The Three Mile Island Nuclear Accident – What Really Happened

This is a sample chapter from our recent book The Dark Horse – Nuclear Power and Climate Change (link to Amazon store). The book was published in 2020. It is our effort to counter the misinformation out there regarding the Three Mile Island accident.

Three Mile Island

Three Mile Island (TMI) ranks as the third most famous and serious civilian nuclear power plant accident. It happened 28th March in Harrisburg, Pennsylvania. Nobody died, and no significant amounts of harmful radioactivity were released into the surrounding areas. Reactor number two had a partial meltdown, and the cost of clean-up was around one billion US dollars. Public communication about the accident was a horrible failure. This caused panic, mistrust and the strengthening of the nascent anti-nuclear movement. Three Mile Island is commonly seen as one of the key reasons for the almost total shutdown of building new reactors anywhere in the western countries during the next 30 years.

The TMI pressurized reactors designed by Babcock & Wilcox had their quirks. The bigger problems were probably the lack of proper training and expertise of many nuclear operators in the fast-growing industry. People who had operated far smaller nuclear reactors in Navy submarines for a couple of years found well-paying jobs in the civilian nuclear industry. Those submarine reactors were sized in the ballpark of 12 MW, which made them much easier to handle than the civilian reactors that were dozens of times larger. For example, a full day after shutdown, a 1.2 gigawatt PWR still produces around 15 MW of residual heat.

The chain of events that led to the partial meltdown started around eleven hours before the accident.^[i] Plant operators were trying to clean up some sophisticated water filters of the secondary water loop. The blockage was stuck, so instead of using pressurized air, the operators forced some water through to remove the resin. A small amount of the pressurized water got past a valve and ended up in the wrong place (instrument airline). Later this water caused the feedwater pumps to turn off, cutting off feedwater to the steam generators. This in turn caused increases in pressure and temperature in the reactor cooling system, which eventually led to an automatic emergency shutdown (SCRAM^[ii]). As the turbines were offline, decay heat started gathering in the primary water loop.

Three auxiliary pumps were automatically activated, but they offered little help, since some valves were closed due to maintenance. This effectively cut off all primary and auxiliary cooling from the core. This closure of the valves was a violation of Nuclear Regulatory Commission (NRC) rules and was later found to be one of the key reasons for the partial meltdown.

As pressure in the primary system kept increasing, it automatically opened the pressure release valve. Normally this valve closes after pressure goes down, but due to a mechanical failure, it remained open. In addition to pressure, cooling fluid also started leaving the system. This mechanical failure of the valve was found to be one of the key factors that led to the accident. The stuck valve went unnoticed by the operators, mainly due to a badly designed status indicator light.^[iii] The operator read the indicator as the valve being closed (as it should be), while in reality it was stuck open. This caused a lot of confusion amongst the operators, since other indicators were acting strange considering that the valve was supposed to be closed. Only the next work shift, seeing the situation with fresh eyes, realized what was happening. By then, it was much too late: 120,000 litres of coolant fluid had leaked from the cooling system, and the damage had been done.

The accident revealed severe shortcomings in communications between officials and the public. Responsibilities were found to be unclear. The public received a constant barrage of conflicting information, which increased panic and caused unnecessary evacuations.

The accident was a result of many unlikely technical problems occurring at the same time. Combined with somewhat lacking operator skills, the result was a partial core meltdown. Nobody got hurt and no significant radioactivity was released to the surroundings. What was released was mainly harmless Xenon-gas, although a small amount of radioactive iodine was also released.

^[i] The article relies on this article in Wikipedia: http://en.wikipedia.org/wiki/Three_Mile_Island_accident

^[ii] SCRAM comes from “safety control rod axe man”  and refers to the last line of safety in the first reactor humans built, whose job it was to cut the rope holding the control rods up with his axe.

^[iii] Some sources say that the indicator was partly covered.