Nuclear Accidents - Essay Example

?Nuclear Accidents The term nuclear accident is described by the International Atomic Agency as a Level 4 to 7 event based on the International Nuclear and Radiological Event Scale which constitute release of radioactive material, reactor meltdown, death from radiation, and risk of public exposure (International Atomic Energy Agency 2009). Compared to other potentially hazardous events, nuclear accidents are quite rare (Cole 2006). However, the risk of a nuclear accident increases by the number of operational nuclear reactors being built around the world. The most notable nuclear accidents so far is the Three Mile Island in 1979 (Chauhan 2008), Chernobyl in 1986 (Coles 2006), and most recently, Fukushima in Japan (Institute for Energy and Environmental Research 2011). Nuclear Fission. Nuclear fission is described as the process of splitting a heavy atom into two or more lighter atoms by bombarding the heavy atom with a slow moving neutron. As the heavy atom splits, a large amount of energy is released. In addition, a number of neutrons are released which in turn splits other heavy atoms. This creates a chain reaction that releases the energy required to generate electricity in nuclear power plants (Washington State Department of Health 2003). As shown in Figure 1, a slow moving neutron hits a heavy atom, in this case Uranium-235 and splits it into two lighter atoms, Krypton-92 and Barium-141, and 3 neutrons. Figure 1. Nuclear fission (FastFission 2010b). In Figure 2, the three neutrons either: (1) gets absorbed by Uranium-238 and does not split the atom; (2) does not hit any other atom, which stops the reaction; or (3) hits another Uranium-235 atom and causes it to split, replicating the previous process. This continuous process of splitting atoms is called a nuclear fission chain reaction. Breeze (2005) described the energy produced as enormous. Theoretically, one kilogram of naturally-occurring Uranium can produce about 140 GWh of energy, which is equivalent to the power output of a 1000 MWh coal-fired power plant operating at full power for approximately 6 days. Figure 2. Fission chain reaction (FastFission 2010). Nuclear Power Plants. To generate electrical power, nuclear fission is harnessed in a way that a controlled nuclear reaction can take place and continue indefinitely. In a nuclear reactor, fission is controlled by the use of boron rods which absorb the neutrons generated, stopping the chain reaction from proceeding. Another crucial component in nuclear reactors is the presence of a moderator, usually in the form of water or graphite. Water slows the neutrons down, making fission possible. Otherwise, fast moving neutrons cannot initiate a fission reaction (Breeze 2005). Figure 3. A typical pressurized water reactor setup (United States Nuclear Regulatory Commission 2007) Figure 3 illustrates how a nuclear power plant works. As Uranium undergoes fission, an enormous amount of energy is generated. This energy is released in the form of heat which raises the temperature of the water inside the reactor. As the water heats up, it is pumped through pipes and flows into a steam generator. As the pipes get in contact with the colder water stored in the steam generator, steam is produced. The steam then drives the turbines connected to the generators, producing electricity. The hot water from the reactor does not come in contact with the water in the steam generator since the hot water is separated by the pipes. This eliminates the risk of radioactive materials going outside the reactor (Breeze 2005). NRX accident in Canada. The NRX reactor was created to demonstrate the use of heavy water or D2O as a moderator for nuclear reactors and served as the forerunner of the Canada Deuterium Uranium or CANDU reactor. It was constructed in Canada during in 1947. The NRX reactors utilized uranium fuel rods contained in a pressure tube cooled by light water flowing between the rods and the pressure tube wall. Meanwhile, heavy water is utilized as the moderator flowing between pressure tubes (Martin 2006). Figure 4. Cross-section of the NRX fuel tube (Collier & Hewitt 2000). On December 12, 1952, an ongoing experiment which involved reducing the flow of light water coolant suddenly went awry. During the test, several control rods were incorrectly being pulled out from the reactor core. The supervisor immediately informed an operator to close the pneumatic valves to stop the rods from completely being pulled out. However, not all control rods re-entered the reactor and got stuck. To make matters worse, a miscommunication between the supervisor and operator caused four safeguard control rods were completely pulled out from the core. This caused the power level to rise exponentially, triggering alarms in the facility (Collier & Hewitt 2000). Attempts to drop the safeguard control rods back into the reactor in order to lower power levels were futile. It was therefore decided to dump the heavy water stored in the calandria into the reactor. It took some time to lower the power to zero since it took time for the heavy water to drain into the reactor core. However, the light water in the cooling system reached boiling point, rupturing the calandria and coolant pipes. This caused an estimated one million gallons of water with 10,000 curies of radioactive fission products to drain into the basement of the facility (Collier & Hewitt 2000). Both the reactor core and calandria were damaged beyond repair and were dismantled and buried. Luckily, personnel exposure was low (Martin 2006). The whole facility was decontaminated and operations resumed after installation of an improved calandria and reactor core 14 months later (Collier & Hewitt 2000). The primary lesson learned from the event was