Design for Values in Nuclear Technology
Safety has always been an important criterion for designing nuclear reactors, but in addition to safety, there are at least four other values that play a key role, namely, security (i.e., sabotage and proliferation), sustainability (i.e., environmental impacts, energy resource availability), economic viability (i.e., embarking on new technology and its continuation), as well as intergenerational justice (i.e., what we leave behind for future generations). This chapter reviews the evolution of generations of nuclear reactors (I, II, III, III, and IV) in terms of these values. We argue that the Best Achievable Nuclear Reactor would maximally satisfy all these criteria, but the safest reactor is not always the most sustainable one, while the reactor that best guarantees resource durability could easily compromise safety and security. Since we cannot meet all these criteria simultaneously, choices and trade-offs need to be made. We highlight these choices by discussing three promising future reactor types, namely, the high-temperature reactor pebble-bed module (HTR-PM), the molten salt-cooled reactor (MSR) and the gas-cooled fast reactor (GFR).
KeywordsSafety Sustainability Security Economic viability Intergenerational justice
In December 2011 Bill Gates announced that he plans to invest one billion dollars to jointly develop a new nuclear reactor with the company TerraPower. This reactor is designed to be less expensive than the current reactors; it must run on abundantly available natural uranium, it must generate little waste, and, perhaps most importantly, “all these new designs will be incredibly safe,” Gates emphasized (BBC 2011).
Gates’ reactor seems to be the ideal nuclear power solution as it enables us to enjoy the benefits of nuclear power without being troubled by any of its drawbacks. So this reactor is assumed to carry low accident risks, to not require any proliferation sensitive enrichment of uranium, and to produce only a small volume of high-level waste. These claims are all made by the manufacturer, who estimates this reactor to be available around 2030.
In designing nuclear reactors, several criteria have played an important role: the possibility and the probability of core failure or meltdown, the kind of fuel needed, the amount of energy produced, the volume and lifetime of the remaining waste after operation, and, last but not least, the possibility of using the reactor to manufacture one of the key ingredients of a nuclear bomb, namely, weapon-grade nuclear material. The latter is perhaps among the oldest issues in nuclear technology. The world’s first nuclear reactor was built in the 1940s to show the feasibility of producing plutonium, which then could be extracted from the irradiated fuel. Although this was primarily intended for the energy generation purposes, plutonium was used shortly after its discovery in the Nagasaki bomb (Seaborg 1962). The dual use of nuclear technology, alternatively known as proliferation, has been a central issue since the beginning of the civil use of nuclear power in the 1950s and the 1960s up until the present.
The aforementioned criteria are referred to as values, since they reflect how we perceive “the good” or how we want the world to be (Scanlon 1998). Values are very important in the design of nuclear reactors. However, we cannot always accomplish all the “goods” at the same time so we need to make choices and trade-offs between the good we find to be more important indicating why we find it more important. It has been argued that the impossibility of accomplishing several values at the same time – or simply value conflicts – has fueled innovation in engineering design (Van de Poel 2009; see also the Chapter “Conflicting Values in Design for Values” in this Volume). In designing nuclear technology, there are at least five main values that play a key role, namely, safety (i.e., public health impacts), security (i.e., sabotage and proliferation), sustainability (i.e., environmental impacts, energy resource availability), economic viability (i.e., embarking on new technology and its continuation), and intergenerational justice (what we leave behind for future generations).
These values should be in balance with each other since they cannot always be simultaneously accomplished. Different societal, ethical, or political considerations could bring one of these values to the forefront. It is particularly interesting to see how nuclear accidents have affected the perception of nuclear safety which, in turn, has determined the evolution of nuclear reactors. For instance, the development of substantially safer nuclear reactors started after the Three Mile Island accident in Pennsylvania in 1979. Since the Fukushima disaster in 2011, “safety” seems again to be the leading value in design.
The chapter is organized as follows: in section “Nuclear Technology,” we will first introduce nuclear power technology and its key component, the nuclear reactor. Special attention will be devoted to the historical evolution of safety as a key value in nuclear technology design; we will also discuss other values that are relevant when designing civilian nuclear technology. Section “Design for Nuclear Values” focuses on the design of several new nuclear reactors and how a preference for different values has resulted in different nuclear reactor designs. Section “Open Issues and Future Work” presents the open issues for further academic endeavor. Conclusions are presented in section “Conclusions.”
Radioactivity was discovered by the end of the nineteenth century. Yet, it had little practical relevance until 1938 when the first fission reaction (i.e., splitting the nuclei by neutrons) was discovered. Since a fission chain reaction releases more than one free neutron, a fission chain reaction could be made self-sustaining. This technology was used in the WWII for the development of nuclear weapons, but soon thereafter the same physical principles were applied for civil purposes. The first non-weapon application was for the propulsion of submarines in 1953. In 1956, world’s first nuclear plant for electric production started operation at Calder Hall in the UK (Tester et al. 2005, Chap. 8). In this section, we briefly introduce nuclear power production. More specifically we will focus on nuclear reactors and the nuclear fuel cycle in which those reactors have a key role. We will focus on the evolution of safety in nuclear reactor design; other relevant values in nuclear technologies will also be introduced.
