专业英语教材

LESSON ONEThe peaceful uses of nuclear energy: technologies of the front and back-ends of the fuel cycleJacques PerceboisAbstractThe principal peaceful application of nuclear energy is that of electricity generation. The nuclear industry is a young one, which is today confronted with difficult choices, essentially because this activity generates fear. This fear is partly related to the generation of electricity in power plants but is particularly present in relation to the transport, reprocessing, management and underground disposal of nuclear waste. This paper examines, respectively, the nuclear technologies available today (1), the future perspectives for nuclear energy on a worldwide basis (2) and the controversial question of the management of nuclear waste (3). Nuclear energy can be considered as an alternative to fossil fuels in the context of policies to reduce emissions of greenhouse gases. The potential technological progress is a key element of the future of nuclear energy; but the crux of the problem remains the long-term management of waste.Keywords: Nuclear technologies; Perspectives of nuclear energy; Management of nuclear waste 1. Nuclear technologies1.1. The principle of nuclear fission (see Percebois, 1989)The cohesion of an atomic nucleus is obtained by the binding energy emitted by each nucleon making it up. However, this binding energy is not the same for all nuclei: it is weak for light nuclei, strong for medium-sized nuclei and average for heavy nuclei. Therefore, if a heavy nucleus is split, the two nuclei resulting from this fission have a total binding energy greater than that of the original nucleus. Thus, during this transformation, a certain quantity of energy is emitted, as it is now necessary to provide more energy to dissociate the two new nuclei rather than to dissociate the initial heavy nucleus: part of the weight of the initial nucleus is in fact transformed into energy (Einstein’s law). The energy thus emitted is kinetic energy of the two resulting nuclei. The situation is the same if, instead of splitting a heavy nucleus, we re-bond two light nuclei: the binding energy of the nucleus resulting from this fusion is greater than that of the original nuclei.Fission takes place when a heavy nucleus absorbs a neutron (a non-electrically charged sub-atomic particle), thereby creating an unstable edifice which splits into two lighter nuclei by emitting 2 to 3 neutrons which in turn can cause fission. During fission, if at least one neutron emitted is absorbed by a new fissile nucleus thereby instigating further fission, then there is said to be a “chain reaction”.However, few elements are naturally fissile. Most natural substances are capable of capturing neutrons without there being either fission or emission of energy: they have simply become radioactive. Naturally, fissile elements are to be found at the top of Mendeleev’s table, i.e. beside the heaviest and therefore most unstable atoms. The most important of the fissile isotopes is U-235which, associated with U-238 to a ration of 1 nucleus of U-235 to 140 nuclei of U-238, constitutes natural uranium. U-238 is fertile: if it comes into contact with a neutron it will absorb it and transform into Pu-239 which itself is fissile. In natural uranium, we find 99.3% of U-238 and only 0.7% of U-235.U-235 + 1 neutron = other atoms (actinides) + 2.5 neutrons + energy.A chain reaction will occur if, during fission, the neutrons emitted come into contact in turn with fissile nuclei which, by splitting, release further neutrons. In order for this to happen, it is necessary to reduce both the speed of the neutrons and have a minimal density of fissile material (critical mass). The principle behind a nuclear power plant consists of controlling the chain reaction in order to obtain a certain quantity of energy over certain duration. The principle underlying the atomic bomb consists, on the contrary, of accelerating the chain reaction in order to obtain the maximum amount of energy at one given time. It should be noted that the disintegration of a gram of U-235 emits as much energy as the combustion of 2 tonnes of petrol or 3 tonnes of coal.In order to construct a nuclear reactor there are two options:● Reactors with thermal neutrons in which the density of fissile material remains low(enrichment of U-235 to approximately 3% or 4% as compared to 0.7% in its natural state). It is necessary, therefore, to slow down the neutrons in order to instigate the chain reaction (facilitate the capture of neutrons) which would imply using a material which slows down neutrons without absorbing them too much.●Reactors with fast neutrons in which the density of fissile material is high, which obviates theneed to slow down the neutrons (principle of fast-breeder reactors).A nuclear reactor is generally composed of:● The core, which holds the fuel and possibly a moderator in order to slow down the neutrons.●A system of control rods, designed to regulate the chain reaction by absorbing neutrons.● A coolant which evacuates the heat given off to the steam generator.