Nuclear reactor core

A nuclear reactor core is that portion of a nuclear reactor containing the nuclear fuel components where the nuclear reactions take place.


Water-moderated reactors

Inside the core of a typical pressurized water reactor or boiling water reactor. are pencil-thin nuclear fuel rods, each about long, which are grouped by the hundreds in bundles called "fuel assemblies". Inside each fuel rod, pellets of uranium, or more commonly uranium oxide, are stacked end to end. Also inside the core are control rods, filled with pellets of substances like hafnium or cadmium that readily capture neutrons. When the control rods are lowered into the core, they absorb neutrons, which thus cannot take part in the chain reaction. On the converse, when the control rods are lifted out of the way, more neutrons strike the fissile uranium-235 (U-235) or plutonium-239 (Pu-239) nuclei in nearby fuel rods, and the chain reaction intensifies.

The heat of the fission reaction is removed by the water, which also acts to moderate the neutron reactions. An alternative form of nuclear fuel would be fissile uranium-233 (U-233) made by the neutron-bombardment of the common thorium-232. Also, fissile uranium-234 (U-234) is found as a trace addition to U-235 wherever U-235 is found. They are both good nuclear "fuels".

Graphite-moderated reactors

There are also Graphite moderated reactors in use.

One type uses solid graphite for the neutron moderator and ordinary water for the coolant. See the Soviet-made RBMK nuclear-power reactor. These were the reactors that were the cause of the Chernobyl disaster.

In a so-called advanced gas-cooled reactor, a British design, the core is made of a graphite neutron moderator where the fuel assemblies are located. Carbon dioxide gas acts as a coolant and it circulates through the graphite removing heat.

Experimental & Developmental Reactors

Several merely experimental or hypothetical nuclear reactor cores are mentioned below.

There have been developmental graphite-moderated nuclear power reactors that were cooled by helium gas. These are no longer in service.

The core of a molten salt reactor is a block of graphite through which holes are bored in which molten salt circulates. The graphite serves as a neutron moderator, it is the solid structure of the reactor. The molten salt that circulates in the channels is both the fuel and the coolant, it contains the fissionable material needed to sustain the chain reaction.

A set of compact nuclear reactors were developed by the United States under the Systems Nuclear Auxiliary Power Program (SNAP). One SNAP reactor, the SNAP-10A was launched into space and was successfully operated for 43 days in 1965.

Aqueous homogeneous reactors cores employ water in which soluble nuclear salts (usually uranyl sulfate or uranyl nitrate) have been dissolved. As the water serves as the solvent for the uranium salts, it serves as the fuel. As it is water, it serves to cool the reactor as well- hence the name 'homogeneous' (as coolant and fuel are one homogeneous substance). The water can be either heavy water or ordinary light water.

In a gaseous fission reactor the reaction takes place in a core which is bounded and created by magnetic field. The fuel is supplied and fission occurs in the gas phase.

See also


  • Nuclear Reactor Analysis, John Wiley & Sons Canada, Ltd.

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