Key Characteristics of Prompt Neutrons
- Prompt neutrons are emitted directly from fission and they are emitted within very short time of about 10-14 second.
- Most of the neutrons produced in fission are prompt neutrons – about 99.9%.
- For example a fission of 235U by thermal neutron yields 2.43 neutrons, of which 2.42 neutrons are prompt neutrons and 0.01585 neutrons are the delayed neutrons.
- The production of prompt neutrons slightly increase with incident neutron energy.
- Almost all prompt fission neutrons have energies between 0.1 MeV and 10 MeV.
- The mean neutron energy is about 2 MeV. The most probable neutron energy is about 0.7 MeV.
- In reactor design the prompt neutron lifetime (PNL) belongs to key neutron-physical characteristics of reactor core.
- Its value depends especially on the type of the moderator and on the energy of the neutrons causing fission.
- In an infinite reactor (without escape) prompt neutron lifetime is the sum of the slowing down time and the diffusion time.
- In LWRs the PNL increases with the fuel burnup.
- The typical prompt neutron lifetime in thermal reactors is on the order of 10-4 second.
- The typical prompt neutron lifetime in fast reactors is on the order of 10-7 second.
Slowing Down and Diffision Times for Thermal Neutrons in an Infinite Medium
Source: Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
Interactive chart – Infinite Multiplying System Without Source and Delayed Neutrons
Press the “clear and run” button and try to stabilize the power at 90%.
Look at the reactivity insertion you need to insert in order to stabilize the system (of the order to tenth of pcm).
Do you think that such a system is controlable?