to be further enhanced. Under the IAEA’s International Project on Innovative Nuclear Reactors and Fuel Cycles (INPRO) a set of objectives, requirements, and criteria have been accordingly formulated for the new reactors (www.iaea.org/INPRO). Most of the new designs are introducing inherent and passive safety features that do not depend upon external source of power or human actions for their operation and safety. In fact, possibility of core melt down accidents are almost not possible in the new designs; and if happens are managed by severe accident management guidelines such that no activity release beyond the acceptable limit can occur at the plant boundary. In our opinion, with advances in technology and a better understanding of the nuclear power, the possibility of acci- dents like Three Mile Island, Chernobyl or Fukushima will ever happen again is practically zero. This paper discusses the design safety features of Indian nuclear reactors against such extreme events. 2. Physics of nuclear safety In a nuclear reactor, electricity is produced by conversion of thermal heat received by the coolant from heat produced by nuclear fission. For fission to occur, a neutron should be absorbed in fissile atomic nucleus such as uranium-235 or plutonium-239. The heavy nucleus splits into two or more lighter nuclei (the fission products), releasing kinetic energy, gamma radi- ation and free neutrons. A portion of these neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on, known as a nuclear chain reaction. Heat is produced due to conversion of the kinetic energy of fission prod- ucts to thermal energy when these nuclei collide with nearby atoms. Some of the gamma rays produced during fission are absorbed by the reactor and produce heat; following termination of chain reaction also, the heat gets generated due to interaction of radiation released from radioac- tive decay of fission products and materials that have been activated by neutron absorption with matter which is called decay heat. A kilogram of uranium-235 releases approximately three mil- lion times more energy when undergoes fission than a kilogram of coal burned conventionally (7.2 × 1013 joules per kilogram of uranium-235 versus 2.4 × 107 joules per kilogram of coal). The heat released due to fission must be removed adequately by a cooling medium; otherwise which would result in heat up of the uranium fuel. This is done by supplying adequate amount of coolant to the core, which continuously removes heat generated due to fission and transports the heat to the turbine for electricity generation. Due to large amount of heat released in the fission process, the nuclear reaction should be controlled so that the heat removal process can be carried out in a stable manner. The power output of the reactor is adjusted by controlling the nuclear fission rate.
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