The evolution of generations of nuclear reactors
Reactor type – acronym
PWR and BWRa
Estimated core damage frequency (CDF)b (per reactor year)
1.6 × 10−7d
4.2 × 10−7e
5 × 10−7f
Type of change in design
Small and incremental compared to BWR
Medium and incremental compared to PWR
Medium to radical
Very radical. Change in reactor technology
Leading values in design
Safety and economic viability
The majority of the world’s 435 reactors still in operation today comprise Gen II reactors. These reactors use light water (1H2O) as a coolant and moderator which is why they are referred to as light water reactors (LWR). Of the LWRs, 75 % are pressurized water reactors (PWR), originally designed for ship propulsion. The remainder of LWRs are boiling water reactors (BWR) (Tester et al. 2005, p. 374).
The Historical Evolution of Safety in Reactor Design
Historically, safety has been one of the important driving forces behind serious changes in reactor design philosophy. Major nuclear accidents seem to have particularly affected people’s thinking about reactor safety. After the core meltdown accident in Three Mile Island in Pennsylvania in 1979, David Lilienthal called upon nuclear technologists to design safer nuclear reactors whose cores could not melt (Lilienthal 1980). This proposal was first only met with skepticism, but it did provoke a discussion on the philosophy of nuclear reactor safety (Weinberg and Spiewak 1984).
Before moving toward designing safer systems, the skeptics first proposed reassessing the probability of core damage in existing TMI-type nuclear reactors. A couple of years before the Three Mile Island accident, the Atomic Energy Commission (AEC) had initiated a new study to coherently assess the safety of nuclear reactors by mapping all the events that could possibly lead to an accident and then assigning probabilities to each single event. The study, officially known as the Reactor Safety Study, was better known as the Rasmussen Report (NRC 1975), and the proposed method was termed probabilistic risk assessment (PRA) (Keller and Modarres 2005). The Rasmussen Report found the core damage frequency of a LWR to be approximately 5 × 10−5 per reactor year. An analysis of the actual precursors to potentially serious events in operating reactors between 1969 and 1979 suggested, however, a more pessimistic probability, namely, 10−3 per reactor year (Minarick and Kukielka 1982). Taking these semiempirical results into account, Spiewak and Weinberg estimated the core damage frequency of all operational reactors in the 1980s to be 15 × 10−5 per reactor year, “within a factor three of the core melt probability” as estimated by the Rasmussen report (Spiewak and Weinberg 1985, p. 436).
In policy-making, an even higher probability of core melt down seems to have become acceptable in the years after, namely 10−4 per reactor year (NRC 1986). This probability corresponds to once in every ten thousand reactor years based, undoubtedly, on the number of reactors in operation in the 1980s (ca. 500) which thus meant that an accident would probably occur once in every 20 years.1 However, serious growth was anticipated during what was known as the Second Nuclear Era in the 1980s; forecasts for as many as 5,000 reactors were made. Ten times more reactor years means that accidents could in principle happen ten times more frequently: a subsequent core melt accident probability of once every 2 years2 was deemed unacceptable in terms of public confidence (Weinberg and Spiewak 1984). Safer nuclear reactors were therefore needed.
Most of the reactors in operation at the time of the TMI accidents were LWR-type reactors. They were originally designed for maritime purposes, the leading design criteria being compactness and simplicity rather than enhanced safety. As these reactors grew bigger for commercial energy production, the possibility of a core melt and its consequences became a serious challenge; i.e., new reactors had greater power capacities and more radioactive inventory. At that time several safety systems were then added to existing designs in order to enhance safety, but they also made the design “immensely complicated” because various additional “electromechanical devices, such as valves, scram rods, emergency pumps, and backup diesels” were then needed (Spiewak and Weinberg 1985, p. 432). This reliance on “external mechanical and/or electrical power, signals or forces” makes active intervention in the event of an incident necessary; the LWR designs of the early days therefore came to be known as “actively safe” (IAEA 1991, p. 10).
The first steps taken toward creating safer systems involved removing “[s]ome potential causes of failure of active systems, such as lack of human action or power failure” (IAEA 1991, p. 10); such systems came to be known as “passively” safe systems. The level of passivity of any system should be considered in terms of the number of external factors that have been removed. It would therefore be best to speak of higher or lower categories of passivity. IAEA illustrates this by giving an example. When a system’s dependence on external power supply has been replaced by an internal power source, such as a battery, to supply active components, we can speak of the system being passive since it does not depend on a potential external failure, but this will probably be the lowest category of passivity (IAEA 1991); higher levels of passivity could be reached by removing reliance on more external factors, for instance, by removing reliance on power sources at all.
