7.9: Chernobyl Nuclear Disaster
Chernobyl (Pripyat, Ukraine, April 26, 1986)
Another major nuclear accident involving a reactor occurred in April 1986, at the Chernobyl Nuclear Power Plant in Ukraine, which was still a part of the former Soviet Union. While operating at low power during an unauthorized experiment with some of its safety devices shut off, one of the reactors at the plant became unstable. Its chain reaction became uncontrollable and increased to a level far beyond what the reactor was designed for. The steam pressure in the reactor rose to between 100 and 500 times the full power pressure and ruptured the reactor. Because the reactor was not enclosed in a containment building, a large amount of radioactive material spewed out, and additional fission products were released, as the graphite (carbon) moderator of the core ignited and burned.
Watch the video above (8:42 in length) and answer the questions below:
- How many working reactors were at the Chernobyl Power Plant site? How many more were under construction?
- Which unit experienced a full meltdown?
- What type of fuel and moderator did Chernobyl use?
- Describe the positive void coefficient? Are Western reactors designed to experience positive void coefficients?
- Compare the basic core size of Fukushima to Chernobyl.
- Compare the basic containment layers of Fukushima to Chernobyl.
- What is the purpose of leaving the top of an RBMK reactor open? Do Western reactors have this design?
- What events led to the explosion of Chernobyl? Was this a chemical or a nuclear explosion?
- Name the two countries that were most affected by the Chernobyl meltdown.
- Did the people who cleaned up Chernobyl have special training in radiation? What is the nickname of these individuals?
- How fast did the USSR construct the first sarcophagus? Why did it need to be replaced?
- Look below to the section of Today's Chernobyl. The EU constructed this massive structure. How much did this cost and how long did it take to build?
- Does Russia still use RBMKs?
- Describe the ghost town of Pripyat today. Just before the accident, an amusement park was constructed to celebrate May Day. Did the people of Pripyat get to enjoy the Ferris Wheel? Would you have gotten on that Ferris Wheel during that time (assuming you were on this planet)?
Basics of the Disaster
On April 26, 1986, a test was scheduled at the Chernobyl Nuclear Power Plant to evaluate residual cooling capacity in the event of a power grid failure. At the time of the accident, the Chernobyl facility used four RBMK (Reaktor Bolshoy Moshchnosty Kanalny or high power channel) reactors to produce a total of 4000 MW of energy. Two more reactors were being constructed to produce additional wattage. RBMKs not only produce a large amount of energy but also produce weapons-grade Pu-239. The excessive power, design, and materials of these reactors have discouraged many countries (the United States) from building them.
Video \(\PageIndex{2}\): Video from the 2019 HBO miniseries " Chernobyl."
Components of the Chernobyl RMBK reactors.
Unlike light water reactors, RBMK reactors use graphite moderators to slow the neutrons of the fission process. This type of material can combust easily. The less enriched U-235 fuel of this reactor produces Pu-239 to be produced. Unfortunately, the Chernobyl reactor technicians felt that removing the new Pu-239 fuel would be too difficult with a full containment covering of unit 4. The new Pu-239 material would be extracted while the reactor continued to produce energy. The high power wattage, combustible moderator, and open containment convinced many other countries to steer clear of building RBMKs.When comparing the differences of RBMK reactors to those typically used in most other countries, view the table below:
| Type of Reactor | Containment | Fuel | Coolant | Moderator |
|---|---|---|---|---|
| PWR | repetitive layers of lead/ concrete and top is closed inside the reactor building | slightly enriched U-235 (2-4%) | LW = light water | LW = light water |
| BWR | repetitive layers of lead/ concrete and top is closed inside the reactor core building |
slightly enriched U-235 (2-4%l |
LW = light water | LW = light water |
| RBMK | layers of lead/concrete and top is open inside the reactor core building to allow extraction of rods while the reactor is producing energy |
low enriched U-235 (<2%) |
LW = light water | Graphite (carbon) |
Before the accident
Nuclear reactors require active cooling in order to remove the heat generated by radioactive decay. Even when not generating power, reactors still generate some heat, which must be removed in order to prevent damage to the reactor core. Cooling is usually accomplished through fluid flow, water in Chernobyl's case.
The problem at the Chernobyl plant was that following an emergency shutdown of all power, diesel generators were needed to run the cooling pumps. These generators took about a minute to attain full speed, which was deemed an unacceptably long time for the reactor to be without cooling. It was suggested that the rotational momentum of the winding down steam turbine be used to power the pumps in the time between shutdown and the generators being ready. A test was devised to test this method in 1982, but the turbine did not prove to be successful in providing the required voltage as it spooled down. Two more tests would be conducted in the following years, but would also be unsuccessful. The fourth test was scheduled to be run on April 25, 1986.
