Haven't had time to read it yet, but here is the article, sans abstract, figures, and references. If anyone wants the PDF, let me know.
Grimes and Nuttall 2010 wrote:In North America and Europe, the development of nuclear power stalled after the March 1979 Three Mile Island accident in Pennsylvania, and until recently the building of additional nuclear reactors was not likely. Yet today, a nuclear renaissance is underway, and globally 52 reactors are under construction (1). How nuclear energy found itself in a state of decline is well documented. Will it continue to move forward and avoid another collapse?
In this article, we assess technological responses and opportunities for nuclear generation technology on two time scales (Fig. 1): first, those of immediate concern and consequence, and second, matters that will dominate in the longer term (beyond about 2030), when nuclear development could once more stall. The immediate future also indicates continued growth of nuclear energy in the Middle East, East Asia, South Asia, and elsewhere.
If the global electricity system is to be largely decarbonized over the first half of this century, then two key challenges must also be surmounted. One will be to develop civil nuclear programs in all parts of the world without risking the proliferation of nuclear weapons technologies (2). The other will be to deal with nuclear waste in as safe a manner as possible. Settling on policy options has proved extremely difficult in many countries for many decades. Technical proposals are available, including deep geological disposal. The assessment and perception of the risks associated with the transport and storage of radioactive wastes will continue to be reconsidered, given growing concerns about unconstrained fossil fuel wastes being emitted into the atmosphere. Newer reactor designs have the promise of creating less waste or waste that has a shorter lifetime, but some storage issues will still need to be resolved.
The Immediate Time Scale
Text Box 1. Specific Global Problems During the Immediate Time Scale
Supply chain for nuclear new build. During the 1990s, the nuclear supply chain (especially for large forged components) had largely wound down to a point where few companies worldwide retained key capabilities. In recent years this trend has started to reverse, but there is still no basis for complacency concerning the nuclear supply chain.
Personnel/expertise. As with the supply chain, the nuclear skills base has also not been replenished, so the nuclear industry suffers from an aging workforce. The problem is particularly acute in areas such as regulatory safety inspectors.
Research facilities. During the past two decades, numerous radiation and reactor test facilities have closed. The absence of such facilities will hamper progress in understanding radiation damage processes in materials and components and the development of alternative fuel cycles for use beyond 2030.
Social factors and waste. The last wave of nuclear power plant construction occurred in a very different geopolitical and social context than today. One consequence is a widespread reluctance to accept the building of new nuclear power stations without a clearly defined policy for nuclear waste management and eventual disposal, not just for future wastes but also for existing ("legacy") wastes.
Plutonium. Legacy plutonium remains problematic in several countries. The use of separated Pu in a LWR via a U/Pu mixed oxide fuel does not rapidly reduce the total inventory of Pu but does convert it from a relatively easily handled oxide powder to a less easily handled (or diverted) radioactive spent fuel assembly. Such considerations are important for international civil nuclear fuel cycles.
For many countries with established nuclear programs, the most immediate challenges are nuclear life extension and how best to renew nuclear generation infrastructure. Such steps are no better than neutral in terms of reducing greenhouse gas emissions, but they can preserve diversity of fuel and technology in the electricity mix, thereby helping to preserve a secure energy supply. However, lifetime extensions demand a thorough and precisely justified safety case. This requires prediction of reactor component materials performance under the extreme conditions experienced—still a great challenge to materials science (3). Therefore, the length of lifetime extensions is uncertain for all current reactor types.
The electricity industries of most current nuclear energy states are dominated by large centralized power stations that transmit over large-capacity grids to end users metered only infrequently. This transmission system is well suited to what is known as large-scale base-load generation and will be continued by the Generation III (Gen III) nuclear power stations currently planned or under construction (Table 1), which will replace Gen II facilities [definitions of the generation classes of nuclear reactors are given in (4)]. Base-load operation means that a power plant is operated at maximum capacity for as long as fueling and maintenance requirements will permit. Nuclear power stations are at their most profitable when operated by this regime (5). In the immediate future, only a few countries will need to consider more flexible nuclear technologies optimized to change their output power with changes in demand; such exceptions result from very high reliance on nuclear energy (for example, in France) or because of substantial energy generation from intermittent renewable energies, such as wind (for example, in Germany) (5).
