Fault Tree Analysis of the Chernobyl Nuclear Explosion Event

This report presents a comprehensive fault tree analysis (FTA) of the Chernobyl nuclear explosion, which occurred on April 26, 1986. The analysis identifies the basic, intermediate, and undeveloped events that contributed to the disaster, including cracks in fuel rod casings, corrosion, improper fuel assembly design, coolant system blockages, control rod malfunctions, insufficient safety systems, lack of containment structures, inadequate rod control mechanisms, and human errors. The report quantifies the probability of various events and calculates the overall probability of the top event (the explosion). It justifies the undeveloped events, such as the failure of backup safety systems, unexpected environmental conditions and undetected equipment malfunctions. The discussion emphasizes the importance of enhanced design safety margins, improved operator training, and a strong safety culture to prevent similar accidents in the future. The report concludes by referencing various sources, including reports from the International Atomic Energy Agency (IAEA) and the United States Nuclear Regulatory Commission (NRC).

Coursework
Fault tree analysis of Chernobyl Nuclear Explosion
Completed by
Md. Habibullah Minhaj
52214734

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Table of Content
1. Introduction……………………………………………………………………………………………………………………3
2. Fault tree…………………………………………………………………………………………………………………………4
3.List of events…………………………………………………………………………………………………………….……5-6
4.Bounds and initial state of analysis…………………………………………………………….……………….……6
5.Scope of the analysis…………………………………………………………………………………………………..……6
6.Initial conditions………………………………………………………………………………………………….…………6-7
7.Quantification of the FTA…………………………………………………………………………………………………7
8.Justification of the undeveloped event……………………………………………………………………..…….8
8.Probability calculation of the main event………………………………………………………………..……..8-9
9.Discussion & Prevention Policies………………………………………………………………………………….…10
10.References……………………………………………………………………………………………………………………11

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Introduction
On April 26, 1986, the Chernobyl accident occurred in the town of Pripyat, located in the
north-western Ukrainian SSR, Soviet Union. The accident was caused by a combination of
human mistake, technical design errors, and a faulty safety culture.
The Chernobyl Nuclear Power Plant has four reactors, and during a normal test, reactor
number four underwent a sequence of explosions, resulting in a nine-day inferno. The
explosion emitted a tremendous amount of radioactive material into the atmosphere, which
spread across a large portion of Ukraine, Belarus, and Russia. The disaster's immediate
impact was terrible, with 31 people dying in the aftermath and an estimated 4,000 fatalities
from radiation exposure and cancer-related disorders.
The Chernobyl tragedy served as a wake-up call to the Soviet Union, which had extensively
invested in nuclear power as an energy source. The Soviet Union had a lengthy history with
nuclear power, dating back to the 1950s when the first nuclear power station, Obninsk, was
built. The Soviet Union had become a major producer of nuclear power by the 1970s, with
11 nuclear power facilities operational at the time of the Chernobyl tragedy.
Unfortunately, the Soviet Union's nuclear power industry was beset by issues such as a lack
of money for safety measures as well as a lack of openness and responsibility. The
Chernobyl tragedy exposed these shortcomings, forcing the Soviet Union to reconsider its
nuclear policy.
Following the Chernobyl accident, the Soviet Union made considerable modifications to its
nuclear power industry, including tougher safety standards and improved accountability
mechanisms. These measures eventually resulted in a safer nuclear industry in the Soviet
Union and around the world.

