PROMPT AND DELAYED NEUTRONS

Reactor Theory (Neutron Characteristics) DOE-HDBK-1019/1-93 PROMPT AND DELAYED NEUTRONS Rev. 0 Page 29 NP-02 PROMPT AND DELAYED NEUTRONS Not all neutrons are released at the same time following fission. Most neutrons are released virtually instantaneously and are called prompt neutrons. A very small fraction of neutrons are released after the decay of fission products and are called delayed neutrons. Although delayed neutrons are a very small fraction of the total number of neutrons, they play an extremely important role in the control of the reactor. EO 3.1 STATE the origin of prompt neutrons and delayed neutrons. EO 3.2 STATE the approximate fraction of neutrons that are born as delayed neutrons from the fission of the following nuclear fuels: a. Uranium-235 b. Plutonium-239 EO 3.3 EXPLAIN the mechanism for production of delayed neutrons. EO 3.4 EXPLAIN prompt and delayed neutron generation times. EO 3.5 Given prompt and delayed neutron generation times and delayed neutron fraction, CALCULATE the average generation time. EO 3.6 EXPLAIN the effect of delayed neutrons on reactor control. Neutron Classification The great majority (over 99%) of the neutrons produced in fission are released within about 10^-13 seconds of the actual fission event. These are called prompt neutrons. A small portion of fission neutrons are delayed neutrons, which are produced for some time after the fission process has taken place. The delayed neutrons are emitted immediately following the first beta decay of a fission fragment known as a delayed neutron precursor. An example of a delayed neutron precursor is bromine-87, shown below.