Several Gen III nuclear energy technologies are ready for immediate deployment (Table 1). They are each the product of experience gained by a few large companies over many years of operating related reactors. In each case, probabilistic safety assessments (PSAs) have demonstrated even higher safety and reliability of these designs (6). PSAs were introduced after the Three Mile Island incident, which did not occur just as the result of one event, but following a sequence of interrelated events (7). A PSA can be used to predict the consequence of a series of events to yield, for example, a core fault accident frequency. These important numbers have continued to improve.
Gen II reactors rely on active processes in the event of a fault occurring (for example, a pump will start or a valve will open to mitigate the problem), but active control systems may themselves incur a fault. Some Gen III systems rely on passive processes (such as natural heat convection and gravity) to prevent irrecoverable damage. However, it remains a major challenge to determine the extent to which a specific design truly incorporates passive safety or the length of time that the passive system can operate reliably.
Gen III designs have also benefited considerably from advances as diverse as three-dimensional computer-aided design, concretes with improved microstructural properties, and new powerful lifting equipment. Large complete plant modules are now built away from the site, lifted into place, and fitted into position. Gen II designs often required reactors to be disassembled and then reassembled inside their containment structure.
In addition to their large generating capacity, the reactors in Table 1 are all water-moderated and use a fuel technology in which stacks of uranium oxide pellets (the fissile material) are sealed inside tubes made from zirconium-based alloys (the cladding). Both the pressurized water reactor (PWR) and the boiling water reactor (BWR) designs use light water to moderate down the energy of the neutrons produced during fission so that they can initiate further fission reactions. These designs advance earlier Gen II PWR and BWR light-water reactor (LWR) designs. In the past 4 decades, the proportion of time during which LWRs were available to generate electricity (their availability factor) increased from around 70% to over 90% (8).
However, LWRs require the 235U/238U isotopic ratio to be enriched by a factor of roughly 5 over that of natural uranium to sustain fission. Although the enrichment process is very energy intensive, more of the uranium atoms undergo fission in enriched fuels, resulting in more energy being extracted per kilogram of fuel (i.e., a high degree of burn-up). Enrichment is not needed in heavy-water designs, because the D atoms capture far fewer neutrons, permitting a chain reaction even when fewer 235U atoms are available for fission. Currently, heavy-water reactor technology is being developed in India as a way to use their domestic thorium reserves (9). However, the best-known heavy water reactor is the Gen II Canada deuterium uranium (CANDU) design, which uses natural uranium fuel and can be refueled while remaining online (LWRs have to be shut down when fuel is exchanged). The CANDU design (including the proposed Gen III advanced CANDU) is based on individual but linked pressure tubes, rather than the monolithic pressure vessel of LWR reactors. This feature makes the CANDU design more complex, but it requires no very heavy forgings like those needed to construct a PWR or BWR reactor pressure vessel. This is an important consideration now, because there is some concern about the ability of global supply chains to source sufficient quantities of heavy forgings for PWR and BWR systems. The burn-up reached in CANDU reactors is, however, much lower than in LWRs.
Burn-up is a crucial economic imperative. Burn-ups in LWRs have been increasing steadily from 20 gigawatt-days per metric ton of uranium (GWd/t) in 1970 to over 50 GWd/t at present (10 GWd/t roughly correspond to 1% of the uranium atoms undergoing fission) (10). Further increases in burn-up will require modifications to existing strategies, but to do so more research must be undertaken to satisfy regulators that the fission products are retained safely and securely in the fuel assembly. This will require cladding that maintains its integrity for longer under greater irradiation dose and fuel that can retain the fission products within its crystal lattice for longer. New alloys with better metallographic texture that reduce the (already low) frequency of cladding breach are being developed (11), as are fuels with larger grain sizes that may provide longer migration paths for fission products, increasing the times before the fission products can penetrate a breach or chemically attack the cladding (12). Nevertheless, the task is one of enormous complexity, given that irradiated fuels contain complex crack patterns, inhomogeneously distributed fission gas bubbles and oxides, and noble metal precipitates (Fig. 2). Advanced modeling techniques can, together with experiment, reveal the chemical processes operating within active fuel and should, in the future, enable more efficient fuel use (13).
