Thin-walled metal tube that forms the outer jacket of a nuclear fuel rod. It prevents corrosion of the nuclear fuel and the release of fission products into the coolant. Zirconium alloys (also called zircaloy, see below) are common cladding materials in commercial nuclear fuel.


In the context of heat transfer, the transfer of heat within a medium through a diffusive process (i.e., molecular or atomic collisions),

Containment structure:

A robust, airtight shell or other enclosure around a nuclear reactor core to prevent the release of radioactive material to the environment in the event of an accident.


Heat transfer by the physical movement of material within a fluid medium.

Cooling time:

The amount of time elapsed since spent fuel was discharged from a nuclear reactor.


That portion of a nuclear reactor containing the fuel elements.


Term used in reactor physics to describe the state in which the number of neutrons released by the fission process is exactly balanced by the neutrons being absorbed and escaping the reactor core. At Criticality, the nuclear fission chain reaction is self-sustaining,

Decay heat:

Heat produced by the decay of radioactive isotopes contained in nuclear fuel.

Decay, radioactive:

Disintegration of the nucleus of an unstable element by the spontaneous emission of charged particles (alpha, beta, positron) or photons of energy (gamma radiation) from the nucleus, spontaneous fission, or electron capture.

Depleted uranium:

Uranium enriched in the element uranium-238 relative to uranium-235 compared to that usually found in nature. Also, uranium in which the uranium-235 content has been reduced through a physical process.

Design basis phenomena:

Earthquakes, tornadoes, hurricanes, floods, and other events that a nuclear facility must be designed and built to withstand without loss of systems, structures, and components necessary to ensure public health and safety.

Design basis threat:

In the context of this study, hypothetical ground assault threat against a commercial nuclear power plant. Some generic elements of the design basis threat are described in Title 10, Section 73.1(a) of the Code of Federal Regulations (10 CFR73.1(a)).

Dirty bomb:

See Radiological Dispersal Device.

Dry storage:

Out-of-water storage of spent nuclear fuel in heavily shielded casks.

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