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-

51

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?

52

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.

53

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

Exercise 2. Match the following Russian and English equivalents.

54

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.

55

Exercise 6. Replace the underlined words with their synonyms from the list below. Use the appropriate form of the verb.

To remove, to release, to disintegrate, to moderate, amount, to regulate, to reduce, to lead to, to maintain, to generate.

1.When the nucleus is split, tremendous energy is liberated.

2.235U is mixed with a much greater quantity of 238U.

3.The size of the reactor core can be decreased.

4.Neutron capture in 238U may result in the formation of 239Рu.

5.233U does not occur in nature but may be produced by exposing 232Th to neutron bombardment.

6.This cross-section is not adequate to sustain a chain reaction in a structure of reasonable size.

7.The function of the cooling system is to take away the heat produced by the fission process.

8.But for the cooling fluid, the reactor would be destroyed as a result of thermal stresses.

9.To maintain a steady chain reaction, it is necessary to control the rate of neutron production.

56

10. The fuel in а natural uranium reactor is surrounded by the material which slows down neutrons rapidly to improve their chances of inducing fission

Exercise 7. Use Reading 5-A to decide if the statements below are true or false. If they are false, correct them. Use expressions of agreement and disagreement below.

1.There is a great difference in the operation of a NPP and a coalburning plant.

2.The fission cross-section is the probability that a fission event will occur.

3.Fission is more likely to occur if it is caused by fast neutrons.

4.Water, heavy water and graphite are moderators which are commonly used in fast neutron reactors.

5.The most urgent function of the coolant is to prevent the destruction of the reactor as a result of thermal stresses.

6.The NPP consists of two principal systems: the Nuclear Steam Supply System and the turbogenerator system.

7.Control rods keep the chain reaction from «dying out», but they

cannot prevent it from «running away».

8. Control rods are made of neutron-moderating materials such as boron.

Reading 5-B

Chicago Pile-1

You are going to read a passage about the world's first man-made nuclear reactor. Five sentences or parts of sentences have been removed from the text. Choose from the phrases and sentences A-F the one which fits each gap (1-5). There is one extra sentence which you will not need to use.

A. led to the atomic pile (nuclear reactor) and the first controlled nuclear chain reaction.

57

B.who actually made uranium "burn".

C.a new fissionable element was produced inside the pile during operation.

D.nor could it be very useful as а power source.

E."The atomic pile works successfully. The nuclear chain reaction has been achieved."

F.it was necessary to mix natural uranium with a large amount of carbon in the form of graphite.

The Italian-American physicist Enrico Fermi is sometimes called the Architect of the Atomic Age. It was Enrico Fermi 1). ________. Не was able to do so by utilizing the fact that the effectiveness of fission neutrons in producing the fission оf U235 nuclei increases considerably when they are slowed down. То slow down the original fission neutrons, 2)._______. А large "pile" of graphite bricks with small pieces of natural uranium included in the structure was constructed in great secrecy under the grandstand of the University of Chicago Stadium.

And on December 2, 1942, Professor A. Compton telegraphed to Dr. V. Bush in Washington, D.C. "The Italian navigator has landed in the New World. The natives are friendly." In the secret language of the Manhattan Project this meant: 3).________.

In the Fermi-pile the natural uranium was so highly diluted by carbon that high efficiency in energy production could not be achieved. Owing to the presence of inactive U238 the chain reaction in the pile could not result in an explosion, 4). ________. But although the energy released in the fission of U235 nuclei was less than the amount necessary to light a small electric bulb, 5). _______. This element was plutonium.

Do you know why the first reactor was called a pile? Read the passage below and find out.

The Chicago Pile-1was named so because the reactor was more or less a pile of elements stacked on top of one another, primarily made of uranium pellets separated by graphite blocks. Rods coated with cadmium were used to control the reaction. The pile of blocks was held together by wood. An "atomic pile" was a code word for a device that in peacetime would be known as a "nuclear reactor".

58

Reading 5-C

Reactor Types

Put each of the following words in its correct place in the passage below.

Breeder, light water, boiling, thermal reactors, pressurized, rare, heavy, coolant, fertile, fast-neutron reactors, gas, fissile.

There are many types of nuclear reactors. Basically, they fall into two broad classes: (a) ______ in which the chain reaction is supported by thermal neutrons, and (b) ______ in which fission is produced by fast neutrons.

Thermal reactors are classified according to the type of (c) ______

used. 80% of the commercial reactors in use are cooled and moderated by ordinary water and are known as (d) ______ reactors. Two major types of light water reactors are (e) ______ water reactors, including a Russian version (VVER), and (f) ______ water reactors. The water-cooled graph- ite-moderated channel type reactor (RBMK, the Chernobyl reactor) is also a light water reactor. Most of the remaining 20% of reactors are cooled by

(g) ______ water, like CANDU (the Canadian Deuterium-Uranium reactor), or (h) ______, like the high-temperature gas-cooled reactor.

Thermal reactors use only one uranium isotope – U235, which is very

(i) ______. Besides, they extract only 1-2 % of the energy from uranium. That is why Russia is interested in fast-neutron reactors which make

full use of natural uranium. Fast-neutron reactors can convert (j) ______

materials Th232 and U238 into (k) ______ U233 and Pu239 and produce (or breed) more fuel than they consume. That is why they are called fast

(l) ______ reactors. Fast breeder reactors, by creating fuel from nonfissile isotopes, can increase available world nuclear fuel resources. The general trend in the future development of Russian nuclear power is toward the construction of fast reactors.

Reading 5-D

What is in Nuclear Power?

Read the text and choose a suitable heading for each part. There is one extra heading. Translate Parts 5 and 6 in writing.

ATo err is human

BThe rest is easy

CThe fuel

59

DSupport and opposition

EBe in control

FWhy the whole is more than the sum of its parts

GEinstein was right

1

Commercial nuclear power has been around for nearly half a century. In 1956, the reactor at Calder Hall in north-west England became the first in the world to supply electricity to a national grid. A year later, in Shippingport, Pennsylvania, the US opened its first commercial plant, while France followed suit in 1959 and the Soviet Union in 1964. From the start, great claims were made for the industry: it was going to produce cheap, clean and reliable power. But its critics saw it as costly, polluting, dangerous in the event of an accident, and closely linked to nuclear proliferation.

2

All nuclear power plants use uranium as fuel. With an atomic number of 92 (the number of protons in its nuclei), uranium is the heaviest element found in considerable amounts in nature. It's radioactive.

All the uranium on Earth was formed from lighter elements during supernova ехрlоsions billions of years ago. Material from these exploded stars was strewn across space, and eventually some was incorporated into new stars and their orbiting planets. So nuclear power here on Earth harnesses energy that has been stored in uranium for more than 4.5 billion years. Uranium is fissile - its atomic nuclei can be split to release energy. Its most common isotopes are U-238 and U-235.

3

In fission, the nucleus of а heavy element splits and energy is liberated. То trigger the fission of а uranium nucleus, it must be struck by а neutron. This particle is temporarily absorbed, making the nucleus unstable. The nucleus then splits to form two smaller nuclei, releasing two or three more neutrons in the process. Some of the energy of the nucleus is liberated in the form of the kinetic energy of the particles and gamma radiation.

Einstein’s famous equation Е= mc2 expresses this fact. The mass of the initial nucleus and the neutron that strikes it is greater than that of the split nuclei and released neutrons, because the missing mass was converted into energy.

60