Ep 13 Chernobyl Nuclear Power Plant Explosion
Engineering News - Printed thermoelectric generators
Engineering Failure - Chernobyl Nuclear Power Plant Disaster
New Safe Confinement
Printed thermoelectric generators - https://techxplore.com/news/2021-02-energy-harvesting-thermoelectric-power.html
Chernobyl HBO – Valery Legasov Explains how an RBMK reactor works - https://www.youtube.com/watch?v=TmIEI4ky-Zc
Scott Manley – Why Chernobyl Exploded – The Real Physics Behind the Reactor - https://www.youtube.com/watch?v=q3d3rzFTrLg
Hi and welcome to Failurology; a podcast about engineering failures. I’m your host, Nicole, and I’m from Calgary, Alberta.
I will be doing a Q&A bonus episode very soon. If there is anything you want to ask me about me, my job as a mechanical engineering consultant, the podcast, or about engineering in general; please send them to @failurology on twitter or email them firstname.lastname@example.org. I will be recording this soon so please send your questions in by Friday February 12th.
This week’s engineering failure is lucky number 13. And what would be more fitting for episode 13 than the Chernobyl nuclear power plant explosion, the worst nuclear disaster in history. For this failure, we’re going to have to hop in our way back machines to 1986, a much different time. The USSR was still a country, and the cold war between the USSR and the USA was just getting underway. Russia didn’t become a country until 1991.
I was born shortly after Chernobyl occurred, but I still learned about it very young. I definitely didn’t understand exactly what transpired, but I knew that something bad happened in Russia that made people sick. There was a program in the mid 90s to relocate children outside of the area on an exchange program. One of our family friends sponsored a young girl, around the same age as me, to live with them for a summer. Her name is Svetlana; and she and I became good friends. We couldn’t really talk to each other as we spoke different languages, but she had a picture book that we would use to communicate. Unfortunately, we didn’t keep in touch after she returned home, but I think of her often, especially while I was working on this episode. Svetlana, if you’re out there, I hope you’re ok and doing well.
The Chernobyl disaster started with what was supposed to be a simple safety test that went horribly, horribly wrong. It directly impacted the lives of thousands, if not millions of people. And it also impacted the future of nuclear power generation, even still to this day. I think nuclear power gets a bit of a bad rap. It’s a fairly complex process that is often misunderstood. That combined with Chernobyl, Three Mile Island, and most recently Fukushima, to name a few, have led to most of the public pushing for less nuclear power. When managed and operated properly, nuclear power is one of the cleanest forms of power generation available. Yes, there is hydro, solar and wind, but they are not as reliable and consistent. Nuclear energy doesn’t emit any greenhouse gases, is carbon free and the output can be controlled to meet demand. That said improper management can lead to big problems and widespread radioactive contamination, like we saw in Chernobyl. But 1986 was a long time ago. I’d like to think we’ve come a long way since then. And that we’ve learned a lot from our mistakes.
Before I get into the episode, I want to mention that I am by no means a nuclear power expert and I learned more researching this episode than I knew before. On top of that, nuclear fission is kind of complicated. That said, it’s very important to me that you, the listeners, understand the failures that I am describing. Some failures have definitely been more challenging to explain than others. But regardless of the failure, I am very conscious of trying to explain things clearly and concisely, and this episode is no different.
As well, there is a lot of information out there on Chernobyl. More than I am able to cover in this episode. So I am going to tell you about how the nuclear reactor worked, what went wrong to cause it to explode, and then some additional information that I found really interesting.
This Chernobyl episode’s going to be a blast, but first, the news.
This week in engineering news; staying within the energy theme, printed thermoelectric generators are harvesting ambient heat to generate power.
Scientists of Karlsruhe Institute of Technology in Germany have developed 3D thermoelectric generators using printable materials. Flexible thermoelectric films and devices are made using an innovative screen-printing process that allows for direct conversion of nanocrystals into flexible thermoelectric devices. This design is scalable and relatively inexpensive compared to previous methods. Initial cost analysis is that the screen printed films can power thermoelectric devices for 2-3 cents per watt. Applications include wearables such as smart watches, fitness trackers or digital glasses. But the design concept is scalable and can also be used for recovery of waste heat in industry and heating systems, or in the geothermal sector. If you want to read more on these printed thermoelectric generators, check out the link on the web page for this episode, or head to failurology.ca.
