Transcript Nuclear Energy

Nuclear Energy
• Nuclear Fission - the splitting of two atoms
• Nuclear fusion – the combining of 2 atoms
• Both processes release energy
Atomic structure
• Atoms are composed of protons , neutrons and
electrons – discovered in the 1930’s
• Protons – positively charged
– q=1.602176487×10−19 C
– m=1.67262158 × 10-27 kg
• Neutrons – no charge
– m=1.6749 x 10-27 kg
• Electron –negative charge
– q =−1.602176487×10−19 C
– m= 9.10938215×10-31 kg
Atomic structure
•
•
•
•
•
•
•
•
Protons and neutrons make up the
nucleus
Electrons orbit the nucleus at specific
distances known as energy levels
Atomic number = number of
protons=Z
Atomic mass number = A = Z+ N,
where N is the number of neutrons
Atomic mass = total mass of the
electrons, protons and neutrons
Atomic weight = the ratio of the
average mass of an atom to 1/12 of
the mass of an atom of carbon-12.
Ion –if an atom loses or gains an
electron and has a net charge
Isotope-atom has the same number
of protons but different number of
neutrons
Atomic Structure
• Atoms are held together by forces
• There are four forces in nature
– Gravity -force between masses
– Electrostatic forces-like charges repel, unlike
charges attract
– Strong nuclear force - causes an attraction
between protons and neutrons
– Weak nuclear force – causes protons to transform
into neutrons and neutrons in to protons
Atomic Structure
• In atoms, the electrostatic force is holding the
electrons to the nucleus, since the electrons
are negatively charged and the protons are
positively charged.
• The protons in the nucleus are being pushed
apart by the electrostatic force since they
have the same charge, but the strong nuclear
force overcomes this repulsion on the atomic
size scales and holds the protons together.
Fission
• In the late 1930s, it was discovered that if a
uranium nucleus was bombarded by neutrons,
it absorbed the neutron and became an
unstable isotope of uranium, which then spilt
into 2 separate atoms (Krypton and Barium)
and emitted more neutrons and gamma rays
Is mass conserved?
• Now the mass of the fission products plus the
excess neutron should equal the mass of the
initial incident neutron and the uranium. But
it doesn’t.
• Where did the mass go?
• Well, remember E = mc2 - mass cannot be
created or destroyed, only converted to and
from energy, so the missing mass must be
converted into energy.
How much energy
• 200 MeV is released per fission event
• The fission of 1 g of uranium or plutonium per
day liberates about 1 MW.
• This is the energy equivalent of 3 tons of coal
or about 600 gallons of fuel oil per day
• No CO2 emissions!
• Vastly superior in terms of energy per amount
of fuel
Self sustaining or chain reaction
• The fission reaction
itself releases neutrons,
these can be used to
fission additional nuclei,
so the elements are
there for a sustained or
chain reaction.
• Tremendous power
capability made this an
ideal weapon.
Fission Bombs
• Created in response to a fear that Nazi Germany
would develop one first, which would tip the
balance of power and possibly the outcome of
WWII in their favor.
• Manhattan Project – Secret US project to develop
a nuclear weapon
• Developed 3 nuclear devices, one with 235U and
two with 239PU.
• One was tested in New Mexico in 1945, the other
two were dropped on Hiroshima and Nagasaki
Japan in August 1945, ending WWII in the Pacific.
Critical Mass
• In order to sustain a chain reaction, one needs
a specific amount of fissionable material,
called the critical mass
• Critical mass is the smallest amount of fissile
material needed for a sustained nuclear chain
reaction.
• Creating the critical mass was one of the
challenges that faced the Manhattan project
The devices
• Fat Man and Little Boy
• Little Boy – device dropped on Hiroshima
• Gun-type device
– One mass of U-235, the "bullet," is fired down a
gun barrel into another mass of U-235, rapidly
creating the critical mass of U-235, resulting in an
explosion.
The devices
•
•
•
•
Fat Man
Tested in New Mexico and dropped on Nagasaki
Used Plutonium rather than Uranium
Implosion style device
– The required implosion was achieved
by using shaped charges with
many explosive lenses to produce
the perfectly spherical explosive
wave which compressed the
plutonium sphere.
Effects of a Fission explosion
•
•
•
•
•
Blast Damage
Thermal radiation
Electromagnetic Pulse
Ionizing radiation
earthquake
Blast Damage
• 40-50% of the total
energy released is in the
blast.
• Most of the destruction
due to blast effects
• Blast wind may exceed
1000 km/h.
Thermal Radiation
• 35-45% of the energy released
is in thermal radiation
• Burns occur
• Eye injures
– Flash Blindness-caused by the
initial bright flash, can last up
to 40 minutes
– Retinal burns – scarring due to
the direct concentration of
explosions thermal energy on
the eye-rare the fireball needs
to be in the direct line of sight
• Firestorms-gale force winds
that blow in from all sides
towards the center of a fire
Electromagnetic pulse
• The nuclear explosion produced
high energy electromagnetic
radiation-Gamma rays.
