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Upcoming

• Quiz Friday, Nov 3 on EFP chapter 1112
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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

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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

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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

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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

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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

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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

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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.

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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

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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(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

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Yucca Mountain
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Transportation scenarios
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Transportation scenarios
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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

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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.

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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

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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