Chapter 19 Nuclear Reactions

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Chapter 19Nuclear Reactions
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The Nucleus

• Remember that the nucleus is comprised of the two
  nucleons, protons and neutrons.
• The number of protons is the atomic number.
• The number of protons and neutrons together is
  effectively the mass of the atom.

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Isotopes

• Not all atoms of the same element have the same
  mass due to different numbers of neutrons in
  those atoms.
• There are three naturally occurring isotopes of
  uranium
• Uranium-234
• Uranium-235
• Uranium-238

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Radioactivity

• It is not uncommon for some nuclides of an
  element to be unstable, or radioactive.
• We refer to these as radionuclides.
• There are several ways radionuclides can decay
  into a different nuclide.

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Alpha Decay

• Loss of an ?-particle (a helium nucleus)

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Beta Decay

• Loss of a ?-particle (a high energy electron)
• Where does the ?-particle come from?

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Positron Emission

• Loss of a positron (a particle that has the same
  mass as but opposite charge than an electron)
  Where does the positron come from?

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Gamma Emission

• Loss of a ?-ray (high-energy radiation that
  almost always accompanies the loss of a nuclear
  particle)

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Electron Capture (K-Capture)

• Addition of an electron to a proton in the
  nucleus
• As a result, a proton is transformed into a
  neutron.

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Neutron-Proton Ratios

• Any element with more than one proton (i.e.,
  anything but hydrogen) will have repulsions
  between the protons in the nucleus.
• A strong nuclear force helps keep the nucleus
  from flying apart.

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Neutron-Proton Ratios

• Neutrons play a key role stabilizing the nucleus.
• Therefore, the ratio of neutrons to protons is an
  important factor.

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Neutron-Proton Ratios

• For smaller nuclei (Z ? 20) stable nuclei have a
  neutron-to-proton ratio close to 11.

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Neutron-Proton Ratios

• As nuclei get larger, it takes a greater number
  of neutrons to stabilize the nucleus.

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Stable Nuclei

• The shaded region in the figure shows what
  nuclides would be stable, the so-called belt of
  stability.

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Stable Nuclei

• Nuclei above this belt have too many neutrons.
• They tend to decay by emitting beta particles.

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Stable Nuclei

• Nuclei below the belt have too many protons.
• They tend to become more stable by positron
  emission or electron capture.

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Stable Nuclei

• There are no stable nuclei with an atomic number
  greater than 83.
• These nuclei tend to decay by alpha emission.

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Radioactive Series

• Large radioactive nuclei cannot stabilize by
  undergoing only one nuclear transformation.
• They undergo a series of decays until they form a
  stable nuclide (often a nuclide of lead).

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Nuclear Transformations

• Nuclear transformations can be induced by
  accelerating a particle and colliding it with the
  nuclide.

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Particle Accelerators

• These particle accelerators are enormous, having
  circular tracks with radii that are miles long.

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Kinetics of Radioactive Decay

• The kinetics of radioactive decay obey this
  equation

• The half-life of such a process is

• Comparing the amount of a radioactive nuclide
  present at a given point in time with the amount
  normally present, one can find the age of an
  object.

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Measuring Radioactivity

• One can use a device like this Geiger counter to
  measure the amount of activity present in a
  radioactive sample.
• The ionizing radiation creates ions, which
  conduct a current that is detected by the
  instrument.

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Energy in Nuclear Reactions

• There is a tremendous amount of energy stored in
  nuclei.
• Einsteins famous equation, E mc2, relates
  directly to the calculation of this energy.
• In the types of chemical reactions we have
  encountered previously, the amount of mass
  converted to energy has been minimal.
• However, these energies are many thousands of
  times greater in nuclear reactions.

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Energy in Nuclear Reactions

• For example, the mass change for the decay of 1
  mol of uranium-238 is -0.0046 g.
• The change in energy, ?E, is then
• ?E (?m) c2
• ?E (-4.6 ? 10-6 kg)(3.00 ? 108 m/s)2
• ?E -4.1 ? 1011 J

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Nuclear Fission

• How does one tap all that energy?
• Nuclear fission is the type of reaction carried
  out in nuclear reactors.

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Nuclear Fission

• Bombardment of the radioactive nuclide with a
  neutron starts the process.
• Neutrons released in the transmutation strike
  other nuclei, causing their decay and the
  production of more neutrons.
• This process continues in what we call a nuclear
  chain reaction.

