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 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.Ī mass may be exactly critical without being a perfect homogeneous sphere. 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). It is possible for a fuel assembly to be critical at near zero power. These examples only outline the simplest ideal cases: These attributes have complex interactions and interdependencies. 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. Fission can also be initiated by neutrons produced by cosmic rays.Ĭhanging the point of criticality If there is uranium-238 ( 238U) present, the rate of spontaneous fission will be much higher. The probability that one such event will cause a chain reaction depends on how much the mass exceeds the critical mass. For example, a spherical critical mass of pure uranium-235 ( 235U) with a mass of about 52 kilograms (115 lb) would experience around 15 spontaneous fission events per second. In this case, k 1.ĭue to spontaneous fission a supercritical mass will undergo a chain reaction.
A population of neutrons introduced to a subcritical assembly will exponentially decrease. When k = 1, the mass is critical, and the chain reaction is self-sustaining.Ī subcritical mass is a mass of fissile material that does not have the ability to sustain a fission chain reaction. When a nuclear chain reaction in a mass of fissile material is self-sustaining, the mass is said to be in a critical state in which there is no increase or decrease in power, temperature, or neutron population.Ī numerical measure of a critical mass is dependent on the effective neutron multiplication factor k, the average number of neutrons released per fission event that go on to cause another fission event rather than being absorbed or leaving the material.