Now on to this week’s engineering failure; Chernobyl Nuclear Power Plant. The worst nuclear disaster ever.
The Chernobyl disaster happened on April 26, 1986 around 1:23am local time. It was a result of an explosion during a safety test of the reactor #4.
The Chernobyl Nuclear Power plant and the nearby City of Pripyat (prip-yaat) were located in northern Ukrainian USSR. In today’s geographic borders, it is in northern Ukraine, at the border with Belarus.
There are only two nuclear energy accidents rated at a seven out of seven on the International Nuclear Event Scale. One is Chernobyl. The other is Fukushima Daiichi (foo-koo-shee-muh dai-ee-chee) which was impacted by the Tsunami in Japan in 2011.
The reactors at Chernobyl were RBMK-Type Nuclear Reactor
RBMK is a Russian acronym that in English means high power channel type reactor
This type of reactor is common throughout the soviet union and 10 of them are still operating today; all retrofitted with a number of safety updates of course. Two of them were even started after the Chernobyl disaster of 1986.
At the time of the Chernobyl disaster, most experts thought a nuclear reactor having a runaway reaction was impossible. Reactors are designed to prevent these circumstances. Well, they were supposed to be.
The Chernobyl nuclear power plant consisted of four RBMK reactors. Each is capable of producing 3200 mega watts of thermal power and 1000 megawatts of electricity. At the time of the disaster, the power plant produced 10% of Ukraine’s power. Construction of the plant and the nearby city of Pripyat to house workers and their families, began in 1970, with reactor #1 coming on line in 1977. The final reactor, #4, the one that exploded and changed nuclear power forever, came online in 1983; 3 years before the catastrophe. There were two more reactors planned, one of which was 70% complete at the time of the explosion, but both were scrapped in the aftermath of the disaster.
Reactor #2 was taken offline in 1991 after a fire broke out caused by a faulty switch in a turbine. Reactor #1 shut down in 1996 and #3 in 2000.
Reactor #1 had a partial core meltdown in 1982 when a cooling valve was left closed after maintenance. The uranium fuel overheated and ruptured. Due to negligent operators, the rupture was not noticed for several hours, resulting in radiation being released from the ventilation stack. You’d think they would have learned from this, but no. Before I get into what happened to reactor #4 in 1986, I’m going to do an overview of the reactor design and how it works.
Conceptually, nuclear power is fairly simple. A process creates heat, which boils water into steam, which turns a turbine and generates power. It’s the process that creates heat, called nuclear fission, that’s quite a bit more complicated. A delicate balance of reactivity is needed to create heat but keep the reactor stable.
The RBMK reactors are made up of a concrete cylinder, filled with several fuel rods, which in this case were made of Uranium, a graphite moderator to maintain the reaction, cooling water that gets heated and spins the turbine, and boron control rods to control the reaction. The control rods of the RBMK reactors were designed with a neutron-moderating graphite section attached to the ends to boost reactor output. While this may work well with the control rods fully inserted, you’ll see later that this design flaw greatly contributed to the explosion.
The uranium atoms from the fuel rods are flying around inside the core. They bump into neutrons and split apart, producing iodine and more neutrons. Those neutrons go flying around and bump into other uranium atoms and split them apart in a chain reaction. This process is called nuclear fission. In order to control the chain reaction from running away, out of control, boron control rods absorb some of the neutrons, limiting the rate of reaction. And the moderator, which in this case was graphite, slows down the fast neutrons and sustains the reaction. So fuel rods start reacting, graphite slows down but maintains the reaction, and boron limits reactivity.
The fission process creates heat, which is harvested to generate electricity. A series of pumps pump water through vertical channels from the bottom, up through the top of the reactor core. As the water reaches the top of the reactor, it has become steam and turns turbines to generate power. The steam then travels through a heat exchanger to cool it down, condenses back to water to start the process again.
The amount of steam, the absorption of the control rods, the temperature of the fuel, and several other factors need to remain in balance to keep the core stable.
