Three Chapter 10

NUCLEAR NOTES

Mass-Energy relationships in Nuclear Reactions

• We have seen that in a nuclear reaction, the total of the Atomic number and mass number on either side of the reaction is constant.
• Nuclear reactions can liberate or absorb energy. The energies involved are huge.
• Einstein in his theory of relativity suggested that mass is just another form of energy.
• He suggested that changes in Mass meant there was changes in energy according to the relation:

E = mc²

• Cockcroft and Walton showed that this relationship is true.
  • In any nuclear reaction, (e.g.: A proton collides with a lithium-7 atom to produce two helium-4 atoms.) the total mass of the products is less than the total mass of the reactants.
  • The difference when substituted into the equation given above, equals the energy released.
  • Mass has been converted into energy.
• Nuclear Energy - The energy produced when mass is converted into energy. The name is used because the conversion occurs in the nucleus.
• There are two types of nuclear reactions which convert mass into energy.
  • fission
  • fusion

Nuclear fission

• By using E = mc² and observational data, it was determined that if heavier elements were split into lighter, energy is released.
• Fission - The splitting of heavier atoms into lighter ones.
  • Because of the positive charge on the nucleus, fission cannot be produced by positively charged particles like a particles.
  • Neutrons must be used to split atoms.
• Fermi in 1934 produced the first fission reaction but was unable to prove it because he was unable to identify the products.
• Hahn and Strassmann (Germans) were the first to prove that fission had occurred. The reaction that had occurred was:
  • Uranium-235 was struck by a neutron to produce barium-144, krypton-90, two neutrons and energy.
• Frisch (Austrian) with others verified this result and coined the word fission.
• The benefits of fission include the tremendous energy released.
• During fission, neutrons are also released. these neutrons can cause other atoms to split and release neutrons which can cause more reactions etc.
• Nuclear Chain Reaction - The process where by the splitting of one atom produces neutrons which split other atoms and so on.
• Natural Uranium is composed of isotopes:
  • U - 238 about 99.3 %
  • U - 235 about 0.7 % (the only type we can split)
  • U - 234 trace amounts
• U - 235 atoms are not abundant enough to sustain a nuclear reaction in a sample of uranium. We must increase their abundance.
• Enrichment - The process of increasing the proportion of U - 235 isotopes in natural uranium.
• In any nuclear reaction, neutrons are lost by:
  • escaping the uranium.
  • Being absorbed by impurities.
• To sustain a chain reaction, enough neutrons have to be produced to cause the next splitting, even though many neutrons are lost.
• In order to produce enough neutrons to sustain a chain reaction a certain minimum mass of
• U - 235 must be present.
• Critical mass - The mass of a radioisotope like U - 235 needed to sustain a nuclear reaction.
• Once critical mass is present, the reaction takes place without outside help. We say, "The material has gone critical."
• If the neutrons are slowed down, we can increase the probability of a neutron encountering a
• U - 235 nucleus.
• Moderator - The substance used to slow down neutrons. (e.g.: graphite, water, heavy water, beryllium).
• A nuclear reaction is controlled by inserting a substance that absorbs neutrons into the uranium (e.g.: boron or cadmium).
• Control rods - the rods which contain the material that absorbs neutrons when inserted into the U - 235 pile.
• If enough neutrons are absorbed, the reaction will stop.
• In a nuclear bomb, there are no control rods, and the reaction continues unchecked.
• With a bomb, the release is:
  • energy
  • neutrons
  • gamma radiation
• Nuclear reactors are like a nuclear bomb that is under control. The heat released is used to produce other forms of energy.

