Chernobyl

By William Drees

The Chernobyl Accident and Lessons Learned

 
Overview

Generally, when people hear the term nuclear disaster, the first event that comes to mind is that of the accident at Chernobyl. This is because Chernobyl is considered by experts to be the worst nuclear accident in history. In the event, between 28-31 people were killed as a result of immediate radiation exposure (Chernobyl Accident 1986, April 2015). The number of people that have since died as a result of long-term radiation poisoning and subsequent cancer is difficult to estimate. In 2006, Cardis, et. al., in a peer-reviewed article, estimated that the accident had caused over 5,000 cases of cancer in Europe alone and expects this number to increase to around 41,000 cases by 2065 (2006).

The Chernobyl accident occurred in Ukraine in the midst of the Cold War in 1986. As a result of the nuclear conflict between the United States and the Soviet Union, a vastly different culture existed in terms of safety, operation, and design of nuclear power plants. Although the United States nuclear energy safety culture is and always has been equivalent or better than other countries, the case of the Soviet Union and the Chernobyl accident revealed the terrible consequences if this safety culture begins to become neglected. As a result, United States citizens and especially those that live near nuclear power plants should push for stricter and more effective regulation and oversight to ensure that a disaster of this magnitude does not occur in the United States.

 
About the Reactor

The accident at Chernobyl occurred when Reactor 4 experienced a critical meltdown. The reactor was based on the RBMK reactor design which was a common design used by the Soviet Union at that time. In fact, a total of 17 reactors of this design were completed in the Soviet Union and Eastern Europe (RBMK Reactors, 2015). The reactor worked by using a pump to flow coolant water over a reactor core where it would turn into steam to drive a turbine and create electricity. As shown in Figure 1 below, the reactor core consisted of fuel rods (red rectangles in Figure 1), graphite moderators (black rectangles in Figure 1) and control rods (red/black long rectangles in Figure 1) which could be raised or lowered to control the power generation rate. According to the operating procedure of the RBMK reactor design, the minimum number equivalent of control rods in the reactor should never drop below 15 (Chernobyl Appendix 1: Sequence of Events, 2015).

 

Figure 1
Figure 1: Simplified RBMK reactor diagram
Source: Chernobyl disaster, 2015. <https://en.wikipedia.org/wiki/Chernobyl_disaster>

 
About the Accident

In the events leading up to the accident, reactor 4 was set to undergo an electrical test to investigate whether, during an electrical outage, the momentum of the steam turbine would be able to generate enough electricity to power the water coolant pumps until emergency generators could reach operational speeds and continue the task (Chernobyl Appendix 1: Sequence of Events, 2015). The night crew on the morning of the accident reduced the power of the reactor in preparation for the test by further inserting the control rods into the reactor core.

The test called for power to be reduced to between 700-1000 megawatts of thermal power (MWT) from the operational 3200 MWT (Engineering Failures – Chernobyl, 10 July 2009). However, it is believed that the inexperienced night crew operators accidentally inserted the control rods too far. This caused a poisoning of the reactor by the build-up Xenon 135. This poisoning caused power to drop to a near shut-down state of 30 MWT and created a highly dangerous situation. This poisoning created a possible positive feedback loop in the reactor core where a small increase in power would cause some of the Xenon poisoning to be burned off, this in turn would cause power to rise even more burning off more Xenon in an uncontrollable loop where power could increase to devastating levels.

In reaction to the severely reduced power level, the operators raised the control rods above the maximum level resulting in the equivalent of just 8 control rods being inserted into the reactor (well below the 15 rod design minimum) (Chernobyl Appendix 1: Sequence of Events, 2015). This raising of the rods increased power to 200 MWT. At this point, the operations team felt it was safe enough to continue with the test even though the power was well below the 700 MWT level that the procedure called for. In continuation with the test, some of the coolant water pumps were shut down. This led to a massive increase in power where, as described above, the usually insignificant reduction in coolant flow rate case a small increase in power which burned off some of the Xenon poisoning leading to the massive increase in power.

In the attempt to try to get the near-runaway reactor under control, the emergency shut down procedure was initiated. This shut down procedure attempted to lower all of the control rods, however, a flaw in the control rod design led to even worse problems. The control rods were designed with graphite displacer tips that, upon entering the reactor core, led to an unintended increase in power before a reduction in power (see Figure 2). As a result, when the emergency shut-down was initiated and the rods were lowered and power was even further increased. This led to an overheating of the reactor core causing a very rapid, extreme build-up of steam pressure that resulted in the reactor bursting and blowing off its top.

When the graphite moderators were exposed to oxygen from the atmosphere, they burned at a very high temperature melting the fuel rods allowing the fuel to combine. This then led to a small nuclear explosion called nuclear fizzle that sent around 6 tons of radioactive material into the surrounding area (see Figure 3) (Chernobyl: Ten Years On, 1 November1995). As shown in Figure 3, the cloud of radioactive material, carried by wind, spread as far as Belgium. The disaster exposed an extremely large number of people to radioactive material and has even left some lands around the reactor uninhabitable today.

