Mass subcritical

In a nuclear weapon, the fissile material is initially subcritical. The challenge is to produce a supercritical mass so rapidly that the chain reaction takes place uniformly throughout the metal. Supercriticality can be achieved by shooting two subcritical blocks toward each other (as was done in the bomb that fell on Hiroshima) or by implosion of a single subcritical mass (the technique used in the bomb that destroyed Nagasaki). A strong neutron emitter, typically polonium, helps to initiate the chain reaction. 

State whether the following statements are true or false. If false, explain why. (a) A subcritical mass of fissionable... 

Subcritical Mass when the quantity of radioactive fuel is insufficient to produce a self-sustaining chain reaction Sublimation process where a substance passes directly from the solid to gaseous phase without going through the liquid phase... 

Examples electron proton neutron, subcritical mass See critical mass. sublimation The direct conversion of a solid to a vapor without first forming a liquid, sublimation vapor pressure The vapor pressure of a solid. 

In the chain reaction shown in Figure 14.5, if there is so little uranium that the released neutrons escape before they have a chance to cause a fission reaction, the reaction stops. The critical mass is the minimum mass of fissionable material needed in order for the reaction to continue. The critical mass concept is the key to the design of a fission-type nuclear weapon. In such a weapon, two smaller-than-critical masses are present but are separated. When these subcritical masses are suddenly combined, the rapidly escalating fission reactions produce an explosion of incredible intensity. 

During World War II an intense research effort (the Manhattan Project) was carried out by the United States to build a bomb based on the principles of nuclear fission. This program produced the fission bombs that were used with devastating effects on the cities of Hiroshima and Nagasaki in 1945. Basically, a fission bomb operates by suddenly combining two subcritical masses of fissionable material to form a supercritical mass, thereby producing an explosion of incredible intensity. 

A different design was used to detonate the bomb dropped on Nagasaki. Plutonium was stored in one large but subcritical mass. It was compressed to a critical density by means of surrounding chemical explosives. When the chemical explosive detonated, the blast forced the bomb material into a density that reached criticality. In either type of design, once criticality is reached the explosion follows in a millionth of a second. 

A sample of fissionable material must have sufficient mass in order for a fission chain reaction to occur. If it does not, neutrons escape from the sample before they have the opportunity to strike other nuclei and continue the chain reaction— the chain reaction never begins. A sample that is not massive enough to sustain a chain reaction is said to have subcritical mass. A sample that is massive enough to sustain a chain reaction has critical mass. When a critical mass is present, the neutrons released in one fission cause other fissions to occur. If much more mass than the critical mass is present, the chain reaction rapidly escalates. This can lead to a violent nuclear explosion. A sample of fissionable material with a mass greater than the critical mass is said to have supercritical mass. Figure 25-18 shows the effect of mass on the initiation and progression of a fission reaction. 

The amount of fissionable matter present determines whether a nuclear chain reaction can be sustained. In a subcritical mass, the chain reaction stops because neutrons escape the sample before causing sufficient fissions to sustain the reaction, o In a supercritical mass, the chain reaction accelerates as neutrons cause more and more fissions to occur. 

Figure 7. Comparison between supercritical mass transfer coefficients (10-200 atm and 35 C) and subcritical mass transfer coefficients (1 atm and 25 C)...

In general, there now exists a variety of scientific subdisciplines, each of which recognizes its inherently multidisciplinary nature, but each of which represents a subcritical mass for attracting sufficient-focused funding to support its needs. These areas include corrosion science, colloid science and interfacial phenomena, passivity and surface films, electrocatalysis, bioelectrochemical and membrane phenomena, electrocrystallization, and others. One unifying theme that emerges from each of these areas, however, is that the forces that determine the structure and properties of the surface and extended interfacial region must be better understood. 

Whether a chain reaction occurs depends on the mass (and thus the volume) of the fissionable sample. If the piece of uranium is large enough, the product neutrons strike another fissionable nucleus before flying out of the sample, and a chain reaction takes place. The mass required to achieve a chain reaction is called the critical mass. If the sample has less than the critical mass (a subcritical mass), too many product neutrons leave the sample before they collide with and cause the fission of another nucleus, and thus a chain reaction does not occur. 

Subcritical mass (too many neutrons escape to keep the reaction sustained)... 

Figure 24.17 Whether a nuclear reaction can be sustained depends on the amount of matter present, in a subcritical mass, the chain reaction does not start because neutrons escape before causing enough fission to sustain the chain reaction, in a supercriticai mass, neutrons cause more and more fissions and the chain reaction acceierates. 

O Reading Check Compare subcritical mass and critical mass. 

Describe what is meant by the terms critical mass, subcritical mass, and supercritical mass. Which is shown in Figure 24.32 How can you tell ... 

It had been clear from the beginning that implosion, by squeezing a hollow shell of plutonium to a solid ball, could effectively assemble it as a critical mass much faster than the fastest gun could fire. What von Neumann and Teller now realized, and communicated to Oppenheimer in October 1943, was that implosion at more violent compressions than Neddermeyer had yet attempted should squeeze plutonium to such unearthly densities that a solid subcritical mass could serve as a bomb core, avoiding the complex problem of compressing hollow shells. Nor would predetonation threaten from light-element impurities. Develop implosion, in other words, and they could deliver a more reliable bomb more quickly. 

In a nuclear fission bomb the critical mass is kept separated into several smaller subcritical masses until detonation, at which time the masses are... 

Figure 21.16 Schematic drawing of an atomic bomb. A conventional explosive is used to bring two subcritical masses together to form a supercritical mass. 

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