Controlling Nuclear Fission

1. Controlling Nuclear Fission

2. Thermal neutrons • Uranium 235 is the main fissile material which we are concerned with. • Uranium-233 and plutonium-239 can also be used but must be produced artificially • U-235, can be split by bombarding it with a slow, or thermal, neutron. • Slow neutrons are called “thermal” because their average kinetic energies are about the same as those of the molecules of air at ordinary temperatures. • krypton and barium are the major products of the process.

3. Fission thermal

4. Since this reaction also releases neutrons, a chain reaction is possible, provided at least one neutron per fission is captured by another nucleus and causes a second fission. • If at least one neutron from U-235 fission strikes another nucleus and causes it to undergo fission, then the chain reaction will continue. • If the reaction will sustain itself, it is said to be critical,.

5. The fission bomb • In an atomic bomb, the number of subsequent fissions is greater than 1 and the uncontrolled chain reaction increases rapidly to an explosion. • This state is achieved simply by bringing enough U235 together. This is the critical mass. • For pure U-235 the critical mass is about 52kg i.e. a sphere of about 17cm • However weapons grade uranium 235 is only around 80% pure.

7. Nuclear Reactors

9. Controlled Fission (the nuclear reactor • In a nuclear reactor the chain reaction is controlled, • the number of neutrons producing additional fission must be exactly 1.0 in order to maintain a steady flow of energy. • The rate of reaction may be controlled in a nuclear reactor by effectively using a “dilute” mixture containing the U-235, U-238,

10. Neutron Moderators • For a sustained nuclear reaction we require slow thermal neutrons which are more easily captured by a fissile nucleus • a neutron moderator is a substance which reduces the velocity of fast neutrons, thereby turning them into thermal neutrons. • atoms which do not absorb neutrons but recoil when struck by fast neutrons are good for this purpose. • The fast neutrons lose kinetic energy in this process. • Light atoms such as graphite recoil more causing the neutrons to lose more energy.

11. “neutron poisons” that is materials which absorb neutrons are used in some processes (hafnium is sometimes used for this purpose) • Nuclei which absorb neutrons are said to have a high neutron capture cross section.

12. Control Rods • Movable control rods containing neutron-absorbing material are often used for fine control of the core of the reactor. • Chemical elements with a sufficiently high capture cross section for neutrons include indium, boron and cadmium.

13. Coolants Coolants in a modern thermal nuclear reactor are responsible for removing the heat generated in the core of the reactor to generate steam elsewhere. The following list gives the properties of the idea coolant • Low melting point. • High boiling point. • Non-corrosive properties. • Low neutron absorption cross section. • Good moderator • Stability in high radiation environment • Thermal stability. • Low induced radioactivity. • Good heat transfer properties

14. Advantages of Water as a Coolant • Water technology well known. • Water is cheap. • Water is very effective moderator of neutron energy • Water has high heat capacity. • Ordinary leakage can be tolerated. • Fission products are contained, not circulated. • Radioactivity of coolant is short-lived if kept pure. .

15. Disadvantages of water • Water must be highly pressurized to achieve even reasonably high temperature without boiling. • The temperature is limited in metallic fuel elements. • Fission product activity in the core builds up to high a level. • Pure hot water is highly corrosive, requires special materials for the core • Heat transfer only moderately efficient. • Water would flash to steam in case of rupture of containment • Water reacts with uranium, thorium, and construction metals under certain conditions.

16. Sodium and sodium mixtures • The heat transfer properties of liquid metals make them attractive reactor coolants. • In addition, liquid metal systems operate at low pressure require the minimum of pumping power and are capable of operating at the high temperatures required to generate steam for modern turbine generators. • Two liquid metals, sodium and mixtures of sodium and potassium have been used extensively in nuclear reactors

17. Carbon Dioxide Advantages • Corrosion by coolant is negligible. • Coolant doesn't react with fuel or with other core materials. • Coolant will not flash into vapor if primary system is ruptured. • Coolant has very low capture cross section. • Any fuel can be used, including natural uranium. • Coolant is cheap. • Ordinary leakage can be tolerated. • Gas turbine may be employed

18. Carbon Dioxide disadvantages • Reactor vessel and heat exchangers large and expensive. • Heat transfer efficiency is low. • Coolant must be pressurized. • Power density is low. • Carbon dioxide dissociates above 300 Celsius.