Nuclear engineering : a conceptual introduction to nuclear power 9780081009628, 0081009623

Table of contents :
1. Fundamental concepts 2. Historical context 3. Fundamentals of radioactivity 4. The fission process 5. The actinides and related isotopes 6. Moderation 7. Cooling and thermal concepts 8. Elementary reactor principles 9. The reactor equation and introductory transport concepts 10. Mainstream power reactor systems 11. Advanced reactors and future concepts 12. Nuclear fuel manufacture 13. Nuclear fuel reprocessing 14. Nuclear safety and regulation 15. Radioactive waste management and disposal 16. Public acceptability, cost and nuclear energy in the future

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Nuclear Engineering

Nuclear Engineering A Conceptual Introduction to Nuclear Power

Malcolm Joyce

Butterworth-Heinemann is an imprint of Elsevier The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States # 2018 Elsevier Ltd. All rights reserved. Portraits courtesy of Graham Lowe, artist and illustrator, www.grahamloweartist.com No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-08-100962-8 For information on all Butterworth-Heinemann publications visit our website at https://www.elsevier.com/books-and-journals

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1.1 SUMMARY OF CHAPTER AND LEARNING OBJECTIVES The aim of this chapter is to introduce the fundamental concepts associated with nuclear engineering that support the detail explored in the chapters that follow. Extensive detail is avoided in preference to a focus on the foundation principles associated with, for example, the order of magnitude of quantitative aspects of the field, the definition of terms used later in the book and the general aspects associated with nuclear reactor design. The concepts and principles selected at this stage are given further elaboration later in the text; they constitute features that in the author’s experience pervade the nuclear engineering discipline irrespective of the specific aspect of the field in which they tend to arise. The subjects discussed in this chapter also represent concepts that are rather specific to nuclear engineering and do not always arise in the study of the other, more general branches of engineering. Many of these stem from the disciplines of nuclear physics and radiochemistry but nonetheless arise frequently in nuclear engineering. The objectives of this chapter are to: • • • • • • •

introduce the main distinctions of nuclear engineering over other engineering disciplines review the structure of the atom and introduce the concept of the atomic nucleus discuss the interplay between the Coulomb force that exists between the protons in the nucleus and the strong nuclear force that holds the nucleus together describe the extreme properties of the nucleus in terms of its density and the miniscule space it occupies in nature introduce the chart of the nuclides, what this represents and the properties of the nucleus that it highlights provide a comprehensive description of the concept of the generic nuclear reactor and its components including fuel, cladding, coolant and moderator introduce the concept of the reactor cycle and the distinction of direct and indirect cycles

Nuclear Engineering. https://doi.org/10.1016/B978-0-08-100962-8.00001-9 # 2018 Elsevier Ltd. All rights reserved.

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1.2 HISTORICAL CONTEXT: ERNEST RUTHERFORD 1871–1937 Among Ernest Rutherford’s first nuclear-related achievements was the proof of there being two distinct forms of radiation emitted by uranium (α and β particles), having separated them with thin layers of aluminium. Along with Robert McClung, he calculated that a significant amount of energy was radiated by radium in the form of α particles and, perhaps most significantly, he observed that the physical and chemical properties of the α particle were consistent with what would come to be known as the nucleus of a helium atom. For this, Rutherford (Fig. 1.1) was awarded the Nobel Prize in Chemistry in 1908. Subsequently, Rutherford observed that the properties of radioactive disintegration were independent of the chemical and physical characters of the emitting substance and were thus atomic in origin, rather than molecular. He defined the correspondence of the α-decay process in terms of the related chemical changes of the parent substance, studied thorium leading to the discovery that it emitted radon, he developed the disintegration theory of radioactive decay (along with Frederick Soddy) and devised a means for detecting single α particles along with Hans Geiger. In 1910, he observed that α particles could be repelled by materials through large angles and hence postulated that the atomic mass was concentrated in the form of a miniscule but extremely dense ‘nucleus’. This theory (albeit with notable improvements relating to concepts derived from the work of Neils Bohr and Werner Heisenberg) is that which is used to this day. This observation set the scene for the cascade of discoveries that followed, including those of the neutron and nuclear fission. Subsequently, along with Henry Moseley, he observed that atoms emitted characteristic X-rays in response to excitation from which a

FIG. 1.1 Ernest Rutherford.

