5Ws Activity

Features of Nuclear Reactors

The nuclear reactor

Control rods

Moderator and coolant (water)

Steel vessel

Fuel pins

Pump

Concrete shield

Water from turbines

Water to turbines

Control rods: Contain elements like Boron or

Cadmium

The

atom

s of

bor

on o

r cad

miu

m

can

abso

rb n

eutro

ns. L

ower

ing

the

cont

rol r

ods

in th

e rig

ht

posi

tion

betw

een

the

fuel

pin

s

ensu

res

that

an

aver

age

of 2

-3

neut

rons

are

abs

orbe

d fo

r ev

ery

nucl

eus

of U

-235

that

fiss

ions

,

keep

ing

the

fissi

on re

actio

n

goin

g at

a s

tead

y sp

eed,

rat

her

than

in a

n un

cont

rolle

d w

ay

They can be lowered down in the core of the reactor

between fuel pins. Their position is automatically controlled by sensors that

measure the flux of neutrons.

When the flux

of neutrons becomes

too high, the bars are

lowered dow

n into the core.

When the flux is decreasing, the

rods are lifted up.

For safety reasons; th

e bars control th

e

flux of neutrons in the core of th

e

reactor. This ensures that th

e chain

reaction is controlled.

The control ro

ds are fully inserted

in the core to shut

the reactor d

own.

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Fuel pins: They are the nuclear fuel. They

generally contain natural or

enriched uranium.

Nat

ural

ura

nium

con

tain

s le

ss

than

1%

U-2

35, w

hich

can

fiss

ion

whe

n it

abso

rbs

a th

erm

al

neut

ron.

Thi

s fis

sion

rel

ease

s

diffe

rent

frag

men

ts, a

mon

g w

hich

othe

r neu

tron

s. T

hese

will

be

abso

rbed

by

atom

s in

the

cont

rol

rods

, U-2

38 (w

ithou

t fis

sion

) or

U-2

35, c

ausi

ng th

e re

actio

n to

cont

inue

.

Situated in the core of the reactor, between the

control rods and surrounded by the moderator and coolant.

When the fission

reaction starts, the

fission products recoil

carrying a lot of KE

. These

products collide with surrounding

atoms, leaving the pins very hot.

Often the uranium is enriched with

additional U

-235 to enhance the fission

reaction. The pins in the re

actor must

have critical m

ass, i.e. th

e

minimum mass able

to produce a self sustaining

chain reaction.

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Moderator/Coolant:

Usually water, but it could be

made of carbon too.

Whe

n th

e ne

utro

n pr

oduc

ed in

the

fissi

on re

actio

n en

ter t

he

mod

erat

or, t

hey

tran

sfer

thei

r KE

to th

e

ato

ms

in th

e m

oder

ator

. In

this

way

the

mod

erat

or

gain

s en

ergy

. The

neu

trons

slow

dow

n un

til th

eir a

vera

ge K

E is

the

sam

e as

the

KE

of t

he m

oder

ator

ato

ms.

At t

his

poin

t, th

e

neut

rons

are

cal

led

ther

mal

neu

tron,

as

they

don

’t

trans

fer a

ny m

ore

KE

to th

e m

oder

ator

.

It surrounds the fuel pinsand if it is water it also act

as a coolant, so it is circulating inside a closed circuit made of steel (steel

vessel).

The w

ater always

circulates in the steel

vessel, gaining energy in the

core and transferring its KE

to

the water from

the turbines to

turn it into steam.

The moderator is enclosed in a steel vessel capable of

resisting the high pressures and temperatures inside the

reactor and also because the coolant becomes ra

dioactive

inside the core. Water is

a good moderator

because if the temperature ris

es too much it

turns into steam and forms bubbles,

so the neutrons cease to collid

e

with the atoms in the moderator and

the chain reaction stops.

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Nuclear waste:

Contain fission products and U-

238 and U-239 atoms

Nuc

lear

was

te b

ecom

es ra

dioa

ctiv

e

due

to th

e co

ntin

uous

bom

bard

men

t

of th

erm

al n

eutro

ns. T

his

caus

es th

e

U-2

38 to

abs

orb

neut

rons

and

turn

into

U-2

39, w

hich

dis

inte

grat

es b

y ra

dioa

ctiv

e

emis

sion

. Als

o th

e fis

sion

pro

duct

s ar

e m

ade

of

isot

opes

rich

in n

eutro

ns a

nd th

eref

ore

are

beta

emitt

ers.

Waste nuclear material is stored in sealed containers for

many years. Sometimes is then buried underground, or under the sea.

The fuel pins are

removed from

the core

after use and they are placed

in cooling ponds until they get

cooler.

The used fuel is re

moved from the fuel cans using

remote control systems, because of th

e intense

radioactivity. This fuel is

also reprocessed to

recover any nuclear fu

el which

didn’t get used in the re

actions.

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