The nuclear reactor core.

The nuclear reactor.

The fuel assembly

The fuel assembly.

A nuclear reactor core is a key component of a nuclear reactor. In reactor physics, the nuclear core is a bounded region, where a neutron multiplication occurs and where chain reactions take place. The reactor core contains especially the nuclear fuel (fuel assemblies), the moderator and the control rods. The core of the reactor contains all the nuclear fuel assemblies and generates most of the heat (fraction of the heat is genarated outside the reactor – e.g. gamma rays energy). The assemblies are exactly placed in the reactor according to a fuel loading pattern.

The core is located inside a reactor pressure vessel and is carried by a core support barrel. A bottom plate of the support barrel is called lower core plate. This structure consist of N holes for fixing fuel assemblies (N is number of fuel assemblies in specific reactor – e.g. 157 for AP1000).

There is a neutron reflector (core baffle or heavy reflector) around the core (around peripheral fuel assemblies), which is usually formed of stainless rings accurately surrounding the nuclear fuel assemblies.

The top of the nuclear core is defined by an upper guide structure assembly. This assembly is made of stainless steel and has many purposes. The upper guide structure assembly exerts an axial force on fuel assemblies (through springs in the fuel assembly), thus defines the exact position of the fuel assembly in the core. The upper guide structure assembly also guides and protects control rod assemblies and in-core instrumentation.

nuclear fuel assembly

The nuclear fuel assembly with the rod cluster control assembly.

Components of a nuclear reactor core

The key components common to most PWR types of nuclear reactor cores are:

  • Nuclear fuel. The nuclear fuel is composed of nuclear fuel assemblies located in exact positions in the reactor.
  • Neutron moderator. The moderator slows down neutrons from fission to thermal energies.
  • Control rods. Control rods are rods, or tubes containing a neutron absorbing material such as boron, hafnium, cadmium, etc., used to control the power of a nuclear reactor.
  • Neutron Reflector. The neutron reflector (core baffle) surrounds the core. The reflector reduces the non-uniformity of the power distribution in the peripheral fuel assemblies and reduces a coolant flow bypass of the core.
  • Burnable neutron poison. Burnable poisons / absorbers are primarily used to control large amounts of excess fuel reactivity, are used to reduce a concentration of a boric acid at the beginning of the fuel cycle.
  • Neutron source. Neutron sources increase a neutron flux during subcritical state.
  • In-core instrumentation. The in-core instrumentation system measures neutron flux distribution and  temperatures in the reactor core. This helps determine safety margins of the reactor.
WWER-1000 nuclear core. The lower support structure, the neutron reflector and 11 fuel assemblies.

Top view of the WWER-1000 nuclear core. The lower support structure, the neutron reflector and 11 fuel assemblies.
Source:; used with permission of АО ОКБ “ГИДРОПРЕСС”

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