Unit 15. 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 in one direction will result in the rapid extinction 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 of the entire structure. 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 borom) which are 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” reaction in which several cylindrical containers filled with pure fissionable material are 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 Cherenkoff’s radiation produced in water by high energy electrons.

Words and word combinations to be remembered

fissionable –расщепляемый, способный к ядерному делению	chain reaction – цепная реакция	maintain – поддерживать, сохранять
the desired level – желаемый уровень	Fermi-pile – реактор Ферми	drop below – опускаться ниже
unsuitable – неподходящий, неподобающий	deviation – отклонение	exceed – превышать, превосходить
unstable – изменчивый, непостоянный, нестабильный	cut-off – прерывание, остановка	push in – вталкивать
steady – устойчивый, равномерный, неизменный, постоянный	dilution – ослабление, спад, разжижение, раствор	pull out – выталкивать, выходить

Ex.1. Translate into English:

Ядерная цепная реакция, расщепляемый, нестабильные условия, небольшое отклонение, поглощающий нейтроны материал, желаемый уровень, превышать, опускаться ниже, узкие каналы, производить плутоний, производство ядерной энергии, электроны высокой энергии, процесс расщепления, не подходить для чего-либо.

Ex.2. Explain how you understand the italicized words and phrases in the text:

to carry on, the sample, steady nuclear energy liberation, the cut-off of the nuclear chain reaction, “control rods”, push in, “alchemical plants”, water tank, deadly nuclear radiation.

Ex. 3. Answer the following questions:

1. What is the main problem in maintaining a steady chain reaction? 2. What are “control rods” used for? 3. What is the design of a “swimming pool” reactor? 4. For what purposes are nuclear power reactions used? 5. What do we call “alchemical plants”? 6. What pure fissionable materials do you know? 7. Is any sample of fissionable material able to carry on a nuclear chain reaction?

Ex. 4. Write a summary of the text (approximately 200 words) using the sentences which express the main ideas of the text. Read your summary and make sure that the sentences are presented in a logical order.