Shigeo Ohki

Abstract Nuclear fuel burnup and reactivity control are important points in the core design of nuclear reactors.

The fuel burnup analysis generally evaluates the time-dependent core power distribution and reactivity by solving burnup equations for the atomic density change of nuclides contained in the fuel as well as solving multi-group diffusion equations for neutron flux distribution and effective neutron multiplication factor. The core power distribution is necessary information for thermal-hydraulic and fuel designs.

The core design for reactivity control predicts reactivity change during reactor operation and determines its optimal control methods based on calculations of reactivity change with fuel burnup, fission product (FP) accumulation (poisoning effect), inherent reactivity feedback by temperature changes of fuel and coolant, etc. Among the general methods available for reactivity control, the insertion and withdrawal of neutron absorbers, generally referred to as control rods, is the approach usually taken for power reactors. A burnable poison, (a nuclide that has a large neutron absorption cross section) or a chemical shim (a neutron-absorbing chemical, usually boric acid, which is concentrated in the moderator or coolant) is employed for reactivity control depending on reactor types.

Fuel burnup and reactivity control based on fundamental theories with numerical expressions will be briefly reviewed in this chapter.