Robert Stieglitz, Joachim Knebel, Walter Tromm...bility and reactivity control, which have to be met...
Transcript of Robert Stieglitz, Joachim Knebel, Walter Tromm...bility and reactivity control, which have to be met...
78. DPG-Jahrestagung, 17.-21.März.2014
Berlin
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International role of nuclear fission energy generation, status and perspectives
Robert Stieglitz, Joachim Knebel, Walter Tromm
Karlsruhe Institute for Technology (KIT), Hermann-v. Helmholtz-Platz 1, D76021 Karlsruhe; [email protected], [email protected]; walter [email protected]
Abstract
The Fukushima incident in March 2011 caused worldwide a change in the perception of nuclear energy generation. Independent from the decision made by individual nations regarding the future use of nuclear energy for electricity generation, the number of nuclear power plants (NPP) operated worldwide has hardly changed. Essential reasons are mainly rising feedstock pric-es, increased energy demands and the simultaneous aspiration to reduce substantially the CO2-emission by fossil fuels. Especially emerging Asian economies are forced to an aggressive exploi-tation of all electricity generating technologies including nuclear to match their societal and eco-nomic demands. Nevertheless, the Fukushima accident initiated worldwide a new quality in the safety assessment and safety culture by considering additional man made or natural disasters. This process is reflected in enhanced bilateral or international co-operations. One of the most striking consequences is that a safe NPP operation demands a continuous retrofitting and evaluation of the plant behavior based on the current state of science and technology, which is part of the German safety practice since the Three-Mile-Island (TMI) incident.
Within this article different new nuclear plant developments with enhanced safety features are presented. Although these concepts as well as their deployment options diverge considerably in design and operational strategy the major nuclear protection goals in terms of confinement, coola-bility and reactivity control, which have to be met by any plant design, remain the same. Regard-ing the operational safety increased computational capabilities allow by means of coupled multi-physics and multi-scale method to identify design weaknesses down to the pin scale of a fuel as-sembly both for steady state and also for plant transients. To master severe accidents the different plant concepts, however, yield to a considerably larger diversity of technical solutions, nearly all of which are based on passive systems that exploit the physical natural laws. A sustainable use of nuclear fuel avoiding large scale deep underground repositories inherently implies a closed fuel cycle and the deployment of fast spectrum reactors, so-called Generation –IV reactors, for which similar nuclear postulations in terms of safety on all levels have to be demonstrated. Within the ar-ticle for both operational safety and severe accident measures examples are presented to illustrate the main functionality and operational principle.
1 Present status of nuclear electricity generation – observations worldwide and in Eu-rope
At present 435 commercial nuclear reactors (NPP) are operating and almost 2/3rd´s of the 72 plants under construction are erected in Asia [1]. More than 75% of the existing reactor fleet is light water reactors and about 85% of the new built belong to the class of pressurized water reactors (PWR). All commercially operated NPP´s produced in 2013 nearly 11.5% of the global electricity production, which is only slightly less than in the previous years. These commercial plants are complemented by approximately 240 research reactors operated in 56 countries and currently nearly 180 civil nuclear powered ships. Remarkable is that the countries engaged in new built or strongly envisaging the use of nucle-ar power as a “nuclear newcomer” belongs either to Eastern Europe or to Asia and the moti-vation to use nuclear power is mainly triggered by their societal decision to rely to a large quantity on industrial production as one major pillar of economic development- or simply as source of future wealth. The specific reasons of those societies range from vast economic de-velopment and rapidly rising electricity consumption, grid independence, fuel independence
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2 Boundary conditions for NPP deployment-Large reactors/ vs. small medium sized reactors
Considering nuclear as an option either as bridging technology or as one major pillar of the energy mix of an individual country, the question of the appropriate reactor size for the base load configuration arises immediately: large reactors (LR) or small scaled modular reac-tors (SMR) ? . The arguments for the choice of LR or SMR may be grouped in social (ac-ceptance, risk perception), political (independence, CO2 limitations), economic (resources, price, risk) or technological (technical maturity, safety performance) criteria. Mainly the deci-sion matrix is composed of mixtures of all these arguments and the ranking is strongly de-pendent on the national boundary conditions.
