Transcript Nuclear reactor

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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 (a neutron is converted to
a proton and an electron) 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
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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
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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 and
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
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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(MCI) 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
Yucca Mountain
Transportation scenarios
Transportation scenarios
What can go wrong?
• Nuclear power plants cannot explode like a
nuclear bomb.
• A bomb needs a critical mass in a confiuration
which is not present in the reactor core.
• Even a deliberate act of sabotage or terrorism
could not cause such an explosion.
• The worst that can happen is a core melt down.
• 2 classes of accidents-Criticality and Loss of
Coolant (LOCA) accidents
Criticality accident
• If the control rods were removed and/or the control
systems failed, a runaway reaction would occur.
• The tremendous heat produced would melt the
containment system and the reactor core would sink
into the Earth
• Radioactive material would enter the ground and be
released as steam (a radioactive cloud) into the air
• The area around the reactor would be highly
contaminated with radioactivity
• The cloud could travel for hundreds or even thousands
of miles, and could spread dangerous levels of
radioactivity around the world.
Loss of coolant accident
• After a reactor is shut down, it is still hot
enough to experience a core melt down if
cooling system fails.
• Emergency coolant systems are in place to
prevent this
• Big part of reactor design is the prevention of
such accidents
Probability
• To determine the likelyhood that such an accident
would occur something called an event tree is
constructed.
• This determines the consequences of a particular
event occurring
• Each component (pump, valves etc) has a failure
probability assigned to it
• Bottom line-most recent studies indicate that for
all 104 reactors operating the US, over their 30
year operating lifetime, there is a 1% probability
of a large release of radioactivity