Sunday, December 27, 2009

Nuclear Fission / Fission and Fusion / Nuclear power

a) Nuclear Fission

The1932 discovery of the neutron by james Chadwick gave physicists an excellent tool for probing the atomic nucleus. Unlike protons or alpha particles, the neutron carries no electric charge and therefore can penetrate the nucleus without having to overcome the coulomb repulsion. Following earlier work by the Italian physicist Enrico Fermi, the German chemists Otto Hahn and Fritz Strassmann in 1938 bombarded uranium (atomic number Z = 92) with neutrons. They were puzzled to find among the reaction products radioactive versions of the much lighter elements barium (Z = 56) and lanthanum (Z = 57) . Physicist Lise Meitner and her nephew Otto Frisch soon interpreted these unusual findings, concluding that neutron bombardment had caused the uranium nuclei to fission into two parts. Word of the discovery spread throughout the world's physics community, and with it the realization that fission represented an energy source many orders of magnitude more potent than chemical reactions. The United States initiated a program to develop fission explosives, hoping to produce nuclear weapons before the Germans did.

It is well known that, Lise Meitner and Otto Hahn. Meitner and her nephew Otto Frisch interpreted Hahn and Strass­mann's experiments as evidence of neutron-induced fission of uranium.

Meitner, an Austrian physicist, had fled to Sweden to escape Hitler. By the 1930s she had become one of the world's most respected nu­clear physicists. Element 109 (meitner­ium) now bears her name.

With the help of the international physics community, many of whom had fled Europe to escape Fascism, the US. effort succeeded. In 1945, only seven years after the discovery of fission, the world's first nuclear explosion was detonated in the New Mexico desert. A few weeks later, fission bombs devastated the Japanese cities of Hiroshima and Nagasaki, bringing World War to an end. In the course of development work on the bomb, scientists led by Enrico Fermi had also constructed the world's first nuclear reactor. Built under the stands of the University of Chicago stadium, the reactor became operational in 1942 .

Painting of the first nuclear reactor, built under the stands of the University of Chicago stadium dur­ing World War 2 is shown, where the man in the pit is manually adjusting a control rod to increase the nuclear reac­tion rate.

1- Chain Reactions

Nuclear fission occurs when a massive nucleus splits into two lighter parts. Although the process can occur spontaneously in a variety of heavy nuclei, such spontaneous fission is extremely rare. More common is induced fission, occur­ring typically when a heavy nucleus absorbs a neutron. (High-energy protons and gamma rays can also induce fission.) In a common fission reaction, a 235U nucleus absorbs a neutron to form a highly excited 236U nucleus. The 236U undergoes vigorous oscillations that deform it into a dumbbell shape as shown.

Neutrons induce fission, and fission itself may release neutrons-the very things needed to induce fission. This fact makes possible a chain reaction, in which each fission event supplies neutrons that give rise to more such events.

For a sustained chain reaction, neutrons from each fission event must, on the average, cause at least one more nucleus to fission. Otherwise the reaction will fizzle to a halt, in which case the configuration of fissile material is called a subcritical mass. If each fission event causes, on the average, exactly one additional fission event, then the reaction continues with a constant rate of energy release; the fissile material then comprises a critical mass. Recall that the average number of neutrons released in 235U fission is 2.47; thus in a critical action most of the neutrons do not cause additional fission. If, on the other hand, an average of more than one neutron from each fission causes additional fission, then the reaction rate grows exponentially with time. In this case the configuration is supercritical.

A supercritical chain reaction with the multiplication factor k = 2 is shown, since two neutrons from each fission cause additional fission.