• The Gamma rays interact with
(scatter) electrons and produce
higher energy electrons.
• Long metal objects (cables, etc)
act as antenna and generate high
voltages and currents, which can
damage or destroy electrical
equipment.
• No known biological effects,
though useful against Sentinels
(The Matrix).
Ionizing radiation
• About 5% of the energy
• In the form of neutrons,
gamma rays, alpha
particles and electrons,
moving at nearly the
speed of light
• Neutrons transmutate
(change the atomic
structure of) the
surrounding matter, often
making it radioactive. This
adds to the radioactive
fallout
What does this have to do with
Nuclear Energy
• It sets the historical context and shows the
power released
• To point out that this is NOT what will happen
if there is an accident in a nuclear power
facility-they do not “blow up” like explosive
devices. More on that later
Nuclear reactor
• In a nuclear power plant, the energy to heat the water
to create steam to drive the turbine is provided by the
fission of uranium, rather than the burning of coal.
• Fuel is 3% 235U and 97% 238U. 235U is an isotope of 238U.
The chain reaction will only occur in the 235U, but
naturally occurring uranium has both present in it.
• The neutrons coming from a fission reaction have an
energy of 2Mev. They are too energetic to sustain a
nuclear reaction in 235U.
• Need to slow them down to energies on the order of
10-2 so they can sustain fission in the 235U
Slowing the neutrons down
• A moderator is used to slow down the neutrons
and cause them to lose energy
• The moderator could be water or graphite
• The lower energy neutrons are called thermal
neutrons
• Some of the neutrons will be absorbed by 235U
instead of causing a fission reaction or by 238U
and resulting in the emission of a gamma ray in
both cases.
• Absorption of a neutron by 238U can result in the
creation of 239Pu which is also fissionable
Creating Plutonium
• So: 238U captures a neutron creating 239U
• 239U undergoes a beta decay in with a half life of 24
minutes and becomes 239Np (Neptunium)
• 239Np then beta decays with a half life of 2.3 days into
239Pu.
• 239Pu has a half life of 24,000 years
• 239Pu can also undergo fission by the slow neutrons in
the core, with an even higher probability
• So as it builds up in the core, is contributes to the
fission reaction
Breeder reactor
• A reactor designed to produce more fuel (usually 239Pu )
than it consumes.
• 239Pu does not occur naturally, and it is more fissile than
235U.
• Leads to the possibility of reactors that can create their
own fuel, and only need limited mounts of naturally
occurring uranium to operate.
• Also leads to the danger of countries creating additional
nuclear fuels for weapons development
– Caution-reactor must be designed to produce weapons grade
plutonium, jut because someone has a nuclear reactor does not
mean they create weapons grade plutonium
Reactor design
•
•
•
•
•
•
•
PWR – pressurized water reactor
Core – where the action is. Fuel assembly is
kept in here (fuel is usually in the form of
fuel rods)
Fuel rods are surrounded by the water which
acts as the moderator. This water is kept
under high pressure so it never boils-it heats
a seconds water source which turns into
steam
Control rods are slid in and out from the top
to control the fission rate-in an emergency
they can be dropped completely into the
reactor core, quenching the fission
Once the steam is generated, this works just
like a fossil fuel power plant
Can run without refueling for up to 15 years
if the initial fuel is highly enriched
Used in submarines and commercial power
systems
Reactor design
•
•
•
•
•
BWR –Boiling water reactor
Core – where the action is. Fuel
assembly is kept in here (fuel is
usually in the form of fuel rods)
Fuel rods are surrounded by the
water which acts as the moderator
and the source of steam
Control rods are slid in and out from
the bottom to control the fission
rate-in an emergency they can be
dropped completely into the reactor
core, quenching the fission. Also,
boron can be added to the water
which also efficiently absorbs
neutron
Once the steam is generated, this
works just like a fossil fuel power
plant
Fuel Cycle
• Fuel rods typically stay in a reactor about 3 years
• When they are removed, they are thermally and
radioactively hot
• To thermally cool them they are put in a cooling
pond.
• Initial idea was that they would stay in the
cooling pond for 150 days, then be transferred to
a facility which would reprocess the uranium an
plutonium for future use.
Nuclear waste disposal
• This idea ran into problems.
• Fear that the plutonium would be easily
available for weapons use halted reprocessing
efforts in 1977
• Note that it is very difficult to extract weapons
grade plutonium from spent fuel rods
• Plan is now to bury the waste deep
underground, in a place called Yucca
Mountain, Nevada
Nuclear waste
•
•
•
•
The spent fuel rods are radioactive
Radioactivity is measured in curies
A curie is 3.7x1010 decays per second
A 1000 MW reactor would have 70
megacuries of radioactive waste once it was
shut down
• After 10 years, this has decayed to 14 MCi
• After 100 years, it is 1.4MCi
• After 100,000 years it is 2000 Ci