Critical Mass For Mac
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If you find our site useful, please chip in. — Brewster Kahle, Founder, Internet Archive. Dear Internet Archive Supporter, I ask only once a year: please help the Internet Archive today. The average donation is $45. If everyone chips in $5, we can end this fundraiser today.
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Critical Mass Agency
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As part of a re-creation of a 1945 using the; a is surrounded by blocks of. The original experiment was designed to measure the radiation produced when an extra block was added. Instead, the mass went supercritical when the block was placed improperly by being dropped.
A critical mass is the smallest amount of material needed for a sustained. The critical mass of a fissionable material depends upon its properties (specifically, the ), its, its, its, its purity, its, and its surroundings. The concept is important in.
Contents. Explanation of criticality 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 population.
A numerical measure of a critical mass is dependent on the 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. When k = 1, the mass is critical, and the chain reaction is self-sustaining. A subcritical mass is a mass of fissile material that does not have the ability to sustain a fission chain reaction.
A population of neutrons introduced to a subcritical assembly will exponentially decrease. In this case, k 1. Due to a supercritical mass will undergo a chain reaction. For example, a spherical critical mass of pure will have a mass of 52 kg and will experience around 15 spontaneous fission events per second. The probability that one such event will cause a chain reaction depends on how much the mass exceeds the critical mass. If there is present, the rate of spontaneous fission will be much higher.
Fission can also be initiated by neutrons produced. 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). 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 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. 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 of the 238U 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 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 ). 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 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 further reduces the mass needed for criticality. A common material for a neutron reflector is 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. This increases the efficiency. 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), because 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. 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 configuration. Critical size The critical size is the minimum size of a nuclear reactor core or nuclear weapon that can be made 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 reactor core is less than a certain minimum, too many fission neutrons escape through its surface and the chain reaction is not sustained. Critical mass of a bare sphere. If two pieces of subcritical material are not brought together fast enough, nuclear predetonation can occur, whereby a very small explosion will blow the bulk of the material apart.
Until detonation is desired, a must be kept subcritical. In the case of a uranium bomb, this can be achieved by keeping the fuel in a number of separate pieces, each below the either because they are too small or unfavorably shaped. To produce detonation, the pieces of uranium are brought together rapidly. In, this was achieved by firing a piece of uranium (a 'doughnut') down a onto another piece (a 'spike'). This design is referred to as a.
A theoretical 100% pure 239Pu weapon could also be constructed as a gun-type weapon, like the Manhattan Project's proposed design. In reality, this is impractical because even 'weapons grade' 239Pu is contaminated with a small amount of 240Pu, which has a strong propensity toward spontaneous fission. Because of this, a reasonably sized gun-type weapon would suffer nuclear reaction before the masses of plutonium would be in a position for a full-fledged explosion to occur. Instead, the plutonium is present as a subcritical sphere (or other shape), which may or may not be hollow. Detonation is produced by exploding a surrounding the sphere, increasing the density (and collapsing the cavity, if present) to produce a configuration. This is known as an. Prompt criticality.
Main article: The event of fission must release, on the average, more than one free neutron of the desired energy level in order to sustain a chain reaction, and each must find other nuclei and cause them to fission. Most of the neutrons released from a fission event come immediately from that event, but a fraction of them come later, when the fission products decay, which may be on the average from microseconds to minutes later. This is fortunate for atomic power generation, for without this delay 'going critical' would always be an immediately catastrophic event, as it is in a nuclear bomb where upwards of 80 generations of chain reaction occur in less than a microsecond, far too fast for man, or even machine, to react. Physicists recognize two points in the gradual increase of neutron flux which are significant: critical, where the chain reaction becomes self-sustaining thanks to the contributions of both kinds of neutron generation, and, where the immediate 'prompt' neutrons alone will sustain the reaction without need for the decay neutrons. Nuclear power plants operate between these two points of, while above the prompt critical point is the domain of nuclear weapons and some nuclear power accidents, such as the.
A convenient unit for the measurement of the reactivity is that suggested by: that of the and cents. See also. References.