The first ideas on ways of making nuclear reactors safer focused on changes in the design of the LWRs; these changes were considered to be “incremental changes” that did not drastically affect the design philosophy (Firebaugh 1980). Below we will first discuss the rather small changes made in LWR design for the sake of safety. In contrast to these small changes, more substantial change is also conceivable. With these concepts, rather than adding incremental safety features, the design would emerge from a different safety philosophy, namely, that of inherent safety. Such drastic changes are hailed the transformation of the technological regime in the design of nuclear reactors (Van de Poel 1998).
The notion of inherent safety in reactor design shows some similarities with the design of chemical processes. Designing for inherent safety in chemical processes (and plants) has been introduced by Kletz (1978) and it entails that “we should avoid or remove hazards rather than add on protective equipment to control them” (Edwards and Lawrence 1993, p. 252). The same rationale has been adopted by the IAEA (1991, p. 9): that is, “inherent safety refers to the achievement of safety through the elimination or exclusion of inherent hazards through the fundamental conceptual design choices made for the nuclear plant.” Before a power plant can be declared completely inherently safe, all these hazards have to be eliminated, but that is simply not feasible. We therefore speak, instead, of degrees of inherent safety. Thus, when in a reactor an inherent hazard is eliminated, it is inherently safe with respect to that specific hazard; for instance, when no combustible materials are used, a reactor is inherently safe against fire regardless of whatever incident or accident might occur (IAEA 1991). One important piece of rationale behind reactor design is the notion that new reactors should be made inherently safe in terms of being resistant to meltdown or core damage; more will be said about this below.
Nuclear Fuel Cycles
Producing electricity requires more than a nuclear reactor to supply heat to a turbine. There are many steps required prior to electricity production (front end) and after reactor operation (back end). The whole process is called a nuclear fuel cycle and it starts with the mining and milling of uranium ore and ends with the possible treatment of the waste product and its geological disposal. There are now two ways to produce nuclear power, through open and through closed fuel cycles. Both methods use a LWR and both use uranium as fuel. Natural uranium contains two main isotopes, i.e., 235U and 238U. Only the first isotope (235U) is fissile and is used in LWRs as fuel, but it only constitutes 0.7 % of all natural uranium; this is why uranium is enriched by which we increase the fraction of the fissile isotope 235U to 3–5 % for energy production in LWR. Irradiating uranium produces other materials, including plutonium (239Pu) and other fissile and non-fissile plutonium isotopes as well as minor actinides. Actinides are elements with similar chemical properties. Uranium and plutonium are the major constituents of spent fuel and so they are known as major actinides. Neptunium, americium, and curium are produced in much smaller quantities and are thus termed minor actinides. Fission products are a mixture of radionuclides that will decay to a nonhazardous level after approximately 250 years.
In the open fuel cycle, an isotope of uranium (235U) is fissioned – split – in the reactor. The spent nuclear fuel is then designated for disposal underground and will take 200,000 years to become stable. The required storage time is dominated by plutonium. As stated above, less than 1 % of the uranium ore consists of the fissile isotope 235U. The major isotope of uranium (238U) is non-fissile and needs to be converted into a fissile material for energy production: plutonium (239Pu). In the closed fuel cycle, spent fuel undergoes a chemical process to separate useable elements, including the not irradiated uranium fuel as well as the plutonium produced during irradiation; this chemical treatment is referred to as reprocessing. During reprocessing, uranium and plutonium isotopes in the spent fuel are isolated and recovered. Recycled uranium could either be added to the beginning of the fuel cycle or used to produce Mixed Oxide Fuel (MOX) that is used as fuel in some nuclear reactors. The waste remaining after reprocessing is referred to as high-level waste (HLW), and it has a radiotoxicity higher than that of natural uranium for approximately 10,000 years dominated by the minor actinides.
Values in Nuclear Engineering Design
Values are relevant to many of the choices that we make, also with regard to the design of technology; they reflect our understanding of the rightness and wrongness of those choices. The term value indeed has definitions that extend beyond philosophy and ethics. That said, the focus of this chapter is confined to the moral values that deal with how we want the world to be. We should not, however, confuse values with the personal interests of individuals; values are the general convictions and beliefs that people should hold paramount if society is to be good (Van de Poel and Royakkers 2011). Indeed, “the good” might be perceived differently by different individuals. In the following paragraphs, we will give definitions of these values, as they have been presented by the relevant international nuclear organizations. We believe that contention often arises more from how different values should be ranked in terms of their importance (moral or otherwise) than from how a single value is conceived of.
