Light Water Reactor Low-Enriched Uranium Fuel

Fuel fabrication for light (regular) water power reactors (LWR) typically begins with receipt of low-enriched uranium (LEU) hexafluoride (UF6) from an enrichment plant. The UF6, in solid form in containers, is heated to gaseous form, and the UF6 gas is chemically processed to form LEU uranium dioxide (U0₂) powder. This powder is then pressed into pellets, sintered into ceramic form, loaded into Zircaloy tubes, and constructed into fuel assemblies. Depending on the type of light water reactor, a fuel assembly may contain up to 264 fuel rods and have dimensions of 5 to 9 inches square by about 12 feet long.

Light Water Reactor Mixed Oxide Fuel

MOX fuel differs from LEU fuel in that the dioxide powder from which the fuel pellets are pressed is a combination of U0₂ and plutonium oxide (Pu0₂). The NRC was directed by Congress to regulate the Department of Energy's (DOE's) fabrication of MOX fuel used for disposal of plutonium from international nuclear disarmamentagreements.

Nonpower Reactor Fuel

Nonpower reactors are much smaller reactors that do not generate electrical power but are used for research, testing, and training. Nonpower reactors can include research reactors and reactors used to produce irradiated target materials. The fuel design varies with the reactor type and manufacturer. Plate-type fuel consists of several thin plates containing a uranium mixture clad with aluminum. Another fuel is in the shape of rods and consists of a uranium and zirconium/hydride mixture. There are also compact, self-contained, low-power (less than 5 watts) tank-type reactors. Although use of highly en­riched uranium (HEU) fuel can reduce the size of a nonpower reactor, the NRC adopted a policy of discouraging use of HEU fuel.

Other Types of Fuel Fabrication Facilities

NRC also regulates some fuel fabrication facilities that have DOE contracts to down-blend HEU with other uranium to create LEU reactor fuel. The HEU being blended down to lower enrichment comes from Russian or U.S. weapons programs as part of an international arms control agreement.

Safety Concerns at Fabrication Plants

Chemical, radiological, and criticality hazards at fuel fabrication facilities are simi­lar to hazards at enrichment plants. Most at risk from these hazards are the plant work­ers. These facilities generally pose a low risk to the public.

WORDLIST

Barrier - диафрагма

porous membrane - пористая мембрана

mishandling - несоблюдение правил эксплуатации

trains - цепочка,

cascade - каскад, ступень

low-enriched uranium (LEU) - низкообогащенный уран

mixed oxide (MOX) fuel - смешанное оксидное яд. топливо

nonpower reactor - неэнергетический реактор

plate-type fuel - пластинчатое яд. топливо

tank-type reactor - корпусной ядерный реактор

hazards - опасные факторы

irradiated target materiall - облученный материал мишени

depleted uranium - обеднённый уран

EXERCISES:

I. Translate the following word combination into Russian:

commercial light water reactors, natural uranium ore, tank-type reactor, highly enriched uranium fuel, uranium mixture clad with aluminum, irradiated target mate­rial, research reactor, fuel fabrication facilities, nonpower reactor, nuclear disarmament agreements, mixed oxide fuel, uranium dioxide powder, enrichment plant, low-enriched uranium.

II. Translate the following sentences into Russian:

a) The vast majority of all nuclear power reactors in operation require enriched uranium fuel in which the content of the U-235 isotope has been raised to about

3,5 % or slightly more.

b) Because uranium needs to be in the form of a gas before it can be enriched, the U-308 is converted into the gas uranium hexafluoride (UF6) at a conversion plant.

c) The enrichment process removes 85% of the U-238 by separating gaseous uranium hexafluoride into two streams: enriched and depleted uranium.

d) The depleted uranium has remains of uranium (usually less than 0/0%) and it is used in mental form in yacht keels, as counterweights, and as radiation shielding, since it is 1.7 times denser than lead.

e) Enriched UF6 is transported to a fuel fabrication plant where it is converted to uranium dioxide (UO2) powder and then pressed into small pellets.

f) A small number of reactors are gas- cooled reactors and do not require uranium be enriched.

g)After uranium has been used in a reactor to produce electricity it is known as spent fuel and may undergo a further reprocessing.

II. Translate the following text into English:

Ядерный топливный цикл.

