Chernobyl 10/14/21

Science with Richard Bleil

Today, when people think nuclear disasters, they probably think Fukushima in Japan simply because it’s the most recent disaster of this type. Caused by a tsunami, this disaster is already a decade old, having occurred in 2011. But one of the earliest meltdowns of a nuclear power plant was Chernobyl, in the USSR, in 1986.

Preceding Chernobyl was the Three Mile Island nuclear disaster in the state of New York, in 1979. Although mostly covered up, some consider this disaster worse than Chernobyl, although it did not result in a complete meltdown as Chernobyl did. In 1979, I was just learning how to become a pimply faced driver at sixteen and more interested in Melissa the cheerleader than I was in world events. I do recall the news stories from Three Mile Island, though, a series of understated and calm briefs reporting simply, “a report of radioactive steam escaped Three Mile Island again today. Experts say this presents no danger to people as…” Aside from these releases of steam (which I believe were to reduce pressure inside of the plant) I don’t recall any further details. Because of the Jet stream, I’m guessing that this steam was swept out over the ocean, which is why there was minimal injuries, at least to humans. But for those who are old enough to remember, prior to 1979 it seemed like fish all had just the single eye.

By 1986, I had completed my bachelor’s degree in chemistry and was in that two-year sliver of time in my life when I was working as an analytical chemist before starting my doctoral program. In those two years, I had worked at two different companies. The second company was part of an international delegation to go to Chernobyl to look into what happened just before I started working there. Obviously, I was not part of this committee, but it was interesting hearing at least part of what they had found.

In principle, nuclear power is easy to understand. I’ve written before that to generate electricity all you need is a way to turn turbines with coils of wire in a strong magnetic field. Much like fossil fuel plants, nuclear power plants use super heated steam under extremely high pressure for this feat. More than enough heat is released to create this steam (in a closed system so the irradiated water is never released back into the environment, at least in principle) as unstable isotopes decay in a nuclear process. So a highly purified “core” of uranium or plutonium is at the center of the plant, but the decay must be carefully controlled.

As one atom of fuel decays, it releases three neutrons. If one of these neutrons hits the nucleus of another unstable atom, it will also decay releasing three more neutrons. Uncontrolled, this becomes explosive as more and more atoms decay, releasing more and more energy and resulting in an atomic explosion. To control this, “control rods” are inserted into the fuel to “capture” some of these neutrons to prevent the uncontrolled breakdown. If the want to slow the reaction down so it doesn’t go out of control, they insert the rods deeper into the core, and if the reaction is not generating enough heat, the pull them out a little bit.

It’s a very delicate and difficult balance to keep the reaction going just fast enough to generate the amount of heat needed. Much of this is computer controlled, and the plants are constantly being inspected for safety. In Three Mile Island, safety tags from these inspections covered warning lights on the control panels that had been lit for an estimated three days before anybody noticed them leading to the disaster, at least in part.

In Chernobyl, the inspection included an emergency shutdown drill. The shutdown went flawlessly. The control rods were fully inserted, and sure enough, the temperature and pressure both dropped as they should have. In starting the plant back up, the government had expectations and “milestones” as to how quickly the plant should be able to get back online. The plant was falling behind, and the managers decided they needed to allow the core to heat up more quickly to keep up with these milestones. As they were falling behind, they turned more and more of the automated systems off. By the time the disaster happened, nearly all of the emergency automatic shutdown systems had been taken offline, and the accelerating nuclear reactions simply couldn’t be stopped in time to prevent the disaster.

To protect the public, American nuclear power plants encase the core in lead. In Chernobyl, a massive carbon lid weight 100 tons (2,000,000 pounds) was used. Any material can be used to shield nuclear particles, but the less dense, the heavier it has to be, so the choice to use carbon necessitated such a mass. Unfortunately, the heat from the meltdown was enough to cause this two-million-pound carbon dome to catch on fire further complicating the clean-up efforts. I know that this massive dome burned for years after the disaster, but I do not know if it is still burning today. But what is happening to day is an uptick in neutron radiation.

What remains of the fuel core, the carbon dome and the control rods are still buried in Chernobyl. Radioactive levels had been declining over the decades, but in recent years the radiation counts have again been increasing. Nobody is quite sure why, and knowledge of this has been a source of concern for many people.

I’m hardly an expert, but I know that the reactions of the meltdown are still occurring. If carbon remains of the dome, I’m guessing it’s still burning. The nuclear core is still undergoing decay and releasing heat. The lead control rods are probably molten, and if any oxygen is getting to them they are likely reacting as well creating forms of lead oxide. So what could be happening? Any of these things could lead to the increased neutron counts. Perhaps a portion of the lead dome is burning away creating holes or thin spots where the neutrons can escape. Perhaps the molten lead has flowed below the nuclear core and no longer blocking the neutrons. Perhaps the core, in its decay path, is at a point where it has created elements that are decaying, at least for a time, faster than the elements prior (nuclear decay creates new elements as the original element literally falls apart, often ending in lead).

What I can tell you is that there is no need for panic. There will be no atomic explosion. What we have is a very complex mix of chemical and nuclear processes occurring in Chernobyl that I couldn’t begin to piece together, but an atomic explosion requires highly pure fuel. As the elements are changing, and as debris no doubt continues to mix in the fuel core, and as the uranium and plutonium transmute to other elements, the core is becoming less concentrated, not more. It’s a chaotic system, not well understood, but any damage that will occur will be significant only in the local region, not global. If people have been reclaiming ground surrounding Chernobyl, they may have to move back again, but that should be the extent of it.

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