Nuclear Chemistry Chapter 25

1
Nuclear ChemistryChapter 25
2
Characteristics of Chemical Nuclear Reactions

• Chemical Reactions
• Occur when bonds are broken and formed
• Atoms remain unchanged, though they may be
  rearranged
• Involve only valence electrons
• Associated with small energy changes
• Reaction rate is influenced by temperature,
  pressure, concentration, and catalysts

• Nuclear Reactions
• Occur when nuclei emit particles and/or rays
• Atoms are often converted into atoms of another
  element
• May involve protons, neutrons, and low-orbit
  electrons
• Associated with large energy changes
• Reaction rate is not normally affected by
  temperature, pressure, or catalysts

3
Balancing Nuclear Equations

• Rubidium undergoes electron capture to form
  krypton. Show the balanced equation.
• Reactant 81Rb 0e
• 37 -1
• Product 81Kr 0g (x-ray)
• 36 0

4
Balancing Nuclear Equations

• Oxygen-15 undergoes positron emission. Show the
  balanced equation.
• Reactant 15O
• 8
• Product 15N 0b
• 7 1

5
Balancing Nuclear Equations

• Thorium-231 becomes Protactinium-231. Show the
  balanced equation and identify the type of
  radioactive decay.
• Reactant 231Th
• 90
• Product 231Pa 0b
• 91 -1

6
Uranium

• Uranium is a naturally radioactive element that
  can be found in the crust of the Earth.
• This element, quite abundant in many areas of the
  world, is naturally radioactive.
• Certain isotopes of uranium can be used as fuel
  in a nuclear power plant.
• The uranium is formed into ceramic pellets about
  the size of the end of your finger.
• By bombarding uranium with neutrons, neptunium
  can be synthesized, which decays into plutonium
• 238U 1n ? 239U ? 239Np 0b
• 92 0 92 93 -1
• 239Np ? 239Pu 0b
• 93 94 -1

7
Conservation of Mass

• Matter is neither created nor destroyed.
• This is true, with the caveat that matter can be
  converted into energy (and vice versa) according
  to the equation
• DE Dmc2
• DE change in energy,
• Dmchange in mass,
• cspeed of light (3.00x108 m/s)
• Thus, ANY reaction that has a consumes or
  produces energy will also consume or produce an
  accompanying quantity of mass.
• Thus, the total conversion of 1kg of matter
  yields an equivalent of 1 x (3x108)2 9x 1016
  joules - this is approximately the energy output
  of a 200 MW power station running for 14 years!

8
Binding Energy The Mass Defect

• Recall for nuclei to be stable there must exist
  a strong nuclear force between the nucleons that
  is short range, attractive, and can overcome the
  coulomb repulsion of the protons.
• Now suppose we assemble a nucleus of N neutrons
  and Z protons.
• There will be an increase in the electric
  potential energy due to the electrostatic forces
  between the protons trying to push the nucleus
  apart
• but there is a greater decrease of potential
  energy due to the strong nuclear force acting
  between the nucleons and attracting them to one
  another.     
• As a consequence, the nucleus has an overall net
  decrease in its potential energy.
• This decrease in potential energy is called the
  nuclear binding energy
• The decrease per nucleon is called the binding
  energy per nucleon.
• The loss of this energy is, by the mass-energy
  relation, equivalent to a loss of mass called the
  mass defect.

9
The variation of binding energy per nucleon with
atomic mass number
So how is energy released in stars? This can be
explained by a graph of the binding energy per
nucleon against atomic mass number A
10
Releasing Nuclear Energy

• The curve reaches a maximum at iron, which,
  because of its high binding energy per nucleon,
  indicates that the protons and neutrons are very
  tightly bound and iron is a very stable nucleus.
• Beyond iron, the binding energy per nucleon falls
  slightly as A increases towards the more massive
  nuclei.
• Two processes can release energy from the nucleus
  of an atom. They are nuclear fission and nuclear
  fusion.