Атомная энергетика - это сложное производство, включающее множество промышленных процессов, которые вместе образуют топливный цикл. Сущест­вуют разные типы топливных циклов, зависящие от типа реактора.

Обычно топливный цикл состоит из следующих процессов. В рудниках добы­вается урановая руда. Руда измельчается для отделения диоксида урана, а радиоак­тивные отходы идут в отвал. Полученный оксид урана (желтый кекс) преобразует­ся вгексафторид урана - газообразное соединение. Для повышения концентрации урана-235 гексафторид урана обогащают на заводах по разделению изотопов. За­тем обогащенный уран снова переводят в твердый диоксид урана, из которого изготавливают топливные таблетки. Из таблеток собирают тепловыделяющие элементы (твэлы), которые объединяют в сборки для ввода в активную зону ядер­ного реактора АЭС.Извлеченное из реактора отработанное топливо имеет высо­кий уровень радиации я после охлаждения на территории электростанции отправ­ляется вспециальное хранилище. Предусматривается также удаление отходов с низким уровнем радиации, накапливающихся в ходе эксплуатации и технического обслуживания станции. По истечении срока службы и сам реактор должен быть выведен из эксплуатации (с дезактивацией и удалением в отходы узлов реактора). Каждый этап топливного цикла регламентируется так, чтобы обеспечивались бе­зопасность людей и защита окружающей среды.

III. Translate the following text into English:

Атомная электростанция (АЭС) |- электростанция, в которой атомная (ядер­ная) энергия преобразуется в электрическую энергию. На АЭС внутренняя энер­гия, выделяемая при делении ядер некоторых тяжелых элементов (U-235, Р-239Х используется для получения водяного пара. Последний, как и в обычных тепловых электростанций, приводит во вращение вал паровой турбины и турбогенератора.

Генератором энергии на АЭС является атомный реактор. Ядерный реактор- устройство, в активной зоне которого осуществляется управляемая цепная реакция деления ядер тяжелых элементов, в результате которой происходит контроли­руемое выделение ядерной энергии. Ядерные реакторы используются:

- для выработки электрической энергии;

- для научных исследований;

- для воспроизводства ядерного топлива Ядерные реакторы различаются:

- по энергии нейтронов, вызывающих деление ядер: ядерные реакторы на тепловых (медленных) и быстрых нейтронах;

- по характеру распределения ядерного топлива: гомогенные и гетерогенные;

- по используемому замедлителю: графитовые, водо-водяные и др.;

- по назначению: энергетические, исследовательские и т.д.

V. Translate the following extract into English:

Комплекс работ в отрасли по производству ядерного топлива образует ядерно-топливный цикл (ЯТЩ который реализуется на технологически связанных пред­приятиях и в объединениях, выполняющих крупномасштабные работы по добыче и переработке сырья (прежде всего уранового), производству ядерных материалов и веществ в широком ассортименте (в том числе уникальных и особо чистых), изготовлению из них узлов, агрегатов и систем для атомной энергетики, ядерного оружия и установок военного назначения, строительству и эксплуатации крупных атомных объектов, переработке и регенерации облученного ядерного топлива на основе безопасных технологий обезвреживания радиоактивных отходов, их лока­лизации и захоронения.

Ядерно-топливный цикл (ЯТЦ) охватывает деятельность большой группы предприятий атомной промышленности, изготовляющих в едином цикле ядерное топливо для отечественных атомных станций, а также для поставки на зарубеж­ные рынки. Условия развития ядерной энергетики в ближайшее десятилетие в значительной мере определяются тенденциями в совершенствовании ЯТЦ.

ЯТЦ является фундаментом ядерной энергетики и производства ЯБП (ЯЗ). Этим обеспечивается целостность атомной отрасли.

LESSON #4

NUCLEAR FISSION REACTORS

Weapons weren't the only possibilities open to nuclear scientists and engineers at the end of the 1930's. While nuclear fission chain reactions and thermonuclear fusion were clearly ways to unleash phenomenal destructive energy, they could also provide virtually limitless sources of useful energy. By controlling the same nuclear reactions that occur in nuclear weapons, people have since managed to extract nuclear eneTgy for constructive uses. In the half-century since their conception, nuclear fission reac­tors have developed into a fairly mature technology and have become one of our major sources of energy. Nuclear fusion power remains an elusive goal, but efforts continue to harness this form of nuclear energy as well.