1.4 THE NUCLEAR LANDSCAPE

3

number, their atomic number, could be assigned which was related to the chemical properties of the corresponding element. Rutherford also observed that some light elements could be disintegrated by the effect of α particles, providing the first evidence of deliberate transmutation of one element to another.

1.3 INTRODUCTION A significant distinction of nuclear engineering from other branches of engineering is that nuclear systems deal with material that is or has the potential to become radioactive. In this context, many of the subsystems and general concepts of engineering science that an engineer needs to be aware of in a nuclear context correspond directly to those in general engineering disciplines. However, there are some very important issues that are unique to the nuclear engineering field; these arise because of the requirement to manage, process and be aware of nuclear materials and the radiation that can arise from them. Often the nuclear properties of only a relatively small number of the isotopes are exploited in most nuclear engineering systems. While the objective of nuclear operations might often be simple in terms engineering fundamentals, this can be complicated significantly by the imperative to manage the risk associated with the radioactivity of the substances involved to levels that are safe and considered acceptable to society.

1.4 THE NUCLEAR LANDSCAPE

1.4.1 ATOMIC RADIATION AND NUCLEAR RADIATION Radiation can arise from processes associated with the electron shell structure of the atom; for instance, we are perhaps all familiar with the electromagnetic spectrum of radiation that encompasses everything from the frequencies received and transmitted by mobile phones, the infrared responsible for the images fire crews use to find people with heat-seeking cameras, through to the visible photon spectrum with which we see the world and on to the ultraviolet and X-ray components. These examples are not the nuclear radiation that we are concerned with in this book and in the wider nuclear engineering discipline because the origin of this radiation is atomic rather than nuclear. Nuclear radiation is, by definition, emitted by processes in the atomic nucleus rather than those in the electron shell structure of the atom that surrounds the nucleus. This distinction is important because where, for example, processes in the nucleus are associated with the emission of electromagnetic radiation, the potential exists for the emission of radiation at much higher energies (at shorter wavelengths) than that generally associated with processes at an atomic level. This can have implications as to how we might manage and protect ourselves from the risk posed by excessive radiation exposure. There is also the additional and important possibility that the nucleus might emit particles as forms of radiation. These can be highly ionising and tend not to occur in the case of the atom (electrons are an exception). Nuclear radiation has the potential to ionise the matter that it interacts with and the extent to which this occurs is dependent on the energy, mass and charge of the radiation. Relatively significant amounts of energy can be imparted to matter by ionising radiation on a microscopic scale, and this has the potential to change the composition of the substances in which it is deposited. This can, in turn, change

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the properties of these materials; where said material is living tissue the effect of ionising radiation can cause it to behave differently or even to kill the cells of which it is comprised. However, it is important to appreciate that for the vast majority of nuclear processes and operations, the radiation environment experienced by people working in it bears no difference to natural background levels of radiation. These comprise those sources that we are all subject to, largely unavoidably, largely as a result of radiation emitted from naturally radioactive minerals present in the Earth’s crust and from sources in outer space. One further important distinction is that while atomic processes of radiation emission, such as fluorescence and phosphorescence rarely have lifetimes longer than a few minutes or hours, nuclear radiation can be associated with the decay of atomic nuclei that span enormous ranges in lifetime, from picoseconds through to many billions of years. Notwithstanding the possibility of transmuting longlived radioactive isotopes into others with shorter lifetimes that we will discuss in Chapter 15, it is not possible to change the lifetime of a radioactive substance. For this reason, many requirements in nuclear engineering are associated with the management of radioactive materials to ensure that people are protected from the risk of harm, while we harness the potential of these materials for the benefit of civilisation. The phenomena that astonished Rutherford little more than a hundred years ago occurred when a minority but nonetheless a significant number of α particles were detected being reflected from a thin metal foil. He is said to have remarked that it was as if he had ‘fired a 15-in. shell at a piece of tissue paper and it came back’. This demonstrated that almost all of the mass of the atom, and thus the vast majority of all of the mass of visible matter, must be concentrated in a small and dense nucleus. This was in great contrast to the more dilute and dispersed arrangement that had been widely postulated at that time but which had not been proven outright. From the extensive research that followed Rutherford’s observation based on nuclear scattering, it was possible to infer the dimensions of the nucleus. This led to the model of the atom that is now accepted universally and that has been the basis for much scientific discovery and related engineering that followed in the 20th century.