In the event of an emergency, the reactor is shut down, but the cooling has to continue because the reactor is still producing heat. This is because the uranium, iodine, and neutrons flying around are radioactive, slowly decaying and releasing energy. So the pumps that distribute cooling water through the reactor have to keep running, even when the reactor that powers them shuts down. There are backup generators to run the pumps, but they take a full minute to get up to speed, which is too long. The theory was that while the steam generator stopped, the turbine would spin down, but was spinning enough to generate enough power to keep the pumps running for 1 minute until the diesel generators were up and running. This had never been successfully demonstrated on reactor 4 and a test was planned to demonstrate that the safety system worked. Reactor 4 was scheduled to be brought offline anyways so they scheduled the test for the time of the shutdown.
But, on the day of the test, the power grid needed more energy than expected, and reactor 4 couldn’t shut down on schedule, delaying the test. Reactor 4 was designed to operate at 3200 mega watts of thermal power, but for most of the day before the accident, it was running at 1600 megawatts. Now, remember the iodine that’s produced as part of the fission process? When the Uranium splits, it creates iodine and neutrons. The iodine on its own has a small probability of absorbing a neutron, so it doesn’t impact reactivity too much. But it also has a half life of 6.5 hours and then decays into xenon, which has very high neutron absorption. When the reactor is operating at full power, the reaction rate is balanced and the xenon is burned away. BUT, big but here, when the reactor is operating at reduced power, like it was for the full day preceding the accident, the xenon is not burned away and instead builds up, causing what is known as xenon poisoning. Essentially, the xenon is absorbing all of the neutrons floating around, meaning less uranium atoms are split, less heat is generated and the reactor power continues to decrease until all of the xenon is eventually burned away.
For the safety test, to see if the turbines could spin long enough to keep the cooling pumps running while the diesel generator started, the reactor was supposed to be operating at 700 megawatts. Power reduction from 1600mw to 700 mw began at 11:10pm on April 25th. About an hour later, after a shift change, the operators stalled the reactor and power levels dropped to 30mw, too low to run the test. The operators, not realizing that the reaction would not increase due to the xenon poisoning, tried to manually increase the reaction rate. They did this by pulling out the control rods, remember those things that absorb neutrons and stop the reactivity from increasing out of control? There were more than 200 rods in the reactor core, and they pulled all but 8 of them all the way out. Now, I am by no means an expert in nuclear power, in fact most of what I know about fission, I learned researching this episode. But even I can tell that removing almost all of the control rods was a bad idea, a very bad idea. The space left behind from the control rods was filled with water, which is a poor neutron absorber. Without the control rods to absorb the neutrons, the reaction began to increase, and burned away the xenon pretty quickly. And with no xenon or control rods to absorb the neutrons, the reactivity continued to increase, running out of control.
The operators, recognizing that the reaction had increased, tried to reinsert the control rods, but they took a full 18 seconds to fully descend into the core because they had to displace the water that took their place. On top of that, remember that the bottoms of the control rods were graphite which actually helped or sustained the reaction. During the emergency insertion of the control rods, there was a spike in the reaction at the lower part of the core. This had been seen before on other RBMK reactors, but was counter intuitive and unknown to the reactor operators at Chernobyl.
In the normal operating scenario, the steam production takes place near the top of the reactor. But because the power rose so quickly, and the control rods hadn’t yet been fully reinserted, most people believe that the water flash boiled in the channels near the bottom of the reactor, rupturing the pipes, causing a small explosion. A second larger explosion may have been due to water being dissociated into hydrogen & oxygen by heat and combustion elsewhere. The final power reading recorded for the reactor was 33 giga watts, which is ten times higher than the design power to the station. That is just what was measured; some estimate the power spike was actually much much higher.
I know, that was a lot of information, so I am going to summarize it quickly. The reactor was operating at half power for the day before the test, causing a buildup of xenon, referred to as xenon poisoning which reduced reactivity and the power generated. When the operators tried to reduce power for the test, they stalled the reactor and tried to increase reactivity in order to run the test. In an effort to do so, the operators pulled out the control rods allowing more neutrons to go flying about splitting atoms, not realizing the reactor was full of xenon which limited reactivity. Once the xenon quickly burned off, the reactivity rose very fast. To try to prevent the reaction from running out of control the operators tried to re-insert the control rods to stabilize the reactor, but the graphite on the bottom of the rods caused the power to spike, generating steam in the lower section of the reactor, causing a small explosion that triggered a larger explosion.
This all happened around 1:23 am on the morning of August 26th, 1986.
