Nuclear criticality safety

A nuclear criticality accident occurs from operations that involve fissile material and results in a sudden and potentially lethal release of radiation.

Nuclear criticality safety practitioners attempt to prevent nuclear criticality accidents by analyzing normal and credible abnormal conditions in fissile material operations and designing safe arrangements for the processing of fissile materials.

Controls (requirements) on process parameters (e.g., fissile material mass, equipment) result from this analysis.

These controls, either passive (physical), active (mechanical), or administrative (human), are implemented by inherently safe or fault-tolerant plant designs, or, if such designs are not practicable, by administrative controls such as operating procedures, job instructions and other means to minimize the potential for significant process changes that could lead to a nuclear criticality accident.

If a fissile body has a given size and shape but varying density and mass, there is a threshold below which criticality cannot occur.

Hence hydrogenous materials including oil, polyethylene, water, wood, paraffin, and the human body are good moderators.

Reflection: When neutrons collide with other atomic particles (primarily nuclei) and are not absorbed, they are scattered (i.e. they change direction).

If the change in direction is large enough, neutrons that have just escaped from a fissile body may be deflected back into it, increasing the likelihood of fission.

Good reflectors include hydrogen, beryllium, carbon, lead, uranium, water, polyethylene, concrete, Tungsten carbide and steel.

[2] Physicochemical Form: Consists of controlling the physical state (i.e., solid, liquid, or gas) and form (e.g., solution, powder, green or sintered pellets, or metal) and/or chemical composition (e.g., uranium hexafluoride, uranyl fluoride, plutonium nitrate, or mixed oxide) of a particular fissile material.

In all but very simple cases, this usually requires the use of computer programs to model the system geometry and its material properties.

Computer codes used for criticality safety analyses include OPENMC (MIT), COG (US),[3] MONK (UK),[4] SCALE/KENO (US),[5] MCNP (US),[6] and CRISTAL (France).

[7] Traditional criticality analyses assume that the fissile material is in its most reactive condition, which is usually at maximum enrichment, with no irradiation.