● A heat-utilization system: A turbine in the case of a submarine reactor for propulsion,generator for electricity production as with a plant connected to the grid.All nuclear reactors are the result of several compromises. It is necessary to use either highly enriched fuel or else a moderator. It is necessary to find a moderator which is effective at slowing down the neutrons without absorbing them too much. In practice, nuclear reactors are defined by three factors: fuel, coolant and moderator. Military choices (the bomb and the development of reactors for submarines) strongly influenced options chosen today for peaceful uses.1.2. Categories of nuclear energy productionThere are four principal categories of nuclear power plants:1. Plants with natural uranium—graphite—gas: Graphite is a good moderator which slows down neutrons without absorbing them too much, thereby allowing to maintain a chain reaction in fuel where fissile uranium is only present at 0.7%. The coolant, CO2 (generally pressurized) is apoor moderator and presents the advantage of being inexpensive (the first nuclear reactor which was started up, that of Enrico Fermi, was of this type). The French nuclear industry first used this category of reactor, until 1969. It was also the British choice with the Magnox variant or AGR. 2. Water reactors (light or heavy water)—graphite—enriched uranium: Heavy water is the best known moderator but is a costly product as it is necessary to separate the two isotopes of water—hydrogen and deuterium (light water only contains 160 mg of heavy water or deuterium/kg). This heavy water may also be used as a coolant in reactors which use enriched uranium. The most common in this category of reactor is RBMK (Chernobyl-type) reactors developed in the ex USSR and which use graphite as a moderator, light (boiling) water as a coolant and enriched uranium as fuel.3. Light (boiling or pressurized) water reactors—enriched uranium: Light water, inexpensive, is used both as moderator and as coolant. However, light water is an inefficient moderator in that it absorbs lots of neutrons. This is why it is necessary to use enriched rather than natural uranium: the fuel contains 3–4% of U-235(instead of 0.7%). This is a considerable disadvantage as it is necessary to build installations for enrichment which are both costly and also large consumers of electricity. Enrichment can take place in several different ways: by gaseous diffusion, by gas centrifuge or by laser. Two types of plants can be round in this category: pressurized water reactors (PWR) and boiling water reactors (BWR). These are by far the most common reactors in the world, particularly the PWR. This category is to be round in Russia and in the ex-Eastern bloc countries under the name of VVER.4. Plants with fast neutrons, or fast breeder reactors: There is no moderator and, therefore, it is necessary to use a fuel which is rich in fissile materials. A mixture of Pu-239 (20%) and U-238 (80%) is used. Plutonium is preferable to U-235 for three reasons: (1) it releases more neutrons at the time of fission thereby facilitating the maintenance of the chain reaction and allowing a large proportion of the U-238 to be transformed into plutonium 239, (2) this category of reactor allows plutonium obtained from reactors of the first category to be re-used (Pu-239 is no longer waste but rather become fuel. Note that since this fast-breeder reactor category has been abandoned, some of the Pu-239 is mixed with U-235 in PWR reactors, as MOX fuel), (3) without Pu-239 it would be necessary to highly enrich U-235 to operate this category of reactor. The coolant used is liquid sodium, which withstands heat well and which absorbs few neutrons. The principal advantage of this type of reactor (apart from the re-use of certain waste) is to have a conversion coefficient which is greater than one.During the chain reaction there are more fissile atoms created than there are destroyed. The fission of Pu-239 emits three neutrons which either come into contact with plutonium (thus maintaining the reaction) or are absorbed by U-238 which is then transformed into fissile Pu-239. Thus, with SuperPhenix the annual consumption of 900 kg of fissile materials was accompanied by an annual transformation of 1100 kg of fertile material into fissile material (rendering a conversion coefficient of 1.2). This method makes extremely good use of the 238 isotope which represents 99.3% of natural uranium.Currently, one particular category dominates the nuclear industry—light water reactors with their various sub-categories: PWR, BWR and VVER. A total of 564 electro-nuclear reactors dispersed in the world between 1951 and 2000, thus representing a cumulated generating capacity of 384GWe. The annual number of connections to the grid culminated around 1985. The worldwide nuclear power park is now located across 32 countries and totals a thermal power of356 GWe, 80% of which is in OECD countries (98GWe in the United States with 104 reactors; 63GWe in France with 59 reactors and 43GWe in Japan with 53 reactors). Some of the reactors connected have already been removed from the network. Out of the 443 reactors in service, 347, or 78%, are light water reactors. It is noteworthy that the worldwide nuclear park is rather “young” and that the life span of plants may be longer than was initially thought, where there is “rejuvenating” investment. For example, in France, 40% of the installed capacity is over 15 years old and 60% is less than 15 years old. The average intended life span of a reactor is 30–35 years but this may be extended to 45 or even 60 years where certain conditions are met.2. Future perspectives for nuclear energy2.1. Background explanationThe development of nuclear energy was slightly less that hoped in the 1970s, after the first oil crisis. Major doubts persisted and the role of nuclear energy in the overall world energy situation is today rather modest. Nuclear energy only represents at worldwide level 7% of the primary energy commercialized in 2000, as compared to 40% for petrol, 26% for coal and 24% for natural gas (the remainder being hydraulic energy). This percentage of nuclear energy should