that the reliable operation of control rods should be maintained at all times. Operator error and mechanical problems encountered during the incident were complicated by the fact that boiling water caused neutrons to escape into the environment (Collier & Hewitt 2000). Windscale Fire, UK. The Windscale reactor was Britain’s first nuclear reactor and was also the site of Britain’s worst nuclear disaster. Initially constructed to create nuclear material for the British A-bomb, it was then commissioned to produce material for the British H-bomb project (Dwyer 2007). Figure 5. Design of Windscale Pile No. 1 (Wikimedia Commons 2011). In October 10-11, 1957 the Windscale Number 1 pile caught fire, causing the release of radioactive material into the atmosphere. The Windscale reactor utilized graphite as moderator and used natural uranium as fuel source. Air was used as coolant and are driven by huge fans, with the exhaust flowing into 125 meter high chimneys with filters installed at the top (Cooper, Randle, & Sokhi 2003). The problem occurred during a routine shutdown of the reactor to release accumulating Wigner energy in the graphite moderators. Wigner energy is created through the displacement of atoms in graphite which creates heat. On October 10, 1957 operators were alerted of increased radioactivity in the reactor ten times the normal level. Four hours later, visual inspection of the reactor revealed red hot fuel cartridges that swelled up and jammed the fuel channels, making discharge impossible. Carbon dioxide was used to extinguish the flames, but to no effect (Collier & Hewitt 2000). The operators faced a serious crisis. Letting the fire burn out would cause widespread radiation in a large portion of Britain. On the other hand, using water on the reactor might cause an explosion and death of personnel. Finally, the operators took a gamble and used water to put out the fire. (Dwyer 2007). The reactor went back to a cold state. However, since the reactor was cooled by pumping air into the fuel channels, radioactive material found its way through the chimney and into the atmosphere. The filters installed can only remove half of the particulate matter and is entirely ineffective in filtering noble gases and radioactive materials. It was estimated that xenon and krypton gas, as well as 20,000 curies of radioactive iodine-131 was released into the atmosphere (Collier & Hewitt 2000). Wood (2007) identified three primary factors that contributed to the disaster. First, the Windscale reactors were originally designed to use natural uranium. However, in 1953, the use of very slightly enrich uranium was introduced. This caused the heat rating to go beyond the design limits. Second, the thermocouples used in the reaction were positioned incorrectly, causing faulty readings. Third, the lack of technical documentation, poor coordination and communication among departments within the organization hampered the operators’ attempts to create a timely solution. Chernobyl. The RBMK reactor is a vertically-oriented direct-cycle pressure tube reactor which used graphite as moderator. The core is made up of graphite blocks and zirconium tubes containing reactor fuel and numerous control and shut-off rods. The fuel used in the reactor is zirconium-clad uranium oxide enriched to 2 percent (Wood 2007). Figure 6. Design of RBMK Chernobyl Reactor 4 (World Nuclear Association 2011) Based on its design, the RBMK reactor’s power coefficient at various power levels is influenced by reactor coolant conditions. At high power levels the positive reactivity effect of increased coolant void is offset by the negative reactivity effects of increased fuel temperatures. Thus, power rises can be effectively controlled. However, this is not the case when the power level goes down to less than 20 percent as the reactor becomes unstable and difficult to control. Due to this condition, standard operating procedure does not allow sustained operations at power levels below 20 percent (Wood 2007). In April 26, 1986 a safety test in designed to see how the Chernobyl turbines would perform during loss-of-power emergencies went out of control, initiating what is to become the worst nuclear disaster in history. At the time, the USSR already had 3 RBMK reactors in operation. Chernobyl Reactor 4 is only one test away from being operational and the Soviet government has been pressuring to expedite the completion of the project. To ensure that the test is conducted immediately the Soviet government sent a representative to observe the operation (Lusted 2011). The test was simple. The first step involved lowering the reactor power level to the extent that the turbines could be cut-off from the reactor. After disengaging the turbines, engineers will check how long the turbines will continue to operate without electrical power. To prevent the emergency core cooling system from identifying the test as a real accident it was decided that it will be temporarily shut down. This caused the cooling pumps to shut down, preventing it from pumping water into the reactor in case the heat goes beyond the allowed limits (Lusted 2011). As the power level was going down, a miscalculation on the control rod setting caused the power to go down to less than 1 percent full power which was well below the 20 percent limit required to sustain a stable reactor. At this point raising the power to stabilize the reactor was difficult as the reactor became unstable. In addition the water level in the reactor went dangerously low, raising the temperature in the cooling pipes. Despite the serious condition of the reactor it was decided that the test will proceed as planned (Wood 2007). As a result, the cooling pipes burst from the extreme pressure. A few minutes later a second explosion