Loss of Coolant

1
Loss of Coolant

• PWR, BWR, CANDU
• Gas-Cooled, Na-Cooled
• Pebble Bed, GEN IV

2
Why is coolant loss in a nuclear reactor bad?
3
Complexity More Breakdowns
4
Loss of Coolant Accident (LOCA)

• Component(s) breakdown
• Interruption of normal coolant flow
• Reactor shutdown
• Decay heat buildup
• Alternative systems needed to prevent damage to
  reactor

5
Operational States

• Normal Operation
• Continuous Power Generation
• Operational Transients
• 10 per reactor year
• Shifts in steady-state conditions
• Startup
• Shutdown
• Maintenance Routines

• Upset Conditions
• 1 per reactor year
• Not normal operating event
• Expected occurrence
• Lightning strike
• Power lines broken
• Pump failure
• Loss of feedwater
• Little or no damage
• No radiation release

6
Operational States

• Emergency Events
• 1 in 100 reactor years
• Some damage to power plant components
• Break in reactor pipes
• Relief valves stuck open
• Electrical Fires
• No radiation release

• Limiting Fault
• 1 in 10,000 reactor years
• Design Basis Accident
• Radiation release possible
• Earthquake
• Plane impact
• Unprotected, Beyond DBA
• Asteroid impact
• Act of God

7
Engineered Safety Systems

• Main safety systems
• tripping the fission reaction
• Emergency core cooling system (ECCS)

• Safety achievement
• Duplication
• Multiple sensors, processors, and control devices
  to monitor reactor
• Multiple safety systems to prevent and/or contain
  accidents
• Diversity
• Monitoring different parameters to confirm results

8
Design Basis Accident

• Postulated events that cause limiting faults
• Earthquake
• Plane impact
• Flood
• Terrorist activities
• Volcanic eruption

• Beyond DBA
• Events too unlikely to occur for that reactor
• Simultaneous DBA events
• Asteroid impact
• Geological upset in an area without history for
  that type of event
• Dinosaur stampede

DBA
9
LWR Conditions

• Upsets
• Loss of coolant through relief valve
• Addition of extra coolant through pump
• Changes in feedwater conditions
• Improper operation of reactor controls

• Emergency events
• Valves stuck open
• Small breaks in steam line
• Loss of flow from all reactor coolant pumps
• Limiting faults
• Large break in steam line
• Large break in coolant pipe
• Steam generator rupture
• Main coolant pump failure
• Failure of control rods

10
PWR Heat Regulators

• Accumulators
• Pressurized water vessels
• High-Pressure Injection System (HPIS)
• Low injection rate of high pressure water
• Low-Pressure Injection System (LPIS)
• High injection rate of low pressure water

• ECCS Sumps
• Recirculate water which has escaped from the
  primary circuit
• Power-Operated Relief Valve (PORV)
• Release valve to release built up steam and
  energy from the reactor
• Depressurize vessel

11
BWR Heat Regulators

• High-Pressure Corespray System (HPCS)
• Regularly spray water from storage tank or
  suppression pool onto core and fuel for any
  pressure level
• Automatic Depressurization System (ADS)
• Release vessel water into a suppression pool
• Lowers vessel pressure

• Low-Pressure Corespray System (LPCS)
• Spray water from suppression pool to remove heat
  at low pressures
• Low-Pressure Coolant Injection System (LPCI)
• Pump water from suppression pool for long-term
  heat removal

12
CANDU

• Headers
• Distributors of D2O to and from core
• Emergency Coolant Injection System (ECI)
• Separate system to supply H2O to the reactor
• Similar HPIS and LPIS designs

• Disadvantages
• Horizontal tubes can get steam bubbles
• Reactivity increases in areas where no coolant is
  present
• Advantages
• Lower operational temperature and pressure
• Significant heat transfer to moderator (when
  present)

13
Gas-Cooled Conditions

• Operational Transients
• Startup and Shutdown
• Online refueling
• Upsets
• Loss of site power
• Turbine trip
• Steam fault
• Failure of gas circulators

• Emergency Conditions
• Interruption of electricity supply to power
  station
• Reactor trip
• Loss of boiler feedwater
• Steam line break
• Water entering reactor
• Limiting Fault
• Rupture of containment
• Loss of control rods
• Blocked channel flow

14
Sodium-Cooled Fast Reactor

• Operational Transients
• Slow response of molten sodium to heat input
• Upsets
• Similar to water- and gas-cooled reactors
• Emergency Conditions
• Similar to water- and gas-cooled reactors

15
Gen IV Reactors

• LOCA concerns are vital in the design and
  construction of new nuclear power plants
• Extra safety features are implemented into the
  Gen III and Gen IV reactor designs to protect
  against LOCA accidents and prevent reactor core
  meltdown

16
Examples and Problems 4.1

• LOCA in a PWR

17
Examples and Problems 4.2

• Inlet Pipe Rupture in a Magnox Reactor

18
Examples and Problems 4.3

• Pumps On or Pumps Off?
• What Events Occur When the Main Circulating Pumps
  are Stopped (Path 1) or Left Operating (Path 2)
  During a Small LOCA Event?
• Core Coverage
• Core Damage
• Heat Transfer
• What ShouldYou Do?

19
Examples and Problems 4.3

• Other Problems
• What about a large LOCA event?
• What could you do to prevent the core from
  melting?
• Additional Analysis
• If you had knowledge of the location of the
  break, how would that affect your decision?
• What measures could be taken to avoid future LOCA
  events of the same type?