Истории и востоковедения
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- C. Activities
| Nuclear reactors
A sample of fissionable material smaller than the "critical size" is unable to carry on a nuclear chain reaction. If the size of the sample is exactly critical, the number of neutrons produced in each generation is the same as that produced in the previous one, resulting in steady nuclear energy liberation. The original Fermi-pile and its later modifications maintain nuclear reactions at the critical size level. It must be mentioned in this connection that the conditions of "criticality" are extremely unstable: a small deviation (отклонение) in one direction will result in the rapid extinction (уменьшение количества нейтронов) of fission neutrons and the cut-off of the nuclear chain reaction, whereas a deviation in another direction will lead to a rapid multiplication of the fission neutrons and the melting (плавление) of the entire structure. 100 Thus, the important problem in maintaining a steady chain reaction is that of regulating the rate of neutron production and of keeping the chain reaction from "dying out" or "running away". This is achieved by using "control rods" made from neutron-absorbing materials (such as boron) which arc automatically pushed in or pulled out from narrow channels drilled through the reacting fissionable material as soon as the rate of neutron production drops below or exceeds the desired level. We have already mentioned that Fermi-piles were unsuitable for purposes of nuclear power production because of the high dilution of uranium by carbon; they should be considered rather as "alchemical plants" in which plutonium is produced. For the purpose of nuclear power production, we use the controlled nuclear chain reactions in pure fissionable materials, such as U235 or Pu234, which can be run at quite high temperatures. In the so-called "swimming pool" reactor (реактор бассейнового типа) in which several cylindrical containers filled with pure fissionable material arc placed at the bottom of a large water tank, the water circulating through the tank carries away the heat produced in the fission process and also protects the observer from the deadly nuclear radiation. The color of the water turns blue as a result of the so-called Cherenkoffs radiation produced in water by high-energy electron. (Adopted from www.studfiles.ru ) C. Activities I. Read the text one more time and give an adequate translation of the sentences marked with an asterisk. II. Summarize the passage and present your report in oral. |