Contrary to safety regulations, the reactor building roof was constructed with combustible materials. The explosion of reactor #4 resulted in at least 5 fires on the roof of reactor #3. While this was going on, the chief engineer wouldn’t allow them to shut down reactor #3 and the operators were given respirators and potassium iodide tablets and told to keep working. At 5am the night shift chief shut down reactor #3 anyways.
The exterior fires were extinguished by 5am and reactor #3 was spared. But many firefighters received high doses of radiation. The inside of reactor 4 burned for 2 weeks after the explosion.
Some unprotected workers died from radiation exposure within three weeks.
Bubbler pools – two floors of bubbler pools below the reactor acted as a large water reservoir. The floor between the bubbler pools and the reactor acted as a steam tunnel. Smoldering graphite, fuel and other materials started to burn through the reactor floor, mixing with molten concrete and risking a serious steam explosion. The bubbler pools had to be drained. The valves were located in a flooded corridor, but volunteers in wetsuits and respirators were able to open the valves. The volunteers miraculously survived and one continues to work in the nuclear energy industry today.
The firefighters who initially arrived on scene were not warned about the radioactive smoke or debris; they weren’t even aware it was a reactor fire. The firefighter brigade Lieutenant who was in command of the scene died 13 days later of acute radiation sickness.
Acute radiation sickness is caused by exposure to high amounts of ionizing radiation in a short period of time. It can affect the brain, thyroid gland, lungs, GI tract, bone marrow and blood vessels, as well as the skin. Symptoms can start immediately and last for months. They include nausea, vomiting, headache, fever, skin blistering and burns, seizures, internal bleeding and in severe cases, death. Radiation exposure has also been linked to cancer, fertility issues, birth defects and other serious health issues.
The city of Pripyat was not immediately evacuated. The plant was run by authorities in Moscow and the government of Ukraine didn’t receive up to date information about the disaster. The evacuation finally started at 2pm on April 27th, 18 hours after the explosion. By this point, most people would have been exposed to varying levels of radiation. The evacuation zone at first was 10km radius. Ten days after the explosion, the area was expanded to a 30km radius. Today the exclusion zone around Chernobyl spans 2,600km2. That said, there are about 180 people that live permanently in the exclusion zone; they refuse to leave.
By april 28th, the radiation set off alarms at a nuclear power plant in Sweden, over 1000km away. After Sweden confirmed the radiation originated elsewhere, they contacted the USSR who initially denied a nuclear accident. How very Russian of them…. But when the Swedish government said they were going to file an alert with the International Atomic Energy Agency, the Soviet government admitted that an accident took place at Chernobyl.
The first public announcement of the disaster at Chernobyl was at 9:02pm on April 20th and it said “there has been an accident at the Chernobyl Nuclear Power Plant. One of the nuclear reactors was damaged. The effects of the accident are being remedied. Assistance has been provided for any affected people. An investigative commission has been set up.” That’s it.
Spread of radioactive materials
It’s estimated that about 400 times more radioactive material was released from Chernobyl than by the Hiroshima and Nagasaki bombs combined.
100,000 square kilometers of land were contaminated, impacting Belarus, Ukraine and Russia the most.
Low levels of contamination were detected all over Europe
Because the power plant was located next to the Pripyat River, which drains into the Dnieper reservoir system, drinking water was impacted in Kiev and part of Ukraine for several years after the disaster.
A 30m deep underground barrier was built to prevent contaminated groundwater from entering the Pripyat river
Around 30 people died from the immediate blast and acute radiation trauma in the seconds to months after the reactor explosion. However, it’s estimated that as high as 60,000 deaths worldwide were related to the radioactive material released following the Chernobyl disaster. But that number is highly contested because it’s extremely hard to prove that radiation exposure caused those deaths.
Design of the initial sarcophagus structure started 24 days after the disaster and construction lasted 206 days.
The sarcophagus is a concrete structure to entomb the reactor and reduce radioactive dust from being released into the atmosphere.
It was built to lock in 200 tons of radioactive lava-like corium (a fuel containing material that is created from a nuclear reactor meltdown), 30 tons of highly contaminated dust, and 16 tons of uranium and plutonium
They initially tried to use robots to clear the roof of the reactor so they could build the sarcophagus, but the robots failed due to high levels of radiation on their electronic controls. So they used humans, wearing heavy protective gear, and who could only spend 40-90 seconds on the roof tops due to the high doses of radiation. The soldiers were only supposed to go on the roof once, but some reported doing it 5 or 6 times. In the end it took 5000 men to remove the roof on the debris.
