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Reactor Accident Concerns - I

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Reactor Accident Concerns - I

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    1. Reactor Accident Concerns - I Severe Accidents in Water-Cooled Reactors

    2. International Nuclear Event Scale (INES) After the Chernobyl accident, prompt public awareness of a nuclear event was deemed necessary Similar ideas Richter scale (earthquake) Beaufort scale (wind) Terror security alert

    3. International Nuclear Event Scale (INES) Major accident (7) Large radiation release across countries Possibility of widespread health and environmental effects Serious accident (6) Large radiation release locally Accident with off-site risks (5) Radiation release locally Severe damage to most of reactor core

    4. International Nuclear Event Scale (INES) Accident mainly in installation (4) Small radiation release locally Possible local food control Some damage to reactor core Acute worker health effects Serious incident (3) Small radiation release locally No action needed High radiation exposure on-site to workers Further failure could lead to an accident

    5. International Nuclear Event Scale (INES) Incident (2) Events that do not cause safety concerns but do cause safety reevaluation Anomaly (1) Events that do not pose a risk but indicate the necessity of further safety provisions Below scale (0) No safety significance

    6. Core Damage 1st Barrier = Fuel Matrix + Cladding Loss of Cooling or Power Increase Swelling or Burst Fuel Can Molten Material at 1200-1400C Exothermic Steam/Zirconium Reaction above 1100C Zircaloy Melts at 1700C Molten Material Blocks Cooling Channels and Collects at Vessel Bottom

    7. Reactor Pressure Vessel 2nd Containment Barrier Ferritic Steel Structure 4-8 in thick, weighing >300 tons Failure of Vessel Overpressurization Damage to Support Structure Creep Failure from Overheating Shock from Steam or Hydrogen Explosions

    8. Reactor Core & Vessel

    9. Reactor Containment 3rd Barrier = Containment Building Steel & Reinforced Concrete ~4-foot thick Subatmospheric Pressure Failure Hydrogen Combustion (Heat & Pressure) Gradual Overpressurization Basemat Melt-through

    10. Multiple Layers of Protection

    11. Sandias Rocket Sled Track 10,000-ft Track for High Speed Testing 2,000-ft Railroad Track for Very Large Items F-4 Plane Crash into Simulated Reactor Concrete Wall (video clip)

    12. Severe Accident Avoidance Multiple Decay-Heat Removal Systems Depressurization Facility Spray Heat Removal System Double-Wall Containment Catalytic Recombiners to Reduce Hydrogen Concentration Spreading Chamber for Molten Fuel

    13. Palo Verde - PWR

    14. EPR = Redundancy

    15. Metal Forgings

    16. AP1000 = Passive Cooling

    17. Steam Explosions Rapid Mixture of Two Liquids where to Temperature is Greater than the Boiling Point of the Second Liquid Detonation Possible Molten Fuel Ejected into Coolant Water Can Cause a Shock Explosion Equivalent to 200 kg TNT Unsure the Complete Impact of Steam Explosions within the Core

    18. Overpressurization Study Sandia National Labs 1/4-scale concrete model of a nuclear power plant containment vessel Small leaks at 2.5 times design pressure Maximum pressure of 3.1 times design pressure

    19. Debris Beds Cooling Debris Prevents Remelting and Overheating of Barriers Depends on Multiple Variables Bed Particle Size Pathway for Coolant to and through the Bed Bed Depth System Pressure

    20. Hydrogen Gas Created during Chemical Reactions Most Important is Oxidation of the Zirconium Cladding of the Fuel Can Burn or Detonate Can have Nitrogen Atmosphere to Reduce Oxygen Content of Air Catalytic Recombiners to React Hydrogen and Oxygen To Form Steam Igniters to Burn Small Hydrogen Bubbles

    21. Basemat Melt-Through Debris Bed Melts through the Concrete Floor and Bedrock beneath the Building Penetration Depth is Limited Long-Term Cooling and Containment Difficult if the Containment Building is Ruptured

    22. Preventing Melt-Through

    23. Less Piping

    24. Compact Control Design

    25. Examples and Problems 6.1 Total Decay Heat from a Reactor Calculate the total decay heat released from 1 kg of iodine-131. Each decay releases 0.5 MeV (9.12 x 10-14 J) Half-life of I-131 is 8 days What fraction of energy is released in the first 30 days of decay?

    26. Examples and Problems 6.1 Number of total atoms N = 6.022x1026/131 = 4.6x1024 atm/kg Total energy E = 4.6x1024 x 9.12 x 10-14 J E = 0.419 TJ Number of atoms at 30 days Where l = ln 2 / T = 0.8664 day-1

    27. Examples and Problems 6.1 N30 = 0.0743 N0 Therefore 92.6% of the original atoms have decayed and released their energy within the first month

    28. Examples and Problems 6.1 Other Problems Decay from other isotopes Absorption of heat into coolant or structure material of reactor Additional Analysis Account for efficiency of heat removal and its effective distribution to various portions of the core and surroundings support materials Compare heat generation to conventional explosives, radiators, geothermal, etc.

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