As mentioned earlier, safety has played a key role in the developments of civilian nuclear technology; the detrimental health impacts of ionizing radiation were known long before the deployment of nuclear power in the 1950s (Clarke and Valentin 2009). The notion of safety is sometimes used in absolute terms (safety as an absolute, as equated to no harm) and sometimes in relative terms (safety in terms of reducing the possibility of harm). Due to the many uncertainties we deal with in engineering design, safety is often interpreted in relative terms (Hansson 2009). This is certainly the case when addressing radiation risk, particularly since it is the accumulation of ionizing radiation that can have health impacts (see also the Chapter “Design for the Value of Safety” in this volume). The philosophy of radiation protection is “to reduce exposure to all types of ionizing radiations to the lowest possible level” (ICRP 1959, p. 10). The underlying rationale is that we reduce the level of radiation such that we eliminate or at least reduce the probability of detrimental effects. So, the “health objective” prescribes that the “deterministic effects are prevented, and the risks of stochastic effects are reduced to the extent reasonably achievable” (Valentin 2013, p. 19).
In short, safety as a value refers here to those concerns which pertain to the exposure of the human body to radiation and its subsequent health effects.
Due to the longevity of nuclear waste, safety is a value that relates to future generations as well. The safety of future generations has been one of the concerns from the early days of nuclear power production. The Nuclear Energy Agency states that we should offer “the same degree of protection” for people living now and in the future (NEA-OECD 1984). The IAEA reiterates this in its Safety Principles where it states that nuclear waste should be managed in such a way that “predicted impacts on the health of future generations will not be greater than relevant levels of impact that are acceptable today” (NEA-OECD 1995, p. 6).
In the IAEA’s Safety Glossary, nuclear security is defined as “any deliberate act directed against a nuclear facility or nuclear material in use, storage or transport which could endanger the health and safety of the public or the environment” (IAEA 2007, p. 133). One can argue that “security” as defined here also refers to the safety considerations discussed above. We shall, however, keep the value of “security” separate in our analysis so as to be able to distinguish between unintentional and intentional harm. We define “security” as the protecting of people from the malicious intentional harmful effects of ionizing radiation resulting from sabotage or proliferation. Thus security variously relates to nuclear theft and unauthorized access, to the illegal transfer of nuclear material or other radioactive substances at facilities (IAEA 2007, p. 133), and also to the dissemination of technical know-how or facilities that could lead to the proliferation of nuclear weapons. Proliferation threats arise either from using highly enriched uranium (HEU) which has been enriched up to 70 % (and higher) or from producing or separating weapon-grade plutonium in reprocessing plants; more will be said about this in section “Design for Nuclear Values.”
Sustainability is one of the most discussed and perhaps most contested notions in the literature on nuclear power. It is not our intention to enter into those discussions here and certainly not to assess the degree of sustainability of nuclear power. One common and influential definition concerning sustainable development is the Brundtland definition that emphasizes the ability of present generations to meet their own need without compromising the ability of future generations to meet their needs (WCED 1987; see also the Chapter “Design for the Value of Sustainability” in this volume). In nuclear power production and nuclear waste management, this definition at least relates to two specific issues, namely, the state of the environment as posterity bequeaths from us – referred to as environmental friendliness – and the availability of natural (nonrenewable) energy resources on which future generations’ well-being relies, referred to as resource durability.
The value of environmental friendliness relates to the accompanying radiological risks to the environment. Radiological risks, as perceived in this chapter, express the possibility or rather probability that radioactive nuclides might leak into the biosphere and harm both people and the environment. Issues that relate to the harming of human beings have already been subsumed under the heading safety. The effect of the same radiation on the environment and nonhuman animals is subsumed here under the heading of environmental friendliness.
Whether we should protect the environment for its own sake or for what it means to human beings is a long-standing and still ongoing discussion in the field of environmental philosophy. In the anthropocentric (human-centered) approach, this notion would solely encompass those aspects of the environment that are relevant to human health. The non-anthropocentric approach would address the consequences of radiation in the environment without making reference to what this means for human beings.
Various UN policy documents, including IAEA publications, interchangeably refer to both approaches. We do not intend to take a stance on this matter either. We preserve the value of “environmental friendliness” as a separate value in order to allow for a broader number of views to be reflected with this set of values. Those who would follow the anthropocentric approach will then simply merge this value with the value of “safety .”
If we now consider the period from the industrial revolution up until the present, it would be fairly straightforward to conclude that the availability of energy resources has played a key role in achieving (and sustaining) people’s well-being. The appropriate consumption of nonrenewable natural resources over time is one of the central issues of sustainability; “later generations should be left no worse off […] than they would have been without depletion” (Barry 1989, p. 519). Since it would be irrational to expect the present generation to leave all nonrenewable resources to its successors and since replicating such resources is not an option either, it has been argued that we need to offer compensation or recompense for depleted resources “in the sense that later generations should be no worse off […] than they would have been without depletion” (Barry 1989, p. 519). The value of resource durability is therefore defined as the availability of natural resources for the future or as the providing of an equivalent alternative (i.e., compensation) for the same function.
The next value that we shall discuss in relation to sustainability is that of economic viability. One might wonder whether economic issues have inherent moral relevance and whether it is justified to present economic durability as a moral value. We can safely assume that the safeguarding of the general well-being of society (also, for instance, including issues of health care) has undeniable moral relevance. However, in our understanding of economic viability in this chapter we do not refer to general well-being but only to those aspects of well-being that have to do with nuclear energy production and consumption. With this approach economic aspects do not have any inherent moral relevance; it is what can be achieved through this economic potential that makes it morally worthy.