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Nuclear Fission

• If there are not enough radioactive nuclides in
  the path of the ejected neutrons, the chain
  reaction will die out.
• Therefore, there must be a certain minimum
  amount of fissionable material present for the
  chain reaction to be sustained Critical Mass.

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Nuclear Reactors

• In nuclear reactors the heat generated by the
  reaction is used to produce steam that turns a
  turbine connected to a generator.

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SCRAM

• The sudden shutting down of a nuclear reactor,
  usually by rapid insertion of control rods,
  either automatically or manually by the reactor
  operator. May also be called a reactor trip. It
  is actually an acronym for "safety control rod
  axe man," the worker assigned to insert the
  emergency rod on the first reactor (the Chicago
  Pile) in the U.S. http//www.nrc.gov/reading-rm/ba
  sic-ref/glossary/scram.html

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Nuclear Reactors

• The reaction is kept in check by the use of
  control rods.
• These block the paths of some neutrons, keeping
  the system from reaching a dangerous
  supercritical mass.

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Fissionable Material

• fissionable isotopes include U-235, Pu-239, and
  Pu-240
• natural uranium is less than 1 U-235
• rest mostly U-238
• not enough U-235 to sustain chain reaction
• to produce fissionable uranium the natural
  uranium must be enriched in U-235
• to about 7 for weapons grade
• to about 3 for reactor grade

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Nuclear Power Plants - Core

• the fissionable material is stored in long tubes,
  called fuel rods, arranged in a matrix
• subcritical
• between the fuel rods are control rods made of
  neutron absorbing material
• B and/or Cd
• neutrons needed to sustain the chain reaction
• the rods are placed in a material to slow down
  the ejected neutrons, called a moderator
• allows chain reaction to occur below critical mass

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Pressurized Light Water Reactor

• design used in US (GE, Westinghouse)
• water is both the coolant and moderator
• water in core kept under pressure to keep it from
  boiling
• fuel is enriched uranium
• subcritical
• containment dome of concrete

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Cooling Tower
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Nuclear Power

• Nuclear reactors use fission to generate
  electricity
• About 20 of US electricity
• The fission of U-235 produces heat
• The heat boils water, turning it to steam
• The steam turns a turbine, generating electricity

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Nuclear Power Plants vs. Coal-Burning Power
Plants

• Use about 50 kg of fuel to generate enough
  electricity for 1 million people
• No air pollution

• Use about 2 million kg of fuel to generate enough
  electricity for 1 million people
• Produces NO2 and SOx that add to acid rain
• Produces CO2 that adds to the greenhouse effect

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Concerns About Nuclear Power

• core melt-down
• water loss from core, heat melts core
• China Syndrome
• Chernobyl
• waste disposal
• waste highly radioactive
• reprocessing, underground storage?
• Federal High Level Radioactive Waste Storage
  Facility at Yucca Mountain, Nevada
• transporting waste
• how do we deal with nuclear power plants that are
  no longer safe to operate?

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Fusion
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Nuclear Fusion

• Fusion is the combining of light nuclei to make a
  heavier one
• The sun uses the fusion of hydrogen isotopes to
  make helium as a power source
• Requires high input of energy to initiate the
  process
• Because need to overcome repulsion of positive
  nuclei
• Produces 10x the energy per gram as fission
• No radioactive byproducts
• Unfortunately, the only currently working
  application is the H-bomb

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Nuclear Fusion

• Fusion would be a
• superior method of
• generating power.
• The good news is that the
• products of the reaction are
• not radioactive.
• The bad news is that in order to achieve fusion,
  the material must be in the plasma state at
  several million kelvins.
• Tokamak apparati like the one shown at the right
  show promise for carrying out these reactions.
• They use magnetic fields to heat the material.

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Radiation Exposure
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Medical Uses of Radioisotopes,Diagnosis

• radiotracers
• certain organs absorb most or all of a particular
  element
• can measure the amount absorbed by using tagged
  isotopes of the element and a Geiger counter
• use radioisotope with short half-life
• use radioisotope low ionizing
• beta or gamma

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Medical Uses of Radioisotopes,Diagnosis

• PET scan
• positron emission tomography
• C-11 in glucose
• brain scan and function

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Medical Uses of Radioisotopes,Treatment -
Radiotherapy

• cancer treatment
• cancer cells more sensitive to radiation than
  healthy cells
• brachytherapy
• place radioisotope directly at site of cancer
• teletherapy
• use gamma radiation from Co-60 outside to
  penetrate inside
• radiopharmaceutical therapy
• use radioisotopes that concentrate in one area of
  the body