1. What is the main problem in maintaininga 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 reactors used?
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 ofthe fission neutrons and the melting (плавление) 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 "controlrods" made from neutron-absording 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 ofthe high dilution of uranium by carbon; they should be considered rather as "alchemicalplants" in which plutonium is produced. For the purpose of nuclear power production, we use the controlled nuclearehain 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 electrons.
• Explain how you understand the italicized words and phrases in the text.
• Give an adequate translation of the sentences marked with an asterisk.
• Give a summary of the passage in English or in Russian.
• Give a free translation of the text.
У каждого элемента есть радиоактивные изотопы, которые либо существуют в природе, либо возбуждаются искусственно бомбардировкой устойчивых ядер такими частицами, как протоны, альфа-частицы или нейтроны. Частица, однако, не будет поглощаться ядром мишени, если ее скорость не будет соответствовать (correspond with) одному из энергетических уровней (level) ядра. Тяжелые ядра, имеющие больше энергетических уровней, чем легкие, осуществляют захват частицы с большей вероятностью, и этот факт помогает объяснить значение урана, тория и других очень тяжелых атомов в ядерном исследовании.
Так как нейтрон не заряжен, на него не влияют заряженные электроны и протоны, и поэтому более вероятно, что он, а не другая частица, будет захвачен. В случае захвата нейтрона массовое число ядра увеличится и ядро станет неустойчивым и радиоактивным. По мере того как излучение продолжается, уровень радиоактивности уменьшается экспоненциально, и время, которое требуется для того, чтобы он достиг половины своей первоначальной (исходной) величины, известно как период полураспада изотопа (half-life).
HOMEWORK
(to be done in writing)
1. Translate into Russian.
1. The predicted accuracy was found to be difficult to obtain in practice.
2. Light was assumed to leave a source as an indefinite number of particles travelling in straight lines.
3. Collisions between electrons and neutral molecules in solids appear to be frequent.
4. The moon appears to be a particularly good gravitational antenna.
5. The new lasers were expected to work in principle at least at a very high efficiency in comparison with other lasers.
6. An intense beam of light is observed to emerge from the mirror ends of the crystal: the entire process is called light amplification by stimulated emission of radiation (hence the acronym "laser").
7. Solid-state lasers already exist and although they have not had the spectral purity of gas lasers, this situation seems to be changing.
8. At first the concept of charge density in an atom seems to bear little resemblance to Bohr's picture of an electron orbiting the nucleus, but the two views are closely related.
2. Translate into English.
1. Считают, что эти величины находятся в хорошем согласии с экспериментальными величинами.
2. По-видимому, высота остается одной и той же в течение всего периода времени.
3. Не наблюдалось, чтобы эти вещества обладали радиационными свойствами.
4. Оказывается, лазеры широко используются в медицине и в промышленности.
5. Маловероятно, чтобы он решил это уравнение.
6. Известно, что на движение электронов влияют эти столкновения.
7. Известно, что радиоактивность была случайно открыта в 1896году французским физиком Беккерелем.
UNIT THIRTEEN
GRAMMAR: GERUND
• Sentences to be translated.
1. The possibility of water being converted into ice is evident.
2. The teacher objected to our measuring this volume in advance.