期末題雙語講稿

Chapter

3Nuclear

Energy

and

ItsEnvironmental

Impact(核能及其環(huán)境影響)3.1

Nuclear

Fission

and

Nuclear

Power（核裂變和核電廠）3.2

Nuclear

Fusion（核聚變）3.1

Nuclear

Fission

and

NuclearPower1. Introduction

to

Nuclear

Fission

andNuclear

PowerNuclear

fission:

nucleus

splits

into

two

daughter

nuclei,with

a

very

large

release

of

energy.Nuclear

power:

any

nuclear

technology

designed

to

extractusable

energy

from

atomic

nuclei

via

controlled

nuclearreactions.the

only

method

in

use

today

is

through

nuclearfission.Nuclear

power

plant

generates

electricity

in

a

mannersimilar

to

a

coal-burning

power

plant.heating

a

fluid

(usually

water)generates

steam

(directly

or

indirectly)drives

turbine

blades

connected

to

a

power

generatorDifference

——

the

method

for

heating

the

water:nuclear

reactions

vs.

coal

combustionIsotopes

–

elements

with

the

same

number

of

protons,different

number

of

neutrons

(and

different

atomic

mass)Example

-

Uranium

(U)

=

92

protons

in

nucleus...238U

(146

neutrons)235U

(143

neutrons)?Radioactivity

and

radioactive

decay

(half-life)Not

all

isotopes

are

stableRadioactive

isotopes

undergo

radioactive

decay

toeventually e

a

stable

nucleusEisxoatomppesle同-位23素8pUroetovne質(zhì)nt子uanlleyuterosn

s中ta子blread2io0a6cPtivbity(l放ea射d性)decay

衰變half-life

半衰期2.

Nuclear

FuelNuclear

fuel

uses

the

isotope

of

uranium,

radioactive235U,

the

only

isotope

of

uranium

that

readily

splits.—

Pure

uranium

ore

is

mostly

composed

of

238U

(only0.7%

235U),

which

does

not

split

readily.—Thus,

most

nuclear

fuel

needs

to

be

concentrated(enriched)

before

it

can

be

used

(3%~4.5%).Nuclear

weapons

require

much

higher

enrichment

in235U>>WEAPON

GRADE

nuclear

material—

enrichment15%~20%

235U.

A

big

step

up

from

preparation

of

reactorfuel.After

enrichment,

the

uranium

is

formed

intoceramic

pellets

which

are

then

encased

in

metalfuel

rods.Bundles

of

rods

are

put

into

a

stainless

steel

reactor vessel.Fuel

rods

remain

in

the

reactor

core,

undergoing fission.At

below1%

the

fuel

is

no

longer

efficient

for

use

in

thereactorpellet

小球encase裝入stainless

steel

不銹鋼vessel

容器For

nuclear

fission:A

neutron

(non-charged particle)

strikes

the

nucleus

of a

235U

atomThis

causes

it

to

split

apart,releasingheat2-3

other

neutrons,

andvarious

kinds

of

fissionproductsThe

released

neutrons

collide with

other

235U

nuclei, repeating

the

process

in

a chain

reaction

(self-supporting)strike

撞擊collide

碰撞Usually,

in

nuclear

reactors,

operators

control

thischain

reaction

so

it

does

not

get

out

of

control.UseControl

rodsRods,

composed

of

either

cadmium

or

boron,

areinserted into

the

core

to

capture

neutronsControl

the

chain

reaction

when

the

reaction

accelerates

too quickly.ModeratorUsed

to

slow

neutrons

and

encourage

more

interaction

with uraniumfuel“Critical

mass”

–

the

amount

of

U

required

tomaintain

a

chain

reaction

in

a

nuclear

reactorGenerally,

completely

out

of

control

reactionswould

lead

to

an

explosion

(as

in

nuclearweapons)Reactor

materials

are

not

concentrated

enough

to

explode.An

out

of

control

chain

reaction

would

lead

to overheating

of

the

core,

causing

the

fuel

rods

and vessel

to

melt“Meltdown”

=

supercritical

mass

-

Chernobyl,

Ukrainereactor

反應(yīng)堆

fuel

rod

燃料棒

control

rod

控制棒

core

堆芯

moderator

慢化劑

shielding

屏蔽Fuel

rods

contain

pellets

of

fuel

for

a

nuclear

reactor

.Control

rods

control

the

chain

reaction.Liquid

water

acts

both

as

a

coolant

and

as

a

moderator.Moderator

slows

down

the

neutrons

to

increase

the

fissionprobability.—the

probability

of

fission

is

maximal

when

the

neutron

energy

is

closeto

the

average

energy

of

the

surroundingmolecules.Breeder

reactor（增殖反應(yīng)堆）Breeder

reactor

extends

the

nuclear

fuel

supply

byconverting

238U

to

239Pu.The

conversion

is plished

by

irradiating

uranium

withneutrons.Efficient

production

of

239Pu

requires

a

“breeder”

