nuclear chemistry chapter 25
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DESCRIPTIONNuclear Chemistry Chapter 25. 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 - PowerPoint PPT Presentation
Characteristics of Chemical & Nuclear Reactions Chemical ReactionsOccur when bonds are broken and formedAtoms remain unchanged, though they may be rearrangedInvolve only valence electronsAssociated with small energy changesReaction rate is influenced by temperature, pressure, concentration, and catalystsNuclear ReactionsOccur when nuclei emit particles and/or raysAtoms are often converted into atoms of another elementMay involve protons, neutrons, and low-orbit electronsAssociated with large energy changesReaction rate is not normally affected by temperature, pressure, or catalysts
Balancing Nuclear EquationsRubidium undergoes electron capture to form krypton. Show the balanced equation.Reactant: 81Rb + 0e 37 -1Product: 81Kr + 0g (x-ray) 36 0
Balancing Nuclear EquationsOxygen-15 undergoes positron emission. Show the balanced equation.Reactant: 15O 8 Product: 15N + 0b 7 1
Balancing Nuclear EquationsThorium-231 becomes Protactinium-231. Show the balanced equation and identify the type of radioactive decay.Reactant: 231Th 90 Product: 231Pa + 0b 91 -1
UraniumUranium 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
Conservation of MassMatter 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, Dm=change in mass, c=speed 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!
Binding Energy & The Mass DefectRecall: 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 energyThe 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.
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
Releasing Nuclear EnergyThe 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.
Nuclear FissionIn 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.
Fission Inside Nuclear Reactors235U + 1n 236U 92Kr + 1n + 141Ba + 1n 92 09236 0 56 0Each 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.
Critical MassIt 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.
Pressurized Water Reactor
Components of a Nuclear ReactorFuel 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 resultsCf. 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
Producing Electricity from Nuclear ReactorsIn 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 turbineThe turbine blades spin the shaft of a generator.Inside the generator, coils of wire and magnetic fields interact to create electricity
Turbine & Generator
Converting Water to SteamThe energy needed to boil water into steam is produced in one of two ways: by burning coal, oil, or gas (fossil fuels) in a furnaceby 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.
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.
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.
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.
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-s