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Nuclear 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
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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 Characteristics of Chemical & Nuclear Reactions
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
Balancing Nuclear Equations • Oxygen-15 undergoes positron emission. Show the balanced equation. • Reactant: 15O 8 • Product: 15N + 0b 7 1
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
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
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, • 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 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.
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 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.
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.
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.
Critical Mass • It isn’t 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.
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 isn’t 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
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
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.
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.
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.
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-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.
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 98°C. • It has a wide working temperature since it does not boil until 892°C. • 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.
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 500°C 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.
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.
The Tokamak Reactor • To satisfy the conditions of thermonuclear fusion, using deuterium-tritium fuel, • the plasma temperatureT must be in the range 1~3×108 K, • the energy confinement timetE must be about 1~3 s and • the densityn must be around 1~3×1020 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/
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.
Trinity 1945 • On July 16, 1945, at 5:29:45 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.
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.