stabilize or else diminish around 2020. However, the situation is very different from one country to another. Thus, nuclear energy represents more than 78% of the electricity production in France as compared to only 16% at a global level. France has 17% of the installed nuclear capacity in the world and 55% of the installed capacity in the European Union. Numerous countries do not use nuclear energy and some of those countries which still use it have envisaged phasing out this energy source. Such contrasts can be explained by a number of reasons, and the future of nuclear energy is subject to multiple considerations:1. Economic criteria: The nuclear industry is a very capital-orientated one and requires sophisticated technologies. The economic competitiveness of nuclear kWh is, therefore, dependent on the price of substitutes (kWh produced with natural gas, fuel oil or coal), it is also dependent on the cost of capital (interest rates) and on the importance of the market concerned. It is generally necessary to opt for large-scale units to benefit from economies of scale, therefore nuclear is not appropriate where the electricity network is of modest proportions. Therefore the nuclear kWh is competitive in France when compared with the kWh produced from imported gas. This is no longer the case in the United States, or at least in certain regions, when it is in competition with local high-quality coal from open-air mines. The opening up of the electricity industry to competition is not favourable to nuclear energy which requires investments on a longer term than those necessary for other energies (gas, in particular) (see Percebois, 1997).2. Environmental considerations: Two major concerns have emerged little by little and explain why certain nuclear programmes have been challenged. These are the risk of accidents and the problem of how to deal with nuclear waste. After the Three Mile Island accident in the United States in 1979 and that of Chernobyl in Ukraine in 1986, the fear of an accident taking place greatly modified the image of nuclear energy in public opinion. The management of waste produced by the nuclear industry was not originally a sizeable preoccupation. It has become little by little the major worry with the increased attention attached by public opinion to environmental issues. First there is the risk of proliferation of nuclear weapons using certain waste generated bypower plants (plutonium). Most importantly, there is the management of long-term storage of high-level waste. But at the same time, nuclear energy can be considered as an alternative to fossil fuel in the context of policies to reduce emissions of greenhouse gases, giving rise to a certain ambivalence when the environmental dimension of this form of energy is discussed.3. Technological criteria: The potential technological progress is a key element of the future of nuclear energy. This concerns both the production of electricity from better and more efficient nuclear fuel and the procedures for the reprocessing of waste at the back-end of the nuclear fuel cycle.The potential technological progress is based on the reactors themselves. Certain technologies will allow the burn-up rate of fuel and the electrical output of reactors to be considerably improved. Other types of more “revolutionary” progress are in the pipeline, which will allow the development of more reliable reactors using safer fuel. It is even envisaged to construct reactors capable of burning a sizeable proportion of the most dangerous fuel. Amongst technologies being developed are the EPR or European Pressurized Reactor, the RHR1 and the RHR2. The feasibility study of the EPR has now been completed but its industrial development is still uncertain. This reactor has a capacity of 1530MWe and is capable of burning UOX or MOX fuel (with a maximum of 50% MOX fuel). Its life span would be 60 years and its generation capacity would be much superior to that of current reactors. The RHR1 would be capable of burning part of the plutonium. It could be of modest size and low-power reactors could become competitive. The RHR2 would be capable of burning actinides including plutonium, and would thus allow a sizeable reduction in the final waste to be stored. However, its development could not take place before 2040 (see the Charpin report). All of these reactors would have increased safety as compared to those currently in operation.New fuels are also envisaged, besides UOX and MOX: APA fuel (advanced plutonium fuel) would allow multi-recycling of the plutonium resulting from the combustion of MOX fuel (around 2020) and MOX Th which is a mix of plutonium oxide and thorium oxide. However, the substitutes for nuclear energy are also currently benefiting from potential technological improvements. This is in particular the case with plants with a combined cycle gas turbine or with coal plants on fluidized beds. The relative competitiveness, therefore, of the diverse technologies for the production of electricity depends on multiple factors and it is evident that technical progress is one of the essential variables to be taken into consideration.4. Political or strategic considerations: These considerations are present in relation to nuclear energy essentially because the civil nuclear industry is a direct descendant of its military counterpart, and also because the fear of proliferation of nuclear weapons from waste generated in electro nuclear plants has become a major worry. Political powers can not ignore the reactions (sometimes irrational) of public opinion and the abandon of certain nuclear programmes following a public referendum