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 in a time dependence of neutron population and therefore the power level of the reactor. Such situation may take place, for a number 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 requires attention and adjustment to the day-to-day operation of the 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 be able to predict the time behavior of the neutron population in a reactor core driven by changes in reactor multiplication, which is not a circumstance that is totally controlled by the operator of power plant and reactor core. Furthermore, variables such as indirect accessibility to control such fuel temperature or coolant density distribution throughout the reactor do have impact to the situation. However, these variables depend on the reactor power level and hence the neutron fluxes itself. Additionally, the study of the time dependence of the related process, which is involved with determining the core multiplication as a function of power level of the reactor multiplication, is the subject of our study in this chapter, and it is called nuclear reactor dynamics. This usually involves with detailed modeling of the entire nuclear steam supply system, which is part of feedback system as well.