11
Nuclear Fission

• In nuclear fission a massive nucleus such as
  uranium splits in two to form two lighter nuclei
  of approximately equal mass.
• This happens on the falling part of the curve so
  that mass is lost and binding energy released
  when very heavy elements fission to nuclei of
  smaller mass number. Nuclear fission is
  responsible for the release of energy in nuclear
  reactors and atomic bombs.

12
Fission Inside Nuclear Reactors

• 235U 1n ? 236U ? 92Kr 1n 141Ba 1n
• 92 0 92 36 0 56 0
• Each fission of Uranium-235 releases additional
  nuetrons. If 1 fission reaction produces 2
  neutrons, these 2 neutrons can create 2
  additional fission reactions each.
• This is a self-sustaining process called a chain
  reaction!
• Both the of fissions and amt of energy release
  increase extremely rapidly.
• The explosion from an atomic bomb represents the
  results of an uncontrolled chain reaction.

13
Critical Mass

• It isnt enough just to have a sample of
  fissionable material, like uranium-235.
• You must also have a critical mass of your
  material.
• If there is not a sufficient amount of mass, the
  released neutrons will dissipate before finding
  another unstable nucleus with which to react.
• No chain reaction will form and the reaction will
  be unsustainable.
• The amount of mass necessary to sustain a chain
  reaction is called the critical mass.
• Below this amount is called the subcritical mass.
• Above this amount is called the supercritical
  mass.
• Supercritical masses cause rapid acceleration of
  the reaction and can lead to a violent explosion.

14
Pressurized Water Reactor
15
Components of a Nuclear Reactor

• Fuel Elements Usually pellets of uranium oxide
  (UO2) arranged in corrosion-resistant tubes to
  form fuel rods. The rods, enriched with 3
  uranium-235, are arranged into fuel assemblies in
  the reactor core.
• Control Rod cadmium, hafnium, or boron rods
  absorb excess neutrons, controlling the reaction
  within the reactor. (Secondary shutdown systems
  involve adding other neutron absorbers, usually
  as a fluid, to the system.)
• If the reaction isnt properly controlled,
  disaster results
• Cf. Three Mile Island (U.S. 1979), Chernobyl
  (Ukraine, 1986)
• Moderator This is material which slows down the
  neutrons released from fission so that they cause
  more fission. It may be water, heavy water
  (deuterated), or graphite (carbon).
• Coolant fluid circulating in the reactor core,
  serving to lower the reaction temperature
  usually water

16
Producing Electricity from Nuclear Reactors

• In America today, nuclear energy plants are the
  second largest source of electricity after coal
  -- producing approximately 21 of our
  electricity.
• With the exception of solar, wind, and
  hydroelectric plants, all others including
  nuclear plants
• Convert water to steam
• The steam spins the propeller-like blades of a
  turbine
• The turbine blades spin the shaft of a generator.
• Inside the generator, coils of wire and magnetic
  fields interact to create electricity

17
Turbine Generator
18
Converting Water to Steam

• The energy needed to boil water into steam is
  produced in one of two ways
• by burning coal, oil, or gas (fossil fuels) in a
  furnace
• by splitting certain atoms of uranium in a
  nuclear energy plant.
• Nothing is burned or exploded in a nuclear energy
  plant.
• Rather, the uranium fuel generates heat through
  fission.

19
Fast Breeder Reactors

• Under appropriate operating conditions, the
  neutrons given off by fission reactions can
  "breed" more fuel from otherwise non-fissionable
  isotopes.
• The most common breeding reaction is that of
  plutonium-239 from non-fissionable uranium-238.
• The term "fast breeder" refers to the types of
  configurations which can actually produce more
  fissionable fuel than they use, such as the
  LMFBR.
• This scenario is possible because the
  non-fissionable uranium-238 is 140 times more
  abundant than the fissionable U-235 and can be
  efficiently converted into Pu-239 by the neutrons
  from a fission chain reaction.
• France has made the largest implementation of
  breeder reactors with its large Super-Phenix
  reactor and an intermediate scale reactor
  (BN-600) on the Caspian Sea for electric power
  and desalinization.