from the reactor itself rocked the facility, sending radioactive material out of the reactor and through the roof. Large chunks of red hot graphite rained down all around the compound. As air entered the damaged reactor, it ignited a fire inside the reactor (Lusted 2011). Figure 7. Chernobyl Reactor 4 after the accident (World Nuclear Association 2011) It would take some time for firemen to extinguish the flames and the extensive cleanup that followed took months. Ultimately, the reactor had to be entombed in thick concrete to prevent further radiation from spreading. The accident at Chernobyl was a combined result of faulty reactor design and poor decision making of the operators (Lusted 2011). Natural nuclear accidents. In 1972 a French scientist observed that uranium mined from a mine in Oklo, Gabon was deficient in uranium-235. The isotope ratio in the uranium from Oklo resembled those from spent fuel produced by nuclear reactors. After further investigation scientists have concluded that a natural fission process occurred in the Oklo uranium deposits around 2 billion years ago. Water seeped into the bed of uranium and acted as moderator, effectively initiating nuclear fission. The same water also served as regulator, preventing the whole Oklo reactor from exploding. As the heat turned water to steam, it stopped the fission reaction. The fission process restarted as water seeped back in and the whole cycle went on for about one million years, until the concentration on uranium-235 became too low to sustain a reaction (Kotz, Treichel & Townsend 2009). Meanwhile, volcanic rocks used in building construction in Italy were found to have high uranium content. In addition, radon gas was found to be emanating from these rocks, causing lung cancer. Pumice miners and workers developed pneumoconiosis after they were repeatedly exposed to pumice dust (Marti & Ernst 2005). Conclusions. Since the dawn of the nuclear age, man has harnessed the power within the atom to his advantage. Initially developed for wartime purposes, nuclear energy has found its way towards peaceful and more beneficial applications such as power generation and medicine. However, hard lessons had to be learned along the way as evidenced by the aforementioned nuclear disasters. The combination of faulty design, environmental factors, and most importantly, errors in human judgment had contributed to the demise and detriment of people by the thousands, if not millions. The mere radioactive nature of the elements involved should always be taken into consideration since these radioactive isotopes will still exist long after this generation or the next generation is gone. The recent events in the Fukushima plant in Japan for example, pose a serious health risk comparable if not exceeding, the damage in Chernobyl in 1986. The current situation in Japan involves multiple reactors in close proximity to each other making the situation complicated. In a letter written by Anatoly Dyatlov, the engineer who supervised the fatal test in Chernobyl, to a friend, he recounted what happened on that fateful day: “It seemed as if the world was coming to an end… I could not believe my eyes. I saw the reactor ruined by the explosion. I was the first man in the world to see this. As a nuclear engineer, I realized all the consequences of what had happened. It was a nuclear hell (Lusted 2011).” References Breeze, PA (2005) Power generation technologies. Newnes, Oxford. Chauhan BS (2008) Environmental studies, University Science Press, New Delhi. Cole E (2006) Health emergency planning: a handbook for practitioners, The Stationery Office, Norwich, UK. Collier, JG & Hewitt, GF (1987) Introduction to nuclear power. Taylor & Francis, London. Cooper, JR, Randle, K & Sokhi, RS (2003) Radioactive releases in the environment: Impact and assessment. John Wiley & Sons, West Sussex, UK. Dwyer, P (2007) Windscale: A nuclear disaster, viewed 13 April 2011, http://news.bbc.co.uk/2/hi/science/nature/7030281.stm Fast Fission (2010a) Fission chain reaction, viewed 12 April 2011, http://upload.wikimedia.org/wikipedia/commons/9/9a/Fission_chain_reaction.svg FastFission (2010b) Nuclear fission, viewed 12 April 2011, http://upload.wikimedia.org/wikipedia/commons/1/15/Nuclear_fission.svg International Atomic Energy Agency (2009) INES the international nuclear and radiological events scale users’ manual, International Atomic Energy Agency, Vienna. Institute for Energy and Environmental Research (2011) Radioactive iodine releases from Japan’s Fukushima Daiichi reactors may exceed those of Three Mile Island by over 100,000 times, viewed 11 April 2011, http://www.ieer.org/comments/Fukushima_IEER_press_release_2011-03-25.pdf Kotz, JC, Treichel, P & Townsend, JP (2009). Chemistry and chemical reactivity, vol. 2. Thomson, Belmont, CA. Lusted, MA (2011) Chernobyl disaster. ABDO Publishing, Edina, MN. Marti, J & Ernst, G (eds) (2005) Volcanoes and the environment. Cambridge University Press, Cambrisdge. Martin, JE (2006) Physics for radiation protection: a handbook. Wiley-VCH Verlag, Weinheim, Germany. United States Nuclear Regulatory Commission (2007) The pressurized water reactor, viewed 12 April 2011, http://www.nrc.gov/reading-rm/basic-ref/students/animated-pwr.html Washington State Department of Health (2003) Fact sheet #47 nuclear fission, viewed 11 April 2011, http://www.doh.wa.gov/ehp/rp/factsheets/factsheets-pdf/fs47nucfis.pdf Wikimedia Commons (2011) Windscale reactor, viewed 14 April 2011, http://upload.wikimedia.org/wikipedia/commons/f/fd/Windscale-reactor.svg World Nuclear Association (2011) Chernobyl Accident, viewed 15 April 2011, http://www.world-nuclear.org/info/chernobyl/inf07.html Wood, J (2007) Nuclear power. The Institution of Engineering and Technology, Herts, UK. Read More