On September 20th, 2019 — one year ago today as I write this — the infamous Three Mile Island (TMI) nuclear power plant was permanently shut down. TMI Unit-2 has been shuttered since the partial meltdown in 1979, an event described as the “most serious accident in U.S. commercial nuclear power plant operating history.” The accident caused significant shifts in public understanding and perception of nuclear power, and the effects of those five days in 1979 are still felt today.
When a nuclear power plant is officially shut down, the decommissioning process begins. Federal regulators — specifically the Nuclear Regulatory Commission (NRC) — require that nuclear plant sites must be cleaned up within 60 years of initial shutdown, so the clock is ticking and there is a lot of work to be done. Currently, 19 commercial nuclear power reactors are undergoing the decommissioning process in the U.S. What exactly does it take to safely close a nuclear power plant, and why does it take so long to do so? Additionally, what does closing a nuclear power plant mean for the future of clean and renewable energy?
|The Three Mile Island nuclear power plant. Image credit: Exelon Generation.|
Nuclear reactors are designed to achieve one simple goal: heating water. The heated water turns into steam, which drives a turbine that turns a generator to produce electricity. TMI is a pressurized water reactor, meaning that the water heated by the reactor is kept under extremely high pressure. The essential working parts of the system are the reactor, water, steam generators, steam turbine, pumps that circulate water through the system, and the pressurizer.
Nuclear power is possible because of fission, the splitting apart of nuclei. All nuclear reactors in the U.S. that produce commercial electricity are powered with Uranium fuel. When free neutrons strike a Uranium nucleus, it splits apart, and most of that energy is directly converted into heat. As these free neutrons strike other Uranium atoms, it creates a chain reaction, and nuclear reactors are designed to sustain this reaction to continuously produce energy. Steam escapes as the only emission from the cooling towers, which is not only critical for producing electricity, but for removing the intense heat that the reactor water carries.
|Schematic of a pressurized water reactor. Image Credit: energy.gov.|
Decommissioning Options The biggest challenge in the decommissioning process is to manage the radioactive fuel safely and efficiently. The spent nuclear fuel must be removed from the reactor and transferred to a used fuel pool before finally containment in dry storage facilities. Power plant operators have three possible choices for decommissioning: DECON, SAFSTOR, and ENTOMB.
DECON is a process of immediate dismantling; the radioactive materials are decontaminated to levels such that they can be moved off-site and stored elsewhere. (In 1987, the Nuclear Waste Policy Act designated Yucca Mountain in the remote Nevada desert as the US national repository for spent nuclear fuel). SAFSTOR — the option chosen by TMI officials and is also known as “deferred dismantling” — means that the nuclear facility is maintained and monitored over a long period of time to allow the radioactivity to decay before the facility is dismantled. (Waiting 50 years allows the radiation to decay to 1% of the original contamination levels). Lastly, the ENTOMB process means that the radioactive contaminants are permanently encased on-site — “entombed.” No NRC-licensed facility has ever chosen this course of action.
TMI Unit 1 will be decommissioned according to the SAFSTOR plan of action.
Phases of Decommissioning
There are three main phases of decommissioning: transition, major decommissioning and storage, and the final license termination.
|Generalized decommissioning timeline. Image credit: NRCgov on YouTube.|
The major decommissioning and storage activities can begin ninety days after the NRC receives this report; these include permanent removal of major components, such as the reactor vessel and steam generator. According to the SAFSTOR timeline, this stage involves maintaining the facility while waiting for more radioactive decay. This creates a much safer environment for the decommissioning workforce who will still be working on-site. Exelon — the company that owns and operates the facility — estimates that most of the spent fuel pool will be moved to dry cask storage by the end of 2022 (Exelon has plans to construct an Independent Spent Fuel Storage installation facility on-site), and the ultimate dismantling of the cooling towers is estimated for 2074.
A Changing Energy Landscape
The consequences of climate change are having more immediate and disastrous effects in our daily lives, making our aspirations for reliable and clean energy sources even more necessary and urgent. Although TMI and other nuclear power plants are closing, nuclear power remains a critical component of our country’s broad spectrum of energy capabilities. The 1979 accident indeed had a lasting effect on the nuclear power industry in the U.S. writ large, and the more recent expansion of fracking has also brought significant competition to the nuclear power industry. All these factors, intersecting with and influenced by political and economic forces over time, have accumulated in significant changes to our modern energy landscape. Only time will tell how these changes will shape the future of nuclear power.
“What’s going on in this video? Our science teacher claims that the pain comes from a small electrical shock, but we believe that this is due to the absorption of light. Please help us resolve this dispute!”
(We’ve since updated this article to include the science behind vegan ice cream. To learn more about ice cream science, check out The Science of Ice Cream, Redux)
Over at Physics@Home there’s an easy recipe for homemade ice cream. But what kind of milk should you use to make ice cream? And do you really need to chill the ice cream base before making it? Why do ice cream recipes always call for salt on ice?