This is why we present the value of economic viability in conjunction with other values. First and foremost, economic viability should be considered in conjunction with resource durability. In that way it relates to the economic potential for the initiation and continuation of an activity that produces nuclear energy. As we shall see in the following sections, some future nuclear energy production methods might require serious R&D investments for further development; particularly those new methods that are based on new types of reactors which would require serious investment prior to industrialization. Economic viability could also become a relevant notion when we aim to safeguard posterity’s safety and security by introducing new technology for the reducing of nuclear waste lifetime. In general, economic viability is defined here as the economic potential to embark on a new technology and to safeguard its continuation in order to maintain the other values.
Concerns about depleting the Earth’s resources and damaging the environment have triggered a new debate on the equitable sharing of goods over the course of generations; this is referred to as intergenerational justice. The main rationale is that we should consider justice in what we leave behind for generations to come after us. There are two ways in which intergenerational justice relates to nuclear power production and to waste management. First of all, assuming that this generation and those that immediately follow will continue depleting uranium, a nonrenewable resource, there will be evident intergenerational justice considerations to bear in mind. Secondly, the production of nuclear waste, and its longevity in terms of radioactivity, signifies substantial present benefits with deferred costs. In nuclear waste management this notion of justice across generations has been influential, particularly in promoting geological repositories as final disposal places for nuclear waste.3 Also in designing nuclear reactors and their surrounding systems, intergenerational justice has proven to be a relevant value. This will be elaborated in section “Design for Nuclear Values.”
Design for Nuclear Values
Section “Nuclear Technology” briefly presented the development of nuclear reactors design and introduced four main values that play a key role in designing reactors and their surrounding systems, such as nuclear fuel cycles. In this section, we will first operationalize these values by specifying how they relate to different phases of the nuclear fuel cycle. In this way we can assess fuel cycles based on the presented values. More importantly, we will focus on how these values have played a role in the design of nuclear technology, both in designing new fuel cycles and the associated nuclear reactors.
Designing Nuclear Fuel Cycles
In the interests of brevity, we will not elaborately discuss the operationalization of these values for the assessment of the two fuel cycles, but we shall briefly explain how this could be effected.4 What is particularly important in this analysis is that we link the impact of different steps in the fuel cycle to the values presented and evaluate the extent of those impacts. In so doing we should distinguish between the impacts for both the present and future generations. Let us illustrate this by taking an example in which we shall operationalize the value “ safety.”
In this way we can evaluate the existing fuel cycles based on the values and how they have been operationalized to relate to specific steps in the fuel cycle. Elsewhere we argued that each fuel cycle would promote certain values and sometimes, as a result, undermine other values. So choices need to be made between these values. So, the open fuel cycle seems to be preferable from the perspective of present generations, since it creates fewer safety and security risks and is less costly. The closed fuel cycle is, on the other hand, more beneficial from the point of view of future generations, because it reduces the long-term safety concerns of waste disposal while helping to extend nonrenewable resources farther into the future. At the same time, the closed cycle creates more short-term safety and security concerns and economic burdens. The choice of a given fuel cycle should thus be made by weighting the moral relevance of each values in a temporal sense (Taebi and Kloosterman 2008).
This ex-post analysis shows that when policy-makers opt for a specific fuel cycle, these value trade-offs are made implicitly. This analysis gains more relevance when we include the values in ex ante analysis of what we deem to be a desirable future nuclear technology. This approach accounts for values through the design process and it is referred to as Value Sensitive Design (Friedman 1996). It constitutes an attempt to uphold human values with ethical importance as design criteria, so that we can proactively think and guide future technologies (Friedman and Kahn 2003).
Let us now take the following example to illustrate this point. As mentioned above, the waste emanating from the open and closed fuel cycles is radiotoxic for either 200,000 or 10,000 years. Societies might find it desirable to further reduce the waste lifetime in order to enhance the value of long-term safety. If we were to incorporate such societal desire into technological development, there is one waste management method that would be particularly interesting, namely, the partitioning and transmutation (P&T) method. It could in principle reduce the waste lifetime by approximately a factor of 20 to 500–1,000 years. Its feasibility has already been shown at lab level, but the relevant technologies surrounding multiple reprocessing and fast reactors still need to be further improved. Fast reactors – accelerator-driven systems (ADS) could alternatively be used – are applied to create higher energy neutrons, which are capable of fissioning a greater number of isotopes, including the minor actinides, in the reprocessed spent fuel. This would help reduce the waste lifetime (IAEA 2004). Proactive thinking in terms of the values at stake could then help determine how to incorporate the value of safety, long term and otherwise, in nuclear waste management while elaborating on the implications for the other values at stake. P&T creates additional burdens for present generations in the form of the safety issues derived from additional nuclear activities, security issues emerging from the multiple recycling of plutonium, and economic burdens for the further development of the technology, including the required R&D funding. This brings us back to the fundamental question of which value should be preferred and for what reasons. More specifically, can additional burdens upon present generations sufficiently be justified?5
In sum, in this subsection we argued intergenerational justice is inevitably an important value when we are to choose between different fuel cycles. We further showed that in answering the thorny question of “what justice exactly entails for future generations,” we need to assess the impacts of fuel cycles in terms of the aforementioned value, namely, safety, security, sustainability, and economic viability.