If uranium prices rise and concerns about future uranium availability increase, then such factors will, in fuel cycles where the fuel is not reprocessed, drive the need for greater efficiency obtained by higher burn-up. Uranium prices are, however, unlikely to be high enough to prompt a resurgence in reprocessing, although key reprocessing competencies should be preserved to keep options open.
Higher burn-up would also reduce the volume of waste, because fewer spent fuel assemblies are generated, or—if reprocessing is used—produce considerably less intermediate-level waste. The total inventory of radiotoxic species is, however, not reduced by the same extent. This is because each kilowatt-hour (kWh) of energy generated requires roughly the same number of fission events, whether that be via low or high burn-up fuel, and gives rise to nearly the same number of fission products.
Beyond 2030
Text Box 2. Issues Beyond 2030
Fuel availability. A widespread global first wave expansion and its growing demand for uranium resources will by 2060 have made today’s once-through uranium fuel cycle increasingly unsustainable. Rising uranium prices will prompt renewed interest in nuclear fuel reprocessing, innovative fuel cycles, or gaining uranium from unconventional sources. It may even become desirable to recycle earlier generations of spent LWR fuel held in long-term storage.
Life extension. This will be needed by 2060 for Gen III reactors built in the immediate time frame.
Design of future plants. Plants must be designed to enable a later three-way choice between life extension, reactor replacement, and full power station decommission and rebuild. The capacity of waste repositories for decommissioning waste in the second half of the century will become a factor.
New degradation mechanisms. They will be discovered for materials that were necessary to facilitate a plant with much longer life than was the hitherto the case.
Regulation and inspection. Both national and international regulators will experience greater demands, in part because of requirements for different reactor types, aging fleet, and repository build and management.
Public acceptance. A major nuclear expansion is very likely to require green-field developments in places with little or no nuclear heritage. This could be a major difficulty regarding public acceptance. It will be necessary for waste repositories to be under construction, not just being planned.
Skills. The young engineers of the 2030s are currently in preschool. We are likely to face a second wave of skills difficulties in the future unless we act now to ensure that science and engineering subjects grow in popularity within the school system.
The second phase will be driven mostly by the need to decarbonize electricity supply; however, because developments in nuclear engineering take so long to find their way into nuclear power stations, it is crucial to consider the implications or consequences now. In advanced economies, the main challenge will be to decarbonize heating and transport, through either hydrogen fuel or electricity, possibly placing further stress on electricity supply. Furthermore, supply from individual generators—e.g., from wind farms or solar panels—to the electricity system will probably be far more intermittent than it is today. Nuclear power could potentially fill short-term gaps in electricity supply. However, at present, the costs involved in the construction of new nuclear plants do not favor it for an energy gap–filling role, despite the technology’s low-carbon credentials (5).
The second phase of nuclear technology has the potential to overcome these challenges by moving beyond the electricity generation business. It is well-suited to high-temperature industrial process heat applications (at, for example, 850°C) for the direct thermochemical production of hydrogen as a future vehicle fuel. This latter approach forms part of the Gen IV VHTR (very high temperature reactor) concept (14). Competing models for a thermochemical hydrogen economy include hydrogen production via electrolysis or an electricity-only low carbon transport system using battery energy storage. The widespread use of battery cars would help to mitigate volatility in the system caused by the large-scale deployment of intermittent renewable energies and might restore the cost benefits of nuclear energy in the 2030s. Nuclear power could also be widely used for desalination, another efficient way to use surplus power in an electricity system dominated by base-load generation and with fluctuating demand.