The First Reactor and the First Atomic Bomb

• Einstein, Fermi and others moved to the U.S. because of the political climate in Europe during World War II.
• These scientists convinced President Roosevelt to give financial assistance to build the first reactor.
• The project was called the Manhattan Project.
• Fermi led the project and with others built the first reactor in a squash court at the University of Chicago in December 1942.
• The reactor consisted of:
  • graphite blocks
  • blocks of uranium were inserted into holes in the graphite blocks
  • cadmium strips were inserted into the pile to absorb neutrons and control the reaction.
• Radiation levels were monitored as layer upon layer was added.
• At 57 layers, neutron intensity increased dramatically (A chain reaction was occurring).
• Cadmium strips were quickly inserted to shut the reaction down.
• The project continued in order to build a bomb which would defeat Germany and Japan.
• At Oak Ridge Tennessee, U - 235 was enriched and collected as well as Plutonium - 239. The purpose was to build three bombs.
• July 16, 1945, one U- 235 bomb was tested in the desert in New Mexico.
• The other U - 235 bomb was dropped on Hiroshima on August 6, 1945. The Pu - 239 bomb was dropped on Nagasaki on August 8, 1945.
• Thousands of people died from the effects of heat, high energy neutrons, gamma radiation and the shock wave.
• Of the people who survived, many later died of cancer.

Nuclear Power Reactors

The Chain Reaction

• 99% of uranium is U - 238 which is not fissionable.
• Less than 1 % of the uranium is U - 235 which we can fission.
• A natural sample of uranium is unlikely to cause a chain reaction because:
  • most free neutrons are absorbed by U - 238 to produce Pu - 239.
  • the remainder of neutrons are moving too quickly to cause U - 235 to fission.
• A fast neutron will not lose energy to a U - 238 atom on collision (The collision is similar to that of a marble hitting a bowling ball).
• If a fast neutron strikes lighter molecules, or atoms (e.g.: water molecules), energy may be transferred
• If a neutron strikes a lighter molecule and loses energy, it is more likely to cause fission of a U - 235 atom.
• Moderator - a substance that slows down neutrons.
• Water is a good moderator, but often takes neutrons out of the reactor.
• Heavy water does not take the neutrons out of the reactor since heavy water is oxygen combined with deuterium (Deuterium already has an extra neutron, and so is less likely to absorb more neutrons.).
• Deuterium - a hydrogen atom with a neutron in the nucleus.
• To prevent too many neutrons from being produced, control rods made of materials like cadmium are inserted into the reactor.
• Control rods - rods inserted into a nuclear reactor to absorb neutrons to prevent the reaction from getting out of control.

A Simplified Nuclear Power Plant

• Fission reactions occur inside uranium fuel bundles.
• The fuel bundles are inside of the reactor core inside of a containment building.
• The building shields the environment from the radiation.
• Water acts as a moderator to slow down neutrons and as a coolant for carrying heat out of the core.
• To start the fission control rods are raised.
• Once one fission occurs, the moderator slows down the neutrons, which collide with other nuclei (U - 235) to produce more fissions etc. The chain reaction has started.
• The rate of fission is controlled by raising or lowering the control rods.
• The kinetic energy of the fission products and radiation raises the temperature of the core and moderator.
• The moderator is under pressure, and so it does not boil or vaporize. It is super heated.
• The moderator is pumped to a heat exchanger which transfers heat to normal water.
• The normal water boils and turns to steam.
• The steam expands through a turbine which is connected to a generator which produces electricity.
• Not all the heat will generate electricity. Some is lost to the environment by pumping the water into lakes and rivers, and by transfer to the air in cooling towers.
• Differences in reactors occur in types of moderators and fuels used.
• The American uses U - 235 and a water moderator.
• The British and Russian reactors use U - 235 and a graphite moderator
• The Canadian uses natural uranium oxide and heavy water (D₂O).

Candu Reactor

• Candu - Canadian Deuterium Uranium reactor.
• The Candu uses heavy water as the moderator and coolant. Otherwise it is setup as described above.
• The heavy water makes the Candu one of the safest reactors.
• If excessive heat builds up, the moderator can be drained, and neutrons are no longer slowed and therefore natural uranium will no longer fission.
• The Candu can be constructed on a larger scale
• The Candu can be refueled without being shut down.

