Fission and Fusion / Nuclear power

3) Fission and Fusion
Nuclear energy is 10 million times more concentrated than chemical energy. One consequence is the awesome destructive power of nuclear weapons.
Nuclear energy plays a major role in the natural universe, powering the Sun and other stars. On Earth, applications of nuclear energy became a serious possibility in the 1930s, a reality in the 1940s, and a fact of life in the 1950s. Since then, the use of nuclear energy for both peaceful and military purposes has increased substantially. Proliferation of nuclear weapons throughout the world presents a serious threat to human survival. On a more positive note, nuclear fission supplies more than 15% of the world's electrical energy, and nuclear fusion raises the prospect of a cleaner, safer nuclear power source with nearly limitless fuel resources. The curve of binding energy leads to the possibility of nuclear energy release through fusion-the joining of lighter nuclei to form heavier ones nearer the peak of the binding energy curve. Fusion in the Sun provides the energy that sustains life on Earth. Fusion energy released in our thermonuclear weapons, on the other hand, has the potential to destroy that life. For nuclei heavier than iron, energy release can occur if a nucleus splits, or fission, into two lighter nuclei. In contrast to fusion, nuclear fission seems to be of little consequence in the cosmic scheme of things. A self-sustaining fission reaction did occur naturally in what is now a uranium mine in Africa some three billion years ago, and it has provided useful information on the long-term movement of nuclear waste through the environment. Technologically, though, nuclear fission is important: it is the one major energy-releasing nuclear reaction that we can now sustain in a controlled manner to produce electricity, while uncontrolled fission reactions play an important role in nuclear weapons.
The curve of binding energy , shows regimes where fusion and fission can result in the release of nuclear energy .

Fission and Fusion :

a) Nuclear Fission

b) Nuclear Fusion

Why nuclear energy ? / Nuclear power

2- Why nuclear energy?

World reserves of coal are, in theory, large enough to produce the electricity we shall need for more than a hundred years. However, it is likely that more and more of the coal mined in the future will be converted into the more valuable liquid fuels and so will not be available for electricity generation. There are also environmental and other problems associated with the increased mining and burning of coal . The difference in the heat value of uranium compared with coal and other fuels is important (though both are used at about 33% thermal efficiency in the power station). A one million kilowatt (1,000 MWe) power station, operating at 80% capacity, consumes about 3.1 million tonnes of black coal each year, or about 24 tonnes of uranium (as UO2) enriched to about 4% of the useful isotope (U-235). This requires the mining of over 200 tonnes of natural uranium which may be recovered from, say, 25-100,000 tonnes of typical uranium ore.

a) Economics The difference in fuel requirements between coal fired and nuclear power stations also affects their economics. The cost of fuel for a nuclear power station is very much less than for an equivalent coal fired power station, usually sufficient to offset the much higher capital cost of constructing a nuclear reactor. Consequently, in practical terms, electricity from nuclear reactors in many regions is competitive with electricity produced from coal, even after providing for management and disposal of radioactive wastes and the decommissioning of reactors. As gas prices rise and coal faces the prospect of economic constraints on its emissions, nuclear energy looks increasingly attractive.

b) Electricity generation - the future fuel mix For most countries the questions that need to be answered are: What are our likely electricity requirements? What forms of generation are available to us? Which combination will affordably provide our needs with maximum security, and the least harm to our population and environment? In mid 2001, there were 31 countries of varying size, political persuasion and degree of industrial development, which included nuclear power in their energy mix and were operating nuclear reactors. Over 16% of the world's electricity is being produced by more than 440 reactors, with 30 more under construction. Belgium, China, France, Germany, Hungary, India, Japan, Russia, Switzerland, UK and USA are just some of the countries with major nuclear energy programs. No country would want to be too dependent on a single energy source. For many it is therefore not a question of coal or nuclear for their main supply of electricity, but a combination of both, with as much help as possible from renewable sources, and back-up from gas.

c) Alternatives to nuclear electricity No technology is absolutely safe or without environmental effects. We should therefore compare the production of electricity from nuclear energy with the other options available to us. Burning coal in power stations is still the major source of electricity worldwide, followed by hydro, uranium and gas. A 1000 MWe light water reactor uses about 25 tonnes of enriched uranium a year, requiring the mining of some 50,000 tonnes of uranium ore. By comparison, a 1000 MWe coal-fired power station requires the mining, transportation, storage and burning of about 3.2 million tonnes of black coal per year. This creates around 7 million tonnes of carbon dioxide not to mention sulfur dioxide, depending on the particular coal. Solid wastes from a coal-fired power station can be substantial and cause environmental and health damage. Many people are concerned about the possible warming of the earth through enhancement of the greenhouse effect. About half of this is due to steadily increasing carbon dioxide in the atmosphere over the past 150 years, largely from the burning of fossil fuels, particularly coal.

Nuclear Fusion / Fission and Fusion / Nuclear power

b) Nuclear Fusion
The curve of binding energy, that is shown before, indicates that nuclear energy can be released either by fission of heavy nuclei or by fusion of light nuclei. The binding-energy curve is steepest near its left end, showing that the most energy per nucleon is released by fusing the very lightest element-hydrogen. Indeed, the fusion reactions powering the Sun and many other stars begin with the fusion of hydrogen to form deuterium: 11H +11H ------------> 21H + e+ + (0.42 MeV) (1) a positron (e+) and neutrino (ע) are also released, and the total energy liberated in the reaction is 0.42 Mev. Deuterium then fuses with hydrogen to form the. helium isotope 32He: 11H + 21H ------------> 32He + γ (5.49 MeV). (2) Here γ represents a gamma ray, and the quantity in parentheses is the total energy released. Helium-3 nuclei from two such reactions then react to give a single 4He nucleus and a pair of protons; this event liberates 12.86 MeV: 32He +32He ------------> 42He + 211H (12.86 MeV). (3) There is one additional energy-producing reaction associated with these events: the positron from reaction (1) annihilates with an electron, forming two gamma rays with a total energy of 2mc2 or 1.022 MeV as follows: e+ + e- ------------> 2γ (1.022 MeV). (4) The reactions (1 to 3) constitute the proton-proton cycle. In the full cycle, reactions (1) and (2) occur twice for each occurrence of reaction (3). The net effect, including two occurrences of the annihilation reaction (4), is to con­vert four protons and two electrons into a single helium-4 nucleus; a total of 26.7 MeV is released in the process. In massive stars,42He is then the building block for the formation of still heavier nuclei by additional fusion reactions .
Although ordinary hydrogen (11H) is abundant, the reaction (1) has a low cross section and so does not occur readily. Terrestrial fusion research has therefore focused on reactions involving the heavier hydrogen isotopes. of most immediate interest are deuterium-deuterium (D-D) and deuterium-tritium (D-T) reactions, listed below along with the energy released in each: 411H + 2e- ------------> 42He + 26.7 Me V
The two possible outcomes of the D-D reaction have nearly equal probability. 21H + 21H ------------> 32He + 10n (3.27 Me V) (5) 21H + 21H ------------> 31H + 11H (4.03 Me V) (6) 21H + 31H ------------> 42He + 10n (17.6 Me V). (7)
The electrical repulsion between nuclei makes it difficult to bring them close enough to fuse. In potential-energy terms, nuclei must overcome the potential barrier associated with the electrostatic force before they can drop into the deep potential well of the stronger but shorter-range nuclear force . Potential-energy diagram for two nuclei, showing electro­static potential barrier and deep well as­sociated with attractive nuclear force is shown.
Nuclei can quantum-mechanically tunnel through the potential barrier when they lack sufficient energy to overcome it. Although the possibility of tunneling lowers the energy needed to initiate fusion, that energy still remains high. In the Sun's core, for example, fusing nuclei approach one another with energies of the order of 1 Kev, corresponding to a temperature of 15 MK.
In terrestrial applications, the high temperatures required for fusion pose two problems: first, how to achieve those temperatures and, second, how to contain the fusing material. The stars solve both problems with their immense gravitational fields. A star is born as interstellar material-mostly hydrogen and helium-collapses under its own gravity; the resulting compression heats the material to fusion temperatures. Once fusion has begun, the star settles into an equilibrium in which the high pressure of the fusing material is balanced by the gravitational force.
Gravitational confinement is not possible in terrestrial fusion applications, and we must find another means of confining the fusing material. For net fusion energy production, confinement must last long enough for the energy produced by fusion to exceed the energy needed to heat the material. The heat input required is proportional to the number of nuclei being heated, or, to the density, n. However, the rate of fusion energy production per unit volume is proportional to the square of the density. You can see this by considering half the nuclei as targets to be struck by the other half. If you double the number of target nuclei alone, you double the fusion rate. Doubling the density doubles the number of targets and the number of projectiles hitting them, and therefore quadruples the fusion rate. The total energy produced by fusion is, in turn, given by the fusion rate multiplied by the time τ during which fusion is occurring. Requiring that the fusion energy, proportional to n2τ, exceed the heating energy, proportional to n, gives a minimum value for the quantity nτ- the product of density and confinement time-that must be met in an energy-producing fusion device. This condition on nτ is called the Lawson criterion. For the D-D and D-T reactions the Lawson criteria are approximately nτ > 1022 s/m3 (D-D) nτ > 1020 s/m3 (D-T).
The factor-of-l00 difference between these two Lawson criteria shows that D-T fusion is much more readily achieved. The Lawson criterion offers a choice in the design of a fusion device: Strive for a high plasma density with a short confinement time or a lower density with a longer time. Two distinct approaches emphasize these two possibilities. Iner­tial confinement relies on the inertia of the reacting particles-that is, on their inability to be accelerated instantaneously away from the reaction site to provide confinement; very short confinement times are required in this scheme. Inertial confinement occurs in fusion weapons and in particle-beam and laser ­fusion devices. In magnetic confinement, the more traditional approach to controlled fusion, complicated magnetic field configurations confine the fusion plasma at lower densities but for longer times. However the Lawson criterion is met, it is of course also necessary to surpass the critical ignition temperature.
1- Fusion Reactors
In order to produce useful power from the fusion of nuclei several conditions are re­quired. In particular the particle density and confinement time must satisfy Lawson's criterion. There are two basic approaches to confining a plasma to achieve Lawson's criterion. In the magnetic confinement tech­nique, a Iow particle density is compensated for by a rela­tively long (1s) confinement time. In a system based on inertial confinement, the particle density is high but only for a short (1 ns) time.
Magnetic Confinement
Controlled fusion research began in the 1950s with magnetic confinement of the fusion plasma. The Tokamak is a magnetic confinement device invented in the USSR. The plasma in a Tokamak is confined by the combination of magnetic fields. The toroidal field Bt and the poloidal field Bp produce a net field whose lines are helical as shown in the figure.
A strong toroidal field, Bt , is produced by about 20 coils wrapped around the perimeter of a torus. A weaker poloidal field, Bp, is pro­duced by a large current (106 A) that is induced in the plasma by a different, time-varying field generated by coils in the same plane as the torus. The resultant magnetic field lines are helical and serve to confine the plasma. If the plasma were to come into contact with the walls of the containment chamber, the plasma would lose energy and cool down. Furthermore, impurities would be released into the chamber and would severely curtail the operation of the reactor.
The first job of any magnetic confinement scheme is to create a magnetic configuration that keeps plasma away from the relatively cool walls of the device. Plasma particles can reach the walls in three general ways: (1) If mag­netic field lines penetrate the walls, particles spiraling along those field lines may hit the walls. This mechanism is known as end loss. (2) Collisions among particles, and inhomogeneities in the magnetic field, result in particles drifting across the field lines toward the walls of the fusion device. (3) Plasmas are notoriously unstable. A wide variety of waves can propagate in plasma, and some of these waves can grow exponentially at the expense of particle energy, resulting in gross distortion of the plasma and field configuration that lets plasma hit the walls.
The following figure shows the plasma loss in magnetic confinement. (a) End losses occur when field lines intersect the device walls. (b) Curvature of the field lines results in cross-field drifts. (c) Large-scale instabilities distort the plasma and magnetic field. Here the so-called sausage ,instability causes alternate narrowing and bulging of the plasma column.
	The most promising magnetic confinement devices eliminate end loss alto­gether, using a toroidal magnetic field whose field lines do not penetrate the device walls : A toroidal fusion device has no end losses, since its mag­netic field lines don't penetrate the device walls. The toroidal shape of the Tokamak Fusion Test Reactor at Prince­ton University shows clearly in this photo taken inside the device during its assembly.
The initial heating of the plasma is accomplished by the induced current mentioned above. Then, beams of high­ energy neutral particles (accelerated as ions and then neu­tralized) are injected into the plasma to deliver about 20 MW, thereby further raising its temperature. Radio fre­quency coils are also used to heat the plasma. The 14.1 MeV neutrons from the D-T reaction (7) are absorbed by a molten lithium "blanket" surrounding the containment chamber. The thermal energy deposited in this blanket can than be used to produce steam for a conven­tional electrical generator. The tritium produced in the reac­tions n + 7Li ------------> 3H + 4He + n n + 6Li ------------> 3H + 4He can be extracted and reused.
The Tokamak fusion test reactor (TFTR) at Princeton as shown, has operated with a particle density n = 3 x 1019 m-3 at a temperature such that kT = 1.5 keV, and a confinement time τ = 300 ms. Therefore the product in Law­son's criterion is nτ ≈ 1019 s/m3. In order for such a reactor to produce a 1000-MW electrical output, it would require a plasma temperature such that kT = 15 keV and the Lawson product to be nτ > 1020 s/m3.
In a driven fusion reaction, energy is continuously sup­plied to the plasma. However, in a D-T reaction, 20% of the kinetic energy is carried away by the alpha particle. These parti­cles may also be used to heat the plasma. If the plasma reaches the ignition temperature, it becomes self-sustain­ing.
Inertial Confinement
In the inertial confinement approach the fuel is in the form of tiny pellets, of diameter less than 1 mm, that contain a mixture of deuterium and tritium. In the NOVA system at Livermore, California, 0.1 ns pulses from 10 neodymium-doped glass lasers (operating at 1.05 µm) deliver about 200 kJ in 1 ns to each pellet. (This corre­sponds to a power of 2 x 1014 W, which is greater than the generating capacity of all the stations in the US!) The sur­face of the pellet vaporizes. As it expands, it sends a shock wave inward, which increases the density of the core by a factor of 103 and raises its temperature to over 108 K. This occurs within 1.5 ns, before the particles are able to dis­perse. That is, they are confined by their own inertia. The density of the pellet reaches 103 g/cm3 and its pressure reaches 1012 atm (1017 Pa)-which is greater than the pres­sure in the interior of stars. In a sense these are tiny hydro­gen bombs. A continuous supply of power would be pro­duced by fusing about 20-50 pellets each second. Charged particles, ions or electrons, may also be used in­stead of laser beams.
2- Prospects for Fusion Energy
There are several desirable features of fusion power. Deuterium (D) is easily extracted from sea water, where its concentration is 1/6500 of normal hydrogen atoms. Al­though tritium (T) is scarce and costly ($20 per kg), it can be produced by the bombardment of Li by neutrons, as we noted earlier. A "runaway" reaction is not possible be­cause of the Iow quantity of fuel present at any time. If the magnets or other systems fail, the plasma simply disap­pears. Radioactive wastes are less of a problem than with fission reactors. Tritium is toxic, but it has a relatively short half-life of 12.3 y. If fusion reactors become viable, we will have tapped the energy source of the stars.
When work on fusion power began in the 1950s, researchers confidently pre­dicted that limitless energy sources would be available in a few decades. But plasma confinement and heating have proved more complex and subtle than expected, and the newer inertial confinement scheme has revealed its own technical problems. Nevertheless, the promise of nearly unlimited energy-a gallon of seawater equivalent to more than 300 gallons of gasoline remains, and progress toward controlled fusion continues. An important milestone was reached in 1991, when the Joint European Torus in England produced some 2 MW of fusion power for several seconds.
Once controlled fusion proves scientifically feasible, there will remain formidable engineering challenges in the design of a practical fusion power plant. The intense neutron fluxes from D-T fusion cause severe degradation of the materials comprising the reaction chamber walls.
Furthermore, neutron­capture reactions produce radioactive isotopes within the walls, greatly compli­cating maintenance procedures. Although fusion does not produce the problem of radioactive materials present in fission products, handling of radioactivity especially from D-T reactions-is still a formidable problem. Heat from D-T fusion would be extracted by a heat-transfer medium, then used to drive a conventional steam turbine and generator. The figure shows a possible design for a D-T fusion power plant. The first practical fusion power plants are likely to use D-T fusion because its ignition temperature and Lawson criterion are much lower than for D-D fusion. But the D-D reaction promises cleaner and more efficient power product­ion. With D-D fusion there is no radioactive tritium fuel. And a look at the D-D reactions (Equations, 5&6) shows that one of the reactions produces protons instead of neutrons. Thus there is less neutron-induced radioactivity. Furthermore, high-energy protons can be extracted and passed through a mag­netohydrodynamic generator, a device that uses electromagnetic induction to convert charged-particle energy directly into electricity. Use of MHD generators would bypass the conventional steam cycle and greatly increase the thermody­namic efficiency of the power plant. Even as they strive to make D-T fusion a reality, many fusion researchers have their eyes on a more distant future where D-D fusion provides much of our energy.

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.

Energy Sources / Nuclear power

1- Introduction: Energy Sources Energy can be considered in two categories - primary and secondary. Primary energy is energy in the form of natural resources, such as wood, coal, oil, natural gas, natural uranium, wind, hydro power, and sunlight. Secondary energy is the more useable forms to which primary energy may be converted, such as electricity and petrol. Primary energy can be renewable or non-renewable: Renewable energy sources include solar, wind and wave energy, biomass (wood or crops such as sugar), geothermal energy and hydro power. Non-renewable energy sources include the fossil fuels - coal, oil and natural gas, which together provide over 80% of our energy today, plus uranium. In this century, the only energy resources available for economic large-scale electricity generation are likely to be gas, coal and nuclear. Oil has generally become too expensive to use for electricity and it has the great advantage of being a portable fuel suitable for transport. Wherever possible it is conserved for special uses, such as transport and in the petrochemical industry. Gas can be seen in the same way as oil, as being too valuable to be used for uses such as large-scale electricity generation. But after the oil price shocks of the 1970s, increased exploration efforts revealed huge deposits of natural gas in many parts of the world and today these are extensively used for power stations. The main virtue of gas however is that it can be reticulated safely and cheaply to domestic and industrial users and burned there to provide heat very efficiently. It is also a valuable chemical feedstock. Coal is abundant and world production is about 3.5 billion tonnes per year, most of this being used for electricity. It dominates the scene, and produces 38% of all electricity worldwide, while uranium produces 16%. Uranium is also abundant, and technologies exist which can extend its use 60-fold if demand requires it. World mine production is about 35,000 tonnes per year, but a lot of the market is being supplied from secondary sources such as stockpiles, including material from dismantled nuclear weapons. Practically all of it is used for electricity.

Firewood 16 MJ/kg Brown coal 9 MJ/kg Black coal (low quality) 13-20 MJ/kg Black coal 24-30 MJ/kg Natural Gas 39 MJ/m3 Crude Oil 45-46 MJ/kg Uranium* - in light water reactor 500,000 MJ/kg Table (13): Energy Conversion Typical Heat Values of Various Fuels (MJ = Megajoules), * natural U

Nuclear power / Safety applications of radiation

Introduction

Nuclear power is generated using Uranium, which is a metal mined in various parts of the world.

The first large-scale nuclear power station opened at Calder Hall in Cumbria, England, in 1956.

Some military ships and submarines have nuclear power plants for engines.

Nuclear power produces around 11% of the world's energy needs, and produces huge amounts of energy from small amounts of fuel, without the pollution that you'd get from burning fossil fuels.

1- Introduction: Energy Sources

2- Why nuclear energy?

3) Fission and Fusion:

a) Nuclear Fission

b) Nuclear Fusion