Assembling a critical mass of uranium doesn't always cause a nuclear explosion. In feet, it's rather hard to cause a big explosion. The designers of the atomic bomb had to assemble not just a critical mass but a supercritical mass and they had to do it in much less than a millionth of a second. That's not something that happens easily or by ac­cident. It's much easier to reach a critical mass slowly, in which case the uranium wul simply become very hot. It may ultimately explode from overheating, but it will not vaporize everything in sight.

This slow assembly of a critical mass is the basis for nuclear fission reactors. Their principal product is heat, which is often used to generate electricity. Fission reactors are much simpler to build and operate than fission bombs because they don't require such purified fissionable materials. In fact, with the help of some clever tricks, nuclear reac­tors can even be made to operate with natural uranium.

Let's begin by showing that a fission chain reaction doesn't always lead to an ex­plosion. What's important is just how fast the fission rate increases. In an atomic bomb, it increases breathtakingly quickly. At detonation, the fissionable material is far above the critical mass so the average fission induces not just one, but perhaps two, subsequent fissions. With only about 10 ns (10 nanoseconds) between one fission and the two it in­duces, the fission rate may double every 10 ns. In less than a millionth of a second, most of the nuclei in the material undergo fission, releasing their energy before the material has time to blow apart.

But things aren't so dramatic right at critical mass, where the average fission in­duces just one subsequent fission. Since each generation of fissions simply reproduces itself, the fission rate remains essentially constant Only spontaneous fissions cause it to rise at all. The fissionable material steadily releases thermal energy and that energy can be used to power an electric generator.

A nuclear reactor contains a core of fissionable material. Because of the way in which this core is assembled, it's very close to a critical mass. Several neutron-absorbing rods, called control rods, which are inserted into the reactor's core, determine whether ifs above or below critical mass. Pulling the control rods out of the core increases the chance that each neutron will induce a fission and moves the core toward supercriticality. Dropping the control rods into the core increases the chance that each neutron will be absorbed before it can induce a fission and moves the core toward subcriticality.

A nuclear reactor uses feedback to maintain the fission rate at the desired level. If the fission rate becomes too low, (the control system slowly pulls the control rods out of the core to increase the fission rate. If the fission rate becomes too high, the control system drops the control rods into the core to decrease the fission rate. It's like driving a car. If you're going too fast, you ease off the gas pedal. If you are going too slowly, you push down on the gas pedal.

The car driving analogy illustrates another important point about reactors. Both cars and reactors respond relatively slowly to movements of their controls. It would be hard to drive a car that instantly stopped when you lifted your foot off the gas pedal and leaped to supersonic speed when you pushed your foot down. Similarly, it would be im­possible to operate a reactor that immediately shut down when you dropped the control rods in and instantly exploded when you pulled the control rods out.

But reactors, like cars, don't respond quickly to movements of the control rods. That's because the final release of neutrons following a fission is slow. When a 235U nucleus fissions, it promptly releases an average of 2.47 neutrons which induce other fissions within a thousandth of a second. But some of the fission fragments are unstable nuclei that decay and release neutrons long after the original fission. On average, each 235U fission eventually produces 0.0064 of these delayed neutrons, which then go on to induce other fissions. It takes seconds or minutes for these delayed neutrons to appear and they slow the response of the reactor. The reactor's fission rate can't increase quick­ly because it takes a long time for the delayed neutrons to build up. The fission rate can't decrease quickly because it takes a long time for the delayed neutrons to go away.

To further ease the operation of modern nuclear reactors, they are designed to be stable and self-regulating. This self-regulation ensures that the core automatically be­comes sub critical if it overheats. As we'll see later on, this self-regulation was absent in the design of Chernobyl Reactor Number 4.