2.1 Economic considerations
Large NPP’s require a considerable capital investment per MWh/unit including all costs for projection, deployment, operation & maintenance (O&M) and finally decommissioning. Additionally nuclear power utilization demands a long term strategy of the energy policy and its development. Due to these high financial exposures, the long pay back times (envisaged duration of the investment 80-100 years from planning to decommissioning) present a high investment risk if entirely financed by private shareholders. Compared to coal, RES or gas fired plants the capital costs amount to about 55% [6] and hence the capital intensive invest-ment represents a strong exposure to market risks aside from other critical aspects as political frame (licensing, inspection, regulations,…) and social factors as e.g. public acceptance. As a consequence, private operators in a liberalized market often based on competition and some-times with priority access of other energy sources require a stable energy politics environ-ment. In contrast to purchase a LR there are numerous arguments for deploying SMRs as identified by [6, 7, 8] such as the need for flexible power generation for wider range of users and applications; the replacement of aging fossil fired units; the potential for enhanced safety margin through inherent and/or passive safety features; the economic consideration-better affordability freedom in upgrading; potential for integration innovative energy systems: cogeneration & non electric applica-
tions (desalination, process heat) and hybrid energy systems composed of nuclear with RES. But, according to numerous studies [6, 8] SMR are not significantly cheaper than LR´s and moreover, the capital return time is even larger than for larger reactors although they may offer a higher decision flexibility to expand their unit size. Additionally, SMR´s cannot be conceived as a simple scale reduction of a LR. Also the power output of several SMR to the grid cannot be simply considered as the sum of the modules; the SMR technology presents an entirely different product with respect to fuelling, operation but mainly with respect to the safety features and the applied technology. Among these technology issues their safety behav-ior is due to the smaller dimensions considerably different. SMR have usually a smaller spe-cific power density than LR allowing the use of a set of passive measures to master essential safety functions or even a full encapsulation of the reactor. Nevertheless, SMRs fulfill in some markets already an essential role as to act as base load source in remote regions or as grid stabilization in regions with moderate energy consumption like in China or India.
2.2 Current situation of NPP deployment
More than 95% of the currently operating reactor fleet belongs to the class to generation II plants, which in principle have been designed in the sixties and seventies. Also the present-ly installed generation III reactors are mainly evolutionary designs of Gen-II systems. The major reason for this development may be conceived as a risk minimization strategy of the
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shareholders. The comparability of Gen-II and operating Gen-III plant enables to a considera-ble extend the use of the accumulated experience of the currently operating fleet and therefore it facilitates the licensing aspects. The designs rely on well proven physics principles and no technological leaps are required. All aspects together yield for the operating Gen-III reactors a similar performance and sustainability as for Gen-II units. Another class of Gen-III reactors currently under construction in USA and China are very innovative; they rely mainly on novel passive safety features to assure core coolability e.g. in case of Loss-of-Coolant (LOCA) ac-cidents and to remove the residual heat. What are the peculiarities of the operating Gen-III reactors?
There are essentially two drivers for the new Generation III plants, which arise from both hardened design objectives and economic design objectives. The design objectives may be subdivided in two classes- the nuclear safety and the public acceptability. Regarding nu-clear safety in Generation III units severe accident measures have been already integrated in the design to attain considerably lower core damage frequencies and a significant reduction of potential radiological consequences. Another essential feature is that external events and haz-ards are considered in design and emergency management measures, which end up in a more robust safety architecture. In order to attain public acceptability, the design is devoted to min-imize the environmental impact for all operational stages and to prevent situations, in which off-plant areas are submerged to any emergency planning.
Especially the competition with other sources hardened the economic objectives. In the front row here is the profitability of the project, which in turn demands plant availabilities of more than 90% along the whole life-time, short re-fuelling and outage durations resulting in long cycle length and reduced investments caused by design simplifications and short erection times. According to this list, LR´s are preferred to SMR units. Another economic aspect is the investment protection, which translates into anticipated operation times of at least 60 up to 80 years and a low difficult-to-repair failure rate, which in turn demands to credit mainly for proven technologies. The latter argumentation chain holds mainly for liberalized markets, where temporal economic ups or downs even at low interest rates shall allow for profits for the shareholders within a reasonable time. The frame for NPP development today is conduct-ed in contrast to former times by a set of regulations, standardizations and requirements elabo-rated in the international context of utilities [9], technical survey organizations (TSO) [10], worldwide co-operations and collaborations as well as international institutions like the nucle-ar energy agency (NEA) and the IAEA [11]. All these regulations are publicly available and continuously updated.
3 Safety concept of an NPP
3.1 General safety approach
The major protection goals for NPP´s have not been changed since the early days and scope only three aspects: confinement of the radionuclide inventory; coolability at any time irrespective of origin and source and control of reactivity. This protection goals led to the implementation of a defence in depth (DiD) strategy, for dif-ferent levels are assigned to specific reactor states from 1 to 5, see . The challenge is to pro-vide enough margins between the different levels of safety to prevent cliff edge effects. The subsequent safety demonstration is characterized by a risk informed safety strategy, in which at first the protection goals are transferred into fundamental safety functions to be provided by the individual plant system design. The individual demonstration is conducted by both proba-bilistic and deterministic methods, in which for the latter a set of initiating events (PIE) are postulated and their progression is analyzed.
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Stieglitz, Knebel & Tromm
12
operations and organizations such as the WANO (World Association of Nuclear Operators) aiming to get consensus on common safety design criteria for NPP, to share operation experi-ence collected in each country and in the IAEA e.g. the event notification reports, event anal-ysis reports and to foster mutual exchange of professionals and technical support for safety-relevant issues.