1.4.2 THE NUCLEUS The nucleus is composed of an approximately equal number of protons and neutrons and these are known collectively as nucleons. The exception is the case of hydrogen, the lightest isotope of the lightest element, which has a nucleus composed of just one proton. Protons are positively charged, and neutrons (as their name suggests) are neutral; each chemical element is distinguished by the corresponding number of protons, and this corresponds to the atomic number. Each isotope of a given element has the same number of protons but differs in terms of the number of neutrons it possesses. An atom comprises a nucleus surrounded by a number of electrons equal in number to that of the protons in the nucleus. The charge and mass data for neutrons, protons, electrons and photons are given in Table 1.1.

Table 1.1 Fundamental Properties of the Major Subatomic Particles Proton Mass (kg) Charge (c)

Neutron 27

1.673
10 +1.602
1019

1.675
10 0

Electron 27

Photon 31

9.11
10 1.602
1019

-

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FIG. 1.2 A schematic diagram of the 7Li atom, by way of example, to illustrate atomic composition. The scale of the nucleus as depicted is magnified by a factor of approximately 10,000 to render it visible. Note: electrons are shown as discrete entities whereas they are better approximated as diffuse charge clouds, with protons shown as black and neutrons white for the purposes of this schematic illustration. Diagram not to scale.

Since the protons have like charges, they are subject to the force of Coulomb repulsion acting to force them apart; this form of the electromagnetic force is generally weaker at the short ranges associated with the dimension of the nucleus than the force that binds the nucleus together. The latter, known as the strong nuclear force, is one of the four fundamental forces (along with gravitation and the weak interaction) but which is attractive at dimensions of the order of the size of the nucleus, typically of radius r where r < 1015 m. For the case of isotopes that are not susceptible to radioactive decay, these nuclei exist in a state of stable equilibrium. In this state, the protons are repulsed by one another but the nucleus is held together as a whole by the cohesive, strong force. A schematic illustration of a lithium atom is given in Fig. 1.2. Rutherford’s discovery that matter is concentrated into miniscule nuclei remains profound because it challenges our everyday experience of the density of matter that we are familiar with. The observation of nuclei of a diameter of, say, 1015 m suggested something very different indeed. Given the size of the atom of the order of 1010 m and given the mass of the electron of 9.11
1031 kg, it is clear that the density of the nuclear material that constitutes 99.95% of the atomic mass but only 1/100,000 of the atomic dimension must be extremely high, that is, of the order of 1018 kg m3. Substances with the highest densities witnessed in our day-to-day experiences are of the order of 104 kg m3 (e.g. for the case of lead or tungsten), and hence, it is clear that the density of nuclear material is extremely high. From this observation, we can conclude that the strong nuclear force is indeed very strong because it acts to keep the protons and neutrons in such a tight bundle. Also, it is short ranged not acting much further beyond the dimensions of the nucleus itself. In addition to the density, the nature of solid matter implied by the scale of the nucleus is also a little counter-intuitive. If the atomic nucleus were represented on this page to be the size of the head of a pin, the electrons surrounding it that constitute the size of the atom would be 100 m from the position from which you are reading, if represented on the same scale. It is clear from this observation that the vast majority of matter, that is, 99.999999999999%, is actually free space.

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1.4.3 THE CHART OF THE NUCLIDES The chart of the nuclides is obtained by plotting the number of protons (Z) versus the number of neutrons (N) for all known isotopes (both stable and radioactive), where A ¼ N + Z is the atomic mass. This is shown in Fig. 1.3. In a similar way that the group structure of the periodic table of the elements can infer general chemical properties, the chart of the nuclides provides valuable insight into the properties of the nucleus, both on a general and also at a detailed level. A prominent feature of this summary of all matter is that, because of the isotopic variety that exists for many elements, it composes of several thousand nuclides. Some of these are very short-lived indeed and, in some extreme cases, only exist for the time it takes to travel through a particle accelerator at close to the speed of light. Second, the majority of isotopes that exist are radioactive; only the central spine of the chart, represented in black in Fig. 1.3, corresponds to the stable proportion of nonradioactive species. The chart resembles a cloud around this central spine; the spine highlights what is often referred to as the valley of stability. Either side of this valley are the radioactive isotopes. The chart narrows to a limit for very low masses and also for very high masses where, in both cases, there are fewer known isotopes for each element than in the central region. At the high-mass extreme, the valley of stability breaks up with the most massive isotopes not having a stable isotopic variant at all; this region is a key area of interest for nuclear engineers as it is where the actinide series of isotopes resides. A subtle feature of the chart of the nuclides is that the line along the valley of stability is not straight. Rather, it bends because of the natural trend for heavier isotopes to have a higher proportion of neutrons than they do protons, and hence, the following proportionality holds between Z and N, Z∝ N p

