Chornobyl: A case study

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Chornobyl A case study
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The Site
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Reactor characteristics

• RBMK-1000 BWE
• 1000 MWe
• Enriched 2-3 UO2 Zr clad
• 11.8 m x 7 m
• 211 control rods (min required in for safe
  operation is 30)
• Steam feeds two turbines
• Side mounted ion chambers to monitor power level
• Located at the same height around the core

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Schematic
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• Bearing steel
• Water pipes
• Lower steel structure
• Side Biological shield
• Graphite moderated
• Drum separator
• water./ steam piping
• Steel structure
• Crane
• Central floor
• Side floor
• Controls the tightness
• of the fuel jacket
• 13. Main circulation pump
• 14. Induction manifold
• 15. Pressure manifold

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Design Flaws

• Positive Void coefficient
• Graphite on the leading edge of the control rod
  has less moderation than the water it displaces
  (over moderation reduces the number of neutrons
  available to fission)
• Reactor protection system that was too slow
• 18-21 secs to drive the rods in
• Elena the reactor vault top (1000 ton
  biological shield) was not protected from over
  pressurization
• No secondary containment

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Accidents used for Design Basis

• LOCA in a cold reactor
• Reactor-Core steam collapse
• One or more controls rods stick
• 300mm pipe break in the primary coolant loop
  blow out panels 2.1 rem infant thyroid dose at
  the 3 km (sanitary boundary)

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Beyond design basis

• Fuel damage was not addressed in the design
• Loss of containment

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The experiment

• Experiment to see how the plant would respond to
  a turbine/ generator trip concurrent with a loss
  of offsite power
• Objective of the test was to determine whether
  the residual energy in the turbines would be
  sufficient to supply electricity to essential
  plant equipment and maintain adequate coolant
  flow through the core until the diesel emergency
  power supply came on-line

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Chronology (military time)

• April 25 1986
• 106 unit 4 was placed into maintenance status
• April 26 1986
• 0028 Initaition of Power Level drop to 700MWt
• 100 power level stabilized then power level
  collapses to 30 MWt due to Xe-135
• 123 control rods withdrawn to compensate and
  power level stabilized at 200 MWt
• The reactor has less than 30 control rod in
• 12304 closed the turbine valves
• 12340 operator drives control rods in to shut
  the reactor down

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Between 12340 and 12345

• Reactor power level increase
• by a factor 36 in 5 sec
• Localized Fuel Failure
• Prompt critical/ Steam generation/ Explosion/
  Hydrogen/Zr-water reaction
• Witness say there were 2 explosions
• Elena - 1,000 ton biological shield blown off
• Caused the control rods to jam at the halfway
  point
• Building structure destroyed, 30-40 tons of TNT,
• Plume,
• 35 separate fires in and around unit 4

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Chronology

• 130 the firefighters are called out and
  reinforcements from Pripyat and Chornoblyl are on
  the way
• Morning of 0500 graphite is now on fire
• Drop Dolomite boron carbide(40 T) sand and
  clay(1800 T) and lead (2,400 T) to smother fire,
  filter plume and control criticality
• April 27 1400 Pripyat was evacuated
• Eventually all towns within 30 km were evacuated

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Chronology

• May 9, 1986 graphite fire was extinguished
• May 9, 1986 work began on a massive reinforced
  concrete slab with heat exchanger to be built
  under unit 4.
• Mid May plans for the sarcophagus are made
• Sarcophagus Completed in November 1986

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Unit 4 - after
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The Plume ( 2 types)

• Immediate release resulting from explosion
• Narrow band 80 km long 1-5 km in width
• Weak surface wind indeterminate direction
• 1500m 8-10m/s wind
• Fuel plume
• Accounted for only 10 of the fuel released
• Burn-up characteristics
• ratios of isotopes determine where in the core
  the explosion started

• The release as a result of the fire
• Detectable amounts in Eastern Europe, Scandinavia
  and North America
• 1800-2300K
• Convective plume
• 1 km on April 26
• 600 m

