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					Nuclear Chemistry

L. Scheffler
  The Nucleus




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.
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
Radioactive Nuclei

 The nuclides of some of the
  isotopes of certain isotopes
  unstable, or radioactive.
 They are known as radionuclides.

 Radionuclides can decay into a
  different nuclide by several
  different methods.
Spontaneous Radioactivity
    Spontaneous radioactivity involves
     unstable nuclei which decay with the
     eventual formation of a stable nuclei.
    Thus some nuclei are stable and others
     are not, for example 12C and 14C , 3H
     and 40K. The isotopes in these
     examples have the same number of
     protons but differ in the number of
     neutrons.
 
      Stability Factors
     Two important factors determine
     nuclear stability
1.   The mass number (which is the total
     number of nucleons in the nucleus)
2.   the neutron to proton ratio.
       Stability Factors
                                            180

                                            160
   In the nucleus,
    positively charged
                                            140

    protons repel each                      120




                             # n eu trons
    other.                                  100

   As the number of                        80
    protons in the nucleus                  60
    increases, the forces
    of repulsion between                    40

    the protons increases                   20
    substantially.                           0
   More neutrons are                             0   50               100   150

    required to maintain                                   # protons
    stability
         Alpha Emitters
   If a radionuclide has a lower neutron to proton
    ratio than for a stable nucleus, it falls to the right
    of the stable nucleus
   To increase the neutron to proton ratio back to a
    stable ratio, it undergoes alpha decay.




   Releasing an alpha particle raises the neutron to
    proton ratio since the number of neutrons is
    greater than or equal to the number of protons
         Beta Emitters
   If a radionuclide has a higher neutron to proton
    ratio than for a stable nucleus, it falls to the left of
    the stable nucleus
   To reduce the number of neutrons back to the
    stable ratio it undergoes beta decay.
   A neutron disintegrates with the emission of a high
    speed electron known as a beta particle:




    A proton is produced from the neutron. This raises
     the number of protons by one and reduces the
     number of neutrons by one.
    The neutron to proton ratio is lowered until it
     reaches a stable value. Then no further radioactive
     occurs
Other Forms of Nuclear Decay
   If a radionuclide has a lower neutron to
    proton ratio, a proton can also be
    transformed into a neutron by either
    positron emission or electron capture.
        Heavy Nuclei Are Always
        Unstable
   If the total number of nucleons exceeds 209, the
    limit for stable nuclei, the nucleus always lies
    beyond the stable limit
    The nucleus is always radioactive.
   Several types of radioactive decay are involved
    in order to reach the stability.
    For example, the radioisotope 238U undergoes a
    sequence of fourteen radioactive decay steps
    before forming the final product 206Pb.
     Nuclear Binding Energy

  When a nuclear reaction occurs the mass
  of the particles is not conserved.
 The early quantum physicists were able to
  demonstrate that there is a small change
  in mass when a particle is moving. It is
  given by the formula
        .

   .
Kinetics of Radioactive
Decay
 Nuclear transmutation is a first-
  order process.
 The kinetics of such a process, you
  will recall, obey this equation:

             Nt
          ln    = kt
             N0
        Kinetics of Radioactive
        Decay
   The half-life of such a process is:

                   0.693
                         = t1/2
                     k
• 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.
     Kinetics of Radioactive
     Decay
• A wooden object from an archeological site is
  subjected to radiocarbon dating.
• The activity of the sample that is due to 14C is
  measured to be 11.6 disintegrations per second.
  The activity of a carbon sample of equal mass
  from fresh wood is 15.2 disintegrations per
  second.
• The half-life of 14C is 5715 yr. What is the age of
  the archeological sample?
  Kinetics of Radioactive
  Decay
First we need to determine the rate
constant, k, for the process.
             0.693
                   = t1/2
               k
             0.693
                   = 5715 yr
               k
            0.693
                   =k
           5715 yr
   1.21  10−4 yr−1 = k
  Kinetics of Radioactive
  Decay
Now we can determine t:
               Nt
            ln    = kt
               N0
             11.6
          ln
             15.2 = (1.21  10−4 yr−1) t

          ln 0.763 = (1.21  10−4 yr−1) t
          6310 yr = t
.

   .
Energy in Nuclear
Reactions
 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.
       Energy in Nuclear
       Reactions
 There is a tremendous amount of energy
  stored in nuclei.
 Einstein’s famous equation, E = mc2,
  relates directly to the calculation of this
  energy.
      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
26-8 Nuclear Fission
       Nuclear Fission
   How does one tap all that energy?
   Nuclear fission is the type of reaction carried
    out in nuclear reactors.
Nuclear Fission
   Enrico Fermi 1934.
      In a search for transuranium elements U
       was bombarded with neutrons.
       emission was observed from the
       resultant material.
   Otto Hahn, Lise Meitner and Fritz Stassman
    1938.
      Z not greater than 92.
      Ra, Ac, Th and Pa were found.
      The atom had been split.
       Nuclear Fission

235U
 92
           1
       + 1 0n     → Fission fragments + 3 0 n + 3.2010-11 J
                                          1




                Energy released is 8.2107 kJ/g U.
This is equivalent to the energy from burning 3 tons of coal
Nucleons more tightly bound in
Fission Product Nuclei – Gives 200
Mev Energy per Fission
Nuclear Power Plants
Worldwide Nuclear Power
Reactors
 There are 440 nuclear power
  reactors in 31 countries.
 30 more are under construction.