WORDLIST:

Critical mass критическая масса

nuclear fission reactor ядерный реактор деления

nuclear fusion power термоядерная энергия

nuclear explosion ядерный (атомный) взрыв

slow assembly медленно действующая сборка

purified fissionable material очищенный

(высококонцентрированный) делящийся материал (вещество)

reactor core активная зона ядерного реактора

neutron-absorbing rod поглощающие стержни нейтронов

control rod управляющий стержень

supercriticality сверхкритичность; сверхкритическая масса

feedback (control) обратная связь управления

delayed neutron запаздывающий нейтрон

self-regulation автоматическое регулирование

primary circuit (loop) первичный(внутренний) контур

heat exchanger теплообменник

secondary circuit вторичный (внешний) контур

reactor vessel корпус реактора

fuel assembly топливная сборка

EXPRESSIONS:

To operate with natural uranium - работать на природном уране; toskm the responseofthe reactor-замедлять ответные действия(срабатывание) реактора.

Pulling the control rods out of the core increases the chance that each neutron цill induce fission and moves the core toward supercriticality. - При извлечении управляющих стержней из активной зоны увеличивается вероятность того, что каждый центров вызовет деление, что приведёт к образованию сверхкритичности реактора.

EXERSJSES

I. Translate the following sentences:

a) Nuclear fission reactors have developed into a fairly mature technology and have become one of our major sources of energy.

b) Their principal product of a fission reactor is heat, which is often used to gener­ate electricity.

c) Fission reactors are much simpler to build and operate than fission bombs be­cause they don't require such purified fissionable materials.

d) Several neutron-absorbing rods, called control rods, which are inserted into the reactor's core, determine whether it's above or below critical mass,

e) Both cars and reactors respond relatively slowly to movements of their con­trols.

f) It would be impossible to operate a reactor that immediately shut down when you dropped the control rods in and instantly exploded when you pulled the control rods out.

g) A nuclear reactor uses feedback to maintain the fission rate at the desired level.

h) Pulling the control rods out of the core increases the chance that each neutron will induce fission and moves the core toward super criticality.

//. Translate the following information into Russian:

Heavy atomic nuclei are not so stable as light ones, because in the former the repulsive forces exerted by the protons looses the structure of the nucleus. For this reason it is pos­sible to cause fission of heavy nuclei - such as those of 235 U - by bombarding them with free neutrons. The "fission products" travel at considerable velocity, collide with matter somewhere in the reactor, and give off their kinetic energy as heat. This is the conversion of nuclear energy into heat In addition to the fission products and heat formed in the fission of uranium, however, two fresh neutrons are also formed, which in turn can cause the fission of more uranium atoms. In this way the chain reaction is initiated. A neutron strikes the 235 U nucleus and briefly forms the intermediate product 236 U, which disintegrates spontane­ously into strontium and xenon. In order to be able to utilize these neutrons, which are emit­ted from the parent nucleus at high velocity, for the further fissile processes, they have to be slowed down ("moderated"). Low velocity neutrons are much better suited to split atoms than high velocity neutrons are. The slower neutrons can interact with the uranium nucleus for a greater length of time, whereas faster neutrons are in the vicinity of the nucleus for too short a time to initiate the fission process. The velocity of the neutrons is moderated by causing them to collide with light atoms, large number of which must be incorporated in the reactor for this purpose. Materials consisting of such light atoms are, for example, graphite and water. The neutrons which have been slowed down in this way will then cause fission of further 235 U nuclei. Each fission process gives birth to fresh electrons, so that the chain reaction is self-sustaining and the reactor is consequently kept in operation.

III. Translate into Russian:

Pressurized-water reactor is the simplest form of thermal reactor, in which water serves as the coolant and also as the moderator (i.e., the substance that is used to re­duce the velocity of the fast neutrons produced by nuclear fission).The pressure in the primary circuit is so high, and the boiling point of the water consequently so raised, that no stream can form in the reactor core. The pressure, and therefore the attainable temperature, is limited by the technically practicable dimensions of the reactor ves­sel. Ordinary water as well as "heavy" water (deuterium oxide) may be used as the coolant. The water in the primary circuit is kept in circulation by pumping. The heat absorbed in the core is transferred by means of a heat exchanger to the secondary cir­cuit, where it is utilized to raise steam which drives turbines, which in turn drive the generators for producing electricity.

The reactor fuel consists of slightly enriched uranium dioxide (average 3% 235 U), which is enclosed sealed zircaloy (a zirconium alloy) tubes. One hundred eighty such fuel rods are combined into one fuel assembly. There are 121 fuel assemblies in the reactor core. 27 control rods are uniformly distributed over the core, which are inserted into it from above.