The experiment was devised in such a way that if it had gone as planned, the disruption and danger to the plant would be very minimal. First, the reactors would be brought down to low power, between 700 and 800 megawatts. Then the steam turbine would be run up to full speed and then turned off. The power generated by the winding down generators would then be measured to determine if it was sufficient to power the cooling pumps in the time before the diesel generators got up to full speed.
By 1986, the plant had been running for two years without the implementation of a method to keep the cooling pumps running continuously following an emergency shutdown. This was an important safety measure that the plant was lacking, which presumably gave the plant managers a considerable amount of urgency in completing another test.
Preparing for the test
The experiment was scheduled to run during the day shift of 1985, while the night shift would only have to maintain cooling of the radioactive decay in the shut-down plant. However, another power generator nearby unexpectedly shut down, necessitating the need for the Chernobyl plant to delay the test and continue producing power. The experiment would be resumed at 11:04 PM, by which time the day shift had departed and the evening shift was about to leave. This meant that the experiment would be conducted in the middle of two shifts, leaving very little time for the night shift employees to be briefed about the experiment and told what to do.
The power reduction of reactor 4 to 700 MW was accomplished at 00:05 AM on the 26th of April. However, the natural production of a neutrino absorber, Xenon-135, led to a further decrease in power. When the power dropped to about 500 MW, the night shift operator committed an error and inserted the reactor control rods too far. This caused the reactor to go into a near-shutdown state, dropping power output to around 30 MW.
Since this was too low for the test, it was decided to restore power by extracting the control rods. Power would eventually rise and stabilize at around 200 MW.
The operation of the reactor at such a low power level would lead to unstable temperature and flow. Numerous alarms and warnings were recorded regarding emergency measures taken to keep the reactor stable. In the time between 12:35 and 12:45 AM, alarm signals regarding thermal-hydraulic parameters were ignored in order to preserve the reactor's power level.
The test continued, and at 1:05 AM extra water pumps were activated in order to increase the water flow. The increased coolant flow rate led to an increase in the coolant temperature in the core, reducing the safety margin. The extra water flow also led to a decrease in the core's temperature and increased the neutron absorption rate, decreasing the reactor's power output. Operators removed the manual control rods in order to maintain power.
All these actions led to the reactor being in an unstable state that was clearly outside safe operation protocol. Almost all the control rods had been removed, which reduced the effectiveness of inserting safety rods in an emergency shutdown. The water was very close to boiling, which meant that any power increase would cause it to boil. If it started boiling, it would be less effective at absorbing neutrons, further increasing the reactor's power output.
Conducting the test
The experiment was started at 1:23:04 AM. The steam to the turbines was shut off, causing the turbines to start spooling down. Four of the eight cooling pumps were also shut down. The diesel generator was started and began powering the cooling pumps after at 1:23:43. Between this time, the four pumps were powered by the slowing steam turbines. As the turbines slowed down, their power output decreased, slowing the cooling pumps. This leads to the increased formation of steam voids in the core, reducing the ability of the cooling water to absorb neutrons. This increased the power output of the reactor, which caused more water to boil into steam, further increasing the reactor's power. However, during this time the automatic control system was successful in limiting power increase through the insertion of control rods.
Video \(\PageIndex{3}\): Chernobyl nuclear accident why happened? Animated Demonstration
At 1:23:40, a button was pressed that initiated the emergency shutdown of the reactor and the insertion of all control rods. It is believed that this was done as a routine method to shut down the reactor to conclude the experiment and not as an emergency measure.
The process of inserting the control rods was initiated, but it took about 20 seconds for the rods to be completely inserted. A flawed design in the graphite-tip control rod meant that coolant was displaced before the neutron absorbing material could be fully inserted and slow down the reaction. This meant that the process of inserting the control rods actually increased the reaction rate in the lower half of the core.
Video \(\PageIndex{2}\): Trailer video from the 2019 HBO miniseries " Chernobyl."
A massive power spike occurred, causing the core to overheat. Some of the fuel rods fractured, causing the control rods to become stuck before they were fully inserted. Within three seconds the core's power output rose to above 500 MW. According to the simulation, it is estimated that power output then rose to 30 GW, ten times the normal power output. This was caused by the rising power output causing massive steam buildup, which destroyed fuel elements and ruptured their channels.
It is not possible to know precisely what sequence of events led to the destruction of the reactor. It is believed that the steam buildup entered the reactor's inner structure and lifted the 2000 ton upper plate. This steam explosion further ruptured fuel channels, resulting in more coolant turning into steam and leaving the reactor core. This loss of coolant further increased the reactor's power. A nuclear excursion (an increasing nuclear chain reaction) caused a second, even more, powerful explosion.
The explosion destroyed the core and scattered its contents in the surrounding area, igniting the red-hot graphite blocks. Against safety regulations, a flammable material, bitumen, had been used in the roof of the reactor. When this was ignited and scattered into the surrounding area, it started several fires on reactor 3. Those working there were not aware of the damage that had been done and continued running the reactor until it was shut down at 5:00 AM.