To complete the sarcophagus – which was the largest civil engineering task in history – it took over 250,000 construction workers who all reached their official lifetime limits of radiation
After it was built, decontamination liquidators washed buildings with Bourda, a sticky polymerizing fluid DeconGel, designed to entrain radioactive dust. Once it was dry, it could be peeled off and compacted like carpet rolls for burial.
First they built a cooling slab under the reactor to prevent hot nuclear fuel from burning a hole in the base. Then they built the structure, including concrete protective walls around the perimeter and between reactor 3 and 4, a cover over the turbine hall and reactor and installation of a ventilation system c/w filtration to prevent radioactive material from escaping
The original sarcophagus would only last 20-30 years before requiring restorative maintenance work
The roof beams were at risk of collapse
Rain-induced corrosion of support beams threatened the integrity of the structure
Water was leaking through holes in the roof, becoming radioactively contaminated and then seeping through the reactor floor, into the soil
New safe confinement
So in the 2010s a new safe containment was built to enclose the original sarcophagus. It was moved into place in Nov 2016, and completed at the end of 2018
The new safe containment was designed to prevent release of radioactive contaminants, protect the reactor from external influence, facilitate disassembly and decommissioning of the reactor, and prevent water intrusion
It’s intended to last for 100 years and cost 1.5 billion euro to build
The containment is an arch shaped steel structure 150m long with 13 arches and an internal height is 92.5 m.
The structure is made from tubular steel members with three-layer sandwich panel cladding, and internal polycarbonate panels cover each arch to prevent radioactive particles from accumulating on frame members
Warm, dry air is circulated between the inner and outer roof sections to prevent condensation, reducing corrosion and preventing water from dripping inside
The new safe containment was constructed 180 metres west of reactor 4 and slid into place; pushed on Teflon pads by hydraulic pistons and guided by lasers, moved into place over two weeks
Radioactive dust is monitored by hundreds of sensors, workers carry two dosimeters which measure radiation, they have a daily and annual radiation exposure limit and their dosimeter bees if the limit is reached and they have to leave site, the annual limit may be reached in 12 minutes on the roof of the 1986 sarcophagus and a few hours if they are working around the chimney
So there you have it, the Chernobyl Nuclear Power Plant Disaster. A flawed control rod design, but mostly management and operational issues and deficiencies were to blame for the worst nuclear disaster in history. What was supposed to be a simple safety test, turned into a major event that impacted the future of nuclear power generation for decades. Even today there is a lot of hesitation from the general public to support any kind of nuclear power. I’m not going to sit here and say that it’s completely safe. There are obviously some risks. But if those risks are appropriately managed, nuclear power is one of the cleanest power generation options we have. On top of the Three Mile Island accident in 1979, the 1986 Chernobyl explosion made many people skeptical of nuclear power. Even today, after many studies finding that nuclear power plants are the safest way to make reliable electricity. Looking back at Chernobyl, some scientists believe that between five and ten times too many people were moved away from the Chernobyl area between1986 and 1990. And it was the government’s overreaction that resulted in public fear of nuclear power. When the Fukushima Daiichi accident occurred in 2011 from the tsunami, the scientists found it difficult to justify moving anyone away from the reactor based on the grounds of radiological protection. But I think the overreaction instinct comes from the fact that the same fuels used in nuclear power plants were used to make very destructive bombs. And it’s really hard to separate those two things. Not to mention the fact that the ins and outs of nuclear power are not well known and often misunderstood. Nobody died from radiation exposure at Three Mile Island or Fukushima. In fact, many thousands more people die from air pollution caused by coal burning than radiation. I guess it’s that old saying of better the devil you know than the devil you don’t.
Check out the podcast page, link in show notes, for photos and sources from this week’s episode; or visit failurology.ca. On there you will find an image of the Chernobyl nuclear power plant, the new safe containment, and an RBMK process flow design, which I think helps make sense of the reactor process.
If you’re enjoying what you’re hearing, please rate, review and subscribe to failurology, so more people can find it. And if you want to chat with me, or send me questions for the Q&A episode, my twitter handle is @failurology or you can email me at email@example.com.
Thanks everyone for listening. And tune in next week to hear about the Sampoong Department Store Collapse; the worst peacetime disaster in South Korean history. But more on that next week. Bye everyone, talk soon!