(1.1)

Z Z = 82 N = 126

Z = 50 N = 82 Z = 28 N = 50

Z = 20 Z=8

N = 28 N = 20

N

N=8

FIG. 1.3 The chart of the nuclides, stable isotopes depicted in black. Taken from the National Nuclear Data Centre web site: http://www.nndc.bnl.gov/chart/.

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where p is less than 1 since the trend of the valley of stability is sublinear. At low isotopic masses, the chart is linear with Z  N and hence p tends to unity. The departure from linearity becomes particularly apparent for higher mass isotopes reflecting a particularly salient nuclear property that, as isotopic mass increases, stability is favoured by isotopes for which N > Z rather than for N ¼ Z as might be expected on the basis of the trend for light isotopes. Why is this? Our knowledge of the intricacies of the properties of nuclear matter is based on many years of complex research investigations by generations of nuclear scientists. The results of these studies lead us to conclude that the properties of the nucleus are dependent on many different structural phenomena, a full discussion of which is beyond the level of this text. However, one feature is of specific relevance to nuclear engineering: as proton number increases from light to heavy masses (thus moving up the vertical axis in Fig. 1.3), the force of electrostatic repulsion between the protons increases. The same is not true of the nuclear force (as discussed in Chapter 4 with reference to nuclear binding energy) because its influence per nucleon extends only to nucleons that are nearest neighbours and thus it does not generally increase with A per se. Consequently, as atomic mass increases, the repulsive action of the Coulomb force competes with the strong nuclear force more effectively. The significance of this in the context of nuclear energy is that the nuclei of very heavy nuclei are weakly bound relative to their lighter cousins, with the repulsion of the abundance of protons at these masses acting to push them apart and destabilise them. While, as mentioned earlier, the structure and behaviour of nuclei at the upper mass extreme is generally very complicated, the instability due to the large number of protons is common to all isotopes to a greater or lesser degree, as A ! ∞. Hence, a natural propensity towards the complete breakup of the nucleus is observed at these extremes of mass. However, it is also true that a universal swing towards catastrophic breakup of all nuclei at this extreme is not observed; rather, the nuclei of some isotopes breakup readily, others can be encouraged to do so by excitation while others favour a variety of other modes of decay instead. Such is the complexity of the underlying nuclear structure at this extreme that only broad generalisations can be made with confidence, with neighbouring isotopes often exhibiting stark contrasts in behaviour to one another.

1.4.4 UNITS OF ENERGY ON A NUCLEAR SCALE It was clear to the early discoverers of ionising radiation that the energies associated with the radiation they observed were vanishingly small relatively to their everyday experiences associated with, for example, bulk heat transfer and the motion of everyday objects. The traditional units of energy that had been adopted to describe the thermodynamic phenomena central to the industrial revolution were vast by comparison. Derived from the experiments of the day with electromagnetics (given this is how many new isotopes had been discovered and remains a popular way of detecting nuclear radiations), quantities associated with the ionisation of gases and the influence of evolved charge on thin metal filaments, as used in early electrometers, were used in many cases to describe what was observed in the context of ionising radiation. In particular, a convention based on the relationship between the energy E acquired by an electron with charge e accelerated by a potential difference V is used as follows: E ¼ eV

(1.2)

which defines the unit of the electronvolt or eV. This unit is adopted universally to quantify the very small subatomic energies associated with microscopic nuclear processes, particularly those that yield

