Nuclear Nuclear FundamentaFundamentals Part IIls Part II

Harnessing the Power of the Atom

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

Goal: Determine Goal: Determine FunctionsFunctions

Primary LoopPrimary Loop

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

Any Questions?Any Questions?