 They account for 16% of the
  world’s electricity.
 They produce a total of 351
  gigawatts (billion watts) of
  electricity.
World Nuclear Power
Plants
            How a Nuclear Reactor
            works
   235U fissions by absorbing a neutron and producing 2 to 3
    neutrons, which initiate on average one more fission to make a
    controlled chain reaction
   Normal water is used as a moderator to slow the neutrons since
    slow neutrons take longer to pass by a U nucleus and have more
    time to be absorbed
   The protons in the hydrogen in the water have the same mass as
    the neutron and stop them by a billiard ball effect
   The extra neutrons are taken up by protons to form deuterons
   235U is enriched from its 0.7% in nature to about 3% to produce

    the reaction, and is contained in rods in the water
   Boron control rods are inserted to absorb neutrons when it is
    time to shut down the reactor
   The hot water is boiled or sent through a heat exchanger to
    produce steam. The steam then powers turbines.
Nuclear Reactors Design
The Core of a Reactor
Inside a Nuclear
Reactor
   Steam outlet
    



   Fuel Rods
    

   Control Rods
    
Nuclear Fission from Slow
Neutrons and Water Moderator
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.
Breeder Reactors
   Fertile reactors produce other
    fissile material.
         238U
          92
                + 1 0n →
                    1        239U
                              92


                 →               -1 
         239U        239Np   +    0
          92          93


                 →               -1 
         239Np       239Pu        0
          93          94     +
Production of Plutonium (Pu)
in Nuclear Reactors
 239Pu    is produced in nuclear reactors by
    the absorption of a neutron on 238U,
    followed by two beta decays
   239Pu also fissions by absorbing a

    thermal neutron, and on average
    produces 1/3 of the energy in a fuel
    cycle.
   239Pu is relatively stable, with a half life

    of 24 thousand years.
   It is used in nuclear weapons
   It can be bred for nuclear reactors
        Disadvantages of Breeder
        Reactors
   Liquid-metal-cooled fast breeder
    reactor (LMFBR).
     Sodium becomes highly radioactive in
      the reactor.
     Heat and neutron production are high,
      so materials deteriorate more rapidly.
     Radioactive waste and plutonium
      recovery.
        • Plutonium is highly poisonous and has a long
          half life (24,000 years).
Advantages of Nuclear
Power:
1. Clean (no air pollution, including greenhouse
gases)
2. Safe compared with other fuels
3. Price competitive with fossil fuels
4. 100 year supply of 235U
5. Infinite supply (>10,000 years) of 238U (if we use
        breeder reactors)
Nuclear Fusion
             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.
       Nuclear Fusion
   Fusion produces the energy of the sun.
   Most promising process on earth would be:
                      3        4
               2
               1H   + 1H   →   2 He   + 1n
                                        0

   Plasma temperatures over 40,000,000 K to
    initiate a self-sustaining reaction (we can’t do
    this yet).
   Lithium is used to provide tritium and also act as
    the heat transfer material – handling problems.
   Limitless power once we start it up.
Tokomak
                                      Fission is the release of                              D-T Fusion                      Fusion is the release of
                                                                                                               4He
                                      energy by splitting heavy                    D                           3.52 MeV      energy by combining two
                                      nuclei such as Uranium-235                   T
                                                                                                                Neutron
                                                                                                                14.1 MeV     light nuclei such as
                                      and Plutonium-239                                                                      deuterium and tritium

                                                                                  • The goal of fusion research is to
How does a nuclear plant work?
                                                                                    confine fusion ions at high
• Each fission releases 2 or 3
                                                                                    enough temperatures and
  neutrons
                                                                                    pressures, and for a long enough
• These neutrons are slowed down
                                                                                    time to fuse
  with a moderator to initiate more
                                                                                  • This graph shows the
  fission events
                                                                                    exponential rate of progress over
• Control rods absorb neutrons to             Controlled Fission Chain Reaction     the decades                                        Confinement Progress
  keep the chain reaction in check
                                                                                        There are two main confinement approaches:
                                                  The energy from the
                                                  reaction drives a steam
                                                                                                                           • Magnetic Confinement uses strong
                                                  cycle to produce
                                                                                                                             magnetic fields to confine the
                                                  electricity
                                                                                                                             plasma
                                                                                                                           • This is a cross-section of the
                Nuclear Power Plant                                                                                          proposed International Thermo-
                                                                                                                             nuclear Experimental Reactor
Nuclear Power produces no greenhouse gas emissions; each year                                                                (ITER)
U.S. nuclear plants prevent atmospheric emissions totaling:
•5.1 million tons of sulfur dioxide                                               • Inertial Confinement uses powerful
•2.4 million tons of nitrogen oxide                                                 lasers or ion beams to compress a
•164 million tons of carbon                                                         pellet of fusion fuel to the right
                                                                                    temperatures and pressures
Nuclear power in 1999 was the cheapest                                            • This is a schematic of the National
source of electricity costing 1.83 c/kWh                                            Ignition Facility (NIF) being built at
compared to 2.04 c/kWh from coal                                                    Lawrence Livermore National Lab
Nuclear “Accidents”
       Three Mile Island – partial meltdown
                           due to lost coolant.



       Chernobyl – Fault of operators and
                   testing safety equipment
                   too close to the limit.


       France – safe operation provides
                80% of power requirements
                for the country.
        26-10 Effect of Radiation
        on Matter
   Ionizing radiation.
      Power described in terms of the number of
       ion pairs per cm of path through a material.

                     P > P > P

       Primary electrons ionized by the radioactive
        particle may have sufficient energy to
        produce secondary ionization.
 Radiation Dosage
1 rad (radiation absorbed dose) = 0.001 J/kg matter


1 rem (radiation equivalent for man) = radQ

 Q = relative biological effectiveness
Table 26.4 Radiation
Units
.

   .
.

   .

				
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