Журавлева Сборник текстов для подготовки аспирантов-физиков 2011

МИНИСТЕРСТВО ОБРАЗОВАНИЯ И НАУКИ РОССИЙСКОЙ ФЕДЕРАЦИИ

НАЦИОНАЛЬНЫЙ ИССЛЕДОВАТЕЛЬСКИЙ ЯДЕРНЫЙ УНИВЕРСИТЕТ «МИФИ»

В.И. Журавлева

СБОРНИК ТЕКСТОВ ДЛЯ ПОДГОТОВКИ АСПИРАНТОВ-ФИЗИКОВ

К СДАЧЕ КАНДИДАТСКОГО ЭКЗАМЕНА

Рекомендовано к изданию УМО «Ядерные физика и технологии»

Москва 2011

Содержание

Unit 1____________________________________ 4 Unit 2____________________________________8 Unit 3____________________________________12 Unit 4____________________________________15 Unit 5____________________________________18 Unit 6____________________________________22 Unit 7____________________________________24 Unit 8____________________________________29 Supplementary Reading ____________________30

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Unit 1

Warming up activities

Do you know anything about Kuiper Belt? Have you heard any theories about it? Do you have your own one? Tell the rest of the group about it and prove that it is true. What field of astronomy does your department deal with? This text is about one of these theories. Do you agree with it? If not, why?

Kuiper Belt may be born of collisions

Diversity of objects and frequency of those in pairs not easily generated by standard models of planet formation.

October 6, 2010

By Rick Lovett

The cold and shadowy fringe of the solar system known as the Kuiper belt is generating increasing debate among scientists as data accumulates on the growing population of objects discovered there. Now, two new studies of Kuiper belt objects presented October 5 at a meeting of the American Astronomical Society's Division for Planetary Sciences in Pasadena, Calif., may reveal a crucial hole a prevailing model of the solar system's early history.

One of the scientists challenging established theory is Michael Brown of the California Institute of Technology in Pasadena. Rather than growing incrementally from small precursors, as has been conventionally believed, he argues the largest Kuiper belt objects formed in a series of collisions between objects of roughly equal size - a process Brown describes as "pyramidal growth." Evidence for this, Brown says, comes from recent discoveries that large Kuiper belt objects, which can reach diameters in excess of 2,000 kilometers, have widely disparate densities. Some seem to be comprised almost entirely of rock, with densities as high as 3.0 grams per cubic centimeter. Others have densities so

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low they appear to be almost entirely water ice punctuated with void spaces. Pluto, the best-known Kuiper belt object, lies midway between these extremes, with a density of about 2.0 grams per cubic centimeter. If all these bodies accreted from multitudes of small precursors, Brown says, they should all have densities representing the average composition of the protoplanetary nebula in which they formed. "To get something that large, you would have had to accrete from a very large swath of the outer solar system," he said. "You would think they would be some of the most uniformly composed objects in the solar system." Instead, there "is about as big a variation as you can get."

Brown believes that this wide variation is a sign that the biggest Kuiper belt objects were produced not from gradual accretion, but from a small number of collisions among large objects, beginning with ones on the order of 500 kilometers in diameter. In each of the collisions, most of the mass stuck together to form a new, larger object, with rest blowing off into space - the amount varying with the size and power of each impact. Because the blown-away material is primarily ice, this means that some large Kuiper belt objects could have been built from a small number of really big collisions among increasingly ice-depleted bodies, while others might have been formed from smaller, less powerful collisions that allowed more ice to remain. "You can have different combinations," he said.

Neptune not guilty

Brown's argument that current theory is lacking was bolstered by a study presented by Alex Parker, a graduate student in astronomy at the University of Victoria in British Columbia, Canada. Parker examined binary objects in the cold classical Kuiper belt, a region on the edge of the Solar System, 6 to 7 billion kilometers from the Sun.

Traditional theory finds it hard to explain how these objects formed there because, that far out, their accretion would have been too slow to have been finished during the known life of the solar system. Thus, in a theory known as the Nice model (for Nice, France, where it was first proposed), scientists suggested that these objects formed closer to the

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Sun, where faster growth was possible. Then they were flung outward by dramatic shifts in the orbits of the outer planets, most importantly Neptune.

