Editing Critical mass (section)

==Changing the point of criticality==
The mass where criticality occurs may be changed by modifying certain attributes such as fuel, shape, temperature, density and the installation of a neutron-reflective substance. These attributes have complex interactions and interdependencies. These examples only outline the simplest ideal cases:

===Varying the amount of fuel===
It is possible for a fuel assembly to be critical at near zero power. If the perfect quantity of fuel were added to a slightly subcritical mass to create an "exactly critical mass", fission would be self-sustaining for only one neutron generation (fuel consumption then makes the assembly subcritical again).

Similarly, if the perfect quantity of fuel were added to a slightly subcritical mass, to create a barely supercritical mass, the temperature of the assembly would increase to an initial maximum (for example: 1&nbsp;[[Kelvin|K]] above the ambient temperature) and then decrease back to the ambient temperature after a period of time, because fuel consumed during fission brings the assembly back to subcriticality once again.

===Changing the shape===
A mass may be exactly critical without being a perfect homogeneous sphere. More closely refining the shape toward a perfect sphere will make the mass supercritical. Conversely changing the shape to a less perfect sphere will decrease its reactivity and make it subcritical.

===Changing the temperature===
A mass may be exactly critical at a particular temperature. [[neutron cross section|Fission and absorption cross-sections]] increase as the relative neutron velocity decreases. As fuel temperature increases, neutrons of a given energy appear faster and thus fission/absorption is less likely. This is not unrelated to [[Doppler broadening]] of the <sup>238</sup>U resonances but is common to all fuels/absorbers/configurations.  Neglecting the very important resonances, the total neutron cross-section of every material exhibits an inverse relationship with relative neutron velocity. Hot fuel is always less reactive than cold fuel (over/under moderation in [[Light-water_reactor|LWR]] is a different topic). Thermal expansion associated with temperature increase also contributes a negative coefficient of reactivity since fuel atoms are moving farther apart. A mass that is exactly critical at room temperature would be sub-critical in an environment anywhere above room temperature due to thermal expansion alone.

===Varying the density of the mass===
The higher the density, the lower the critical mass. The density of a material at a constant temperature can be changed by varying the pressure or tension or by changing crystal structure (see [[allotropes of plutonium]]). An ideal mass will become subcritical if allowed to expand or conversely the same mass will become supercritical if compressed. Changing the temperature may also change the density; however, the effect on critical mass is then complicated by temperature effects (see "Changing the temperature") and by whether the material expands or contracts with increased temperature. Assuming the material expands with temperature (enriched [[uranium-235]] at room temperature for example), at an exactly critical state, it will become subcritical if warmed to lower density or become supercritical if cooled to higher density. Such a material is said to have a negative temperature coefficient of reactivity to indicate that its reactivity decreases when its temperature increases. Using such a material as fuel means fission decreases as the fuel temperature increases.

===Use of a neutron reflector===
Surrounding a spherical critical mass with a [[Neutron reflector#Neutron reflector|neutron reflector]] further reduces the mass needed for criticality. A common material for a neutron reflector is [[beryllium]] metal. This reduces the number of neutrons which escape the fissile material, resulting in increased reactivity.

===Use of a tamper===
In a bomb, a dense shell of material surrounding the fissile core will contain, via inertia, the expanding fissioning material, which increases the efficiency. This is known as a [[tamper (nuclear weapons)|tamper]]. A tamper also tends to act as a neutron reflector.  Because a bomb relies on fast neutrons (not ones moderated by reflection with light elements, as in a reactor), the neutrons reflected by a tamper are slowed by their collisions with the tamper nuclei, and because it takes time for the reflected neutrons to return to the fissile core, they take rather longer to be absorbed by a fissile nucleus.  But they do contribute to the reaction, and can decrease the critical mass by a factor of four.<ref>Serber, Robert, ''The Los Alamos Primer: The First Lectures on How to Build an Atomic Bomb'', (University of California Press, 1992) {{ISBN|0-520-07576-5}} Original 1943  "LA-1", declassified in 1965, plus commentary and historical introduction</ref>  Also, if the tamper is (e.g. depleted) uranium, it can fission due to the high energy neutrons generated by the primary explosion.  This can greatly increase yield, especially if even more neutrons are generated by fusing hydrogen isotopes, in a so-called [[Boosted fission weapon|boosted configuration]].