Three Mile Island: The Causes of the Meltdown (Fall 2012)

March 28th, 1979 is a very important day in United States history. No one died or was injured on this eventful day, but the events that took place on this day continue to impact the daily lives of every single American. On this spring day near Dauphin County, Pennsylvania the Three Mile Island nuclear reactor suffered the largest nuclear reactor failure in United States history. The disaster of this event continues to influence energy policies throughout the world, as well as the public’s view of the safety of nuclear reactors. There were many small problems that eventually led to the complete meltdown, but mechanical failures, human error, and faulty design of the reactor were the leading factors in this meltdown.

The Three Mile Island reactor has two separate reactors, TMI-1 and TMI-2. On this day, TMI-1 was shut down for routine maintenance. TMI-1 was in the process of being refueled and was free of the uranium fuel rods. This proved lucky as this disaster would have been much worse that both reactors been operating at the time of the failure. The meltdown occurred inside TMI-2, which was operating at ninety-seven percent power that day.

To better understand the events that occurred at Three Mile Island, some knowledge of the facility and how the reactor works is needed. Both of the reactors at Three Mile Island were manufactured by Babcock and Wilcox. This meant that both of the reactors were pressurized water reactors or PWRs. In this type of nuclear reactor water is pumped through a pressure vessel. This water is kept under very high pressure. This high pressure water then passes through the core where it is then heated to about six hundred degrees Fahrenheit. However, because this water is under extreme pressure it does not boil. Within the TMI-2 reactor was about a hundred tons of uranium. This uranium was encased in 36,816 thin, twelve-foot-long fuel rods. This radioactive core was housed in a pressure vessel that was thirty-six feet high with nine inch thick steel walls. As the nuclear reaction generates heat, water circulating through the PWR in the primary loop becomes heated and proceeds to one of the large steam generators. There were two steam generators at the TMI-2, each were seventy-three feet high. In these steam generators, the heat from the primary loop water is transferred to the secondary loop. The water in the secondary loop is not under pressure and is able to boil. The steam produced here is the steam that runs the turbine to generate the electricity. The water in both loops condenses and re-circulates throughout the reactor again. This is the basic setup of the Three Mile Island nuclear reactor and will help understand the disaster that the plant.

At four in the morning on March twenty-eighth, maintenance workers were working to clear a blockage in one of the eight polishers within the secondary loop. These polishers remove impurities within the water after it has been condensed back to the liquid state. Then, for reasons that have still not been determined to this day, the pumps that supplied the secondary loop shut down. Then, the polisher bypass valve that should have opened to allow for the water already within the loop to drain failed to open. This is the first significant malfunction within the plant and set off the chain reaction of events that ultimately led to the disaster.

Once the secondary pumps shut down, many more systems shut down in response. The pumps for the main water feed that sent water to the steam generators then shut down. After these pumps shut down, the turbine also shut down, which it was designed to do. Once the turbine shut down the entire plant stopped generating electricity and shut down as well. Also according to design, as soon as the turbine shut down, the auxiliary water pumps came on to supply the steam generator with water, but that water never reached the steam generator. Two valves had accidentally been left in the closed position. This is the first failure caused by human interaction within the Three Mile Island disaster.

The chain reaction of machine malfunctions was just beginning. Without water in the steam generators, the entire secondary loop shut down and water stopped flowing. Heat and pressure then began to rise within the primary loop due to the fact that the steam generators were no longer removing heat from the water. This caused the control rods to enter the core and stop the production of heat due to nuclear fission. Eight seconds had elapsed since the polisher pumps shut down.

Even though the control rods were stopping the production of new heat within the reactor, decay heat remained within the system. A pilot-operated relief valve or PORV opened to allow for the heat, and pressure caused by the heat, to escape, but the valve failed to close again and was stuck in the open position. This allowed for large volumes of cooling water to drain from the reactor. This failure of the relief valve to close again is the primary cause of the meltdown at Three Mile Island. This was not the first time the PORV had been stuck open in TMI-2. It was also a chronic problem with all the nuclear plants designed and built by Babcock and Wilcox.

This chronic design flaw is a strong history force behind the failure at the Three Mile Island nuclear plant. The science and technology of nuclear reactors were not nearly what they are today, and the methods used by Babcock and Wilcox had great impacts on the nuclear industry. Babcock and Wilcox knew about the faulty valve, but made no attempt to fix the problem at TMI-2. If the company would have shown any effort in attempt to alleviate the issue, nuclear energy in the United States would be in a much different position. Also, had Babcock and Wilcox learned from the previous failures and re-designed that valve, the whole situation would have been averted as well. These mistakes in the design and construction in the nuclear plants designed by Babcock and Wilcox changed the course of nuclear power within the United States forever.

