Reactor Vessel

MODULE 8

COMPONENTS OF NUCLEAR

REACTORS

10/31/2012 Center for Nuclear Studies 1

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


Reactor is inside a large containment

building

NUCLEAR STEAM SUPPLY SYSTEM





PWR CYCLE T-S DIAGRAM


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.


HEAT REJECTION

COOLING TOWERS


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).


NUCLEAR REACTORS

COMPONENTS OF NUCLEAR REACTORS

• Fuel

• Moderator

• Coolant

• Reflector

• Blanket

• Control Rods

• Shielding

• Reactor Vessel


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


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.


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


• 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


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.


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.


NUCLEAR REACTORS

Installation of beryllium reflector for the

High Flux Isotope Reactor (HFIR) at Oak

Ridge National Laboratory.


NUCLEAR REACTORS


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.


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.


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


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


NUCLEAR REACTORS


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Cutaway view of

reactor vessel


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Control rod drivers


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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


NUCLEAR REACTORS

TOP NOZZLE BLOCK


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.


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


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.


NUCLEAR REACTORS


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.


NUCLEAR REACTORS


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


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.


NUCLEAR REACTORS


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• 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


NUCLEAR REACTORS

EPR reactor core barrel EPR reactor core barrel


NUCLEAR REACTORS

EPR REACTOR CORE BARREL


NUCLEAR REACTORS

EPR REACTOR CORE BARREL


NUCLEAR REACTORS

• -The core barrel

supports the weight of

the core

• -The core barrel hangs

by the barrel flange


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


NUCLEAR REACTORS


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Core Outlets


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.


NUCLEAR REACTORS

EPR reactor core support plate


NUCLEAR REACTORS

PWR


NUCLEAR REACTORS


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PWR

Nuclear reactor

vessel

internals


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.


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”


NUCLEAR REACTORS

• -Lower vessel

• - 8 nozzles with 45

degree separation


NUCLEAR REACTORS

Concrete support

pads

• The nozzles are

supported by

four concrete

pads which

support the

weight of the

entire vessel


NUCLEAR REACTORS

Reactor vessel installation


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.


NUCLEAR REACTORS

Reactor Head Bolts


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.


NUCLEAR REACTORS


Reactor Vessel

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