This short sequence indicates the reactor floor and steam tanks overlaid over the explosion crater. It is an extract from the full video "Chernobyl - an inside look 3d" (Copyright , Tadpolefarm )
Radiation Levels and Fallout
In the worst-hit parts of the reactor building, radiation levels were high enough to cause fatal doses in a matter of minutes. However, all dosimeters available to the workers did not have the ability to read radiation levels so high and thus read "off-scale." Thus, the crew did not know exactly how much radiation they were being exposed to. It was assumed that radiation levels were much lower than they actually were, leading the reactor crew chief to believe that the reactor was still intact. He and his crew would try to pump water into the reactor for several hours, causing most of them to receive fatal doses of radiation.
Since containment failed at Unit 4 Reactor, particle and wave radiation emerged from the burning power plant. Winds carried the fallout to various places in the European continent. Click on this previous OER page for a review of radioactive fallout.
Concentration of Radiation around Unit 4
Radiation is measured with a variety of units. In order to understand biological damage, click on this previous OER page and read about Sieverts (Sv) and Rem. (rem). Also, refer to your learning management (LMS) for problems involving rem and Sv conversions.
Evacuation of Pripyat
Pripyat, a city nearby the power plant, was not immediately evacuated. At first, the government denied that the reactor had exploded and insisted that it was only a small accident. By April 27, though, investigators were forced to acknowledge that the reactor had exploded and ordered Pripyat to be immediately evacuated. The process of moving over 40,000 people from this area was started around 2 p.m. following the nuclear explosion. Over 1200 buses from Kyiv, transported this city's inhabitants beyond the exclusion zone.
On April 28, Sweden's Forsmark Nuclear Power plant detected the presence of I-131 of their radiation detectors. Late into that night, Moscow news reported that an accident had occurred at the Chernobyl Nuclear Power plant. Within an hour of this announcement, Denmark had also detected isotopes that result only from nuclear fission.
Fatalities and Health Effects
At the time of the accident, around 600 people were employed at the Chernobyl Nuclear Power Plant. When the explosion occurred, two workers immediately died. Two decades of radiological health studies performed by the World Health Organization (WHO) have determined that approximately 134 Chernobyl employees were exposed to high-dose radiation (.8-1.6 Gray). These workers would be diagnosed with having Acute Radiation Sickness (ARS). Of the 134 ARS victims, 28 of these individuals passed within the next three months.
In the 2011 report entitled, "Health effects due to radiation from the Chernobyl accident," the United Nations Scientific Committee on the Effect of Atomic Radiation (UNSCEAR), states that 530,000 of the reactor's recovery workers were exposed to ionizing radiation dosages between 20-500 mSv. This test group was initially studied from April 1986 through 1990. Currently, health monitoring of these individual continues. Studies have determined that workers who were exposed to over 200mSv were more likely to develop leukemia. Radiation Induced cataracts were also noted in workers that were exposed at levels lower than 200 mSv.
Over 115,000 people who lived in the Ukraine and Belarus evacuated after the meltdown of Unit 4. Radiation levels reported for this particular group were around 30 mSv. As for other parts of Europe, radiation exposure from the Chernobyl meltdown was considered to be less than yearly background radiation (1 mSv).
UNSCEAR reports that there is a significant increase in diagnoses of thyroid cancer that can be attributed to I-131. Over 6,000 people (with less than 1% mortality rate) in Belarus and the Ukraine have developed this cancer during the study. Exposure to this radioactive isotope could have been due to direct fallout exposure or indirect means (drinking milk from cows who ingested I-131 or eating foods grown in radioactive soil).
There seems to be little or no affect in regards to fertility impairment with any of the evacuees. As for birth defects, residents of Belarus and the Ukraine have reported more birth defects since the accident occurred. UNSCEAR has stated that this increase could be that individuals are more likely to report any type of birth defect now than any before the meltdown occurred. Due to the nature of the accident and evacuation effort, many citizens of Belarus and Ukraine have reported an increase in mental health conditions.
Temporary Containment
Later the reactor was encapsulated in steel and concrete, a now-decaying structure known as the sarcophagus. The construction of this covering occurred in May of 1986 and was completed by November of that same year. Not long after construction, the protective covering was cracked and released more radiation.
Today's Pripyat
Today, radiation levels are still higher than normal in the areas surrounding the plant but have dropped considerably from the levels that they were at twenty years ago. It is now considered safe to visit the areas immediately surrounding the plant for short periods of time. However, it is estimated that it will take 20,000 years for reactor 4's core to be completely safe.
Today's Chernobyl
Previous issues - Unit 1- meltdown ( immediate shutdown, restarted, and finally shut down in 1996). 1986-full meltdown of Unit 4. 1992- Unit 2, fire in turbine room, shut down due to Unit 4 publicity and Unit 3 ran until 2000.
The cake plate moved over reactor 4 in November 2016.