 

Figure 2
Figure 2: Flawed control rod design with a graphite displace at the tip of the rod
Source: Safety Series INSAG-7 The Chernobyl Accident: Updating of INSAG-1, 1992. <http://www-pub.iaea.org/MTCD/publications/PDF/Pub913e_web.pdf>

Figure 3

Figure 3: Map of the spread of the radioactive material through Europe
Source: Chernobyl Radioactive Contamination Cloud, n.d. <http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/cherno.html#c1>

 
Soviet Union and U.S. Response

After the disaster, the Soviet Union began a complete overhaul of its nuclear program and safety culture. They implemented a number of design and operation changes on the other operating RBMK reactors in an effort to make them safer and prevent another nuclear disaster. These changes included design changes to the control rod tip design to eliminate the unintended increases in power output, eliminating or minimizing all possible positive feedback loops where a subtle can have a disastrous effect, stricter operator training, and design changes to prevent operation outside of the maximum/minimum constraints (RBMK Reactors, 2015). As a result of all of these changes to increase reactor safety, the other 16 operating RBMK reactors have not experienced any of the issues that plagued Chernobyl reactor number 4 – in fact, 9 are still in operation today (RBMK Reactors, 2015).

The United States response to the disaster was quite different from the Soviet Union’s. The NRC concluded, “the lessons learned from Chernobyl fell short of requiring immediate changes in regulation” (
Background on Chernobyl Nuclear Power Plant Accident, 12 December 2014). As a result, no changes were made to United States nuclear policy at the time. This was due to the Soviet Union’s nuclear program and safety culture being far more lenient that that of the United States. The NRC belied that it had laws, procedures, and guidelines already in place to prevent a nuclear disaster similar to this from occurring and so far in 2015, this has proven true.

 
Moral of the Disaster

The disaster at Chernobyl provides some significant insight for the public who are in fear of the dangers of nuclear power. The disaster showed that when regulation becomes lenient on its enforcement of safety regulation and fails to keep safety as its top priority, disaster can occur like the one at Chernobyl, Ukraine in 1986. As a result, citizens should ensure that this type of volatile situation does not occur in the United States by pushing for stricter safety regulations, more thorough and effective safety inspections, and harsher punishment when these regulations are violated. This will help ensure that nuclear power will be able to provide safe, clean energy for the public.

 
References

Chernobyl Accident 1986. (April 2015). Retrieved from http://www.world-nuclear.org/info/Safety-and-Security/Safety-of-Plants/Chernobyl-Accident/

Cardis, E, et. al. (15 September 2006). Estimates of the cancer burden in Europe from radioactive fallout from the Chernobyl accident. Retrieved from http://journals.ohiolink.edu.proxy.lib.ohio-state.edu/ejc/article.cgi?issn=00207136&issue=v119i0006&article=1224_eotcbirfftca

RBMK Reactors. (2015). Retrieved from http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Power-Reactors/Appendices/RBMK-Reactors/

Chernobyl Appendix 1: Sequence of Events. (2015). Retrieved from http://www.world-nuclear.org/info/Safety-and-Security/Safety-of-Plants/Appendices/Chernobyl-Accident—Appendix-1–
Sequence-of-Events/

Engineering Failures – Chernobyl. (10 July 2009). Retrieved from http://engineeringfailures.org/?p=1

Chernobyl: Ten Years On. (1 November 1995). Retrieved from https://www.oecd-nea.org/rp/chernobyl/chernobyl-1995.pdf

Background on Chernobyl Nuclear Power Plant Accident. (2014, December 12). Retrieved from http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/chernobyl-bg.html

 
Image References

Chernobyl disaster, 2015. <https://en.wikipedia.org/wiki/Chernobyl_disaster>

Safety Series INSAG-7 The Chernobyl Accident: Updating of INSAG-1, 1992. <http://www-pub.iaea.org/MTCD/publications/PDF/Pub913e_web.pdf>

Chernobyl Radioactive Contamination Cloud, n.d. <http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/cherno.html#c1>

3 thoughts on “Chernobyl

  1. but my question is, what are some of the ways we could prevent another diaster happening like Chernoby? (sorry if ths is spelled wrong i cant see my computer text

    • to prevent a disaster as the chernobyl they needed to have a better supervision of all that goes on in the site. What actually happend was that when they performed the experiment on reactor 4 they could have informed all people of the circumstances and they should have had safety regulations. this disaster teaches us to improve the upcoming constructions in the future. thats what i think

    • The RBMK reactor has some significant design flaws and differences to reactors in the United States and other places. Due to using graphite as their moderator, their reactor had a positive temperature coefficient of reactivity. This means that as temperature rises so does the amount of reactivity and fission in the core. This design makes the reactor inherently unstable. Reactors in the United States have a negative temperature coefficient so if the fuel and coolant temperature rise, which could result in fuel damage, it adds negative reactivity to the core and tries to shut down the reaction. Additionally, the poison control rods, designed to absorb the neutrons from fission and shut the reactor down, had graphite tips which added positive reactivity to the core and resulted in the coolant vaporizing and the steam explosion.
      Additionally, the Chernobyl reactor utilized only slightly enriched uranium for their fuel, so the reactor had to be enormous to achieve their goals for the plant. Another issue was the fact that they did not have a reactor containment structure around their reactor core. This would have helped in containing the core when they had issues. Other plants around the world have containment structures around their cores.
      There are so many issues that happen with nuclear disasters that result in learnings at other reactor sites. Although their design is so different than other plants other nuclear plant still take what is learned from their failures and develop new strategies and systems to prevent them.
      This reactor is an incredibly poor design and had many flaws beyond what I wrote here. I would assume that the Soviet Union used this design with these flaws because of the costs of other designs.

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