Description of Prompt Neutrons

Prompt Neutrons

Prompt fission neutrons and prompt gamma rays are emitted when excited primary fission fragments release their energy to reach a more stable configuration. This happens shortly after the fission of the nucleus on two fission fragments. Studying prompt neutrons and gamma rays is important for deepening our understanding of the nuclear fission process and core design calculations or radiation shielding calculations.

Most of the neutrons produced in fission are prompt neutrons. Usually, more than 99 percent of the fission neutrons are prompt neutrons. Still, the exact fraction is dependent on the nuclide to be fissioned and is also dependent on an incident neutron energy (usually increases with energy). For example, fission of ²³⁵U by thermal neutron yields 2.43 neutrons, of which 2.42 neutrons are the prompt neutrons, and 0.01585 neutrons (0.01585/2.43=0.0065=ß) are the delayed neutrons. The production of prompt neutrons slightly increases with incident neutron energy.

Prompt neutrons are emitted within 10^-14 seconds. This is very important because it is very fast and causes a fast response of the reactor power in case of prompt criticality. Therefore nuclear reactors must operate in a prompt subcritical, delayed critical condition. All power reactors are designed to operate in delayed critical conditions and are provided with safety systems to prevent them from ever achieving prompt criticality.

The reactor’s response on the reactivity insertion is determined by a neutron generation time (not by emission time), which is the average time from a prompt neutron emission to a capture that results only in fission.

Neutron Production - Prompt Neutrons — Most of the neutrons produced in fission are prompt neutrons. Usually, more than 99 percent of the fission neutrons are prompt neutrons. Still, the exact fraction is dependent on certain nuclides to be fissioned and is also dependent on an incident neutron energy (usually increases with energy).

Table of key prompt and delayed neutrons characteristics. Thermal vs. Fast Fission

References:

Nuclear and Reactor Physics:

J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467
G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.

Advanced Reactor Physics:

K. O. Ott, W. A. Bezella, Introductory Nuclear Reactor Statics, American Nuclear Society, Revised edition (1989), 1989, ISBN: 0-894-48033-2.
K. O. Ott, R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 0-894-48029-4.
D. L. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993, ISBN: 0-894-48453-2.
E. E. Lewis, W. F. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 0-894-48452-4.

See previous:

Prompt vs. Delayed

See above:

Prompt Neutrons

See next:

Key Characteristics