12. Reactor Dynamics

In order for nuclear fission power to operate at a constant power level, the rate of neutron production via fission reactions must be exactly balanced by neutron loss via absorption and leakage. If we deviate from this simple balancing role, it would cause time dependence of neutron population and therefore the power level of the reactor. Such situation may take place, for a number of reasons, such as reactor operator may have a requirement to change the reactor power level by temporarily altering the control fuel rod so it will change the core or source multiplication or there may be long-term changes in core multiplication due to fuel depletion and isotopic buildup. Other examples may also be encountered that require attention and adjustment to day-to-day operation of reactor, such as unforeseen accident or failure of primary coolant pump system, etc. The topic of nuclear kinetic reactor as we have learned in the previous chapter is handling this situation by allowing us to predict the time behavior of the neutron population in a reactor core driven by changes in reactor multiplication, which are not circumstances that are totally controlled by the operator of power plant and reactor core. Furthermore, variables such as indirect accessibility to control such as the fuel temperature or coolant density distribution throughout reactor do have impact to the situation. However, these variables depend on the reactor power level and hence the neutron fluxes itself. Additionally, study of the time dependence of the related process, which is involved in determining the core multiplication as a function of power level of the reactor multiplication, is subject of our study in this chapter, and it is called nuclear reactor dynamics. This usually involves detailed modeling of the entire nuclear steam supply system, which is part of feedback system as well.