Nuclear Fundamentals Part II

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Nuclear Fundamentals Part II

• Harnessing the
• Power of the Atom

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Topics To Cover

• Components of Reactor
• Reactor Core
• Reactor Vessel
• Supporting Components
• Pressurizer (Pzr)
• Reactor Coolant Pumps (RCPs)
• Steam Generators (S/Gs)
• Shielding
• Control of Reactor

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Goal Determine Functions
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Primary Loop
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Core Assembly

• Fuel Assembly stores, supports, and isolates
  fuel
• Plate UO2 clad with Zr metal very thin to allow
  for effective heat transfer
• Sub-Assembly group of edge-welded plates w/
  fluid channels between
• Cell group of several sub-assemblies w/ control
  rod fluid in center
• Core collection of cells

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Core Assembly

• Primary Coolant removes heat produced by fission
  in fuel
• Naval reactors use water (effective, easily
  replaceable, does not radiate)
• Typical outlet temp 500 oF
• Typical inlet temp 450 oF
• Typical temp w/in core ?

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Pressure Vessel

• Purpose provides structural support for Rx core
  directs flow of coolant thru core
• Closure Head removable cover on top of pressure
  vessel
• Closure bolts hold down
• Uses seal to prevent leakage
• Houses Control Rod Drive Mechanisms (CRDM)

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Control Rods and CRDMs

• Electronically position control rods
• Supported by closure head
• Control Rod
• Considered neutron leakage
• Coarse Adjust for Reactor Power
• Hafnium
• High sa (sa sc sf)
• High sc
• Low sf

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Control Rods

• Shutdown with all rods lowered, Rx cannot go
  critical
• Startup lift control rods to reduce leakage
  until Rx is critical continue to lift until temp
  of moderator reacts to rod height changes -gt let
  moderator control power
• SCRAM quick shutdown of Rx drop rods to bottom
  vice electronically lower (SuperCritical Reactor
  Ax Man)

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Pressurizer (Pzr)

• Purpose maintains primary coolant in subcooled
  state (prevent boiling) and provides surge volume
  for power transients
• Operates at saturation conditions to allow for
  steam space (NO other part of primary at
  saturation conditions)
• Uses electric heaters/spray to maintain high temp
  pressure

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Pressurizer (Pzr)

• If Pzr not used
• Boiling in reactor core reduces ability to remove
  heat (mass flow rate and heat capacity reduced)
• Boiling in pumps causes cavitation -gt loss of
  flow through core

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Reactor Coolant Pumps (RCP)

• RCP circulates primary coolant through the core
• Multiple RCPs for redundancy
• Hermetically sealed (no leakage)

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Steam Generator (S/G)

• S/G acts as heat sink for reactor and produces
  steam for MS system
• Shell and tube heat exchanger
• Moisture Separators
• Non-nuclear side called the Secondary

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Shielding

• Serves two purposes
• Reduce radiation outside reactor compartment to
  protect personnel
• Reduce radiation inside reactor compartment to
  protect instruments/equipment

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Shielding

• All contained within RC to minimize radiation
• Pressure Vessel Core
• Pressurizer (Pzr)
• Reactor Coolant Pumps (RCPs)
• Steam Generators (S/Gs)

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Controlling Fission

• To control fission, must control slowing down
  /thermalizing of ns (some leakage) -gt control
  rods and moderator used
• Moderator
• Substance used to slow down ns by elastic
  collisions
• Qualities of a Good Moderator
• high ss (scattering cross-section)
• low sa (absorption cross-section)
• atomic mass close to neutron (ie hydrogen)

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Controlling Fission

• Navy uses water as moderator (H2O)
• Temperature of water determines amount of
  interaction
• Temp water becomes more dense
  causes more collisions ns travel shorter
  distance to get thermalized less chance of
  leakage more fission power

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Reactor Plant Control

• Core reactivity/power is inversely proportional
  to moderator temperature (negative temperature
  coefficient)
• Operating AA1/3 -gt AAIII Ordered
• Open throttles -gt Steam demand -gt Tc
• Tave -gt density moderator -gt greater
  chance neutrons will collide with H2O

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Reactor Control

• Higher prob. that neutron will thermalize and
  cause fission -gt fission rate -gt Rx power
• Th -gt Tave -gt density moderator -gt more
  chance of fast leakage
• Lower prob. that neutron will thermalize -gt
  fission rate -gt reactor power
• Tave returns to steady state

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Reactor Control

• Overall, REACTOR POWER FOLLOWS STEAM DEMAND
• While Tave will remain roughly constant from
  steady-state to steady-state, Th and Tc will
  change depending on steam demand

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Any Questions?