20
Breeding Plutonium-239

• Fissionable plutonium-239 can be produced from
  non-fissionable uranium-238 by the reaction
  illustrated.
• The bombardment of uranium-238 with neutrons
  triggers two successive beta decays with the
  production of plutonium. The amount of plutonium
  produced depends on the breeding ratio.

21
Plutonium Breeding Ratio

• In the breeding of plutonium fuel in breeder
  reactors, an important concept is the breeding
  ratio, the amount of fissile plutonium-239
  produced compared to the amount of fissionable
  fuel (like U-235) used to produced it.
• In the liquid-metal, fast-breeder reactor
  (LMFBR), the target breeding ratio is 1.4 but the
  results achieved have been about 1.2 . This is
  based on 2.4 neutrons produced per U-235 fission,
  with one neutron used to sustain the reaction.
• The time required for a breeder reactor to
  produce enough material to fuel a second reactor
  is called its doubling time, and present design
  plans target about ten years as a doubling time.
• A reactor could use the heat of the reaction to
  produce energy for 10 years, and at the end of
  that time have enough fuel to fuel another
  reactor for 10 years.

22
Liquid-Metal, Fast-Breeder Reactor

• The plutonium-239 breeder reactor is commonly
  called a fast breeder reactor, and the cooling
  and heat transfer is done by a liquid metal.
• The metals which can accomplish this are sodium
  and lithium, with sodium being the most abundant
  and most commonly used.
• The construction of the fast breeder requires a
  higher enrichment of U-235 than a light-water
  reactor, typically 15 to 30.
• The reactor fuel is surrounded by a "blanket" of
  non-fissionable U-238.
• No moderator is used in the breeder reactor since
  fast neutrons are more efficient in transmuting
  U-238 to Pu-239.
• At this concentration of U-235, the cross-section
  for fission with fast neutrons is sufficient to
  sustain the chain-reaction.
• Using water as coolant would slow down the
  neutrons, but the use of liquid sodium avoids
  that moderation and provides a very efficient
  heat transfer medium.

23
LMFB Reactor Diagram
24
Liquid Sodium Coolant

• Liquid sodium is used as the coolant and
  heat-transfer medium in the LMFBR reactor.
• That immediately raised the question of safety
  since sodium metal is an extremely reactive
  chemical and burns on contact with air or water
  (sometimes explosively on contact with water).
• It is true that the liquid sodium must be
  protected from contact with air or water at all
  times, kept in a sealed system.
• However, it has been found that the safety issues
  are not significantly greater than those with
  high-pressure water and steam in the light-water
  reactors.
• Sodium is a solid at room temperature but
  liquifies at 98C.
• It has a wide working temperature since it does
  not boil until 892C.
• That brackets the range of operating temperatures
  for the reactor so that it does not need to be
  pressurized as does a water-steam coolant system.
  
• It has a large specific heat so that it is an
  efficient heat-transfer fluid.

25
The Super-Phenix

• The Super-Phenix was the first large-scale
  breeder reactor. It was put into service in
  France in 1984.
• The reactor core consists of thousands of
  stainless steel tubes containing a mixture of
  uranium and plutonium oxides, about 15-20
  fissionable plutonium-239. Surrounding the core
  is a region called the breeder blanket consisting
  of tubes filled only with uranium oxide. The
  entire assembly is about 3x5 meters and is
  supported in a reactor vessel in molten sodium.
  The energy from the nuclear fission heats the
  sodium to about 500C and it transfers that
  energy to a second sodium loop which in turn
  heats water to produce steam for electricity
  production.
• Such a reactor can produce about 20 more fuel
  than it consumes by the breeding reaction. Enough
  excess fuel is produced over about 20 years to
  fuel another such reactor. Optimum breeding
  allows about 75 of the energy of the natural
  uranium to be used compared to 1 in the standard
  light water reactor.