demonstrates how important such considerations are. However, at the same time, political powers know that the quest for independence in energy matters often involves an acceptance of nuclear energy and for certain developing countries, mastering civil nuclear technologies is a means of acquiring one day military nuclear technology. The situation is very different from one region to another, or even from one country to another: out of the 443 nuclear reactors in operation, 146 are in the European Union, 125 in North America, 92 in Asia and 67 in the Eastern countries. However, certain countries have a large network: the USA, France, Japan, Russia. Efforts to further develop the nuclear network are essentially located in Eastern Europe,OECD countries being more circumspect.2.2. The global situationFor many countries a “wait and see” attitude has been the response to the uncertainty with which they are confronted. This attitude manifests itself by an extension in the life span of reactors in operation. Such extensions allow states to wait until new and more powerful technologies become available and to postpone the public debate thereby avoiding decisions on a politically sensitive subject. The United States, which has the largest nuclear power plant park in the world (30% of installed capacity worldwide) prefer to make “rejuvenating investment” into operational plants in order to extend until 40 or even 60 years the life span of reactors. Gas or coal plants are often more competitive than nuclear plants in light of national resources and the opening of the transport and distribution of electricity sectors to competition does not encourage operators (generally private) to invest in large-scale equipment. However, the nuclear option is the choice of some, and even the federal government envisages a new development of nuclear energy for strategic reasons linked to energy independence. Out of the 15 countries of the European Union, seven make use, albeit in different proportions, of nuclear energy. In 1999–2000, the percentage of nuclear in electricity production was 78.2% in France, 60. 1% in Belgium, 35% in Germany, almost 30% in Spain and Finland, 28.6% in the UK and only 3.1% in the Netherlands.In France, the nuclear option has proved itself one of the pillars of its energy policy since the first oil crisis and the Charpin–Dessus–Pellat report showed how a rapid “rejection” of nuclear energy would be costly. An extension of the life span of operational plants (to 45 years) is one part of the solution pending the development of more powerful technologies.The German government committed itself in January 1999 to abandoning nuclear energy, and an agreement on the closure of all plants was signed on 14 June 2000 with the electricity companies. However, this programmed phase-out will not culminate until 2030, which leaves many options open. The case of Sweden is a little different as a date of closure of nuclear power plants in 2010 was established by referendum in 1980. At the end of 1999, the first of the 12 Swedish reactors was closed, but doubts remain here also about the feasibility of this closure plan.Russia and Eastern Europe represent today almost 13% of installed nuclear capacity in the world. Twenty four plants are in construction but the priority is currently the upgrading of certain reactors (RBMK in particular) to meet Western safety standards.Asia appears to be the most dynamic region in the nuclear field. Asian countries have to face a high demand for electricity in light of the current economic development and demographic dynamism particularly in China.Japan has 53 reactors with a total capacity of 43.5GWe which covers 36% of the national electricity production. Four reactors are under construction.The most promising perspectives for nuclear energy are to be round in China. China’s policy consists of trying several foreign categories of reactor before choosing one which will be the best adapted to its need, whilst all the time maximizing transfers of technology. Currently, there are only two reactors in operation and nine under construction but 25 new reactors could be commissioned between now and 2015. Coal will certainly remain the principal fuel for the production of electricity but the Chinese market represents important potential for the Western nuclear industry.3. The problem of the management of wasteThree methods of management of irradiated fuel (back-end of the nuclear fuel cycle) are currently being used:1. Reprocessing–recycling which involves separating and recovering reusable uranium and plutonium from the spent fuel and conditioning non-recoverable products so they can be put into storage (closed cycle).2. Direct disposal of irradiated fuel (without reprocessing), in deep geological formation and aftera period of surface storage in order to condition them (open cycle).3. Placing waste “in waiting” in order to postpone the decision between reprocessing and direct disposal.Those countries which have opted for the “closed cycle” (reprocessing–recycling) have often done so in the interest of energy independence, as pointed out in the Charpin–Dessus–Pellat report, in order to reduce the risk of fissile material resources running out. Reprocessing also presents the advantage of reducing the volume of final waste and eliminating in particular plutonium, which can be re-used as a fuel (MOX). However, on the other hand, this involves a number of further operations which necessarily increase the risk in relation to transport of waste to reprocessing sites (e.g. La Hague in France). The volume of final waste for storage is certainly reduced but the risk