Quantitatively, the criticality of a fissile mass is described by the multiplication factor k, which is just the average number of neutrons from a fission event that cause additional fission. A subcritical mass has k < k =" 1,">k > 1. The value of k is determined by several factors, including the proportion of fissile isotope, the concentration of neutron-absorbing substances, and the size and configuration of the mass. With k = 2, for example, a single fission event results in two more fissions; they cause a total of four more fissions, then eight, and so on. In general, the number of fissions increases by a factor of k with each successive generation of fission events; therefore, the number of fission events occurring after n such generations is kn. The total number, N, of fission events that occur by the n th generation is the sum N = 1 + k + k2 + k3 + . . . + kn. You may recognize this sum as a geometric series; in any event, you can show by mathematical induction that it has the valu

The average time between successive generations of fission events is called generation time. As the example below indicates, short generation times lead to explosive release of nuclear energy.

2- Fission Reactors

Nuclear reactors have been producing electricity since the 1950s and, in early 2003, there were 441 nuclear reactors operating in 30 countries with a total installed capacity of 359 GW.

FISSION reactors depend on a reaction between neutrons and the atomic nuclei of the fuel for their operation. Uranium, the fuel for almost all reactors, consists princi­pally of two isotopes, uranium-235 and uranium-238. In natural uranium, the fuel for early reactors, those isotopes are in the proportion of 0.7 per cent and 99.3 per cent, respectively, by weight. The enriched uranium used in most currently operating reactors contains about 2.5 per cent of uranium-235.

Energy is released when a uranium-235 nucleus absorbs a neutron and undergoes fission, that is, it splits into two large energetic fragments or fission products, accom­panied by the release of several high energy or fast neutrons and some gamma radia­tion. The neutrons are slowed in the reactor so that they induce further fissions in the uranium-235. Such neutrons are often called thermal neutrons and the reactors that rely upon them thermal reactors. By contrast, when a nucleus of uranium-238 absorbs a fast neutron, it becomes uranium-239, which ultimately decays to form plutonium-239. This will also fission or capture neutrons to form isotopes of additional actinides, such as americium or curium. Consideration is currently being given to fuelling some reactors with mixed oxide fuel (known as MOX), which contains enriched uranium mixed with plutonium recovered from spent fuel by reprocessing. This is seen as a way of recycling fuel and control­ling stockpiles of plutonium that can be used to make nuclear weapons.

When a nucleus, such as 23592U, undergoes fission, it releases neutrons that may be used to initiate fission in other nuclei, thereby creating a chain reaction. In an atomic bomb the chain reaction is uncontrolled; in a fission reactor the chain reaction is controlled.

3- Fuel

Besides 235U, a number of other nuclides can undergo neutron-induced fission; these are said to be fissionable. Fissionable nuclides that will fission with neutrons of any energy-especially relatively low thermal energy-are called fissile. In a fissile nucleus, the potential energy barrier is very low and the required neutron energy is therefore negligible. Only three fissile nuclides are known: they include the uranium isotopes 233U, 235U, and the plutonium isotope 239Pu.

Fuel rod bundles are shown, being lowered into the core of a nuclear reactor. The blue glow is from beta radiation-high-energy electrons-in­teracting with the reactor's cooling water.

By far the most important of these are 235U and 239Pu.

Although 235U occurs naturally, it presently constitutes only about 0.72% of rural uranium (nearly all the rest-99.27%-is 238U). For most uses, uranium must be enriched in 235U, to several per cent for commercial power reactors and often over 90% for weapons. Uranium enrichment is a difficult and expensive process; since the isotopes 235U and 238U are chemically similar, enrichment schemes make use of the very slight mass difference between the two. Among the techniques used are centrifuging, gaseous diffusion, and selective ionization of 235U by lasers.

The basic principle of centrifuge is as follows:

After ionization, acceleration, and selection of single velocity particles, the Uranium ions move into a mass spectrometer region where the radius of the path and thus the position on the detector is a function of the mass.

In gaseous diffusion, uranium hexafluoride gas (UF6) passes through a membrane as shown; the lighter, faster moving 235UF6 (light color) is more likely to get through. After many such cells the desired concentration is reached.

Enrichment technology is highly sensitive because a nation possessing it, can readily produce weapons-grade uranium.