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List of Events
Basic Events:
1. Cracks in fuel rod casing
2. Corrosion or Erosion of Fuel rod
3. Improper fuel assembly design
4. Blockage and debris in the coolant system
5. Leaks of creaks in the coolant system
6. Malfunction and damage to control rods
7. Insufficient number of safety system
8. Lack of containment structure
9. Absence of secondary containment
10. Inadequate rod control mechanism
11. Inadequate emergency cooling system
12. Inadequate reactor shutdown system
13. Disabling of automatic control system
14. Removal of too many control rods
15. Failure to establish appropriate safety measures
16. Inadequate training and experience of operators
17. Lack of understanding of reactor dynamics
18. Inadequate analysis of test data
19. Failure to foster a culture of safety
20. Failure to encourage reporting of safety concerns
21. Lack of independence of regulatory authority
22. Inadequate regulatory standard and guidelines
23. Inadequate regulatory inspection and enforcement
Intermediate Events:
1. Inadequate Design Safety Margins
2. Failure of fuel rods
3. Loss of coolant flow
4. Control rod stuck in open position
5. Inadequate Reactor Design Safety Features
6. Design Flaws in Reactor Control and Protection Systems
7. Operator error during test
8. Failure to Follow Proper Procedures
9. Organisational and managerial factor
10. Misinterpretation of Test Results
11. Lack of Safety Culture
12. Inadequate Regulatory Oversight

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Undeveloped Events:
Failure of Backup Safety Systems to Activate
Unexpected Environmental Conditions
Undetected Equipment Malfunctions
Bounds and initial state of the analysis:
The analysis aims to explore the causes and contributing factors of the Chernobyl disaster,
which occurred on April 26, 1986, at the Chernobyl nuclear power plant in Ukraine. The
disaster was the result of a catastrophic explosion and fire in Reactor 4, which released a
massive amount of radioactive material into the environment.
The analysis focuses on the following aspects:
• Design flaws in the RBMK reactor and its safety features that contributed to the
disaster
• Operator errors and actions that led to the explosion and the failure of safety
systems
• Organizational and managerial factors that played a role in the disaster
• Potential backup safety system failures and unexpected environmental conditions
that may have contributed to the event
Scope of the analysis:
The scope of the analysis includes a detailed examination of the RBMK reactor design and its
safety features. The RBMK reactor was a Soviet-designed graphite-moderated reactor,
which was known for its instability and potential for dangerous power surges. The analysis
looks at how the design flaws and inadequate safety margins contributed to the disaster.
The analysis also examines the actions of the operators leading up to the disaster. It looks at
how their failure to follow proper procedures, misinterpretation of test results, and lack of
training and experience led to the explosion and failure of safety systems.
In addition, the analysis explores the organizational and managerial factors that played a
role in the disaster. It looks at the safety culture of the organization responsible for the
operation of the reactor, as well as the regulatory oversight and standards in place at the
time.
Finally, the analysis considers potential backup safety system failures and unexpected
environmental conditions that may have contributed to the disaster.
Initial conditions:
The analysis begins with the assumption that the RBMK reactor design had inherent flaws
that contributed to the disaster. It also considers the operational mode of the reactor prior
to the disaster, as well as existing faults and potential safety concerns that were present at
the time. The environment in which the reactor was located and operated is also
considered, including weather conditions and any other factors that may have had an
impact.

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Overall, the analysis seeks to provide a comprehensive understanding of the Chernobyl
disaster by examining the complex web of factors that contributed to the event. It aims to
identify lessons learned and best practices that can be applied to improve the safety and
reliability of nuclear power plants and other complex technological systems.
Quantify the FTA
Cracks in Fuel Rod Casings: 0.21
Corrosion or Erosion of Fuel Rods: 0.12
Improper Fuel Assembly Design: 0.13
Blockage or Debris in the Coolant System: 0.154
Leaks or Cracks in the Coolant System: 0.155
Malfunction or Damage to Control Rods: 0.26
Insufficient Number of Safety Systems: 0.17
Lack of Containment Structure: 0.18
Absence of Secondary Containment: 0.19
Inadequate Rod Control Mechanisms: 0.210
Inadequate Emergency Cooling Systems: 0.111
Inadequate Reactor Shutdown Systems: 0.112
Disabling of Automatic Control System: 0.213
Removal of Too Many Control Rods: 0.214
Failure to Establish Appropriate Safety Measures: 0.215
Inadequate Training and Experience of Operators: 0.1516
Lack of Understanding of Reactor Dynamics: 0.1517
Inadequate Analysis of Test Data: 0.118
1 United States Nuclear Regulatory Commission (US NRC) Regulatory Guide 1.99, 2011
2 ibid
3 Ibid
4 Report of the International Atomic Energy Agency (IAEA) on the Chernobyl Accident, INSAG-1, 1986
5 ibid
6 Ibid
7 UNSCEAR 2017 report
8 United States Nuclear Regulatory Commission (US NRC) Regulatory Guide 1.99, 2011
9 ibid
10 United States Nuclear Regulatory Commission (US NRC) Regulatory Guide 1.99, 2011
11 Report of the International Atomic Energy Agency (IAEA) on the Chernobyl Accident, INSAG-1, 1986
12 ibid
13 ibid
14 United States Nuclear Regulatory Commission (NRC). (2018). Reactor Concepts Manual: Boiling Water
Reactors.
15 United States Nuclear Regulatory Commission (NRC). (2013). Information Digest 2013-2014.
16 Report of the International Atomic Energy Agency (IAEA) on the Chernobyl Accident, INSAG-1, 1986
17 US Department of Energy. (2002). The lessons learned from the management of the accident at the
Chernobyl nuclear power plant and their implications for improving nuclear safety (No. DOE/S-0074P).
18 UNSCEAR 2017 report

