Many people already know that Three Mile Island was a near catastrophic meltdown incident. However, many people are not totally aware of the events and consequences of the situation. I will try to lend a bit of information gathered from various online sources.
Background Plant Information
In order to understand what happened at TMI, it is necessary to understand the system that was in operation. The reactor at TMI was a PWR, or pressurized water reactor. The components of this reactor consist of a nuclear core enclosed in a steel pressure vessel. There are different water loops that are used to transfer the heat and cool the core. The first loop is the primary loop, which is entirely isolated from other components, except for the transfer of heat. The secondary loop flow through a heat exchanger where the heat from the primary loop is dumped to create steam which is used to power a steam turbine and generate power. It should be noted that the primary loop is maintained at extremely high pressure to ensure that the water remains in the liquid state and does not evaporate to steam.
The reactor had a number of different failsafes.
- Power Operated Relief Valve. This valve served the purpose of relieving pressure pressure in case there was an extreme buildup in the primary core. The particular model at TMI, a Dresser electromatic, was designed to never fail to open, but had a problem with closing.
- A manual block valve. This valve was added to be used in the event that the electromatic valve wouldn't close. If no extra valve was present and the electromatic valve failed, too much pressure would be released and the water in the primary loop would turn to steam, which would not be able to adequately cool the core.
- High pressure and Low pressure injection systems. These systems were capable of dumping unbelievable amounts of water on the core in the event that a loss of coolant, ie primary loop water, was to occur.
- Containment Dome All of the primary loop components were housed in this structure, which had 12 foot thick concrete walls.
- Emergency pumps. Basically everything had a redundant system in case one should fail.
The Chain Reaction
The entire system was sent on a course of near catastrophe based on a single event that started a chain reaction of coincidental mishaps that could almost never be duplicated. It was as if some hollywood screen writer was behind the scenes, composing each and every thing that could go wrong.
Here is the order and actions of events that nearly caused the total meltdown of Three Mile Island Reactor 2.
- The single event that started it all was the mistaken connection of a rubber hose between an air line and a water line. No one know why this was done for sure, but it probably happened because the person thought they were connecting two air lines. All the fittings for water and air lines were the same. The particular air line that was connected to the water line was a control line for many pneumatic controls of the feedwater.
- The water line was at a higher pressure than the air, so eventually the water crept into the pneumatic valves controlling the feedwater. These valves slammed shut, cutting of the cooling flow of water in the secondary loop. Ironically, this scenario had been designed for and a system was created to make the valves fail in the "as is" position, but this system was never hooked up.
- The closing of the valves immediately caused immense pressure due to the stopped flow. This ruptured the secondary pipes and damaged the flow pump.
- The loss of water through the secondary loop eliminated the cooling for the primary loop, and the temperature and pressure began to rise in the primary loop.
- Emergency feed water pumps were triggered to restore flow to the secondary loop. However, during maintenance, the valves to these pumps were closed and never reopened. Yellow maintenance tags were hanging over the indicator lights, so the operator did not see that they were closed. Although procedure called for immediate verification that they were open, the operator assumed they were open because they were supposed to be open.
- The pressure continued to rise in the primary loop because the there was no feedwater to cool the system. This caused a relief valve to open and steam was expelled into a quench tank in the containment building.
- The valve didn't close, but an indicator light said it had because the light was poorly configured to indicate only that the valve had been commanded to close, not that it actually was closed.
- Pumps were started to replace the water that had been lost in steam due to the relief valve opening. However, the indicators began to show that the water level was rising way too fast. This was, in fact, not the case. The stuck relief valve was causing erroneous water level readings. The emergency injectors were shut down and the water level in the primary loop plummeted.
- Pumps were then started to remove water from the loop. Now water was being pumped out and exiting through the relief valve. It continued to drop.
- Seeing that the feedwater pipe temperature was still rising, it was realized that the emergency feedwater pump's valves were closed. The valves were opened and flow was restored.
- The continuous loss of pressure in the primary loop made the primary water begin to boil. This created problems with the coolant pumps. The pumps had to be shut down so that they wouldn't be destroyed by trying to pump steam. The result was that the core was partially exposed and sitting in stagnant coolant.
- The suspicion that the relief valve was open now arose and a measurement was taken for the temperature at the valve outlet. A high temperature would indicate that the valve was still open, but the technician read the wrong indicator, so the valve remained open.
- The flow from the relief valve eventually overran the quench tank and the containment room became flooded with radioactive water.
- Eventually, the block valve was closed to remedy the relief valve problem. It was too late though, because water from the containment room reached the rest of the building through floor drains.
- A hydrogen bubble formed in the containment room and was sparked by an electrical relay. The room easily withstood the explosion, but it led to the fear of further explosions due to the formation of pure hydrogen in the system.
- A judgement was made that determined venting the hydrogen, along with some radioactive steam, was more favorable than a complete explosion.
- The coolant flow was eventually restored and the drama had ended.
Legal Battles
Over 2,000 personal injury claims were filed by people who felt they became ill because of the exposure to radiation from the accident. Many state reports that the average person was subjected to 1.4 mrem of radioactive material, which is not a very significant amount. The plaintiffs claimed that a highly concentrated plume of radioactive material had left the plant and had landed upon a smaller area of people leading to exposure of over 10 rems, which would be capable of causing cancer.
All suits were dismissed due to the plaintiff's inability to prove that any such concentrated plumes existed or that anyone was subjected to extreme amounts of radiation. In all reasonableness, however, it is unlikely that the radiation lost was neatly distributed among the 2 million people in nice little 1.4 mrem portions.
In the End
Three Mile Island was a terrible accident, but not nearly as terrible as it could have been. It was started by a seemingly insignificant action that lead to a string of mechanical and human errors. Looking back at this situation, it's pretty unbelievable that everything could have gone that wrong. Ultimately, it provides a valuable lesson for
the future of nuclear power.
Information was taken from many sources with factual information. For the best TMI breakdown I've seen, go to www.wowpage.com/tmi/. The sight follows every event.