Plutonium-239, with a half-life of 24,110 years, does not occur in nature. It is produced artificially by neutron bombardment of 238U. This reaction first produces the highly unstable isotope 239U, which undergoes beta decay with a half-life of 23.5 minutes to form 239Np. The 239Np again decays by beta emission, with a half-life of 2.35 days, leaving 239Pu. This sequence of reactions leading to plutonium can be written .

10n + 23892U ------------> 239U

23992U ------------> 23993Np + e- + ע

23993Np ------------> 23994Pu + e- + ע.

Although 239Pu is produced in copious amounts in nuclear reactors, reprocessing spent reactor fuel to extract plutonium is difficult and dangerous. Contamination with other plutonium isotopes further complicates the sep­aration of fissile plutonium. Like uranium enrichment, plutonium separation is a sensitive technology; nevertheless, all the nations except China that are known to have developed nuclear weapons chose plutonium over uranium for their first nuclear explosions. And by the early 1990s, several European countries and Japan had embarked on ambitious plutonium reprocessing programs for commercial power reactors, including intercontinental shipments of plutonium.

A number of other isotopes, most importantly 238U, are fissionable with fast neutrons that bring in enough kinetic energy to overcome the potential barrier. But 238U fission does not result in significant neutron emission, so, a self-sustaining 238U fission reaction is not possible. However, fast-neutron fission of 238U plays a significant role in thermonuclear weapons .

4- Moderator

Naturally occurring uranium consists of 0.7% 23592U and 99.3% 23892U. When a 23892U nucleus absorbs a neutron, it tends to emit a γ ray rather than undergo fission. In contrast, 23592U has a high fission probability for slow neutrons (1 eV or less). The high-energy (≈ 2 MeV) neutrons produced in the fission of 235U must be slowed down before they can induce further fissions. This is accomplished by a material called a moderator. In passing through the moderator the average kinetic energy of the neutrons is reduced to the average kinetic energy 3/2 kT (≈ 0.04 eV at 300 K) characteristic of the temperature of the moderator. In an elastic collision, the maximum transfer of kinetic en­ergy from an incoming particle to a target particle occurs when they have the same mass. Thus protons in water are ideal for this purpose. In passing through water, neutrons are thermalized after about 20 collisions within 10-3S. How­ever, protons tend to combine with neutrons to form deuterons: Ordinary "light" water is thereby converted to heavy water, D2O. If the fuel is natural uranium, then heavy water or graphite can be used as moderators. Light water can still be used as a moderator if the uranium is "enriched" by raising the proportion of 23592U from 0.7% to about 3% or 4%.

One cannot simply mix the uranium fuel with the mod­erator because neutrons within the energy range 5 eV to 100 eV have a high probability of being absorbed by 23892U nuclei (with later γ emission). They would become unavailable for the fission of the 23592U nuclei. Therefore the uranium fuel is packed into zircalloy rods that are arranged in a pattern and immersed in the moderator. Fast neutrons being slowed down find themselves outside the fuel rods as they pass through the 5 eV to 100 eV range. After they have been thermalized they can enter other fuel rods and initiate fis­sion in 23592U nuclei.

5- Critical Size and Control

An important parameter in a chain reaction is the multipli­cation factor, k. This is the ratio of the number of neutrons in one generation of the chain reaction to the number in the previous generation. The production of neutrons is propor­tional to the volume of the fissile material, whereas the leak­age increases with the surface area. When k = 1, the num­ber of neutrons produced is equal to the number that are absorbed or leak away. In this condition the system is said to be critical.

In an atomic bomb, two subcritical masses of uranium (enriched to 50% 23592U) are brought together to form a super­critical mass that explodes within 10-8s. Since the enrich­ment in reactor fuel is much lower (under 4%), a nuclear explosion cannot occur. However, when k > 1, the thermal energy generated by the fission events and the radioactivity of the fission fragments can quickly melt the core, which can then melt the concrete below (a possibility sometimes referred to as the China syndrome). In addition, the moder­ating water would turn to steam and explode, thereby spreading radioactive material.