Designing Nuclear Reactors
In the remainder of this section we will focus on the role values have played in nuclear reactor design. Our focus is on new generations of nuclear reactors. We will take the already operational Gen II reactors as the default situation and show how the value of safety has influenced the design of Gen III and Gen III reactors. Design changes could either be through incremental changes (compared to LWRs) or more radical changes. Table 1 presents a list of the reactors discussed in this section and summarizes the type of proposed changes and indicates the assigned probabilities of core damage. We will also focus on how other values such as sustainability are becoming increasingly important in the design of nuclear reactors and how that changes proposed designs.
Gen III: ABWR
Gen III+: AP1000 and HTR-PM
The most significant improvement in Generation III+ reactors is the inclusion of “some designs of passive safety features that do not require active controls or operator intervention but instead rely on gravity or natural convection to mitigate the impact of abnormal events” (Goldberg and Rosner 2011, p. 8). The improvements in Generation III could either qualify as incremental changes or as radical changes to the existing designs. We will discuss examples from both categories, i.e., AP1000 (a successor to the PWR) and a high-temperature reactor pebble-bed module (HTR-PM) which may be seen as a radically new reactor that takes safety as its primary design criterion and starts from scratch.
The core, reactor vessel, and internals of AP1000 are based on the conventional PWRs, built by Westinghouse. AP1000 is clearly a more passive safety system by using fewer safety valves and pumps and less safety piping and cables; furthermore it employs a (passive) core cooling system with three sources of water “to maintain cooling through safety injections,” passive residual heat removal, and (passive) containment cooling system to “provide the safety-related ultimate heat sink for the plant” (Schulz 2006, pp. 1551–1552). According to the manufacturer, these passive systems would reduce the probability of a core melt in an AP1000 to 4.2 × 10−7, making it almost 200 times less probable than the original PWR (i.e., according to the Rasmussen Report 2.6 × 10−5).
A more radical change in reactor design came with the introduction of HTR-PM. This type of reactor was first built in Germany – AVR (Arbeitsgemeinschaft Versuchsreaktor) – and further developed in South Africa, PBMR (Pebble-Bed Modular Reactor). Developments are now being continued in China. HTR-PM takes various safety and economic goals as its primary design criteria; for instance, the design of the reactor should not require “anyone living near the site boundary to take shelter or be evacuated” following any internal event in the reactor or an external event affecting the condition of the reactor (Koster et al. 2003, p. 232). This safety criterion has further been translated into an economic goal from the point of view that these reactors do not need large exclusion zones for operational purposes. That is again beneficial when it comes to licensing matters and to transporting electricity to populated areas. This safety regime has been termed inherently safe by the IAEA (1991, p. 9). The HTR-PM is further designed to offer levels of radiation safety to workers that would be higher than the international recommended standards (Koster et al. 2003).
To accomplish such levels of safety, two important design changes have been proposed in order to ensure that the reactor does not overheat and to make the fuel resistant to heat. The first change concerns the shape of the reactor. It will be a long cylinder with a small radius. This facilitates natural heat exchange with the environment due to the large reactor surface area. The heat is then transported to the cooling system which has the capacity to passively absorb this heat for more than 72 h (Koster et al. 2003, p. 236). It is important to observe that this safety improvement has an adverse effect on the economic aspects of the reactor because during normal operation, a part of the neutrons sustaining the fission chain reaction will leak out of the core, requiring a slightly higher enrichment of the uranium fuel.
The second change in the design concerns the revolutionary approach to fuel and its cladding; see Fig. 2. HTR-PM fuel consists of fuel spheres containing small coated particles. Each particle consists of a small amount of uranium oxide (i.e., fuel) which is encompassed in four layers of coating. Especially having two layers of pyrolytic graphite and one layer of silicon carbide (SiC) would make leakage of radioactive nuclides (i.e., fission products) substantially less probable, since those layers can withstand very high temperatures and can thus support the integrity of fuel spheres.6
Gen IV Fast Reactor: GFR and MSR
The latest developments in reactor technology are concentrated in Gen IV reactors which are designed to reconcile many different criteria. Firstly, these reactors should help us deploy the major isotope of uranium, the non-fissile 238U, thus enhancing resource durability in order to meet the value of sustainability. One must bear in mind that less than 1 % of all naturally occurring uranium is deployable in conventional thermal reactors, while fast reactors are capable of converting the major isotope of uranium (238U > 99 %) to fissile 239Pu. These reactors are the breeder reactors that breed (or make) new fuel (i.e., 239Pu). During operation this plutonium isotope can be used again as fuel. Other types of breeder reactors could be designed to use the more naturally abundant thorium as a fuel. This kind of reactor, the molten salt-cooled reactor (MSR), will be discussed here. Apart from meeting sustainability requirements, Generation IV reactors are intended to furthermore enhance long-term safety by reducing the volume and lifetime of nuclear waste. The gas-cooled fast reactor (GFR) will now be discussed.