In countries that already have a substantial fraction of their power generated via nuclear plant and appropriate grid infrastructure, economic considerations will lead to a demand for power stations with design lifetimes in excess of 70 years, compared with the 40 to 50 years of presently operating plants, and for fuels that can tolerate much higher burn-up. However, these reactors are likely to experience even more extreme temperature and radiation damage conditions than current newly built reactors, especially Gen IV types (15, 16) (which have remaining design work to be done before they can be built even as prototypes). This will dictate a move from designs based largely on permanent, irreplaceable components toward designs allowing almost complete scheduled replacement of parts, some of which may even be recyclable. This approach will alleviate unforeseen materials aging problems and make possible the acceptance of new technology and their associated cost and safety benefits.
Outside established nuclear countries, flexible nuclear technologies will be especially attractive to match supply and demand locally and in real time, reducing the need for grid infrastructures. This may favor small modular nuclear power reactors (17). For example, a small ship-borne civil power plant called the Academik Lomonosov is under construction at Sverodvinsk, Russia, which incorporates two KLT-40S reactors (18). Another idea that has its origins in naval technology is the fueled-for-life core, that is, a nuclear reactor that never requires refueling. Such a reactor can be delivered as a sealed unit that provides the motive force (probably, but not necessarily, steam from an integrated heat exchanger) to the conventional "island" (the turbine and generator). At the end of its life, which might be as long as 40 years, the reactor is returned to the manufacturer for decommissioning and disposal. Because fuel handling is avoided at the point of electricity generation, the radiation dose to workers would be reduced and monitoring would be much less involved. However, such units will be less efficient than conventional refueled systems, because they would have to work well inside established engineering materials performance parameters to minimize the likelihood of unexpected degradation processes (3). These technologies could play an important role in a global roll-out of proliferation-resistant nuclear power technology, because only the country of origin would have access to the spent fuel. The economics of small and fueled-for-life reactors versus large reactors with scheduled replacement will be a constant issue.
Increasingly there will be concern about the long-term sustainability of the nuclear fuel cycle. Nuclear energy will need to move beyond today’s conventional "thermal" use of uranium fuel, which is based on a once-through cycle. Alternative technologies may burn thorium, plutonium, and minor actinides or may close the fuel cycle, for example, by reprocessing. Some of these options could sustain power production for more than 1000 years. If it is not possible to make such a transition then, nuclear power development after 2030 could stall once again. Such a scenario would be consistent with a preference expressed by some policy-makers, who take the view that today’s Gen III nuclear renaissance is a necessary evil, to be tolerated only until a new, more efficient economy based on renewable energies or fusion can emerge (19).
Making the transition requires novel evolutionary fuel designs for high–burn-up Gen III reactors and revolutionary designs for the extreme conditions and higher burn-up anticipated for Gen IV systems. Already, fuels assembled from small (submillimeter) kernels of UO2 enclosed by concentric shells of pyrolytic graphite and silicon carbide achieve in excess of 19% burn-up of uranium atoms. With their three layers of fuel coating, these are known as TRIstructural ISOtropic, or TRISO, particles (20).
The thermal conductivity of fuels must be improved to reduce the temperature gradient across the fuel, thereby keeping the center of the fuel well below its melting point (UO2 has a poor thermal conductivity). This can be achieved by using either uranium nitride or carbide compounds as fuel or by using a composite. In a composite, a nonfissile second-phase powder, such as MgO or SiC, that has a higher thermal conductivity is mixed with UO2 powder fuel (13).
High-temperature corrosion-resistant fuel cladding materials will be important, for example, silicon carbide fiber—reinforced composite. By providing a high density of damage recombination centers (which can heal radiation damage), nanostructures could well play a role in providing materials with greater radiation tolerance (21), especially for areas in the reactor core that receive very high radiation flux. Nanostructure offers other advantages; for example, oxide dispersion steels containing yttrium oxide nanoparticles have superior creep resistance and are being considered as future reactor pressure vessel materials (22).
We suggest six possible complementary routes to adopting sustainable nuclear energy.
Option 1: Unconventional Uranium
As mined uranium resources are depleted, prices are expected to rise, but there are abundant resources of unconventional uranium, such as uranium phosphates and uranium in sea water (23). At present, extracting such uranium is uneconomic, and only tiny quantities of seawater uranium have been extracted. In such a scenario, it is likely that unconventional uranium will yield a price cap to conventional nuclear fuel. That maximum price is likely, however, to be prohibitively high, and other options may be more attractive.
