MODULE 8
COMPONENTS OF NUCLEAR
REACTORS
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A slow moving neutron induces
fission in Uranium 235
Expanding Chain Reaction
• The fission reaction
produces more
neutrons which can
then induce fission in
other Uranium atoms.
Fission products
• The fission products
shown are just
examples, there are a
lot of different
possibilities with
varying probabilities
• Two common US reactor
types: Boiling Water
Reactor and Pressurized
Water Reactor.
• BWR: P=1000 psi
T=545F
• PWR P=2250 psi
T=600F
• PWR is most common
and is basis of marine
nuclear power.
PWR Plant Cycles
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Reactor is inside a large containment
building
NUCLEAR STEAM SUPPLY SYSTEM
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NUCLEAR STEAM SUPPLY SYSTEM
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NUCLEAR STEAM SUPPLY SYSTEM
PWR CYCLE T-S DIAGRAM
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HEAT REJECTION
COOLING TOWERS
• Cooling towers are used to dispose of waste heat into
the atmosphere. Generally, there are two types of
cooling towers, a “wet” type and a “dry” type.
• WET-TYPE – A wet type cooling tower uses the
process of evaporation for cooling. The surface is kept
saturated.
• DRY-TYPE – A dry type cooling tower uses the
process of convection for cooling and thus requires
more surface area and airflow.
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HEAT REJECTION
COOLING TOWERS
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NUCLEAR REACTORS
• Breeder Reactors – Breeding is the process whereby
more than one fissile isotope is produced per fissile
atom consumed. Therefore, breeder reactors produce
more fissile material than they consume.
• Thermal reactors – A reactor in which the majority of
the fissions which take place are from thermal neutrons
(neutrons with an energy of 0.0253eV).
• Fast Breeder Reactors – A reactor in which the
majority of the fissions which take place are from fast
neutrons (neutrons with energy above 100keV).
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NUCLEAR REACTORS
COMPONENTS OF NUCLEAR REACTORS
• Fuel
• Moderator
• Coolant
• Reflector
• Blanket
• Control Rods
• Shielding
• Reactor Vessel
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Fuel Pellets
• The enriched UF 6 is
converted into UO 2
which is then made into
fuel pellets.
• The fuel pellets are
collected into long
tubes. (~12ft).
• The fuel rods are
collected into bundles
(~200 rods per bundle
• ~175 bundles in the
core
Cladding
• The material that the
fuel rods are made
out of is called
cladding.
• It must be permeable
to neutrons and be
able to withstand high
heats.
• Typically cladding is
made of stainless
steel or zircaloy.
NUCLEAR REACTORS
FLOW PATHS THROUGH VESSEL
Hot Leg
Cold Leg
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NUCLEAR REACTORS
NUCLEAR REACTOR FUEL CONCEPTS
• Conversion - The process of manufacturing fissile isotopes
from abundant nonfissile material.
232
Th(
n,
)
233
Th
233
Pa
233
U
Fertile thorium (Th) is
converted to fissile uranium-
233 through neutron
absorption
238
U(
n,
)
239
U
239
Np
239
Pu
Fertile uranium-238 is
converted to fissile
plutonium-239 through
neutron absorption.
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Moderator
• Neutrons are slowed
down by having them
collide with light atoms
(Water in US
reactors).
• Highest level of
energy transfer occurs
when the masses of
the colliding particles
are equal (ex: neutron
and hydrogen)
Tricks of the trade
• Slow moving (thermal) neutrons are
more effective at inducing fission, but,
fissions produce fast moving electron.
We need to slow neutrons down.
• Fissions typically produce several
neutrons but a linear chain reaction only
needs one. We need to get rid of a
good fraction of our neutrons.
Moderator
• Neutrons are slowed
down by having them
collide with light atoms
(Water in US
reactors).
• Highest level of
energy transfer occurs
when the masses of
the colliding particles
are equal (ex: neutron
and hydrogen)
Control Rods
• Control rods are
made of a material
that absorbs excess
neutrons (usually
Boron or Cadmium).
• By controlling the
number of neutrons,
we can control the
rate of fissions
NUCLEAR REACTORS
COMPONENTS OF NUCLEAR REACTORS
• Moderator – Only used in thermal reactors.
It is used to moderate neutrons to thermal
energies so they can be absorbed, leading to
more fission events. The most effective
moderators are nuclei with low mass
numbers. Examples: water, heavy water,
graphite, beryllium oxide
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NUCLEAR REACTORS
COMPONENTS OF NUCLEAR REACTORS
• Coolant – Used to transfer heat from the
reactor core. For thermal reactors, water,
heavy water, and gases are commonly used
as coolants. In fast reactors, however, liquid
sodium is typically used due to its
exceptional heat transfer properties.