The gas-cooled fast reactor is a fast-spectrum helium-cooled reactor that is designed to make efficient use of the major uranium isotope, but it is also designed with a view to waste management. High-energy fast neutrons enable this reactor to irradiate isotopes that thermal neutrons in conventional thermal reactors (e.g., LWR) cannot fission. This has evidently sustainability benefits for the durability of uranium as an energy source; the term “plutonium economy” refers to the implementation of fast reactors for energy generation purposes.
Molten salt-cooled reactors are probably the most ambitious kind of Gen IV reactors since they seriously depart from conventional reactor technology. The MSR was first proposed as a US aircraft propellant and it is one of the few reactors that can use naturally abundant thorium as a fuel. This reactor will run on a combination of uranium-thorium fluoride mixed in a carrier salt such as beryllium fluoride and lithium fluoride. This salt – that serves both as the fuel and the coolant – will continuously circulate through the reactor core and the heat exchanger to transfer the heat to a second circulation system for electricity production. A part of this fuel/coolant will then go to a chemical processing plant where the fission products will be removed and new fissile material will be added. “This continual processing of the fuel allows operation without refueling outages, and the fluid fuel offers a unique safety feature where the entire fuel inventory can be drained from the reactor in the event of an accident” (Abram and Ion 2008, p. 4328); see the emergency dump tanks in Fig. 3. The combination of corrosive and highly radioactive salt constantly running through the reactor places serious and extreme requirements on the material performance in the piping of the primary circuit and the equipment of the processing plant. Among other technical challenges, serious R&D effort needs to be put into fuel development, molten salt chemistry control, and corrosion study carried out on the relevant materials (DOE 2002, pp. 34–35). Some people maintain that the MSR lies at the boundary of Gen IV technology and is perhaps too ambitious to be industrialized (Abram and Ion 2008).
Open Issues and Future Work
Future endeavors should at least focus on three open issues that will be discussed in this section.
The Inability of the Probabilistic Risk Assessment
The safety of nuclear reactors has been systematically conceptualized since the introduction of probabilistic risk assessment (PRA) as proposed by the Rasmussen group in 1975. This was based on a fault-tree analysis that examined the undesired events and assigned probabilities to each event. Since the Rasmussen Report, the probability of core damage has been the leading criterion in safety studies. Even though this method has clear advantages such as highlighting weaknesses, different probabilities assigned in the literature are not necessarily referring to the same types of events; we cannot therefore always easily compare risks in terms of the calculated probabilities. For example, core damage is a different concept than core meltdown; so we cannot easily compare those two risks in terms of their probabilities. More importantly, without a meltdown radioactive nuclides can also leak into the environment following core damage while having a meltdown does not necessarily mean that there will be leakage into the environment, if the containment of the reactor retains its integrity. When we consider the uncertainties regarding the health impacts of the different types of nuclides and radiation, the complexity of the matter reveals itself in all its glory. To sum up, probabilistic risk assessment is a very good indication of safety, but it is not the final word in the discussion on reactor safety. Yet, PRA is absolutely indispensable when assessing the safety of nuclear reactors and comparing different reactors, even though the accuracy of the current estimation could be questioned in the light of the recent events in Fukushima (Goldberg and Rosner 2011); see also (Taebi et al. 2012, pp. 202–3).
Safer reactors could be realized by including incremental changes in the conventional current light water reactors. These reactors, of which the pressurized water reactors were originally designed to be used in the maritime sector, had simplicity and compactness as their main design criteria. When they were deployed on a large scale for commercial energy production purposes, many safety systems such as valves, emergency pumps, and backup diesels were added to the original design. The paradox of these safety systems was that they simultaneously made the reactors immensely complex and, in the process, unsafe. Incremental changes were proposed to remove these complexities. Reactors could furthermore be made passively safe by means of incremental changes; passively safe reactors reduce the need for human intervention and other external systems, for instance, through the use of emergency cooling systems that are solely based on gravitation, all of which reduce the probability of core damage. Safety could be further improved by bringing radical change to reactor design, for instance, by introducing inherently safe reactors that eliminate inherent hazards through fundamental design changes.
