Archivo por meses: mayo 2018

Prompt Neutron and Delayed Neutron

Most neutrons (99.35 % for 235U fission by thermal neutrons) are emitted immediately by a nuclear fission event. These are called “prompt neutrons.” A few neutrons are emitted a little after nuclear fission occurs and they are called “delayed neutrons.”

Fission reaction and his products
Fission reaction and his products

The delayed neutrons are primarily produced from the decay of the fission products emitting neutrons. The fission products that emit delayed neutrons are called delayed neutron precursors. There are many delayed neutron precursors  and they have different half-lives. The delayed neutron precursors are treated in six groups with different half-lives for analysis of nuclear reactor kinetics.

Delayed neutron data differ from those of the fission nuclides and are between the data of thermal fission and fast fission neutrons. The data need to be used correctly depending on the type of the fuel and neutron spectrum of the reactor.

The delayed neutrons have approximately 0.4-MeV average energy, which is lower than the approximate 2-MeV average energy of the prompt neutrons. Therefore, the fraction of delayed neutrons that is leaked outside the reactor and lost disappear is slightly smaller than that of the prompt neutrons. The fraction of delayed neutrons that contributes to the fission chain reactions is slightly larger than that of the prompt neutrons.

This effect is considered in the analysis of nuclear reactor kinetics. A slightly larger delayed neutron fraction is used than the absolute value “b” depending on the reactor and the effect is shown as “b effective” If the reactor has a large core volume, neutron leakage is very small during moderation and there is almost no difference between them. The “b effective” value depends on the reactor size and neutron spectrum. Although the delayed neutron fraction is low, it slows down the transient change of the reactor and, therefore, it plays a very important role in the reactor control.

Nuclear Reactor Kinetics

Reactor power changes when the temperature and position of the control rods of a nuclear reactor are changed. This change is unique to each reactor,and its characteristics are called “nuclear reactor kinetics.”

The control rods are made of strong neutron-absorbing materials, and when they are inserted into the reactor, the reaction rate of neutron absorption increases. The reactor becomes subcritical and its power decreases. Conversely, the reaction rate of neutron absorption decreases when the control rods are withdrawn; the reactor becomes supercritical and its power increases. The reaction rate of neutron absorption changes when the reactor temperature is changed and, therefore the reactor power changes.

The reactor power is proportional to the number of fission reactions per second in the nuclear reactor. As fission reactions are caused by neutrons, the number of their reactions is proportional to the total number of neutrons in the reactor. However, the number of neutrons varies depending on the neutron production rate due to the fission reactions, the rate of neutron absorption by the nuclear fuel and reactor structure materials, and the rate of neutron leakage from the reactor.

The nuclear reactor kinetics usually explains an increase or decrease in the number of neutrons in the entire core. In other words, the spatial distribution of neutrons is not considered in the core.

Throughout the course the student will learn the main basics about the control and monitoring of a nuclear core reactor.