26
Nuclear Fusion

• In nuclear fusion, energy is released when two
  light nuclei are fused together to form a heavier
  nucleus.
• This happens on the rising part of the graph.
• Nuclear fusion is the principal source of energy
  in stars and fusion can happen if each nucleus
  has sufficient kinetic energy to enable them to
  overcome their mutual repulsion, be captured by
  the strong nuclear force and stick together.
• The minimum temperature required to initiate a
  fusion reaction is 4.0 x108 K.
• In star formation, the kinetic energy to do this
  comes from the conversion of gravitational energy
  into thermal energy by the Kelvin Helmholtz
  contraction.
• In the case of stars like the sun, fusion can
  occur when the temperature of the contracting
  cloud reaches about 8 x 106 K.
• It is because of the high temperatures which are
  needed to give the protons sufficient kinetic
  energy, that these nuclear reactions are also
  known as thermonuclear fusion reactions.
• It is fusion of hydrogen nuclei by thermonuclear
  fusion reactions with a release of binding energy
  that is the primary source of energy generation
  in stars.

27
The Tokamak Reactor

• To satisfy the conditions of thermonuclear
  fusion, using deuterium-tritium fuel,
• the plasma temperature T must be in the range
  13108 K,
• the energy confinement time tE must be about 13
  s and
• the density n must be around 131020
  particles/m3.
• To startup a reactor some means of auxiliary
  heating must be used to attain the minimum
  initial temperature of about 108 K.
• After the ignition of the fuel mixture the plasma
  will be heated by the alpha-particles released in
  the reaction and the source of auxiliary heating
  may be turned off.
• The rate of fusion reactions increases with the
  square of the plasma density.
• However, the density cannot increase above
  certain limits without spoiling the plasma
  stability.
• On the other hand, the energy confinement time
  increases with the density, with the degree of
  plasma stability, and with the plasma volume.
• Balancing these requirements, it is possible to
  determine the minimum size for a reactor, which
  depends on the magnetic configuration adopted.
• http//w3.pppl.gov/dstotler/SSFD/

28
How much energy is released during thermonuclear
reactions?

• 4H ? He energy released
• mass of 4 H atoms 4 x 1.008 4.032 amu- mass
  of 1 He atom 4.003 amutherefore... mass
  defect 4.032 - 4.003 0.029 amuUsing the
  mass-energy relation, the mass converted into
  energy is (0.029 amu x 1.66 x 10-27 kg/amu) x
  (3 x 108 m/s)2 4.33 x 10-12 J or,
  equivalently, 27 MeV.

29
Trinity 1945

• On July 16, 1945, at 52945 a.m., the first
  atomic explosion in history took place at the
  Jornado del Muerto (Journey of Death) trail on
  the Alamagordo Bombing Range in New Mexico. An
  extremely tense group of scientists looked on as
  the bomb, named "Gadget," released its 18.6
  kiloton yield, vaporizing the 100-foot steel
  tower it had been hoisted atop.

30
A-Bomb The Nevada Test
31
Test Able An Air Drop in the Bikini Island
32
Test Baker An Underwater Detonation at the
Bikini Atoll
33
Nuclear Fallout

• The National Cancer Institute recently estimated
  that 10,000-75,000 cases of thyroid cancer in the
  United States were caused by the radioactive
  isotope iodine-131 from Nevada A-bomb fallout.
• In addition to the military personnel exposed to
  high levels of radiation in the vicinity of the
  tests, thousands of U.S. citizens downwind may
  have paid a lethal price for the atomic ambitions
  of their own government.

34
Project Ivy Hydrogen Test Bomb