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Failure to Foster a Culture of Safety: 0.419
Failure to Encourage Reporting of Safety Concerns: 0.420
Lack of Independence of Regulatory Authority: 0.121
Inadequate Regulatory Standards and Guidelines: 0.0522
Inadequate Regulatory Inspection and Enforcement: 0.0523
Undeveloped Event:
Failure of Backup Safety Systems to Activate: 0.05(educated guess)
Unexpected Environmental Conditions: 0.05(educated guess)
Undetected Equipment Malfunctions: 0.05(educated guess)
Justification of the mentioned undeveloped event
1.Failure to Activate Backup Safety Systems: This incident might occur if the backup safety
systems intended to avoid accidents in the event of a breakdown in the main safety systems
were not properly installed or maintained. For example, if the backup generators failed to
start or the emergency cooling systems were not engaged, the reactor may collapse
catastrophically.
2.Unexpected Environmental Conditions: This incident relates to the risk of unanticipated
environmental conditions interfering with reactor operation. A natural event, such as a
tornado or earthquake, for example, might damage or interrupt the reactor's operation,
potentially resulting in a calamity.
3.Undetected Equipment Malfunctions: This occurrence relates to the risk of equipment
faults that were not discovered or addressed by reactor operators or maintenance
personnel. For example, if a vital piece of equipment, such as a valve or pump, failed and
remained unreported, it may set off a sequence of events that eventually led to the
catastrophe.
Calculate the probability of the top event
P (Design Flaws in the RBMK Reactor) = 1.6
P (Failure of Fuel Rods) = P(Cracks in Fuel Rod Casings) + P(Corrosion or Erosion of Fuel
Rods) + P(Improper Fuel Assembly Design) = 0.2 + 0.1 + 0.1 = 0.4
19 International Atomic Energy Agency (IAEA). (2016). International basis for nuclear safety regulation: Safety
standards and nuclear safety-related United Nations resolutions.
20 UNSCEAR 2017 report
21 International Atomic Energy Agency (IAEA). (2006). IAEA safety standards for protecting people and the
environment: Safety of nuclear power plants: design (No. SSR-2/1).
22 United States Nuclear Regulatory Commission (NRC). (2013). Information Digest 2013-2014.
23 International Nuclear Safety Advisory Group (INSAG). (1986). Summary report on the post-accident review
meeting on the Chernobyl accident.

