Can A Nuclear Meltdown Be Stopped?

Few manmade threats inspire as much fear as the potential of a nuclear meltdown. The fact that "Chernobyl," once an obscure Ukrainian town, is now globally recognized as a synonym for catastrophe shows how deeply this fear runs. The meltdown of 1986, which left 28 dead and has since caused genetic mutations in local dogs, became a rallying point for critics of nuclear energy, who fear that similar disasters will occur elsewhere. Nuclear power has immense potential as a carbon-free energy source at a time when the planet desperately needs it, but that potential can't be achieved without the trust of the public. To that end, engineers of nuclear power plants are devising more and more methods of stopping nuclear meltdowns before any damage can be done.

The earliest nuclear power plants were built beginning in the 1950s, and from the very start, mitigating the risk of a meltdown was a top priority. This danger arises when a nuclear reactor overheats, so in order to prevent meltdowns, reactors need powerful cooling systems in place. Historically, these systems have relied on circulating water around the reactor core in order to contain the heat of the radioactive fuel within it, and by-and-large, these systems have been successful. However, the catastrophic event at Chernobyl and the 2011 meltdown of Japan's Fukushima Daiichi Nuclear Power Plant (which announced in 2020 that it would be pumping its radioactive water back into the ocean) both occurred because their respective cooling systems broke. The lessons learned from these disasters are now shaping a new approach to nuclear power.

The core of a nuclear reactor contains hundreds of fuel rods packed with uranium, which generate a tremendous amount of heat through the process of nuclear fission. There are two main systems in place to keep the fission reaction under control. First are control rods made from elements like cadmium and boron that can absorb neutrons produced by nuclear fission. Inserting the control rods into the reactor core stops the nuclear reaction, but there is still an immense amount of residual heat leftover, known as radiogenic heat. That's where the second safety feature — the cooling system — comes into play. The reactor core is surrounded by a fluid coolant, which is continuously circulated by pumps to keep the core from overheating.

Meltdowns happen when the cooling system breaks. The coolant must be kept circulating or else all of it will evaporate, leaving nothing to transfer heat away from the fuel. If that happens, the heat of the uranium can cause the zirconium cladding of the fuel rods to react with the steam inside and produce hydrogen gas, which can build up until the pressure causes an explosion, as happened at Chernobyl and Fukushima. The fuel can reach over 3,600 degrees Fahrenheit, hot enough to melt the concrete and steel surrounding the core. This creates a molten blend of uranium, concrete, and metal commonly known as corium. It's like radioactive lava, and it can melt straight through the floor of a power plant, into the soil.

The International Nuclear and Radiological Event Scale (INES) grades all incidents and accidents at nuclear power plants on a seven-level scale. To date, there have only been two level seven events — Chernobyl and Fukushima — both of which were early-generation nuclear reactors. Chernobyl was built between 1972 and 1977, and Fukushima was even older, built between 1967 and 1971. They used water as a coolant, as have a vast majority of nuclear power plants, but engineers have now recognized that water isn't the best choice for this purpose. This has given rise to a new generation of nuclear power plants, known as Gen IV reactors, which use novel methods of cooling for a far better safety system.

Gen IV nuclear reactors use different types of coolant, such as liquid sodium, molten salts, and helium gas, that won't boil away quickly if their circulation systems go down. This could potentially make nuclear meltdowns a thing of the past, but the technology is still very new. The U.S. government plans to develop next-gen nuclear reactors as early as 2026, which will hopefully lay the groundwork for expanding their use. However, older power plants that rely on water-based cooling systems still present the same concerns as Chernobyl and Fukushima, so as the new generation comes into play, we may see some of the other old stalwarts shutting down or rebuilding in pursuit of a safer era in nuclear energy.