Induced fission by thermal neutrons
Possibility of a chain reaction
The functions of a moderator and the coolant in thermal nuclear reactors
Control of the reaction rate
Factors influencing choice of material for moderator, control rods and coolant
Examples of materials used (details of particular reactors are not required).
- recall that a thermal neutron is a neutron that has energy in the infra red photon range.
- know that if U235 absorbs a thermal neutron (becomes U236) it is very unstable and will split into two (not usually equal) nuclei
- know that a couple (on average 2 to 3) of free neutrons are also produced (these can go on to produce more fissions). The fragments are more stable (energetically viable reaction) and energy is released when this happens.The resulting nuclei are called fission fragments NOT daughter nuclei (that is the terminology in radioactivity!)
- The freed neutrons can go on to produce further fissions, but are usually of too high energy to do this and need to be slowed down. This is done by a MODERATOR (moderates the speed of the neutrons!) such as graphite. It slows the neutron down by allowing multiple interactions (about 50) with the carbon lattice without absorbtion of the neutron into the carbon nucleus - graphite has a 'low cross section for neutrons'.
A chain reaction is a reaction in which the instigator of the reaction is also produced as a product. It is therefore possible for the product of one reaction to go on to take the role of the reactant in a future reaction. Each fission produces neutrons that could go on to produce further fissions so the more atoms you have (greater mass of sample) the more likely that the reaction will continue in a chain reaction. But those neutrons are produced isotropically (equally in all directions) - the production direction is random, so an atom on the surface could well shoot off a neutron out of the Uranium mass and no fissions would then occur from them.
The bigger the surface area of the Uranium sample the more likely that neutrons will be sent out and not be able to make more fissions. As mass increases so does volume and surface area of the sample. A very small mass will have a larger surface area relative to its size than a bigger one so a chain reaction is less likely (greater proportion of its atoms will be on the surface). There is therefore a minimum mass that allows a chain reaction to occur. This is called the critical mass
- it has a critical mass/surface area ratio below which a chain reaction is not viable. As 2/3 neutrons are produced each time a nucleus of Uranium splits the energy produced by reaction would escalate by a factor of about 2/3 at each stage. This would be uncontrolled acceleration of the reaction and be very dangerous (bomb). Control rods of cadmium or boron can be inserted into the reaction vessel to maintain the energy production at the required level. These have a 'high cross section for neutrons' - they absorb the neutrons, taking them out of the reaction preventing further fissions occuriing. The deeper the rods are inserted into the vessel the faster the rate at which energy is being produced will be diminished (more surface area of absorber - more absorbtion) Moderator materials are chosen for low cross section for neutrons - don't absorb neutrons - interact to take kinetic energy from them instead.Control rods are made of materials that absorb neutrons effectively - have a high cross section for neutrons.
Coolant (eg. water or CO2) needs to have a high specific heat capacity so that a large amount of heat energy can be absorbed without it getting too hot
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