IV. Translate the following information into English:

Ядерные реакторы. Промышленные ядерные реакторы первоначально разрабатывались лишь в странах, обладающих ядерным оружием. США, СССР, Великобритания и Франция активно последовали разные варианты ядерных ре­акторов. Однако впоследствии в атомной энергетике сташ доминировать три ocновных типа реакторов, различающихся, главным образом, топливом, теплоно­сителем, применяемым для поддержания нужной температуры активной зоны, и замедлителем, используемым для снижения скорости нейтронов, выделяющихся в процессе распада и необходимых для поддержания цепной реакции.

Среди них первый (и наиболее распространенный) тип — это реактор на обо­гащенном уране, в котором и теплоносителем, и замедлителем является обычная, или «легкая», вода (легководный реактор). Существуют две основные разновид­ности легководного реактора: реактор, в котором пар, вращающий турбины, обра­зуется непосредственно в активной зоне (кипящий реактор), и реактор, в котором пар образуется во внешнем, или втором, контуре, связанном с первым контуром теплообменниками и парогенераторами (водо-водяной энергетический реактор -ВВЭР). Так, в 1950-х годах компании «Дженерал электрик» и «Вестингауз» разра­батывали легководные реакторы для подводных лодок и авианосцев ВМФ США.

Второй тип реактора, который нашел практическое применение, - газоохлаж-даемый реактор (с графитовым замедлителем). Его создание также было тесно связано с ранними программами разработки ядерного оружия. В конце 1940-х -начале 1950-х годов Великобритания и Франция, стремясь к созданию собствен­ных атомных бомб, уделяли основное внимание разработке газоохлаждаемых ре­акторов, которые довольно эффективно вырабатывают оружейный плутоний и к тому же могут работать на природном уране.

Третий тип реактора, имевший коммерческий успех, - это реактор, в котором и теплоносителем, и замедлителем является тяжелая вода, а топливом тоже при­родный уран. В начале ядерного века потенциальные преимущества тяжеловод-ного реактора исследовались в ряде стран. Затем производство таких реакторов сосредоточилось главным образом в Канаде отчасти из-за ее обширных запасов урана.

V. Translate the following information into English:

В активной зоне теплового реактора должен находиться замедлитель - ве­щество, ядро которого имеют малое массовое число. В качестве замедлителя при­меняют графит, тяжелую и легкую воду, бериллий, органические жидкости. Теп­ловой реактор может работать даже на естественном уране, если замедлителем служит тяжелая вода или графит. При других замедлителях необходимо исполь­зовал» обогащенный уран. От степени обогащения топлива зависят необходимые критические размеры реактора и с увеличением степени обогащения они меньше. Существенным недостатком реакторов на тепловых нейтронах является потеря медленных нейтронов в результате захвата их замедлителем, теплоносителем и конструкционными материалами. Поэтому в таких реакторах в качестве замед­лителя, теплоносителя и конструкционных материалов необходимо использовать вещества с малыми сечениями захвата медленных нейтронов.

LESSON # 5

THERMAL FISSION REACTORS

The basic concept of a nuclear reactor is simple: assemble a critical mass of fis­sionable material and adjust its criticality to maintain a steady fission rate. But what should the fissionable material be? In a fission bomb, it must be relatively pure 235 U or 239 Pu. But in a fission reactor, it can be a mixture of 235 U and 238 U. It can even be natural uranium. The trick is to use thermal neutrons - slow moving neutrons that have only the kinetic energy associated with the local temperature.

In a fission bomb, 238 U is a serious problem because it captures the fast moving neutrons emitted by fissioning U235 nuclei. Natural uranium can't sustain a chain reac­tion because its many U238 nuclei gobble up most of the fast moving neutrons before they can induce fissions in the rare 235 U nuclei. The uranium must be enriched, so that it contains more than the natural abundance of 235 U.

But slow moving neutrons have a different experience as they travel through natu­ral uranium. For complicated reasons, the 235 U nuclei seek out slow moving neutrons and capture them with unusual efficiency. 235 U nuclei are so good at catching slow moving neutrons that they easily win out over the more abundant 238U nuclei. Even in ^ natural uranium, a slow moving neutron is more likely to be caught by a 235 U nucleus A than it is by a 238 U nucleus. As a result, it's possible to sustain a nuclear fission chain reaction in natural uranium if all of the neutrons are slow moving.