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NUCLEAR REACTORS
COMPONENTS OF NUCLEAR REACTORS
• Reflector: The reflector is located adjacent
to the reactor core or blanket. It consists of
reflecting material which serves to reflect
neutrons which escape the reactor core.
After collisions with the reflector, neutrons
are returned to the core or blanket.
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NUCLEAR REACTORS
Installation of beryllium reflector for the
High Flux Isotope Reactor (HFIR) at Oak
Ridge National Laboratory.
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NUCLEAR REACTORS
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NUCLEAR REACTORS
COMPONENTS OF NUCLEAR REACTORS
• Blanket – The blanket is a region of fertile
material surrounding the reactor core. It is
used to capture neutrons which escape the
core. Specifically, a blanket is used for
conversion and breeding. Due to the nuclear
reactions which take place within the
blanket and the subsequent heat generated, it
too must be cooled.
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NUCLEAR REACTORS
COMPONENTS OF NUCLEAR REACTORS
• Control rods: Made of neutron-absorbing
material. Control rods are an effective way
of managing the reactor power level since
they absorb neutrons. The rods can be
manufactured as cylindrical in shape, or as
cruciform rods. Typical materials used for
control rods are hafnium, cadmium, boron
steel, and silver alloys.
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NUCLEAR REACTORS
LIGHT WATER REACTORS (LWR)
• Light water serves as the moderator, reflector, and
coolant.
• LWR’s operate at high temperatures due to the
high vapor pressure of water.
• Fuel must be enriched in LWR’s due to the thermal
neutron absorption cross-section of water
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NUCLEAR REACTORS
PRESSURIZED WATER REACTOR
(PWR)
• Water does not boil in the reactor core, therefore heat
exchangers such as steam generators are used.
• Coolant inlet/outlet temperatures: 530 – 600 o F
• System pressure: 2250 psi
• Fuel: enriched UO 2 from 2-5 w/o
• Fuel Pellets: 1cm diameter, 2 cm long
- Pellets loaded into cladding tubes made of stainless steel
or zircaloy.
• Control rods inserted from top.
235
U
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NUCLEAR REACTORS
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NUCLEAR REACTORS
Cutaway view of
reactor vessel
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NUCLEAR REACTORS
Control rod drivers
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NUCLEAR REACTORS
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NUCLEAR REACTORS
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NUCLEAR REACTORS
NUCLEAR FUEL ASSEMBLY
• Each fuel assembly is 17 X 17 array
consisting of:
• 264 fuel rods that house the fuel pellets
• 24 RCCA guide thimble that provides
guidance for RCCA when inserted in
the fuel assemblies
1. RCCA guide thimble attachments
– a) Upper nozzle block
• Removable – allows fuel reconstitution.
– b) Lower nozzle block
Rod Cluster Control Assembly
Hold Down Spring
Upper Nozzle Block
Grid
RCCA Guide Tubes
Lower Nozzle Block
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NUCLEAR REACTORS
TOP NOZZLE BLOCK
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NUCLEAR REACTORS
LOWER NOZZLE BLOCK
• The RCCA thimble are
reduced in size near the
bottom in order to create
a hydraulic “dash pot”.
• The dashpot slows
control rods following a
trip in order to minimize
impact energy.
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NUCLEAR REACTORS
Vantage 5 Fuel Rods
• A fuel rod consists of Uranium Oxide pellets
stacked to approximately 12 feet in height in
a zircaloy tube ( known as cladding).
• The tube is sealed at the lower end by a
welded plug.
• Enrichment of fuel depends on core location.
( Range between 2-4.6%)
• Fuel Rods are pressurized with Helium to
about 100 psig. to prevent collapse of
cladding due to high RCS pressure (reduce
clad creep).
• Helium is used because it is inert and will
not chemically react with the fuel or clad.
• Helium also has good heat transfer
characteristics.
• Zircaloy used as the cladding material
because:
• Good heat transfer characteristics
• Low absorption of neutrons
• High corrosion resistance
• High melting point
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NUCLEAR REACTORS
HOW A SINGLE FUEL PELLET IS SUPPORTED
1. All fuel pellets support each other and
distribute their weight to the rod/ cladding.
2. The rod is supported by the grid assembly.
3. The grid assembly is supported by the
thimble tubes..
4. The thimble tubes are supported by the upper
and lower nozzle blocks.
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NUCLEAR REACTORS
HOW A SINGLE FUEL PELLET IS SUPPORTED
5. The lower fuel assembly nozzle block is
supported by the lower core plate.
6. The lower core plate transmits its load to
the support columns and to the lower core
support forging.
7. The lower core support forging is welded
to the core barrel which supports the
support forging.