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Estimated Release (Fuel)
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Other Radionuclides (not all)
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Total release
May 1986 81,000,000 Ci Decay
corrected May 1996 1,600,000 Ci
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Environmental pathways

• Contamination
• Soil
• Vegetation
• Aquifers
• Long term monitoring
• Clean-up

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The Shelter (Sarcophagus)
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Damage to the fuel channels
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Elephants foot 2 metric tons of melted fuel
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Also the elephants foot
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Problems with the SHELTER

• Shelter built on remaining structure of Unit 4
• Unit 3 shares a wall with Unit 4
• Shelter has cracks/holes 1000m2
• The steps contain damaged structural elements/
  and containers of high level radioactive waste
• Exposure rate after the Shelter built 5-10 mGy/h
  (0.5 to 1 R/hr)

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The Liquidators
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Human Price

• Acute Radiation Sickness (ARS)
• Limited to individuals on site and first
  responders (fire fighters)
• 31 early fatalities - occurred within months
• Approximately 200 were hospitalized
• Dose reconstruction - technically limited
  dosimetry at the site - used clinical symptoms -
  lymphocyte counts -chromosome aberrations

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ARS Dose Estimates (1Gy 100 rads)
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Category Characteristics

• Normal ranges
• Lymphocytes 1,000 to 3,000 per ml
• Platelet count 150,000 to 400,000 per ml
• IV-rapid onset of symptoms
• 100 per ml lymphocytes
• 500 per ml platelets at nine days
• Complicated by thermal burns
• Mucous membranes were compromised making
  breathing and swallowing difficult
• Treated with reverse isolation antibioticsIV
  feeding, platelet transfusions below 1000 per ml

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Categories

• III-rapid onset of symptoms
• 200 per ml lymphocytes
• Mucous membranes were compromised making
  breathing and swallowing difficult
• Skin burns from beta radiation
• Treated with reverse isolation antibioticsIV
  feeding, platelet transfusions below 1000 per ml
• 13 patients in IV and II had bone marrow
  transplants - 11 subsequently died

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Categories

• II- 1 to 2 hours for symptoms
• 500 per ml lymphocytes
• Significant platelet drop low at 21 days
• Some beta burns
• I- slower onset of symptoms
• 100 survived 2 had significant thermal burns

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Stochastic effects

• Thyroid cancer in children
• From milk containing I-131
• Normal incident is 0.5 in 1 million
• 96.4 in million in Gomel region latency period of
  4-5 years
• Leukemia
• Normal incident is 25,000 in 7.1 million (0.003
  in 1 million )
• Expected increase based on modeling is 470 (6.6
  x10-5 in 1 million)
• An increase even among the liquidators has not
  been identified

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The Human Price

• Tetrogenisis
• Threshold is 0.1 Gy
• Severe mental retardation
• Cancer
• Antidotal evidence abounds
• However the spatial and temporal association for
  a particular disease or abnormality does not
  correlate with exposure
• The data is inconclusive
• Dramatic increased in the number of abortions in
  the Soviet Union,Central and Eastern Europe.

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Epidemiology studies

• Need to have an unbiased data set
• Need to have a baseline group (controls)
• Confounded by poor health
• Poverty
• Smoking
• Alcohol
• Lack prenatal care
• Stress
• Pollution

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Things you can analyze

• Reactor Period
• Xenon poisoning
• Reactivity of the control rods
• Dose Est. -if you had site specific data
• Target organs
• How much shielding for the Shelter

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References

• The Chornobyl Accident A Comprehensive Risk
  Assessment, Battelle Press,Columbus OH. 2000
• Introduction to Nuclear Power, Taylor and
  Francis, New York, N. 2000
• Behind the Nuclear Curtain Radioactive Waste
  Management in the Former Soviet Union, Battelle
  Press,Columbus OH. 1997
• http//www.infoukes.com/history/chornobyl
• http//insp.pnl.gov