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What is the amount of fissionable material that must be present in order for a nuclear reaction to take place called?
Wiki User
∙ 14y ago
The critical mass. With an amount of U-235 or Pu-239, the smallest critical mass will be a sphere. For a nuclear reactor, it will be the minimum number of fuel assemblies loaded to produce a chain reaction.
Wiki User
∙ 14y ago
Wiki User
∙ 13y agoThe words fissile and fissionable are not generally taken to mean the same thing. Fissile materials can sustain a chain reaction with low energy neutrons. Fissionable materials can sustain a chain reaction, but only if the energy of the neutrons present is sufficiently high. Critical mass is the amount of material needed, in ideal circumstances, to start a chain reaction without neutrons other than those produced internally. 235U is fissile, and has a critical mass of 52 kg. 238U is fissionable, but there is no critical mass because no amount of uranium-238 suffices to produce a chain reaction.
A mix of 235U and 238U can have critical mass, depending on the percentage of 235U present. Natural deposits of uranium do not have a high enough percentage of 235U to produce critical mass, even if the uranium is purified. But by removing some of the 238U, we produce enriched uranium, which can achieve critical mass. The lower the percentage of 235U present, the higher the amount needed to have a chain reaction.
Other materials present also increase the amount needed, as do such things as the shape of the mass. The minimum amount for a critical mass is a pure sphere. You can decrease the critical mass by surrounding the material with something that reflects neutrons.
Strictly speaking, however, the amount of fissionable material does not determine a likelihood of a chain reaction. You have enough fissile material, or you don't, so it is not a matter of likelihood.
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What is the minimum mass of a fissionable isotope in which a nuclear chain reaction can occur?
A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (e.g. the nuclear fission cross-section), its density, its shape, its enrichment, its temperature and its surroundings.The minimum mass for a particular fissionable isotope will always be a sphere, as that is the shape which will lose the least neutrons for a given mass. The behavior of a sphere can be affected by its surroundings, if a neutron reflector is present this will reduce the amount needed. The easiest way to compare materials is to consider a bare sphere with no reflector. In this case Uranium 235 requires a mass of 52 kg and a sphere of diameter 17 cm. Plutonium 239 requires a mass of 10 kg and a sphere 9.9 cm diameter.The Wikipedia article 'Critical Mass' gives a table of other isotopes, but the above two are of most practical interest
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How is electricity made in a nuclear reactor?
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What is the closest nuclear power plant to Grayson Kentucky?
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What is the minimum mass of a fissionable isotope in which a nuclear chain reaction can occur?
A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (e.g. the nuclear fission cross-section), its density, its shape, its enrichment, its temperature and its surroundings.The minimum mass for a particular fissionable isotope will always be a sphere, as that is the shape which will lose the least neutrons for a given mass. The behavior of a sphere can be affected by its surroundings, if a neutron reflector is present this will reduce the amount needed. The easiest way to compare materials is to consider a bare sphere with no reflector. In this case Uranium 235 requires a mass of 52 kg and a sphere of diameter 17 cm. Plutonium 239 requires a mass of 10 kg and a sphere 9.9 cm diameter.The Wikipedia article 'Critical Mass' gives a table of other isotopes, but the above two are of most practical interest
During interphase the chromosomes are not visible but the nuclear material is present in a granular or network condition?
chromatin
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