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NUCLEAR REACTORS
HOW A SINGLE FUEL PELLET IS SUPPORTED
8. The core barrel is supported by the vessel
flange.
9. The forces on the vessel flange are
transmitted to the vessel.
10. The vessel is supported by 4 concrete pads
under the core inlet/outlet nozzles
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NUCLEAR REACTORS
GRID STRAPS
• All 264 fuel pins in each
assembly are held together
to the guide thimbles by
eight grid straps
• Three intermediate flow
mixers increase turbulence
in the coolant thus heat
transfer is improved. The
flow mixers are located in
each assembly between the
upper three grid straps.
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NUCLEAR REACTORS
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NUCLEAR REACTORS
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NUCLEAR REACTORS
• The Reactor Core consist of 193
fuel assemblies with various
enrichments ranging from about
2.5 - 4.5 %
• The fuel is arranged in a “low
leakage” loading pattern.
• Old fuel and enriched new fuel is
mixed around the periphery of the
core.
• Higher enrichments toward
outside of core decreases chances
of neutrons leaking out of the
core.
• Results in a more uniform axial
flux profile.
CORE LOAD PATTERN
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NUCLEAR REACTORS
EPR reactor core barrel EPR reactor core barrel
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NUCLEAR REACTORS
EPR REACTOR CORE BARREL
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NUCLEAR REACTORS
EPR REACTOR CORE BARREL
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NUCLEAR REACTORS
• -The core barrel
supports the weight of
the core
• -The core barrel hangs
by the barrel flange
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NUCLEAR REACTORS
• -The core baffle
provides shape
and lateral support
for the core
• -The upper head
cooling outlet
allows ½% core
bypass flow for
head cooling
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NUCLEAR REACTORS
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NUCLEAR REACTORS
Core Outlets
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NUCLEAR REACTORS
LOWER CORE PLATE
• Lower core plate is a
Stainless steel plate on
which fuel assemblies
rest.
• Flow holes to ensure
consistent and uniform
flow to each assembly
• Rests on a ledge inside
the core barrel.
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NUCLEAR REACTORS
EPR reactor core support plate
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NUCLEAR REACTORS
PWR
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NUCLEAR REACTORS
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NUCLEAR REACTORS
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NUCLEAR REACTORS
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NUCLEAR REACTORS
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NUCLEAR REACTORS
PWR
Nuclear reactor
vessel
internals
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NUCLEAR REACTORS
COMPONENTS OF NUCLEAR REACTORS
• Reactor Vessel – Houses the reactor core along
with the moderator, coolant, blanket, reflector, and
control rods. Due to the emission of γ-rays from
nuclear reactions, a thermal shield manufactured of
iron or steel to absorb the γ-rays is placed between
both the reflector and inner wall of the reactor
vessel. The entire reactor vessel is enclosed in a
containment structure for shielding purposes and to
protect the public in case of a reactor accident.
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NUCLEAR REACTORS
Reactor Vessel
• The function of the reactor vessel is to
provide a pressurized volume in which the
reactor core can be submerged It also
supports the reactor core and the vessel
internals.
• The vessel is made from manganesemolybdenum
steel lined with 1/8” stainless
steel for corrosion considerations (Boric
Acid).
• The vessel is made up of a lower portion
and a removable head (not pictured).
• Vessel dimensions:
• 44 feet tall
• 14.5 feet in diameter
• Wall thickness:
1. Lower head – 5 3/8”
2. Vessel sides – 8 5/8”
3. Upper head – 7”
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NUCLEAR REACTORS
• -Lower vessel
• - 8 nozzles with 45
degree separation
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NUCLEAR REACTORS
Concrete support
pads
• The nozzles are
supported by
four concrete
pads which
support the
weight of the
entire vessel
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NUCLEAR REACTORS
Reactor vessel installation
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NUCLEAR REACTORS
Reactor Vessel Cross-
Sectional View
• Two O-Rings are placed
between the vessel head and
vessel to prevent leakage.
• Grooves are machined in the
upper head for placement of
the O-Rings.
• Leak detection system
monitors the flange O-Rings
for leakage.
• One tap machined in the
vessel between the O-Rings
that monitors inner O-Ring
leakage.
• The second tap is machined
outside the outer O-Ring.
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NUCLEAR REACTORS
Reactor Head Bolts
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NUCLEAR REACTORS
Vessel Head
Penetrations
• View looking up inside
vessel head
• Vessel head penetrations:
• -78 penetrations
• -53 in use for control rods
• -5 for thermocouples ( 4 used
and 1 spare)
• -1 for head vent
• -19 extra for control rods
when using plutonium-mixed
fuel that could be used in the
future.
• This picture also shows the
stainless steel cladding that is
welded in.
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NUCLEAR REACTORS
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