7/30/2019 Cribsheet 5

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The Key Questionin nuclear engineering:

Nuclear reactors are an important

carbon-free source of power. We

are likely to see a surge in their

construction in the next few years.

Why are some people concerned about nuclear power?The issue: soundbiteUranium used in reactors has been enriched to create a higher concentration of U-235 than occurs in nature. Countries with the technology to enrich uranium

for power can expand their capabilities to create highly enriched uranium for weapons. Some reactors use plutonium, another weapons material, or create it as

a waste product. Due to proliferation concerns, the International Atomic Energy Agency keeps close tabs on how and where nuclear materials are used. Even

regular nuclear wastethe radioactive and heavy metals that are the products of fissionis dangerous, making it a concern everywhere nuclear power is used.

Consultants:PerPetersonandDanielKammen,Professors,NuclearEngeneeringatUCBerkeley

MapData:TheNuclearEnergyInstitute/

TheInternationalAtomicEnergyAgency(IAEA)

GraphData:IAEA

Illustrator:CybuRichli-www.cybu.ch

Writer:JoshuaBraun

How can reactors be designedto work at maximum efficiency,to be safe and self-regulating,and to minimize nuclear waste?

Cribsheet #5

Seeds Tear-outable tool for living in the 21st century

Nuclear Fission

self regulation

Modern reactors are designed to be safe and self-regulating.

In the same way that an airplane is designed to level out

naturally after being jostled, reactors are engineered to

maintain safe levels of chain reaction under any circumstances.

the nuclear reaction

Superheated steam forces its way out of

the steam generator. It cranks a

turbine, which produces

electricity. As the water in the

steam generator boils, it also

siphons heat away from the water

in the pipe flowing to the reactor

vessel. This helps to cool the reactor. Inanother type of nuclear plant, called a

boiling water reactor, the steam powering the

turbine comes directly from the reactor vessel.

steam generation

electricity

generation

Nuclear fission occurs when a neutron strikes the nucleus of

an atom and splits it apart. Most of the nucleus of the

original atom goes to form new, smaller atoms. In

addition, several neutrons may split off from

the original atom during fission. But the

mass of all these neutrons and fission

products does not add up to the mass

of the original atom. Thats because

when the atom splits, some of its mass

is converted directly into energy.

Nuclear reactors and warheads can

employ the uranium isotope U-235.

When the U-235 atom splits, it frees

two or three neutrons, which then

fly off and split other U-235

atoms in close proximity,

starting a chain reaction.

In warheads, the chain reaction multiplies exponentially,

giving off a huge amount of energy at once in a nuclear

explosion. In nuclear reactors, a chain reaction must be

controlled to create a safe, harvestable source of energy.

In the reactor vessel, cylindrical uranium pellets are stacked

into long fuel rods. It is within bundles of these rods that

the chain reaction occurs. Together they are called the

reactor. To keep the reaction under control:

Low concentrations of U-235 are used in the reactor, so

not every neutron will strike a nucleus.

Retractable control rods, made of a material that absorbs

neutrons, can be lowered into the reactor to siphon off

neutrons that would otherwise multiply the chain reaction.

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4

1

2

3

2

Much of the energy released by the chain reaction is

manifested as heat. The fuel rods are placed inside a

pressurized chamber full of water. Because this reactor

vessel is kept at high pressure, the water will reach

temperatures much greater than 100 C.

steam condensation4

After the steam cranks the turbine, it is condensed back

into a liquid as it runs over a pipe filled with cold water

flowing from the cooling tower. It is then recycled into the

steam generator. The water from the cooling tower heats

up during this process and some of it is released as steam.It is then replaced from a nearby body of water.

This superheated water is pumped through a pipe that runs,

like a heating element, through a second chamber called

the steam generator. The steam generator is partially filled

with clean water, which is boiled by the

heated pipe running from the reactor

vessel. The steam generator is also

kept at high pressure (though its

lower than that of the reactor vessel).

1

power needs supplied by nuclear plants

200

350

250

400

450Nuclear

Hydro

1-20

20-40

40-60

60-80

Biomass

Gases

world energy consumption by source

300 Solids

Liquids

control rods condenser

cooling tower

reactor

generator

steam generator

turbine

reactor vessel

pump

BASIC PRINCIPLE

Heat flows from one part of the powerplant to the next. As one part warms up,heat flows out of the previous one,allowing it to cool, so s afe operatingtemperatures are maintained.

50

Year

1970 1975 1980 1985 1990 1995 2000 2004

100

150

0

6.34%2.22%6.75%

23.04%

26.08%

35.09%

This graph shows the steady increase in globalenergy consumption since 1970. Combustibleliquid and solid fuels, like gasoline and coal,remain our dominant sources of energy.

Percentby country

1018JOULES

(EQUIVALENTTO

23.9

MEGATONNESOFOIL)