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ionising radiation. On this basis, a single electron (with a charge of magnitude of 1.6
1019 c) accelerated through 1 V would acquire an energy of 1 eV corresponding to 1.6
1019 J. The normalisation provided by dividing through by the energy acquired by an electron accelerated by 1 V makes for a much easier comparison of energies at these scales without the need for constant reference to many orders of magnitude. It also reduces the potential for mistakes in juggling many very small numbers in calculations. In the context of nuclear engineering, energies in the ‘eV’ domain are actually at the lower end of the general range we tend to encounter; energies in the keV range are often considered intermediate while in the MeV range they would be considered at the middle-to-higher end of the energy spectrum. Let us consider the case of the radioactive isotope potassium-40 (40K). This isotope decays so very slowly that much of it still present naturally in the Earth’s crust from the formation of the universe some 13.8 billion years ago. It is taken up by the food we eat in harmless quantities featuring in our diet particularly via its natural occurrence in bananas. The 40K nucleus of this isotope decays via a number of transitions between quantised energy levels, which results in the emission of electromagnetic radiation in the high-frequency range in the form of γ rays. The photons associated with this exhibit a relatively small number of discrete energies in the range 50 keV through to a few MeV. We will focus specifically on the 1491 keV transition for the purposes of this example. The 1491 keV transition has an equivalent energy in SI units as per, E ¼ 1491
1000
1:602
1019 ¼ 2:39
1013 J

(1.3)

which highlights the extremely small scale of the energies of nuclear radiation relative to what we are used to at a macroscopic level.

1.4.5 NUCLEAR BINDING ENERGY Further to the earlier discussion of the strong nuclear force and its critical role in holding the nucleus together, energy is required to overcome this force to break up the nucleus. Taken to its logical extreme, the energy required to disassemble the nucleus into its constituent neutrons and protons is known as the nuclear binding energy. It might appear quite a sophisticated task to measure this energy if it were necessary to separate a given nucleus using, for example, a particle accelerator. However, the energy that binds the nucleus together is manifest as a difference in mass between the sum of its constituent parts (the mass of the neutrons and protons) and the mass of the bound nucleus. This difference is known as the mass defect and can be determined via Einstein’s famous relationship between energy and mass given in Eq. (1.4). E ¼ mc2

(1.4)

We return to this important concept in more detail in Chapter 3.

1.5 THE GENERIC NUCLEAR REACTOR There are many different types of nuclear reactor: just as for the heat engines that came before them, such as steam engines and combustion engines, there are a variety of processes and arrangements by which the energy from nuclear fission can be harnessed. To aid our understanding of this wide and often

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contrasting field of engineering, it is illustrative to first consider a generic nuclear reactor design, while not being specific about the materials, cycles and processes until later chapters. A nuclear reactor might be defined thus: ‘apparatus in which a nuclear fission chain reaction can be initiated, sustained and controlled for generating heat or the production of useful radiation’. Hence, in the definition of a nuclear reactor, the emphasis is clearly on control, since without this, the very high energy density afforded by a nuclear reaction cannot be dissipated in a form and at a rate that is useful. However, the reader will also note that we do not restrict ourselves to the production of electricity, per se, since there are many reactors in use that are not dedicated to power production, as were the very first reactor systems, but that are used for research and materials applications. Also, note that a reference to scale is not implied because nuclear reactors can vary widely in terms of size depending on the application for which they have been designed. In the most simple of terms, a nuclear reactor is composed of fuel, in which heat is generated as a result of a self-sustaining nuclear chain reaction, and a coolant that is necessary to transport the heat away from the fuel so that this energy can be used to perform useful work. Often a material is also included to reduce the energy of the neutrons sustaining the reaction because this makes it easier to sustain the reaction in relatively dilute quantities of uranium; this substance is known as a moderator. In some reactor designs, the moderator and the coolant are the same substance. The reader should note that in our description of the generic nuclear reactor, and in reference to it later in the chapters that follow, we do not refer to the control mechanisms, emergency instrumentation systems, coolant pumps, pressurisers, driers, condensers and so on. While these components are extremely important to the operation of specific reactor designs, the operation and arrangement of them is too specific to be included in a preliminary, generic overview at this point. For the purposes of this generic basis, it is assumed that: (1) The moderator and coolant are separate substances and not one and the same, although the latter arrangement is a popular and very successful design variant that we shall consider later in this text. (2) The reactor is a heterogeneous design such that the fuel is separated into relatively narrow elements and distributed uniformly throughout the moderator and coolant systems. This is a common feature of all of the low-enrichment power reactor systems that make up the world’s fleet of nuclear power generating systems as illustrated schematically in Fig. 1.4.