But there's one problem with that theory, according to Parker. About 30 percent of these objects are binaries. "The most famous are Pluto and Charon, but there are many others," he says. Painstaking studies of their slow-motion orbits, in which they can take four to 17 years to complete a circuit of each other, reveal that many are so far apart that they are very loosely bound - loosely enough that any interaction with Neptune would have sent them flying in different directions. "So if the Kuiper belt was subjected to this violent event, these [binary] systems should have been destroyed," Parker says. Stephen Tegler of Northern Arizona University in Flagstaff agrees that Parker's finding forces theoreticians back to the drawing board. "We have got to come up with a way to tweak the Nice model, or they formed in situ," he says. But, Tegler notes, "It's a brand new idea and it's up to the rest of us to check this out - to confirm or refute it."

Big hunks falling

One reason for caution is that for Kuiper objects to have emerged in situ would require them to have formed more quickly than traditional theory predicts. It's here that Brown's and Parker's findings support each other. That's because one way c objects might have formed in situ, Parker says, is under an emerging model of planet formation in which turbulences and vortexes in the protoplanetary nebula allow many tiny particles to coalesce extremely rapidly into big ones.

Brown refers to this as "big hunks falling out of the nebula" and thinks it's a way in which large building blocks might have been formed as starting material for his 'pyramidal growth' model. He cautions, though, that the new theory is in its infancy. "No one has really put all the pieces together yet," he says. But, he adds, "We could well be completely rewriting how planets form."

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Vocabulary

precursor - предшественник, продукт предшествующей стадии реакции; лидер (разряда)

swath - валковать; захват; полоса

accretion -) разрастание; прирост; приращение, увеличение bolster - валик под подушкой; балка, брус, перекладина, попере-

чина Syn: beam , timbe; подкладка; втулка, шейка; буфер Syn: buffer; один из завитков ионической капители; up поддерживать, помогать; пособничать, содействовать (злу) Syn: uphold; набивать, наполнять (чем-л.)

fling ( flung ) - бросаться, кидаться, ринуться caution - осмотрительность, осторожность

tweak - щипок Syn: nip , pinch; дёргать, щипать; Syn: pinch; на-

лаживать (механизм) hunk - глыба

Find out the answers

How was the Kuiper Belt formed according to the author? What fact helped to prove it?

What is the density of Pluto?

What is the range of densities in Kuiper Belt? What conclusion can be made from this fact?

What kind of systems can be found in the Kuiper Belt?

What questions would you ask? Think of three questions to ask the group.

There are three parts in the text, each with its own theory. Divide into three groups. Each group tries to persuade the two others that their theory is correct.

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Unit 2

Warming up activities

Think about future sources of energy. Is nuclear power among them? What problem of nuclear power does your department deal with? What are the most serious problems in nuclear energy production? Is solving of the problem just round the corner?

Is Spent Nuclear Fuel a Waste or a Resource?

A new report argues that the world has plenty of uranium but needs to make wise choices about what to do with it once its been depleted in a nuclear reactor

By David Biello September 18, 2010

On September 15, the U.S. Nuclear Regulatory Commission affirmed its expert opinion that spent nuclear fuel could be safely stored on nuclear power plant grounds—whether in pools or dry casks—for "at least 60 years beyond the licensed life of any reactor." That is good news, because there is nowhere else for such waste to go.

As President Obama's Blue Ribbon Commission on America's Nuclear Future continues to ponder what role nuclear power might play in the U.S. electricity supply, a group of scientists, engineers and other experts assembled by the Massachusetts Institute of Technology (M.I.T.) released a report on the nuclear fuel cycle paid for by the nuclear industry. In short, the report finds that uranium resources are not likely to run out in the next century, even if the U.S. alone builds as many as 1,000 nuclear reactors. Therefore, either reprocessing or recycling spent nuclear fuel, as the French and Japanese do, is likely to be a waste of money better spent on improving the lightwater reactors presently in use. The funds could also be used to create a $670-million-per- year research and development program for nuclear power as well as to determine the best fuel cycle over the course of the next several decades. Finally, the global expansion of nuclear power plants should be enabled

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by some form of leasing program for the uranium fuel rods—one up for renewal every decade or so.

