Nuclear Fundamentals Part II Harnessing the Power of the Atom

Topics 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 Functions

Primary Loop

Core Assembly Fuel Assembly: stores, supports, and isolates fuel – Plate: UO 2 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 Assembly Primary Coolant: removes heat produced by fission in fuel – Naval reactors use water (effective, easily replaceable, does not radiate) – Typical outlet temp ~ 500 o F – Typical inlet temp ~ 450 o F Typical temp w/in core = ?

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)

Control Rods and CRDM’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 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 SCRAM SCRAM SCRAM : quick shutdown of Rx; drop rods to bottom vice electronically lower ( S uper C ritical R eactor A x M an)

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) 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 (RCP) RCP: circulates primary coolant through the core Multiple RCP’s for redundancy Hermetically sealed (no leakage)

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”

Shielding Serves two purposes: – Reduce radiation outside reactor compartment to protect personnel – Reduce radiation inside reactor compartment to protect instruments/equipment

Shielding All contained within RC to minimize radiation: – Pressure Vessel & Core – Pressurizer (Pzr) – Reactor Coolant Pumps (RCP’s) – Steam Generators (S/G’s)

Controlling 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 Fission Navy uses water as moderator (H 2 O) 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 Control Core reactivity/power is inversely proportional to moderator temperature (negative temperature coefficient) Operating AA1/3 -> AAIII Ordered Open throttles -> Steam demand -> T c T ave -> density moderator -> greater chance neutrons will collide with H 2 O

Reactor Control Higher prob. that neutron will thermalize and cause fission -> fission rate -> Rx power T h -> T ave -> density moderator -> more chance of fast leakage Lower prob. that neutron will thermalize -> fission rate -> reactor power T ave returns to steady state

Reactor Control REACTOR POWER FOLLOWS STEAM DEMAND Overall, REACTOR POWER FOLLOWS STEAM DEMAND While T ave will remain roughly constant from steady-state to steady-state, T h and T c will change depending on steam demand

Any Questions?