FIG. 1.4 A much simplified, schematic diagram of a generic nuclear reactor design.

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1.5.1 FUEL The first of the generic nuclear reactor components we shall consider is its fuel. Like all engines, energy cannot be generated without a source of fuel and nuclear reactors are no exception. All nuclear reactors in operation in the world today rely on uranium in some form or other (with variety in both isotopic composition and physical form) as the primary source of their fuel. This is because uranium is the only element that occurs in sufficient natural abundance with isotopes that are susceptible to the production of energy via splitting (fission) that is stimulated with slow neutrons. It is noteworthy that thorium, while frequently the focus of study for use in reactors and topical at the time of writing, is not usable directly in reactor systems until converted to sufficient quantities of 233U as discussed in Chapter 11. Similarly, where plutonium is used it is either as a mixture with uranium, in the form of a mixed-oxide (MOX) fuel, or as the direct product of 238U in uranium-fuelled reactors. The naturally occurring uranium isotope of choice for nuclear energy production is 235U because it is this isotope that has the dual benefit of being sufficiently abundant naturally (0.71% wt. of naturally occurring uranium is 235U) and susceptible to stimulated fission. Stimulated or induced fission implies that neutrons are used to provoke the 235U nucleus to split; this is essential in terms of sustaining a reaction based on this phenomenon. Other isotopes, such as californium-252 (252Cf ) exhibit spontaneous fission in which the isotope splits randomly as a form of radioactive decay. This is not very useful for reactor systems because, like other forms of radioactive decay, it cannot be controlled. In this context, we often refer to isotopes such as 235U with this susceptibility to stimulation by slow neutrons as being fissile. Uranium used in reactors and in combination with other fissile isotopes can take on a variety of geometries including: rods, plates and slabs with a variety of chemical forms and isotopic compositions. Early in the nuclear era, materials preparation techniques were more primitive than today. Often in this era, the only option was for the fuel to be used in metallic form. This was a reasonable solution at the time and many of the early reactors used fuel of this type. However, it was soon realised that the vulnerability of the fuel to the effects of heat cycling and exposure to high levels of radiation was a critical limiting factor in terms of reliability and longevity of fuel use in nuclear reactor systems. Consequently, alternatives were developed to appeal to the requirements of long life and tolerance to heat and radiation exposure. Many of the world’s reactors are now reliant on ceramic forms of fuel usually comprising uranium dioxide (UO2). These are more resilient to the rigors of reactor use and provide for longer periods of use than metal fuels, as they are more resistant to radiation damage and high temperature degradation, and have adequate thermal conductivity to access the heat. As reactor use continues to develop, fuels with even greater resilience and accident tolerance are a significant focus of many current research programmes. In the context of nuclear fuel, we shall also refer regularly to the concept of enrichment. As referred to early, the specific isotope of interest in the context of uranium, 235U, is only present naturally in relatively dilute proportions. Consider, for example, if atoms of uranium of natural enrichment were laid out in front of us, approximately only 1 in 140 of them would be 235U. The vast majority of the rest would be 238U that constitutes 99.3% wt. of natural uranium with the remainder being a very small amount of 234U. Neither 238U (nor 234U although its abundance is very small) are fissile to a practically significant level. While these isotopes have other uses, aside from reactor use, they generally serve as a matrix that is relatively inert in which the 235U is held. We will return to this debate to highlight an important exception in Chapter 11 associated with 238U and breeding.

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While sustained fission is feasible in uranium fuels of natural enrichment, that is, 0.71% wt. 235U (this was the main approach for many of the first nuclear reactor designs), it can be difficult to achieve and can limit the choice of the materials used for the other components in a reactor. Fuel of natural enrichment tends to have been desirable in cases where access to enrichment facilities has not been readily available or desirable in terms of national energy policy. Most nuclear fuel currently in use for power production is slightly enriched in 235U as this eases the operation of the reactor and offsets the effect of neutrons absorbed in the coolant and moderator. Fuels of low enrichment in such cases are classified typically as