In order to keep k close to one, control rods of cad­mium, which has a high absorption cross section for ther­mal neutrons, are inserted into the core. By carefully raising them, the condition of criticality can be achieved. If k = 1.01, the time constant τor the increase in the neutron flux is only 0.1 s, which is too fast for human response. The ability to control a reactor depends crucially on a small feature of the fission process. Although almost all neutrons are prompt-they are emitted within 10-8s-about 0.7% of the neutrons are delayed by between 0.2 sand 55 s. The core of a reactor is designed to be critical only when the contribu­tion of these delayed neutrons is included. Through this approach, the time allowed for control of the reactor be­comes greater than human reaction times. In case of an emergency, the control rods are dropped into the core, thereby making it subcritical. However, even after a shut­down, the heat generated by the radioactive decay of the fission fragments continues. In a large reactor the heat pro­duction would drop to about 1 %, say 20 MW, after a day. But this is still very large.

The combination of fuel, moderator, and control rods forms the reactor core; coolant flows through the core to remove the heat generated by fission. Reactor details vary greatly. Most US. power reactors are light-water reactors, using ordinary water as coolant and moderator. The core is encased in a thick steel pressure vessel, and water flows freely among the fuel rods . In contrast, the RBMK design, common in the former Soviet bloc, uses a solid graphite moderator; its water coolant is confined in pipes passing among the moderator blocks . The Canadian CANDU reactor design uses two separate loops of deuterium oxide (heavy water) as moderator and coolant. The high-temperature gas-cooled reactor (HTGR), used in England and elsewhere, has a graphite moderator and gaseous carbon dioxide or helium coolant. There is no clearly superior reactor type. Each has its own economic and safety advantages and disadvantages. US. light-water reactors, for example, must be shut down completely for refueling, a process that takes weeks to months. CANDU and RBMK reactors can be refueled in operation, using robot machin­ery. The CANDU design seems particularly safe from a proliferation standpoint since no uranium enrichment is needed. (However, plutonium produced in Canadian-style heavy-water reactors may have been used in India's successful development of a nuclear explosive.)

Whatever the reactor details, the ultimate purpose of a nuclear power plant is to produce steam that drives a turbine that, in turn, is connected to a generator. Electromagnetic induction in the spinning generator drives electric current that is the power plant's useful product. The second law of thermodynamics limits the efficiency with which heat energy can be converted to mechanical and electrical energy; as a result, about two-­thirds of the energy generated in the core of a typical power reactor is dumped to the environment as waste heat. This heat is extracted in the power plant's condenser, where spent steam from the turbine is condensed to water.

In the simplest water-cooled reactor designs, the core cooling water is itself boiled to produce steam. A US. light-water reactor of this type is called a boiling-water reactor.

Although simple and economical, the BWR has the disadvantage that water circulating through the turbine and condenser is highly radioactive. A more common US. design is the pressurized-water reactor (PWR), in which the core cooling water is kept under pressure to prevent boiling. Heat is transferred from this primary coolant to a secondary loop where water boils and drives the turbine. In the pressurized water reactor shown, the reactor core and the moderating water are contained in the reactor vessel. The moderating water also serves as the coolant in the primary coolant system. In order to prevent the water (T = 315 °C) from boiling, very high pressure (15 MPa or 150 atm) is required. The pipes in the primary cool­ant system pass through a steam generator where water from the secondary coolant system is converted to high­ pressure steam (265 °C, 0.5 MPa) and directed to a turbine, which is connected to an electrical generator.

The primary and secondary cooling systems are closed After the steam passes through the turbine, it is cooled in a condenser with water from a reservoir, such as a river or a lake. The heated water is first cooled by evaporation in towers and then dis­charged back into the reservoir.

