Three Mile Island: A Nuclear Warning from the Past

Explore the 1979 Three Mile Island meltdown, where equipment failures and human errors converged. Uncover the overlooked warnings and the pivotal role of software in this nuclear crisis.

Background:

Nuclear energy and power plants have become a hot-button issue for many people around the world. People often state concerns with health and safety when advocating for the reduction of nuclear energy in society. The nuclear disaster at Three Mile Island is credited with decreasing the tolerance for nuclear power plants and energy globally (Wikipedia Contributors, 2018). The Three Mile Island Nuclear Generating Station near Harrisburg, Pennsylvania first started its operations on September 2, 1974, for Unit 1 and on December 30, 1978, for Unit 2 (Wikipedia Contributors, 2019).

Actual Incident / Disaster:

At approximately 4 a.m. on March 28, 1979, the nuclear reactor at Unit 2 was running at 97% power. The reactor's secondary cooling unit malfunctioned, causing an increase in the temperature and pressure of the primary cooling unit. Those events caused the reactor to shut down, and the abnormal shutdown caused a relief valve to open with the aim of releasing the buildup of pressure in the reactor (World Nuclear Association, 2020). The valve, unfortunately, remained open after the pressure was released from the reactor. In the plant technicians’ control room, however, the relief valve appeared to be closed, which led them to be unaware of the situation at hand. Coolant in the form of steam continued to be released from the valve, setting off a chain of reactions and responses from the plant staff that led to the cores of the reactor overheating, resulting in a partial meltdown of the plant (United States Nuclear Regulatory Commission, 2018). Later in the afternoon, after those events took place, operators at the plant were able to restore the reactor's cooling system. Unfortunately, radioactive gasses from the cooling system started to build up, so the operators attempted to use a series of pipes and compressors to transport the gasses to waste decay tanks, but the compressors started to leak and emitted radioactive material in the form of noble gasses (World Nuclear Association, 2020). The Nuclear Regulatory Commission ordered an evacuation of pregnant women and pre-school-aged children within a 20-mile radius of the plant (Wendorf, 2019). Three Mile Island remained in a state of emergency for five days due to a potential explosion caused by a buildup of pressure from hydrogen gas. The plant's operators had to periodically release radioactive contaminants into the air to relieve the pressure caused by the “hydrogen bubble” (Pell, 2019).

Root Cause:

Immediately following the disaster, the then President of The United States, Jimmy Carter, assembled The Commission on the Accident at Three Mile Island, which was appointed to thoroughly research and determine the root causes of the disaster (Pell, 2019). Other organizations, such as the NRC, lead their own investigations to get to the bottom of what went wrong at Three Mile Island. The investigation mounted by Jimmy Carter’s commission came to the conclusion that a lack of quality assurance, maintenance of the plant, inadequate operator knowledge and training, inaccessibility of safety information, poor management and complacency were the definitive factors that caused the accident at TMI (LiquiSearch, n.d.). It is often understated, however, that the main culprit for the disaster was the faulty relief valve that remained stuck open and gave the plant operators no indication of its malfunctioning, causing the reactor to abnormally shut down and commencing the partial meltdown of the plant (Rosztoczy, 2019).

Possible Prevention:

According to industry expert Zoltan R. Rosztoczy, if the role of the pilot-operated relief valve in the plant’s safety was more carefully considered and if the plant’s safety analysis report could more accurately address loss-of-coolant accidents, the disaster at Three Mile Island might have never happened (Rosztoczy, 2019).

Possible Cause - Requirements:

Requirements are integral to the creation of good software. Requirements are there to provide the framework for a project to be accurately realized. It is evident that the requirements were not documented correctly for the design of the reactor due to The Nuclear Regulatory Commission and the manufacturers at B&W omitting the role that the pilot-operated-relief-valve (PORV) played for the safety of the plant as stated in the Possible Prevention subsection (Rosztoczy, 2019). The manufacturer of the PORV (B&W) was not fully informed of the requirements of the device. The PORV needed to be able to close after exposure to accident loads but was unable to and, as a result, caused the disaster (Rosztoczy, 2019).

Possible Cause - Technology:

It is very difficult to find documented proof that the creation of the PORV (which initiated the disaster) or any of the other components at TMI was rushed. However, it can be easy to conclude that the design and quality assurance of the relief valve was not thoroughly scrutinized due to the NRC and B&W’s inability to discover omissions that would significantly affect the safety of the power plant (Rosztoczy, 2019).

Possible Cause - Testing:

As a Mission Critical system, the pilot-operated-relief-valve should have been subject to extra testing that normal systems usually don't have to undertake to ensure the safety and function of the component due to the vast amounts of lives and capital at stake if there was to be a failure. It is clear to see that the PORV was insufficiently tested against the requirements as the requirements were incomplete, and as a result, all subsequent forms of testing that followed the functional testing it should have received would have been incomplete as well.

Pre-Warning:

There have been multiple well-documented failures of the PORV manufactured by B&W at other nuclear plants before the disaster at TMI. PORV failures took place at five different nuclear plants, the last one of them being at TMI itself from the years 1974 to 1977 (Burchill, 2022). With that many PORV failures in such a short amount of time, it leads to the only acceptable conclusion that the inadequate quality of the PORV was not an honest mistake or oversight, but unfortunately, just another act of negligence by the overly complacent nuclear energy industry (Rosztoczy, 2019).

References

Burchill, W. E. (2022, April 29). Insights from the Three Mile Island accident—Part 1: The accident. Www.ans.org. https://www.ans.org/news/article-3902/insights-from-the-three-mile-island-accidentpart-1-the-accident/

LiquiSearch. (n.d.). Three Mile Island Accident - Aftermath - Investigations. Www.liquisearch.com. Retrieved November 22, 2022, from https://www.liquisearch.com/three_mile_island_accident/aftermath/investigations

Pell, H. (2019, October 7). Three Mile Island: On the 1979 Accident and Its Decommissioning Forty Years Later. Www.aip.org. https://www.aip.org/history-programs/niels-bohr-library/ex-libris-universum/three-mile-island-1979-accident-and-its#

Rosztoczy, Z. (2019). Root causes of the Three Mile Island accident. https://www.ans.org/pubs/magazines/download/article-1151/

United States Nuclear Regulatory Commission. (2018, June 21). NRC: Backgrounder on the three mile island accident. Nrc.gov. https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html

Wendorf, M. (2019, July 7). Three Mile Island - America’s Chernobyl. Interestingengineering.com. https://interestingengineering.com/science/three-mile-island-americas-chernobyl

Wikipedia Contributors. (2018, December 3). Three Mile Island accident. Wikipedia; Wikimedia Foundation. https://en.wikipedia.org/wiki/Three_Mile_Island_accident

Wikipedia Contributors. (2019, September 20). Three Mile Island Nuclear Generating Station. Wikipedia; Wikimedia Foundation. https://en.wikipedia.org/wiki/Three_Mile_Island_Nuclear_Generating_Station

World Nuclear Association. (2020, March). Three Mile Island | TMI 2 |Three Mile Island Accident. - World Nuclear Association. World-Nuclear.org. https://world-nuclear.org/information-library/safety-and-security/safety-of-plants/three-mile-island-accident.aspx