nuclear fundamentals part ii harnessing the power of the atom
TRANSCRIPT
Topics To CoverTopics To Cover• Components of Reactor
– Reactor Core– Reactor Vessel
• Supporting Components– Pressurizer (Pzr)– Reactor Coolant Pumps (RCP’s)– Steam Generators (S/G’s)– Shielding
• Control of Reactor
Core AssemblyCore 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
Core AssemblyCore 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 = ?
Pressure VesselPressure 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)
Control Rods and Control Rods and CRDM’sCRDM’s
• Electronically position control rods• Supported by closure head• Control Rod
– Considered neutron leakage– Coarse Adjust for Reactor Power– Hafnium
• High a (a = c + f)
• High c
• Low f
Control RodsControl 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 -> let moderator control power
• SCRAMSCRAM: quick shutdown of Rx; drop rods to bottom vice electronically lower (SSuperCCritical RReactor AAx MMan)
Pressurizer (Pzr)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
Pressurizer (Pzr)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 -> loss of flow through core
Reactor Coolant Pumps Reactor Coolant Pumps (RCP)(RCP)
• RCP: circulates primary coolant through the core
• Multiple RCP’s for redundancy
• Hermetically sealed (no leakage)
Steam Generator (S/G)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”
ShieldingShielding• Serves two
purposes:– Reduce radiation
outside reactor compartment to protect personnel
– Reduce radiation inside reactor compartment to protect instruments/equipment
ShieldingShielding• All contained within RC to minimize radiation:
– Pressure Vessel & Core– Pressurizer (Pzr)– Reactor Coolant Pumps (RCP’s)– Steam Generators (S/G’s)
Controlling FissionControlling Fission
• To control fission, must control slowing down /thermalizing of n’s (some leakage) -> control rods and moderator used
• Moderator– Substance used to slow down n’s by elastic
collisions– Qualities of a Good Moderator:
• high s (scattering cross-section)
• low a (absorption cross-section)• atomic mass close to neutron (ie:
hydrogen)
Controlling FissionControlling Fission
• Navy uses water as moderator (H2O)• Temperature of water determines
amount of interaction:– Temp water becomes more dense
causes more collisions n’s travel shorter distance to get thermalized less chance of leakage more fission power
Reactor Plant ControlReactor Plant Control
• Core reactivity/power is inversely proportional to moderator temperature (negative temperature coefficient)
• Operating AA1/3 -> AAIII Ordered• Open throttles -> Steam demand -> Tc
• Tave -> density moderator -> greater chance neutrons will collide with H2O
Reactor ControlReactor Control
• Higher prob. that neutron will thermalize and cause fission -> fission rate -> Rx power
• Th -> Tave -> density moderator -> more chance of fast leakage
• Lower prob. that neutron will thermalize -> fission rate -> reactor power
• Tave returns to steady state
Reactor ControlReactor Control
• Overall, REACTOR POWER REACTOR POWER FOLLOWS STEAM DEMANDFOLLOWS STEAM DEMAND
• While Tave will remain roughly constant from steady-state to steady-state, Th and Tc will change depending on steam demand