focussing on uncritical mass

http://en.wikipedia.org/wiki/Critical_mass

The critical size is the minimum size of a nuclear reactor core or nuclear weapon that can be made critical for a specific geometrical arrangement and material composition. The critical size must at least include enough fissionable material to reach critical mass. If the size of the fissionable material is less than a certain minimum, fission neutrons escape through its surface and the chain reaction is not sustained. http://en.wikipedia.org/wiki/Critical_size

[...]

Principles

Seven factors influence a criticality system.
Geometry or shape of the fissile material: If neutrons escape (leak from) the fissile system they are not available to interact with the fissile material to cause a fission event. Therefore the shape of the fissile material affects the probability of occurrence of fission events. A large surface area such as a thin slab has lots of leakage and is safer than the same amount of fissile material in a small, compact shape such as a cube or a sphere.
Interaction of units: Neutrons leaking from one unit can enter another. Two units, which by themselves are sub-critical, could interact with each other to form a critical system. The distance separating the units and any material between them influences the effect.
Reflection: When neutrons collide with other atomic particles (primarily nuclei) and are not absorbed, they change direction. If the change in direction is large enough, the neutron may travel back into the system, increasing the likelihood of interaction (fission). This is called ‘reflection’. Good reflectors include hydrogen, beryllium, carbon, lead, uranium, water, polyethylene, concrete, Tungsten carbide and steel.
Moderation: Neutrons resulting from fission are typically fast (high energy). These fast neutrons do not cause fission as readily as slower (less energetic) ones. Neutrons are slowed down (moderated) by collision with atomic nuclei. The most effective moderating nuclei are hydrogen, deuterium, beryllium and carbon. Hence hydrogenous materials including oil, polyethylene, water, wood, paraffin, and the human body are good moderators. Note that moderation comes from collisions; therefore most moderators are also good reflectors.
Absorption: Absorption removes neutrons from the system. Large amounts of absorbers are used to control or reduce the probability of a criticality. Good absorbers are boron, cadmium, gadolinium, silver, and indium.
Enrichment: The probability of a neutron reacting with a fissile nucleus is influenced by the relative numbers of fissile and non-fissile nuclei in a system. The process of increasing the relative number of fissile nuclei in a system is called enrichment. Typically, low enrichment means less likelihood of a criticality and high enrichment means a greater likelihood.
Mass: The probability of fission increases as the total number of fissile nuclei increases. The relationship is not linear. There is a threshold below which a criticality will not occur. This threshold is called the critical mass.[...]
http://en.wikipedia.org/wiki/Nuclear_criticality_safety

http://en.wikipedia.org/wiki/Criticality_accident
http://en.wikipedia.org/wiki/BORAX_experiments
http://en.wikipedia.org/wiki/Borax