Pressured Water Reactor

• The is the American model
• Uses ordinary water as a moderator and coolant.
• Ordinary water absorbs more neutrons than heavy water, therefore must be fuelled with enriched uranium (U - 235)
• The rest of the reactor may be as previously discussed, or water may boil inside of the reactor, and this water runs the turbines (i.e.: the moderator expands through the turbines.)

Gas Cooled Reactor

• This is the British design.
  • uses graphite moderator and CO_2(g) or He_(g) under pressure as a coolant.
  • Hot gas passes to a boiler to produce steam for the turbines.

Fast Breeder Reactor

• In fission, 2 or 3 neutrons are emitted.
• One of these is used to continue the chain reaction, and the others are absorbed by uranium-238 atoms which are held in a blanket around the core.
• The absorbing uranium-238 atoms are changed to plutonium-239 or uranium-233, both of which are fissionable.
• With this reactor, more fissionable by-products are produced than is used as fuel.
• The fuel is uranium-235.
• Conventional reactors use about 1% of the uranium mined.
• Breeder reactors use about 50% of the mined uranium.
• Breeder reactors are technically complex.
  • It is difficult to separate fissionable products.
  • Plutonium is deadly and can be used in atomic bombs.

Safety and the Nuclear Power Reactor

• There is risk in every form of power generation.
• The design must reduce risk.
• Acceptable levels of risk must be decided on.
• The biggest risk is if the cooling system fails.
• The core temperature rises with a cooling system failure to temperatures above 5000^o C and it melts down.
• This process continues, and releases radiation into the environment.
• The probably of a melt down is low but it is not zero. This is evidenced by the partial melt down at Three Mile Island, and the melt down at Chernobyl.
• The Candu reactor is considered to be one of the safest and most efficient. It has three systems for shutting down the reactor.
  • The heavy water moderator can be dumped by gravity into a storage tank under the reactor core. Neutrons then do not slow down and the reaction stops.
  • Boron can be injected into the moderator. The boron absorbs neutrons and therefore suppresses the chain reaction.
  • Cadmium control rods are held above the reactor core by electromagnetic clutches. If the power fails, they automatically fall into the core to stop the reaction.
• In the Candu reactor, there is a higher risk of a rupture of the moderator tubes.
  • The moderator can become contaminated with radioactive material in the fuel rods.
  • If the break is outside of the core, the superheated heavy water changes to steam and creates pressure in the reactor building.
  • If the pressure in the reactor building is not relieved, the building ruptures and releases the steam and contents to the external environment
• Candu reactors have a central vacuum building.
  • If the pressure is building up inside the reactor building, louvres open, and the steam is drawn into a vacuum building, which has lower air pressure than the reactor building.
  • Water in a tank above the vacuum building is released into the steam which condenses it.
  • This creates an even lower pressure which continues to draw steam into the vacuum building.
  • The reactor building will therefore have inside pressure released and it will remain safe.
  • Emergency coolant flows into the reactor core to keep it from overheating
• For a major nuclear accident to occur, a number of these systems must fail at the same time.

By products of Nuclear Power

• By products of nuclear power - these are high-level radioactive wastes, low-level radioactive wastes and thermal discharges.
  • Most come from the fission process.
  • Uranium-235 fissions to produce radioactive barium, krypton and other products.
  • Neutrons emitted transform uranium-238 into plutonium, americium and other products, most of which have long half-lives.
  • High-level wastes would be those products which produce radiations that are many times that of the radiation released by uranium (barium, krypton, plutonium, americium, and other products)
  • Spent fuel rods are stored under 4.0 m of water for a few years.
  • Some wastes decay to 0.1% of their initial level within 300 years.
  • Plutonium takes 250 000 years and therefore requires safe storage.
• There are two methods for intermediate storage of wastes which have been proposed (25 years approximately)
  • Concrete silos - above ground
  • Water filled pools
• Intermediate storage allows the concentration of radiation to decrease until permanent storage is possible.
• Long term storage (over 100 000 years) has many unsolved problems.
• Some suggested long term storage methods are:
  • burying the wastes deep in salt mines
  • burying the wastes in hard rock like the Canadian shield
• Canada's uranium reserves are 50 to 60 years.
  • This can be extended by producing plutonium, which may cause problems like:
    • contamination of the environment
    • problems of disposal of wastes
    • possibility of theft by terrorists
• Production of power by using fission is limited by our management of the problems.