reactor,operating

with

fast

neutrons.Surrounding

the

reactor

is

a

blanket

of

ordinary

uranium,

inwhich

the

239Pu

is

bred

by

the

fast

neutrons.The

reactor

fuel

must

be

enriched

to

the

extent

of

15~20percent

with

a

fissionable

isotope,

either

235U

or

239Pu.The

main

difference

with

respect

to

an

ordinary

fissionreactor

is

that

the

water

coolant

is

replaced

by

liquidsodium（鈉）.The

sodium

atoms

slow

the

neutron

to

a

much

smallerextent,

while

liquid

sodium

efficiently

carries

away

heatfrom

the

reactor.The

technology

of

the

breeder

reactor

is

farmore

complex

than

that

of

the

ordinary

fissionreactor,

and

the

technology

is

still

beingdeveloped.The

most

formidable

（可怕的）

problem

of

thebreeder

reactor

seems

to

be

associated

with

theplumbing（管道）

of

the

heat

exchanger,

sinceleaksofliquid

sodiumcanbedisastrous（慘重的）.ReprocessingThe

235U

in

a

fuel

rod

of

a

pressurized

light

water

reactorcan

not

be

completely

used

up.After

about

a

year,

the

fuel

rods

must

be

replaced

withnew

ones.The

spent

fuel

can

be

reprocessed

by

chemicalextraction

of

the

fission

products,

separation

of

theaccumulated

plutonium（Pu,

钚）,and

reconcentration（再濃縮）

of

the

uranium.Uranium:

refabricated

（再加工）into

new

fuel

rods Plutonium:blended

into

MOX

(mixed

uranium-plutonium

oxide,

混合氧化物)fuel

rods.Reprocessing:

the

chemistry

is

straightforward,

but

thetechnology

is

complicated.—The

fuel

rods

are

chopped

up

and

dissolved

in

acid—The

solution

is

subjected

to

successive

solvent

extractionand

ion

exchange

to

separate

the

elements.—Remote

handled

intensely

radioactive

material.StorageSo

far,

there

is

no

way

to

permanently

dispose

of

high-level

radioactivewasteChemical

reactions

cannot

destroy

radioactive

wasteradioactivity

is

a

nuclear

processatomic

nuclei

are

unaffected

by

chemical

reactionsThe

only

choice

is

tostore

the

waste

in

a

place

safe

fromgeological

hazards

(earthquakes,

volcanic

activity,

landslides(山崩),creep(蠕動),floods,

and

seeping

water(滲水))and

human

intervention

（人類干預(yù)）allow

them

to

decay

naturally

(over

250,000

years)Many

tons

of

radioactive

materials

haveaccumulated

over

past

decades

and

representproven

hazards

to

humans

and

futuregenerations.High-level

nuclear

waste

needs

to

be

isolated because

high-energy

radiation:Kills

cellsCauses

cancer

and

genetic

mutations（基因突變）Cancause

rapid

death

in

cases

of

high

exposure

levels.Also,

radioactive

decay

produces

heat

and

can damage

crystalline

and

metallic

holding tanks/containers

=

leaks!Two

choicespermanent

disposal

(injection

into

impermeable（不透 水的）

layers)monitored,retrievable（可回收的）

storage

(isolated, safe

storage,but

able

to

pull

out

and

move

until technology

finds

a

better

solution)3.

Change

in

Attitudes

TowardsNuclear

Power1950’s

–

nuclear

power

was

seen

as

the

wave

ofthe

futureClean,

safe,

limitlessSince

1979,

new

concernsPlant

safetyRadioactive

waste

disposalVast

cost

overruns

in

plant

constructionTwo

major

accidents

have

occurred

at

nuclear

powerplants:Three

Mile

Island,

Pennsylvania(1979)Mechanical

and

operator

error

prevented

water

from

cooling

the

coreAllowed

heat

to

build

up

and

partial

meltdown

to

occur.Most

radioactivity

contained

within

the

building,–

small

amount

of

water

with

low-level

radiation

leaked

into

theenvironment.The

building

was

highly

contaminated

but

the

leak

probably

did

littledamage

to

the

environmentBut,

did

damage

the

public

perceptions

of

nuclear

plants

and

theirpotential

hazards.–

Chernobyl,

Ukraine,

1986April26,Siteofthe

most

seriousnuclearaccident

inhistory

1986Due

to

mistakes

and

problems

with

the

cooling

water

system,the

reactor

core

became

supercritical

during

atestexperimentAllowed

temperatures

to

rise

above

3000℃,

causinguranium

fuel

to

completely

melt.Amassive

steam

explosion

occurred

that

(literally)blew

thetop

off

of

the

building.The

graphite

surrounding

the

fuel

rods

ignited

and

burnedfor10

days,

sending

radioactive

gas

into

the

atmosphereThe

fire

was

eventually

put

out

and

the

reactor

core

encasedand

buried

inconcrete.The

plant

design

has

been

criticized

as

being

poor

and

thereare

other

reactors

of

this

same

design

in

Chernobyl

andelsewhere

in

the

former

Soviet

UnionLasting

effects

of

Chernobyl30

people

died

immediately

from

radiation

exposure?

acute

radiation

syndromethousands

died

from

exposure

during

clean-up

operationsmillions

were

exposed

to

high

levels

of

radiationThe

Soviet

governmenthandled

the

crisis

verypoorlyand

delayed

notifying

othercountries

about

the

radiationleak.The

accident

occurred

onSaturdaySwedish

scientists

detected

theradiation

onMonday.The

area

was

evacuatedslowly

and

many

people

wereput

at

riskunnecessarily.Radioactive