As the PORV valve stayed open, coolant continued to flow through the valve. The pressure within the primary loop had been returned to normal, but excess coolant continued to drain. Inside the control room of the plant, the alarm system had begun to activate. A loud horn and several hundred flashing lights on the main control panel were activated, but they supplied little guidance in terms of fixing the problem. Operator Chris Faust is actually quoted in saying “I would have liked to have thrown away the alarm panel. It wasn’t giving us any useful information.” (Walker, page 74) The most important light for this situation was the light indicating whether or the PORV valve was open or closed. If the light was lit, it was believed by the operators to mean that the valve was open. On this day, the light had been lit for a period of time, but then shut off, leading the operators to believe that the valve had closed after releasing pressure. However, an investigation after the disaster discovered that the light was lit when the solenoid controlling the valve was receiving an electric current. The light was not actually indicating the position of the valve. The electric current opened the valve exactly as it was designed to do, but when the current was taken away, the valve should have closed, but it did not. This was another design error made by Babcock and Wilcox that could have prevented this disaster.

As the coolant continued to drain through the stuck PORV valve, decisions made by the human operators of the plant began to make the disaster worse. The operators on duty at the time of the incident were all highly-trained, highly-skilled workers. Most of the plant operators in TMI-2 were trained by the Navy, as most of the Navy’s submarines were nuclear powered. All of the works also received training from Met Ed, one of the owners of the plant, and Babcock and Wilcox. All nuclear plant workers were required to pass examinations administered by the Nuclear Regulatory Commission, or NRC. Therefore, they were trained in operation of the plant and solving minor malfunctions, but they were never trained for a plant malfunctioning at the rate TMI-2 was.

While all of the alarms and lights were indication problems, the operators’ main concern was not the loss of the coolant in the primary loop, but that the pressurizer would “go solid.” (Walker, page 76) This means that the pressurizer would be filled with all water, instead of the usual mix of steam and water. As the heat in the plant began to rise, the Emergency Core Cooling System turned on exactly as designed. Two pumps began to pump thousands of gallons of coolant into the core. However, because the operators were already worried about too much water within the core, the shift foreman, Fred Scheimann, made the decision to shut those pumps off. He also turned off another set of core coolant pumps. At this point, it was still believe that the PORV valve was closed and that there was enough coolant within the system, however there was not. This decision is the greatest example of the history force of the role of a specific individual within the Three Mile Island Meltdown. If Scheimann would have known that the valve was open, he would not have made the decision to shut down the pumps. However, he made his decision based on faulty indicators and without knowledge of the actual status of the coolant within the core.

As coolant continued to poor out from the PORV, relief works arrived at the plant. Brain Mehler was one of the workers who came to the plant in relief. He concluded that the PORV must have been at least partially open. He then made the decision to close the block valve. The block valve was a backup to the PORV. This prevented more coolant from exiting the core and in the end prevented an even greater disaster.

The decision made by Mehler to close the block valve is another example of the role a specific individual had on the Three Mile Island disaster. Had he not made the quick decision to close the valve, the coolant would have continued to empty for who knows how much longer. His individual decision saved the plant from an even greater disaster.

Now that the flow of coolant out of the core had been stopped, the coolant remaining in the system began to turn to steam as the heat within the core began to rise. Eventually, the fuel rods were exposed and the metal began to react with the steam. This reaction not only ruptured the rods, releasing radiation, but hydrogen was also released. It would later be discovered that the top half of the rods melted, as temperatures within the core were estimated to reach temperatures as high as four thousand degrees Fahrenheit or more.

The disaster at the Three Mile Island was the result of many small malfunctions and failures that, complied, lead to the worst nuclear reactor meltdown in United States history. Faulty design, mechanical failure, and human error were the main factors in this disaster. The role specific individuals, both negative and positive roles, influenced the severity of the meltdown, as well as the new science and emerging technology in the nuclear field. Many lessons were learned from the mistakes that were made that fateful morning and nuclear regulations were enacted to ensure that what happened at Three Mile Island would never happen again. Thankfully, the damage caused by this disaster was minor and no lives were lost.

Sources

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