Low Level Radioactive Wastes

• Small amounts of low level waste is produced as fission products and activation products.
• Fission Products - lighter atoms that are produced in the fission process.
  • formed in fuel bundles
  • may escape the reactor through small defects in fuel coverings.
• Activation Products - materials which result from the bombardment of materials with neutrons
  • Bombardment of things like air, coolant, metal in the plumbing system, suspended particles in the coolant, the formation of tritium by bombardment of deuterium in heavy water, etc, results in activation products.
• Elaborate precautions are required to detect and contain radiation leaks.
  • Methods used include the use of detectors.
  • Workers are checked daily for excessive exposure levels.
• Despite the precautions, radiation levels increase around nuclear power plants.
• Most solid and liquid low-level wastes are buried in soil beds that are able to trap radioactive parts and prevent them from being carried into nearby rivers and streams.These low-level wastes can concentrate in the food chain.

Thermal Discharges

• All fossil fuel and nuclear plants require cooling water to condense the steam after it has gone through the turbine
• This water comes from rivers, lakes or oceans.
• The used warmer water is discharged back into the body of water if the body of water is large enough.
• If not large enough, the water must be cooled in cooling towers and ponds and then reused.
• In large bodies of water, the effect is smaller though there may be some change in distribution of species.
• Small bodies may be affected greatly. For example:
  • excessive formation of algae
  • depletion of O₂ in water
• Ways are being investigated to use this waste energy. For example:
  • It could be used to provide heating for industries and homes near the power plant.
  • It could heat soil to increase crop output.
  • It heat green houses

Nuclear Fusion

• Fusion - The process in which light elements combine to produce heavier ones.
• All nuclei are positively charged
• To fuse nuclei, they must be brought as close as 1 X 10^-15 m apart, where the strong nuclear force overpowers the repelling electrical forces.
• To bring the nuclei this close, we must accelerate atoms to very high speeds.
• This is the process in the sun and it occurs at temperatures above 1 X 10⁶ degrees Celsius.
• As with fission, the mass of the products are less than the reactants.
• Mass loss is responsible for energy production according to the equation E = mc².
• The first use of fusion was in the hydrogen bomb.
• A fission bomb explodes and releases high speed neutrons. These neutrons combine with lithium - 6 nuclei which decay into tritium and helium. Because of the high temperature (1 X 10⁸ degrees Celsius) the deuterium (also present in the bomb) and the tritium are moving fast enough to fuse to form helium, a neutron and energy.
• The advantages of fusion power:
  • The fuel is cheap and easily obtained
  • The by-products are relatively harmless
• Controlled fusion has only occurred in bombs.
• There are two methods for attempting to control fusion for the purpose of power generation.
  • Hydrogen is ionized and compressed by strong electric and magnetic fields so as to raise the temperature to produce fusion. This process has produced fusion, but only for short periods of time.
  • High energy lasers focus on a pellet of frozen deuterium. The lasers cause the pellet to implode to produce the high temperatures.
• Fusion reactors generally have consumed more energy than they have produced.

Nuclear energy - An Answer or Challenge

• Most energy now produced is hydroelectric or thermal electric (fossil fuels or uranium).
• Solar energy won't affect us much in the future (Too small a scale)
• In the future, a new type of reactor will probably be developed that uses plutonium (the by-products of the present reactors).
• Fusion is still an elusive energy source and will probably remain so for the next 20 years or so.
• The word nuclear has caused fear in the population. This is mainly due to ignorance.
• The issues related to nuclear power include
  • risk of a system failure
  • risk of coolant system rupture
  • disposal of wastes
  • where to build the plants
  • sale of plants to other countries
  • processing of high level wastes.
• Solutions to these problems are everybody's concern.

January 22, 2014