6- Reactor safety measures

The operation of a nuclear reactor requires the imple­mentation of many safety measures. For example, the reac­tor vessel and the steam generators are in a steel shell, itself housed in a reinforced concrete building. None­theless, the accidents at Three Mile Island (US) and at Chernobyl (USSR) illustrate what happens when proper procedures are not followed. The fission fragments are themselves radioactive. Therefore, even after the uranium fuel has been used, there remains the problem of disposing of these radioactive wastes. Burial in deep salt mines is one possibility. Because the structure within the reactor vessel receives intense neutron bombardment, many elements are "neutron activated," that is, they become radioactive. This limits the useful life of a nuclear reactor to about 30 years.

In CANDU and gas-cooled reactors, heat from the primary coolant is also trans­ferred to a secondary boiling-water loop.

As fission proceeds in a reactor core, the concentration of fission fragments in the fuel increases. Before the 235U is exhausted, these fission products absorb enough neutrons to interfere with the chain reaction. Fuel rods must therefore be replaced at regular intervals. In U.S. light-water reactors, about one-third of the fuel is replaced each year, so a given fuel rod remains in place for 3 years.

In Canadian CANDU and Russian RBMK reactors, coolant circu­lates through pipes rather than in a pressure vessel surrounding the entire reactor. In these reactors, fuel rods can be reached while the reactor is in operation, and refueling takes place on a nearly continuous basis. Another reaction that occurs in the nuclear fuel is the conversion of 238U to plutonium, via the neutron-capture reaction we discussed earlier. As plutonium builds up, it begins to fission in significant quantities. Near the end of a fuel rod's 3-year residency in the reactor core, in fact, only 30% of the energy production is from 235U fission. Most of the rest -54%- is from fission of 239Pu, with the remain­der from other plutonium isotopes.

The outlines for the evolution of fuel in the reactor is shown:

- Evolution of 1000 kg of 3.3% enriched uranium over its 3-year stay in a reactor core. Of the 33 kg of 235U initially present, 25 kg are consumed in fission. In addition, 24 kg of 238U are converted to plutonium and other transuranic isotopes, some of which also undergo fission. The fuel mass remains largely 1000, contaminated with 35 kg of radioactive fission products and lesser amounts of transuranic elements formed by neutron capture.

As indicated, spent reactor fuel is contaminated with highly ra­dioactive fission products. It also contains significant amounts of fissile 235U and 239Pu. Through the expensive, technologically complex, and dangerous tech­nique of nuclear fuel reprocessing, these fissile isotopes can be extracted and used to make new reactor fuel or nuclear weapons. The high cost and threat of weapons proliferation in a plutonium-reprocessing economy have led some nations-notably the United States-to forgo reprocessing. Others have eagerly developed reprocessing technology, and international shipments of plutonium began in 1992. Whether or not nuclear waste is reprocessed, the spent fuel ultimately requires disposal. The presence of long-lived radioactive isotopes means that the material remains dangerous for thousands of years. To date, no entirely satisfactory method of disposal has been developed, although it is gen­erally agreed that underground storage will prove the safest alternative .

Containers of radioactive waste being stored at an un­derground facility in France, which gets 70% of its electricity from nuclear power is shown.

The thermal reactors we have been discussing use uranium fuel enriched at most slightly in 235U. Although some of the 235U comprising the bulk of the fuel is converted to plutonium, most remains energetically useless.

In contrast, a breeder reactor is designed to convert large amounts of 238U to plutonium, "breeding" more fissile fuel. Use of breeder reactors would greatly extend our supplies of fissile fuels, by making much of the 99.27% of natural uranium that is non fissile 238U into fissile 239Pu. For this reason, breeder technology has been pursued in a number of countries, particularly in France and Japan. Breeders operate at higher temperatures and use fast instead of slow neutrons to induce fission. They have no moderator and their coolant is liquid sodium. High temperature and the use of fast neutrons make these reactors less stable and technologically more sensitive than non breeding designs.

Furthermore, a breeder-based power system inherently involves reprocessing and the move­ment of large amounts of plutonium-of which only a few kg suffice to make a fission weapon. Because of these technological, safety, and proliferation con­cerns, the future of breeder reactors is unclear.

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