"For the next several decades in the U.S. the once-through fuel cycle using light-water reactors is the preferred option," said M.I.T. physicist and report co-chair Ernest Moniz at its release on September 16 in Washington, D.C. "Light-water reactors are the workhorse, and there's a lot we can do to improve them." The U.S. employs 104 light-water reactors to generate 20 percent of its electricity today; the reactors moderate uranium fission and the heat it produces with water, which is also boiled into steam to turn an electricity-generating turbine.

M.I.T. nuclear engineer Charles Forsberg, another co-chair of the report, noted that a typical light-water reactor in the U.S. needs 200 metric tons of mined uranium resulting in 20 metric tons of uranium fuel per year. All this uranium represents as little as 2 percent of the final cost of the electricity from that nuclear power plant. Therefore, even if uranium prices doubled or more, the impact on electricity prices would be minimal.

The M.I.T. report predicts that even if the world's fleet of more than 400 nuclear power plants grew to be 4,000 such plants that then operated for a century, the cost of the electricity from those facilities would rise by a mere 1 percent as a result of the increased demand for uranium. "There's no shortage of uranium that might constrain future commitments to build new nuclear plants for much of the century," Forsberg said. This also argues against alternate fissile fuels such as thorium. "What do you get by complicating the fuel cycle by looking at thorium when we have plenty of uranium?" asked M.I.T. nuclear engineer and report co-chair Mujid Kazimi.

The question then becomes what to do with that abundant uranium once it's been fissioned in a nuclear reactor. After all, the spent nuclear fuel still contains fissionable uranium 235 and plutonium 239. "Today, we don't know whether spent nuclear fuel from light-water reactors is waste or a resource," Moniz noted. Forsberg added that the spent nuclear fuel currently awaiting a home in the U.S. could be compared with

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"a super-strategic petroleum reserve. We should be cautious before we throw it away."

But a place to throw such radioactive waste remains necessary. Even though the spent nuclear fuel from the entire U.S. fleet of reactors— roughly 2,000 metric tons per year—requires just two hectares of land to be stored in dry casks, some form of geologic isolation—such as the proposed repository at Yucca Mountain in Nevada—will be needed ultimately. But rather than choosing a site for political reasons, as in the case of Yucca, the M.I.T. report authors argue for selecting a site based on the type of waste to be placed there, the geology that then best shields that type of waste, and even the initial reactor design as a result (to make sure the right kind of waste is made). For example, an entire nuclear cycle involving light-water reactors, reprocessing of the spent fuel, and disposal of small "packages" of highly radioactive nuclear waste in deep boreholes could prove an attractive option, Moniz noted.

Such reprocessing—or even fast-neutron reactors that don't use water to moderate fission and can potentially create more fuel than they con- sume—remain a distant prospect. Since the 1950s roughly $100 billion has been spent on the research and development of such reactors around the world, yet there is currently only one producing electricity—the BN600 reactor in Russia, operational since 1980. And even with such fastneutron reactors, the amount of potentially worrisome material for making nuclear weapons does not change. "Transuranics are not magically changed in terms of their inventory by these things," Moniz said. In fact, the M.I.T. report argues that creating reactors that produce more fuel than they consume may never be necessary. "Light-water reactors are with us for the entire century," Kazimi noted. "They are the backbone of the system." So that leaves the question of proliferation, particularly as many countries in Asia begin to build new nuclear power plants, ranging from the United Arab Emirates to Vietnam. The M.I.T. report argues that a leasing program, in which countries with the capability to enrich uranium fuel supply it to other countries and then take back the spent fuel for disposal in one form or another at the end of its useful life. "One might combine climate and proliferation concerns with a way of attaching carbon credits to new nuclear construction in countries that took cer-

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