Fukushima and the Future of Safety
As stated earlier, safety has always been one of the important driving forces behind serious changes in reactor design philosophy. It is particularly major nuclear accidents that seem to have affected people’s thinking about reactor safety. It was, for instance, the Three Mile Island accident that initiated thinking about passively safe reactors and reducing the influence of operator action. It is now interesting to anticipate how the Fukushima Daiichi accidents might affect the design of nuclear reactors. We maintain that the proposed changes for the next couple of years will probably be incremental in two different respects. Firstly, the protection of the surrounding reactor systems, that is to say, the primary system of all but the oldest reactors in Fukushima withstood both the earthquake and the tsunami in 2011. The damage was caused by the defective external cooling system that was not as well protected as the reactor, all of which accelerated the accidents. Secondly, changes can be expected in the cladding of nuclear fuel. Current nuclear fuel cladding is composed of different zirconium alloys, because of their favorable mechanical and physical properties. An important drawback of metallic cladding is, however, that it can undergo a reaction with water above a certain temperature. This chemical reaction generates hydrogen gas which could, in turn, cause a hydrogen explosion like that seen in the Fukushima reactors. A move toward ceramic cladding is thus to be expected. Reactors are further expected to be made less vulnerable to large external events.
Designing for Conflicting Values
With the introduction of Gen IV reactors, sustainability also became a particularly relevant criterion in design. Indeed, security and economic viability have always played a role in design. We can assert that the Best Achievable Nuclear Reactor would maximally satisfy all these criteria, but as we have seen in the preceding sections, the safest reactor is not always the most sustainable one, while the reactor that best guarantees resource durability could easily compromise safety and security. Since we cannot meet all these criteria simultaneously, choices and trade-offs need to be made.
We highlighted these choices by discussing three promising future reactor types. So, depending on which design criteria will be decisive, drastically different reactors could be proposed. The high-temperature reactor pebble-bed module (HTR-PM) scores best on the criterion of safety because of the radical change it makes to safety design philosophy; core melt down is physically impossible in such a reactor; only core damage can occur. On the other hand, the molten salt-cooled reactor (MSR) scores best on resource durability because it can use naturally more abundant thorium as its fuel; however, this reactor type, until proven differently, scores low on chemical safety, because the highly radioactive liquid fuel is also chemically corrosive. The gas-cooled fast reactor (GFR) also scores high on sustainability since it uses the major isotope of natural uranium, but a GFR would score lowest on security since there is constantly plutonium in its cycle.7 The relatively low score of MSR on security is attributable to the production of a certain isotope of uranium (233U) that could be used for weapon purposes (no enrichment required).
The conflicting design criteria for three important designs for the future of nuclear reactors
In this chapter we introduced five main values that play a crucial role in nuclear design namely, safety, security, sustainability (both in terms of environmental friendliness and resource durability), economic viability, and intergenerational justice. We first elaborated on how each of these values has been perceived in the six-decade-old history of nuclear power and what role they have played in designing nuclear reactors and nuclear fuel cycles.
The main focus of the chapter was on incorporating these values into an ex ante analysis of what we deem to be a desirable future technology. This represents an approach that accounts for human values through the design process and it is referred to as Value Sensitive Design. Thinking in terms of values has already motivated the development of new fuel cycles such as partitioning and transmutation (P&T) and the design of reactors. These developments have however often been focused on one single value; also in nuclear technology safety has been a leading value. VSD aims to proactively balance different values in the design process. In balancing these values, choices need to be made between the values that we find to be important in design. These choices often go back to a fundamental issue that should be addressed proactively and prior to further development of the reactors. Research and R&D funds are scarce and previous experience shows that once policy-makers invest in a certain option, they are not easily inclined to shift focus later on because of the initial investments. Therefore it is crucial to address these ethical value conflicts prior to the further development of each reactor.
Each year, 500 reactor years would pass, which means that based on the probability of 10−4, the expected number of accident would be 5 × 10−2 (i.e., 500 × 10−4) or simply once in every 20 years.
Calculation: 5,000 × 10−4 = 5 × 10−1 or once in every 2 years.
This subsection is mainly drawn from the following publication, in which the role of intergenerational justice in nuclear waste management has been extensively discussed (Taebi 2012).
For an elaborated discussion of the operationalization of the values in fuel cycles, see (Taebi and Kadak 2010).
Please see for an elaborate discussion of this issue (Taebi 2011).
This paragraph is partly based on information provided by the South African company, Pebble-Bed Modular Reactor (Pty), that built PBMR. See: http://www.pbmr.com/contenthtml/files/File/WhynoChernobyl.pdf.
There are two remarks that need to be made. Firstly, it is the authors’ opinion that an MSR would score the best on the sustainability criterion. This is because of the natural abundance and good dispersal of thorium compared to uranium. Secondly, the economic viability is based on a rough estimation made by the authors in which assumptions have been made with regard to the required research funding for the industrialization of these three reactors. HTR-PM with a prototype reactor in China seems to be the farthest ahead in its research, which makes it score best on economic viability, while MSR presumably still requires substantial research.
The authors wish to thank Ibo van de Poel as well as Daniela Hanea for their valuable comments.
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