Материал: Аврамова NUCLEAR ENGLISH 2013

The two principal categories into which nuclear reactors may be divided are: thermal reactors, in which fission is produced by neutrons of thermal energies and fast reactors in which the fission chain reaction is supported by neutrons of energies more than 1 MeV.

Read Passage 2 and define the requirements for nuclear reactors in simple terms. Which of the requirements determine the reactor type?

2.Requirements for Nuclear Reactors

The principal requirements for а nuclear reactor may be summarized as follows:

а) а supply of fissile material (fuel contained in the reactor core); b) in a thermal reactor, a moderator;

с) devices for controlling the rate at which the chain reaction proceeds;

d) arrangements for removing the heat generated by the operation of the reactor;

е) а shield to contain the radiation emitted by the reactor;

f) arrangements for removing the radioactive fission products from time to time.

Read Passage 3 and answer the questions below:

1.Can you name the three fissile materials suitable for use in a nuclear reactor?

2.Which of them occurs in nature?

3.What is the content of 235U in natural uranium?

4.Which fissile materials do not occur in nature? How are they produced?

5.What are 232Th and 238U called? What materials are referred to as

fertile materials?

3.Supply of Fissile Material

There are three fissile materials suitable for use in а nuclear reactor - 235U, 239Pu, and 233U. 235U may be used in the form of a naturally-

occurring element, in which it is mixed with a much greater amount of 238U. The critical amount of uranium for a reactor using the natural metal is several tons and the size of the reactor core may be reduced by using uranium in which the proportion of 235U has been increased, through one of the separation methods. When а uranium reactor is working, neu-

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tron capture in 238U leads to the formation of 239Рu which may be extracted chemically from time to time and stored for use in a plutonium reactor. 233U does not occur in nature but may be made by exposing 232Th to neutron bombardment.

Read Passage 4 and answer the questions below:

1.What is the function of the moderator?

2.Why is it necessary to moderate neutrons to thermal energies?

3.What is meant by the fission cross-section? What is the unit of the fission cross-section? (Please refer to the «Do you know?» section to find out where the term «barn» comes from.)

4.What is the fission cross-section for fast and thermal neutrons?

5.What materials can be used as a moderator?

4.Necessity for а Moderator

Аchain reaction in natural uranium is practicable only if the fast neu-

trons released in the fission process are reduced to thermal energies before they are lost. The probability that one or another type of interaction between, say, а neutron and а nucleus will occur is usually measured in terms of the "cross section" for that particular interaction. The cross section for fission of natural uranium by neutrons of energy 1.5 MeV is 0.29 barns and is not enough to sustain а chain reaction in a structure of reasonable size. For fission by thermal neutrons, however, the cross section is 3.92 barns, which is quite adequate. For this reason the fuel in а natural uranium reactor is surrounded by moderating material which slows down neutrons rapidly to improve their chances of inducing fission and thus continuing the chain reaction. In a typical thermal reactor, the moderator is ordinary water, which also serves as the coolant. Other choices оf a moderator are possible, such as graphite or heavy water.

Read Passage 5 and answer the questions below:

1.What is the power of the reactor limited by?

2.What can happen unless proper cooling facilities are provided?

3.How is surplus heat removed from the reactor?

4.What are the requirements for the coolant?

5.What two purposes does the reactor cooling fluid serve?

6.What parts does the nuclear system consist of?

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5.Cooling and Energy Conversion

The power at which the reactor саn operate is limited only by the capacity of the cooling system to take away the heat which is produced by the fission process. Unless proper cooling facilities are provided, the reactor will disintegrate as a result of thermal stresses. The obvious way to remove surplus heat is to circulate а fluid through the reactor and the requirements for the coolant are easily formulated. It should not capture neutrons or be decomposed by them, it should have а high specific heat and thermal conductivity, and, if а liquid, a high boiling point.

The reactor cooling fluid serves a dual purpose. Its most urgent function, as has been pointed out, is to remove the heat from the core. The coolant then transfers this heat for use outside the core, typically for production of electricity. This fluid may be used directly, or it may heat a secondary fluid, to drive а turbogenerator. In all present nuclear generating systems the final fluid is vaporized water, i.e. steam, although this is not the only possibility. In fact it has become conventional to divide the nuclear system into two parts: the nuclear steam supply system (NSSS), which includes the reactor itself and any associated equipment (such as heat exchangers) necessary to produce steam, and the turbogenerator system that is driven by this steam.

Read Passage 6 and answer the questions below:

1.What is the function of the control?

2.Why is control necessary?

3.What materials are control rods made of?

4.In what way is the chain reaction controlled?

6.Control

The important problem in maintaining а steady chain reaction is that of regulating the rate of neutron production to keep the chain reaction from "dying out" or "running away." This is achieved by using control rods made of neutron-absorbing materials (such as boron) which are automatically pushed in or pulled out of the narrow channels drilled through the reacting fissionable material as soon as the rate of neutron production drops below or exceeds the desired level.

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Exercise 1. Match the terms on the left with their definitions on the right.

Exercise 2. Match the following Russian and English equivalents.

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Exercise 3. Use Reading 5-A to make sure you know the following words and word combinations.

Тепловые реакторы, быстрые реакторы, основные требования к…, источник делящегося материала, замедлитель, теплоноситель, активная зона, управляющие стержни, охлаждающие устройства, защита, отводить избыточное тепло, извлекать, хранить, встречаться в природе, подвергать действию, данное взаимодействие, замедлять нейтроны, поддерживать цепную реакцию, улучшать вероятность, захватывать нейтроны, удельная теплота, теплопроводность, турбогенератор, теплообменник, стало общепринятым, приводить в действие, узкие каналы, падать ниже уровня, превышать уровень, как указывалось, следующим образом, посредством (в терминах), если не…, как только.

Exercise 4. Give the derivatives of the words below.

1.To require – требование/потребность.

2.To supply – запас.

3.To moderate – замедление, замедлитель.

4.To proceed – методика (процедура), процесс, обработка, пе-

реработка.

5.To arrange – расположение.

6.To contain – содержание, контейнер, защитная оболоч-

ка/защитное здание.

7.To suit – подходящий.

8.To occur – встречающийся в природе.

9.To reduce – сокращение.

10.To expose to – воздействие.

11.To practise – практика, практически осуществимая реакция.

12.To measure – мера, измерение.

13.To sustain – самоподдерживающаяся реакция, устойчивое развитие, устойчивость.

14.To cool – теплоноситель, охлаждение.

15.Necessary – необходимость.

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