of nuclear proliferation is increased as the plutonium is separated from the other waste. “Furthermore, it is not certain that management of multiple waste of different natures is simpler than managing one single category of waste. The diversification of waste has advantages and can also have disadvantages as the risks are often independent” (Charpin report, p. 88).The direct disposal of irradiated fuel involves minimum storage of 50 years to allow the highly radioactive but short-lived elements to decrease (during what is called a cooling period). Pools are used for this. Following this, it is necessary to locate a final disposal site in a stable geological formation (the seabed is forbidden under the terms of the OSPAR Convention).The question, therefore, is whether such disposal should be reversible or not. Scientific progress will perhaps allow us one day to reuse this waste under improved conditions or even to burn it and eliminate it altogether. Amongst those countries who carry out reprocessing—recycling on their territory are France, the United Kingdom, Russia and Japan (to a certain extent). Other countries have chosen to have their waste reprocessed abroad: Germany, Belgium, the Netherlands, Switzerland. Other countries have abandoned the notion of reprocessing e.g. the United States and Sweden. Yet other countries have planned to reject this option: Switzerland or even Japan.International opinion, which favoured reprocessing at the end of the 1960s is today much more preoccupied with safety and the risks involved in the transport of irradiated fuel and the proliferation of plutonium. This is why direct disposal is favoured by those countries which plan to phase out nuclear energy or which would prefer not to isolate plutonium for fear of proliferation. This is particularly the case in the United States which, since 1988, no longer reprocesses its waste in order to avoid any risk of proliferation. However, in the absence of disposal sites, industrials store their spent fuel close to plants. For high-level waste the Department of Energy plans to construct a vitrification plant at the military centres of Savannah River and Hanford. For deep disposal, the site at Yucca Mountain (Nevada) has been chosen but will not be operational until2010. In Sweden, the Oskarshamm site 200 km from Stockholm was selected. France has selected a storage site at Bure in the Meuse valley.The risks linked to the disposal of radioactive waste have become a more preoccupying factor than that of risks related to the operation of plants, even if the latter have not disappeared altogether. These major environmental dangers have three major characteristics:●The often irreversible nature of the observed effects. The problem is a very long term one andinvolves future generations.●The worldwide nature of the risk. This problem is no longer a national one but rather is of aglobal nature and any potential solutions must involve international cooperation.●The scope of the scientific uncertainties which remain. The current state of the art does notalways allow us to appreciate the real nature of the risks posed. Hence, the adoption of the “precautionary principle” which involves waiting until further information is available before taking a definitive decision. This also means leaving several options open and favoring preventive actions for conservation purposes.Faced with these risks, a whole regulatory framework has been established at national and international levels. Let us refer, for example, to:1.The Non-Proliferation Treaty (NPT) which is a universal instrument designed to prevent nuclear proliferation. Currently 187 States have ratified it and only four States (including three nuclear states—India, Israel and Pakistan) remain outside the Treaty.2.Various specific international conventions in the field of nuclear safety (in particular, the London Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter or the OSPAR Convention on the Protection of the Marine Environment of the North-East Atlantic).3.Measures recommended by the International Atomic Energy Agency (IAEA) or the OECD Nuclear Energy Agency (NEA), in particular concerning standards to be observed during the fuel cycle.4.The West European Nuclear Regulators’ Association which was established in 1999 and which aims to develop a common approach in the field of nuclear safety in Europe.5.In France, the Act of 30 December 1991 on Radioactive Waste Management represents the legal framework for everything related to the management of the back-end of the fuel cycle.Nuclear energy has a definite future, but it suffers from an unfavorable image in the public eye, both due to the military origins of its development and because a satisfactory solution has yet to be found for the long-term management of radioactive waste which it generates. It is a safe form of energy, and has led to fewer deaths than coal or petrol-based energies, it is competitive in many ways and, paradoxically, it can also help to address certain environmental issues like greenhouse gases. However, the objective risk is one thing and the general perception of that risk is another. Technical progress will certainly find more solutions by making available safer reactors and especially by identifying satisfactory solutions for the back-end of the cycle (reprocessing and disposal). If we wish to avoid the situation where the political powers take irrational decisions in order to satisfy an often badly informed public opinion, it is necessary that nuclear activities take place in a strict and reassuring regulatory framework. Better information of citizens and adoption。