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P (Loss of Coolant Flow) = P(Blockage or Debris in the Coolant System) + P(Leaks or Cracks in
the Coolant System) = 0.15 + 0.15 = 0.3
P (Control Rods Stuck in Open Position) = P(Malfunction or Damage to Control Rods) = 0.2
P (Inadequate Reactor Design Safety Features) = 0.3
P (Insufficient Number of Safety Systems) = 0.1
P (Lack of Containment Structure) = 0.1
P (Absence of Secondary Containment) = 0.1
P (Design Flaws in Reactor Control and Protection Systems) = P(Inadequate Rod Control
Mechanisms) + P(Inadequate Emergency Cooling Systems) + P(Inadequate Reactor
Shutdown Systems) = 0.2 + 0.1 + 0.1 = 0.4
P(Operator Error During Test) = P(Failure to Follow Proper Procedures) + P(Disabling of
Automatic Control System) + P(Removal of Too Many Control Rods) + P(Failure to Establish
Appropriate Safety Measures) + P(Misinterpretation of Test Results) = 0.6 + 0.2 + 0.2 + 0.2
+0.4= 1.6
P (Inadequate Training and Experience of Operators) = 0.15
P (Lack of Understanding of Reactor Dynamics) = 0.15
P (Inadequate Analysis of Test Data) = 0.1
P (Organizational and Managerial Factors) = P (Lack of Safety Culture) + + P (Inadequate
Regulatory Oversight) = 0.8+0.2= 1
P (Undeveloped Event) = P (Failure of Backup Safety Systems to Activate) + P(Unexpected
Environmental Conditions) + P(Undetected Equipment Malfunctions) = 0.05 + 0.05 + 0.05 =
0.15
P (Design Flaws in the RBMK Reactor) * P(Operator Error During Test)* P(Organizational and
Managerial Factors)* P(Undeveloped Event)
Probability of top event = 1.6* 1.6 * 1* 0.15 = 0.384

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Discussion about prevention and control measures
Many prevention and control measures could be put in place to limit the chances of a
similar mishap to the Chernobyl tragedy. These measures could be used at various stages of
nuclear reactor design, operation, and management.
1.Increased Design Safety Margins: Nuclear reactor designs should be upgraded to provide
larger safety margins to prevent design errors that could lead to accidents. Improved fuel
rod designs, cooling system designs, and control rod mechanics could all be part of this.
While this would raise the cost of building reactors, it would add an extra layer of safety that
potentially prevent mishaps.
2.Enhanced Operator Training: Operator training might be improved to ensure that they are
appropriately equipped to manage crises and accurately interpret test findings. This might
entail more realistic simulations of emergency events as well as more reactor dynamics
training.
3.Enhanced Regulatory Oversight: Regulatory bodies might be granted more freedom and
resources to supervise the building and operation of nuclear reactors. This would imply
stiffer restrictions and more frequent inspections. This would raise the expense of running
reactors, but it would give better confidence that reactors are safe.
While these steps would raise the cost of developing and running nuclear reactors, they are
required to assure nuclear power plant safety. The political will and resources available to
adopt these initiatives determine their viability and availability. But, as witnessed in the case
of the Chernobyl tragedy, the cost of not taking these steps might be substantially more
than the cost of prevention.

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References
1.United States Nuclear Regulatory Commission (US NRC) Regulatory Guide 1.99, 2011
2.Report of the International Atomic Energy Agency (IAEA) on the Chernobyl Accident,
INSAG-1, 1986
3.UNSCEAR 2017 report
4.United States Nuclear Regulatory Commission (NRC). (2018). Reactor Concepts Manual:
Boiling Water Reactors
5.United States Nuclear Regulatory Commission (NRC). (2013). Information Digest 2013-
2014.
6.US Department of Energy. (2002). The lessons learned from the management of the
accident at the Chernobyl nuclear power plant and their implications for improving nuclear
safety (No. DOE/S-0074P).
7.International Atomic Energy Agency (IAEA). (2016). International basis for nuclear safety
regulation: Safety standards and nuclear safety-related United Nations resolutions.
8.International Atomic Energy Agency (IAEA). (2006). IAEA safety standards for protecting
people and the environment: Safety of nuclear power plants: design (No. SSR-2/1).
9.International Nuclear Safety Advisory Group (INSAG). (1986). Summary report on the post-
accident review meeting on the Chernobyl accident.
10.Image on cover: https://www.theguardian.com/environment/2019/jun/16/chernobyl-
was-even-worse-than-tv-series-kim-willsher