Chernobyl Disaster

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Chernobyl Disaster
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Chernobyl

• Built in 1978-1979
• Chernobyl

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Ukraine
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Background

• Reacktor Bolshoy Moshehnosty Kipyashiy
• RBMK, a Russian acronym translated roughly means
  reactor cooled by water and moderated by
  graphite

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RBMK Reactor
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Control Rods

• The Control Rods long metal rods which, in the
  Chernobyl reactors, contain boron they help
  control the chain reaction

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Reactor Plant Scenario

1. As the reaction occurs, the uranium fuel becomes
   hot
2. The water pumped through the core in pressure
   tubes removes the heat from the fuel
3. The water is then boiled into steam
4. The steam turns the turbines
5. The water is then cooled
6. Then the process repeats

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April 25, 1986

• 0100 The preparation for the test
• 1347 Lowering of the reactor power halted at
  1,600 MWt
• 1400 The ECCS was isolated
• 2310 The power reduction resumed

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April 26, 1986

• 2400 Operation shift change
• 2428 Power level is now 500 MWt and kept
  decreasing to 30 MWt
• 2440 The operator withdrew some control rods
• 0100 Power had risen to 200MWt
• 0103 Connecting the fourth main cooling pump to
  the left loop of the system 200MWt
• 0107 Connecting the fourth main cooling pump to
  the right of the loop system - this was a
  violation of NOP

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April 26, 1986 (cont.)

• 0119 Increased feed water flow to the steam
  drums and removed more control rods -violation of
  NOP
• 0123 The test was started
• 012310 Automatic rods withdrawn from the core
• 012321 Two groups of automated control rods
  were back to the core

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April 26, 1986 (cont.)

• 012330 Power kept increasing
• 012340 Emergency button pushed
• 012344 Power is at 300000 MWth
• 012348 1st thermal explosion
• 012355 2nd explosion

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Power Chart
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Direct Casualties

• 31 employees and firefighters died
• 134 emergency workers suffered from acute
  radiation sickness

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Indirect Casualties

• By the year 2000 there were 1800 case of thyroid
  cancer in children and adolescent
• High number of suicide and violent death among
  Firemen, policemen, and other recovery workers

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Radiation
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Radioactive Fallout
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System Analysis

• Use of graphite as a moderator
• Lack of a well-built containment structure
• Inadequate instrumentation and alarms for an
  emergency situation
• There were no physical controls that prevented
  the operators from operating the reactor in its
  unstable state

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Human Errors

• Isolation of the emergency core cooling system
• Unsafe amount of control rods withdrawn
• Connection of the four main cooling pumps to the
  right and left of the system

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Human Errors (cont.)

• Mental model
• The operator did not have a good mental model of
  the system itself
• Overconfidence
• By having an electrical engineer on site for an
  electrical test
• No confirmation of cues obtained from the system
• Beta too high
• Many missed signals before the accident

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Accident Prevention
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Recommendations

• Have proper Standard Operating Procedures (SOP)
  for both normal and emergency situations
• Have scheduled trainings and practices for normal
  and emergency situations

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Recommendations (cont.)

• Always have a reactor expert on site
• Have operators confirm any cues from the system
  before making hypothesis or take action
• Have a team work kind of environment such that
  every body is involved

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