Option 2: Reprocessing Spent Fuel for Multiple Mixed U-Pu Oxide Fuel Recycle
This potentially long-lived fuel cycle has been developed over many decades in several countries (especially France, United Kingdom, and Russia). It involves the chemical separation of plutonium, in order to fabricate fuel from mixed U-Pu oxide powders (MOX) (24). However, because Pu separation is a proliferation-sensitive technology, MOX fuel fabrication is likely to be restricted to the nuclear weapons states, although perhaps with increasing international access to such fuels via appropriate global agreements.
Option 3: Critical Fast Reactors
These reactors are more compact and have much higher energy density than today’s nuclear power systems. Consequently the rate at which the neutron density or temperature can change in the event of an accident is faster and therefore a greater engineering challenge. As such, critical fast reactors raise safety and reliability issues beyond those typical of today’s nuclear power plants, especially in the event of a loss-of-coolant accident. Also, because of the greater potential for production of fissile material ("breeding"), such technologies raise security concerns. A substantial advantage is that both 235U and 238U isotopes undergo fission in these reactors, thereby using a much greater proportion of the uranium.
Option 4: Thorium Fuel Cycle
Thorium has the potential to become an important nuclear fuel. It is not fissile itself, but in a reactor, thorium-232 can capture neutrons to yield fissile uranium-233. The thorium fuel cycle can then proceed by either (i) fabricating fuel pellets that contain a mix of thorium-232 and a fissile element (such as uranium-233), (ii) placing a blanket of thorium fuel around a reactor core containing fissile material, or (iii) injecting extra neutrons from a particle accelerator (see option 5). Thorium is several times more abundant than uranium, and a thorium fuel cycle can be developed that produces negligible amounts of plutonium and fewer long-lived minor actinides than a uranium cycle. However, fissile uranium-233 is difficult to extract and handle, because it is produced together with other highly radioactive uranium isotopes, and the performance of thorium fuels is not well understood. The proliferation resistance credentials of the thorium fuel cycle deserve greater scrutiny but appear promising.
Option 5: Accelerator-Driven Subcritical Reactors
Despite their complexity, accelerator-driven subcritical reactors (ADSRs) have potentially useful advantages over conventional critical reactor systems. ADSRs can, in principle, produce thorium-fueled nuclear energy, avoiding the need for fissile materials supplied from other sources. In addition, ADSRs show promise for waste treatment. The process of nuclear transmutation using an ADSR has the potential to reduce quantities of long-lived and highly toxic radioactive wastes quite substantially (25). Lastly, ADSRs offer improved safety and fuel utilization compared with other sustainable second-phase nuclear options.
Option 6: Nuclear Fusion Energy
Nuclear fusion could provide clean energy with enhanced intrinsic safety and abundant fuel resources. However, the technology has not been demonstrated at industrial scale and reliability. Furthermore, it relies on helium coolants (a coproduct of nonrenewable natural gas), although various measures such as cooling with liquid hydrogen have been suggested (26, 27). Fusion is unlikely to move toward commercialization until after 2050. Furthermore, the many commonalities between fusion and fission research—high temperature materials for high radiation environments, fast neutron physics, structural integrity issues—favor a collaborative approach between the fusion and fission communities. Fusion-fission hybrids and fusion-driven fission fuel breeders (28, 29) have been suggested as a route to early commercialization of fusion energy.
Outlook
Nuclear technology is at a crossroads. The community has been tested in recent years as it gears up to renew existing facilities in Europe and North America while continuing or initiating an expansion in other regions. It seems ever more likely that a second larger phase of nuclear development will be required beyond the 2030s to ensure a low-carbon energy future that makes maximal efficient use of nuclear plants and resources. Energy and research policy decisions made now will determine whether we have the capacity to design and develop innovative new systems that contribute to sustainable flexible nuclear energy generation.
Although we are developing other energy generating systems and it is possible that a second larger phase of nuclear development will not be required, it would be unwise at this stage to assume that nuclear energy will not be needed. If we are to generate that option for policy-makers and the energy industries of the 2030s, we must act now.
