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| Fig. 1: Image of Three Mile Island. (Source: Wikimedia Commons) |
Alongside solar energy, wind power, and other forms of alternative energy sources, nuclear energy has become increasingly adopted for powering the modern world. Nuclear energy is the generation of electricity via nuclear reactions. Typically, a nuclear reactor uses the nuclear fission of uranium and plutonium to produce electricity. Nuclear fission is the process by which atoms split and release energy. Commercial nuclear power presently supplies bout 2,600 terawatt-hours of electricity per year (9.4 × 1018 J y-1), making it responsible for 10% of global energy usage. However, nuclear energy is not without dangers, as seen with the Chernobyl disaster and Fukushima.
Sitting in the middle of the Susquehanna River (see Fig. 1), Three Mile Island was a nuclear reactor located near the capital of Harrisburg in central Pennsylvania. In 1979, an accident occurred there that changed the course of nuclear energy in the United States. A loss-of-coolant event resulted in a partial meltdown of the core and the release of radioactive substances into the atmosphere. [1] The Three Mile Island accident is rated as a Level 5 Nuclear event, also known as an accident with wider consequences. Beyond serving as an example of issues with modern nuclear energy, it propelled legal, environmental, and scientific action towards regulating and operating nuclear energy in the safest manner possible. Its consequences have persisted for decades.
In the early hours of March 28, 1979, the Three Mile Island accident began. [1] Initially, failures occurred in the reactor's non-nuclear operations, specifically those of TMI-2 which was running at 97% power. Four men, William Zewe, Fred Scheimann, Edward Frederick, and Craig Faust were responsible for managing the reactor that morning.
The initial problem was connected to the flow of water. [1] With the water pumps not correctly supplying water due to the clogging of pipes, the nuclear chain reactions were shut down by the reactor's safety systems. This occurred before the core became exposed. Due to the failure of the auxiliary pumps and with no feedwater being supplied, the temperature of the reactor coolant system (RCS) increased. This was due to the radioactive decay of fission waste in the fuel rods. The pressure in the RCS then increased by approximately 100 PSI. [2] With a gas bubble at the top of the reactor blocking the flow of water, operators and supervisors were unable to establish a proper cooling to lower the temperature.
The next error occurred in the reactor's pilot-operated relief valve (PORV). The PORV was located at the top of the reactor. The PORV had steam valves that were able to reduce the generator temperature and pressure of the RCS. [1] This resulted in the contraction of the coolant, but PORV did not re-close. As a result, a large amount of the reactor's cooling water drained out. [2] With the PORV being stuck, human error again took place, in that the staff did not realize that the reactor core was losing its cooling water, which was pouring out of the PORV in the form of steam. Having misinterpreted the data they had, the operators encouraged a draining of water onto the floor.
Without circulation of water and no emergency cooling water, the water level in the reactor dropped and the reactor core began to overheat. Following the partial meltdown (nearly 80 minutes of the temperature rising), the reactors coolant pumps formed cavities due to its static pressure falling below its vapor pressure. [2] With the top of the reactor core exposed, reactions formed between steam and zircaloy nuclear fuel, and this released a variety of nuclear substances into the atmosphere. Roughly 43,000 curies of Krypton were released into the environment. [1] Radiation at Three Mile Island was measured by several continuous exposure rate monitors and gamma-ray spectrometry.
The first manned entry into Three Mile Island following the incident took place in July 1980, more than a year following the meltdown. Today, the surrounding areas of Three Mile Island are relatively safe since 99% of the nuclear waste and contaminated debris has been properly disposed of, but the reactor itself does still contain a certain amount of contamination. In fact, the nuclear waste from the incident that was properly disposed of was moved to the Idaho National Lab, where it is being stored in steel containers. The nuclear waste that was not properly disposed of remains at the TMI-2 reactor. [1] In terms of health effects, the radiation was negligible since the 2 million people who lived around TMI-2 during the accident were estimated to have received only 1 millirem above the usual background dose of radiation. [1]
The impact that Three Mile Island has had on United States nuclear policy is significant. [3] The United States has not built a new reactor since the incident. In total, over 51 nuclear reactors were canceled between 1980 and 1984. TMI-2's sister reactor, TMI Unit 1 (TMI-1) stayed operational until 2019, marking over 45 years of operation. [4] While the incident took place in March 1979, cleanup began in August 1979 and ended in December 1993, costing over $1 billion. [5] In summary, the Three Mile Island accident was caused by mechanical errors that were exacerbated by human errors and the accident has had major effects on the nuclear energy industry.
© Siddharth Sharma. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Three Mile Island Accident," U.S. Nuclear Regulatory Commission, June 2018.
[2] C. Lyons, "Three Mile Island," Physics 240, Stanford University, Fall 2010.
[3] J. G. Kemeny et al., "Report of the President's Commission on the Accident at Three Mile Island," U.S. Government Printing Office, 0-303-300, October 1979.
[4] M. Cusick, "40 Years after a Partial Nuclear Meltdown, a New Push to Keep Three Mile Island Open," NPR, 28 Mar 19.
[5] C. Hopkins, "Three Mile Island," Physics 241, Stanford University, Winter 2015.