But the previous paragraph seems to be hypothetical because 235 U nuclei emit fast moving neutrons when they fission. That's why pure natural uranium can't be used in a fission bomb. However, most nuclear reactors don't use pure natural uranium. They use natural uranium plus another material that's called a moderator. The moderator's job is to slow the neutrons down so that 235 U nuclei can grab them. A fast moving neutron from a fissioning 235 U nucleus enters the moderator, rattles around for about a thou­sandth of a second, and emerges as a slow moving neutron, one with only thermal ener­gy left. It then induces fission in another 235 U nucleus. Once the moderator is present, even natural uranium can sustain a chain reaction! Reactors that carry out their chain reactions with slow moving or thermal neutrons are called thermal fission reactors.

To be a good moderator, a material must simply remove energy and momentum from the neutrons without absorbing them. When a fission neutron leaves a good mod­erator, it has only thermal energy left. The best moderators are nuclei that rarely or never absorb neutrons and don't fall apart during collisions with them. Hydrogen (1H), deuterium (2H), helium (4He), and carbon (12C) are all good moderators. When a fast moving neutron hits the nucleus of one of these atoms, the collision resembles that between two billiard balls. Because the fast moving neutron transfers some of its energy and momentum to the nucleus, the neutron slows down while the nucleus speeds up.

Water, heavy water (water containing the heavy isotope of hydrogen: deuterium or 2H), and graphite (carbon) are the best moderators for nuclear reactors. They slow neutrons down to thermal speeds without absorbing many of them. Of these moderators, heavy water is the best because it slows the neutrons quickly yet doesn't absorb them at all. However, heavy water is expensive because only 0.015% of hydrogen atoms are deuterium and separating that deuterium from ordinary hydrogen is difficult.

Graphite moderators were used in many early reactors because graphite is cheap and easy to work with. However, graphite is a less efficient moderator than heavy water, so graphite reactors had to be big. Furthermore, graphite can burn and was partly re­sponsible for two of the world's three major reactor accidents. Normal or "light" water is cheap, safe, and an efficient moderator, but it absorbs enough neutrons that it can't be used with natural uranium. For use in a light water reactor, uranium must be enriched slightly, to about 2-3% 235 U.

The core of a typical thermal fission reactor consists of small uranium oxide (U02) fuel pellets, separated by layers of moderator (Fig. 1). A neutron released by a fission­ing 235 U nucleus usually escapes from its fuel pellet, slows down in the moderator, and then induces fission in a 235 U nucleus in another fuel pellet. By absorbing some of these neutrons, the control rods determine whether the whole core is subcritical, critical, or supercritical. The 238 U nuclei are basically spectators in the reactor since most of the fissioning occurs in the 235 U nuclei.

Fig.2. A cutaway drawing of the fission reactor.

In a practical thermal fission reactor, something must extract the heat released by nuclear fission. In many reactors, cooling water passes through the core at high speeds Heat flows into this water and increases its temperature. In a boiling water reactor, the water boils directly in the reactor core, creating high-pressure steam that drives the turbines of an electric generator. In a pressurized water reactor, the water is urn under enormous pressure so it can’t boil. Instead, it's pumped to a heat exchanger outside the reactor. This heat exchanger transfers heat t6 water in another pipe, which boils to create the high-pressure steam that drives a generator.

When properly designed, a water-cooled thermal fission reactor is inherently stable. The cooling water is actually part of the moderator. If the reactor overheats and the water escapes, there will no longer be enough moderator around to slow the fission neu­trons down. The fast moving neutrons will be absorbed by U238 nuclei and the chain reaction will slow or stop.

WORLIST:

Thermal fission reactor тепловой реактор, реактор на тепловых (медленных) нейтронах

moderator замедлитель; модератор

fast moving neutron быстрый нейтрон

momentum импульс, количество движения

fuel pellet топливная таблетка

sodium dioxide перекись натрия

low melting point низкая точка плавления

heat transfer medium теплопередающая среда

reactivity химическая активность, реактивность

EXERCISES: