Nuclear energy

Nuclear energy has been identified as a potential contributor to reduce the greenhouse effect gases emissions and consequently it should play an important role in the electricity production mix of every country. Developing countries can also find in nuclear energy the solution to power their expected growth. Hence it has become very important to study about nuclear energy. In this assignment we will discuss about how this nuclear energy is produced, and how much energy is released from fission of U-235. For this first we will see how neutron induces fission and how it results in a chain reaction and further we will learn working principle of a nuclear reactor and neutron cycle in thermal neutron reactor.

Contents

Introduction. 3

Neutron induced fission. 3

Asymmetrical fission-mass. 5

Nuclear reactors. 6

Conclusion

Introduction

Atoms are the tiny particles in the molecules that make up gases, liquids, and solids. Atoms themselves are made up of three particles called protons, neutrons, and electrons. An atom has a nucleus (or core) containing protons and neutrons, which is surrounded by electrons. Protons carry a positive electrical charge and electrons carry a negative electrical charge. Neutrons do not have an electrical charge. Enormous energy is present in the bonds that hold the nucleus together. This nuclear energy can be released when those bonds are broken. The bonds can be broken through nuclear fission, and this energy can be used to produce electricity.

In nuclear fission, atoms are split apart, which releases energy. All nuclear power plants use nuclear fission, and most nuclear power plants use uranium atoms. During nuclear fission, a neutron collides with a uranium atom and splits it, releasing a large amount of energy in the form of heat and radiation. More neutrons are also released when a uranium atom split. These neutrons continue to collide with other uranium atoms, and the process repeats itself over and over again. This process is called a nuclear chain reaction. This reaction is controlled in nuclear power plant reactors to produce a desired amount of heat. Nuclear energy can also be released in nuclear fusion, where atoms are combined or fused together to form a larger atom. Fusion is the source of energy in the sun and stars. Uranium is the fuel most widely used by nuclear plants for nuclear fission. Uranium is considered a non-renewable energy source, even though it is a common metal found in rocks worldwide. Nuclear power plants use a certain kind of uranium, referred to as U-235, for fuel because its atoms are easily split apart. Although uranium is about 100 times more common than silver, U-235 is relatively rare.

Neutron induced fission.

The reaction leading to the discovery of fission in Jan. 1939 by O. Hahn and F. Strassman was neutron induced fission of Uranium. In this process a heavy nucleus decays into two fragments of comparable mass. It was discussed by L.Meitner in terms of liquid drop which becomes deformed and beyond a critical deformation is breaking apart into pieces, the fission fragments. The fragment emits secondary neutrons. These neutrons then may induce a second generation of fission events. This process is known as chain reaction. This process is described in fig.1

Figure 1 Scheme of neutron induced fission.

The total energy release in a fission event may be calculated from the difference in the rest masses of the reactants (e.g., ²³⁵U + n) and the final stable products (e.g., ⁹³Nb + ¹⁴¹Pr + 2n). The energy equivalent of this mass difference is given by the Einstein relation, E = mc². The total energy release depends on the mass split, but a typical fission event would have the total energy release distributed approximately as follows for the major components in the thermal neutron-induced fission of uranium-235:

Form of Energy Released	Amount of Energy Released (MeV)
Kinetic energy of two fission fragments	168
Immediate gamma rays	7
Delayed gamma rays	3-12
Fission neutrons	5
Gamma rays	7
Beta particles	8
Neutrons	12
Average total energy released	215MeV

 This energy is released on a time scale of about 10^-12 second and is called the prompt energy release. It is largely converted to heat within an operating reactor and is used for power generation. Also, there is a delayed release of energy from the radioactive decay of the fission products varying in half-life from fractions of a second to many years. The shorter-lived species decay in the reactor, and their energy adds to the heat generated; however, the longer-lived species remain radioactive and pose a problem in the handling and disposition of the reactor fuel elements when they need to be replaced.

Asymmetrical fission-mass

The fission results in two massive fission fragments having equal and opposite momentum. This fission fragments are neutron rich. They have too many neurons to be stable hence each fragment may emit one or two neutrons in a period of less than 10^-15 sec. on an average three neutron are emitted per fission of uranium. These neutrons are called prompt neutrons. From these we can also conclude that these fission fragments must exhibit beta activity to reduce the neutron count.

There are many ways for ²³⁵U to undergo fission. It is found that there are thirty modes of fission. Every mode gives different pair of fission fragments. Most of these fragments are

The fission yield is defined as.

Fig 2. Shows plot of percent fission yield and mass No. (A) for ²³⁵U.

Figure 2

It is seen from fig. 2 that most probable values for the mass no. for the two fission fragments are about A = 95 and A = 139 corresponding atomic number for them are Z = 42 and Z = 57 respectively. For A = 117 yield is minimum which is for symmetrical fission having atomic number Z = 50 and as we know 50 is a magic number.

Nuclear reactors

Nuclear reactors generate energy through fission, the process by which an atomic nucleus splits into two or more smaller nuclei. During fission, a small amount of mass is converted into energy, which can be used to power a generator to create electricity. In order to harness this energy, a controlled chain reaction is required for fission to take place. When a uranium nucleus in a reactor splits, it produces two or more neutrons that can then be absorbed by other nuclei, causing them to undergo fission as well. More neutrons are released in turn and continuous fission is achieved.

Neutrons produced by fission have high energies and move extremely quickly. These so-called fast neutrons do not cause fission as efficiently as slower-moving ones so they are slowed down in most reactors by the process of moderation. A liquid or gas moderator, commonly water or helium, cools the neutrons to optimum energies for causing fission. These slower neutrons are also called thermal neutrons because they are brought to the same temperature as the surrounding coolant as shown in Fig 3

Figure 3 Nuclear reactors

In contrast to most normal nuclear reactors, however, a fast reactor uses a coolant that is not an efficient moderator, such as liquid sodium, so its neutrons remain high-energy. Although these fast neutrons are not as good at causing fission, they are readily captured by an isotope of uranium (U₂₃₈), which then becomes plutonium (Pu₂₃₉). This plutonium isotope can be reprocessed and used as more reactor fuel or in the production of nuclear weapons. Reactors can be designed to maximize plutonium production, and in some cases, they actually produce more fuel than they consume. These reactors are called breeder reactors. A fast-breeder nuclear reactor produces more fuel than it consumes, while generating energy. Conventional reactors use uranium as fuel and produce some plutonium. Breeders produce much more plutonium, which can be separated and reused as fuel. The working of a breeder reactor is shown in Fig. 4

Figure 4 breeder reactor

The sodium surrounding the core flows through a heat exchanger, cluster of thin-walled metal tubes and transfer its energy to a separate stream of sodium. The heat then passes through a steam generator. (If there is a leak and the sodium come into contact with the water or air sodium burns). The steam generated is then used to run the turbine which then produces electricity.

Conclusion

As we have seen nuclear energy is a potential contributor to reduce the greenhouse effect gases emissions and solve energy problems, Nuclear power plants use “nuclear fission” (the process of splitting an atom in two). “Nuclear fusion” (the process of combining atoms into one) has the potential to be safer but has not yet been developed to operate within a large power plant. Nuclear energy comes from uranium, a non-renewable resource that must be mined. Typical fission events release about two hundred million eV (200 MeV) of energy, the equivalent of roughly >2 trillion Kelvin, for each fission event. (When a uranium nucleus fission into two daughter nuclei fragments, about 0.1 percent of the mass of the uranium nucleus appears as the fission energy of ~200 MeV.). It is also observed that most probable values for the mass no. for the two fission fragments are about 95 and 139. And minimum for symmetrical fission which is for A =117

In a fission reactor, neutrons produced by fission of fuel atoms are used to induce yet more fissions, to sustain a controllable amount of energy release. nuclear reactors generally convert the kinetic energy of fission products into heat, which is used to heat a working fluid and drive a heat engine that generates mechanical or electrical power

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