On the opposite side governments and technical survey organizations (TSO) established an international cooperation on a worldwide basis as e.g. through the IAEA (International Atomic Energy Agency). The regulatory authorities of the Western European countries have create the WENRA association devoted to intensify the cooperation and work out standard-ized regulations and safety requirements for the licensing of nuclear power plants.
In addition, a vast bandwidth of worldwide collaborations on dedicated topics exit with-in the nuclear community such as the Global Nuclear Energy Partnership (GNEP), Interna-tional Framework for Nuclear Energy Cooperation (IFNEC), Multinational Design Evaluation Program (MDEP), the Contact Expert Group (CEG) and in Europe for example the European Atomic Energy Community (EURATOM), which themselves are complemented often by bilateral agreements on safety standards and best practice guidelines.
As result of these international activities and the beneficial interaction of manufactures and regulators, new reactors of Gen-III have been developed taking profit of the extensive operational experience of hundreds of NPP of Gen-II, of the advances in nuclear technology, material sciences, computer codes, etc. and considering the overall safety requirements con-tinuously updated and published by the IAEA that reflects the state-of-the –art of science and technology. Hereafter, selected reactor designs without claiming for completeness are briefly described.
The European Pressurized water Reactor (EPR), depicted in Figure 13a, is based on a 4 loop evolutionary PWR design evolving from both the N4 (France) and the Konvoi (Germa-ny) design; its rated power is about 1600 MWel and it consists of 4 train active safety systems, a strong double containment design (primary containment designed for low pressure core melt, Corium spraying area, shield building), protection of the plant against commercial air-plane crash by protected buildings (containment, fuel building, part of the safeguard build-ings) and by physical separation (part of the safeguard buildings, diesels, …). The large core (241 FA) allows for a power upgrade, an economical fuel management allowing for 50% mixed oxide (MOX) core loading and long cycles up to 24 months [20]. Another concept currently deployed successfully by KEPCO Korea is the APR1400, figure 13b. It is also PWR with a rated power around 1400 MWel using a compact core. This reactor design originated from the CE80+ developed by Combustion Engineering in the 80’s (certified in USA in 1996). It is a 2-loop design with 2 steam-generators and 4 pumps having 2-train active safety systems and 4 independent mechanical trains for safety injection systems. The containment consists of a single concrete containment with steel liner with a high re-sistance against earthquakes. In contrast to the EPR, the severe accident management strategy focuses on an in-vessel Corium retention through external reactor vessel cooling by means of water provided from the IRWST and additionally a boric acid make-up pump [21]. Among these two PWR designs shown here, several other PWR are currently erected as the AES fam-ily (Russia), AP1000 (Westinghouse-Toshiba) and others are certified as the ATMEA (MHI-AREVA) and the APWR 1000 (MHI). The interest in light water boiling water reactors (BWR) is considerable smaller than in PWR´s and aside the already licensed plant types such as the AB1600 (Toshiba), ESBWR (General Electric) and KERENA (AREVA), the Advanced Boiling Water Reactor (ABWR from Hitachi-General Electric) is currently erected. The safety philosophy of such reactors regarding the control of severe accidents is similar to the one of a PWR plant, while marginal differences naturally arise with respect to the operational safety due to the diverging principle.
Figur1=douarea aKEPC
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Stieglitz, Knebel & Tromm
15
volume reduces dependent on the technology used by a factor of 10 to 30 (theoretically sub-stantially more) meaning that about 3-7 m3 of vitrified waste in form of glass are the subject of nuclear waste disposal. Due to the heat generation, the glass pellets require about 28 m3 disposal volume in a dedicated canister [29]. The technological progress e.g. volume reduc-tion techniques, abatement technologies, etc. as well as optimization of work flows, a sub-stantial minimization of waste was achieved in the recent decades. Nonetheless, nuclear utili-zation poses a societal challenge since it demands a consistent and enduring waste manage-ment policy to ensure environmentally sound solutions preventing any hazard to both work-ers and general public. Even, abandoning the nuclear energy option for electricity generation, there is a need to preserve the knowledge related to ionizing radiation, radiation physics, radi-oisotopes, etc. due to the large application of nuclear technology in areas not related with electricity generation such as medical diagnostic, automation and control, water treatment, etc. . Moreover, one should relate the numbers of NPP waste production to that of a coal fired power plant of the same size, which produces aside from CO2 about 4.105tons ash a year con-taining heavy metals such as As, Cd, Hg, Pb or Thallium [31], requiring an adequate storage.
The nuclear waste in Germany is continuously monitored by the Bundesministerium für Strahlenschutz (BfS) [32]. The expected amount of nuclear waste to be conditioned in the future in intermediate and final repository is also well known and any time quantifiable.
Summarizing one can state that irrespective of societal decision taken, nuclear energy utilization requires reprocessing, conditioning and transportation to a safe confinement. All these processes are oversight by the regulatory body according to the national nuclear regula-tions. Regarding the waste disposal. there are several options feasible either in the temporal and the spatial frame, necessitating societal acceptance and simultaneously matching safety constraints. Regarding the temporal time window, the choice is at first an intermediate storage deciding in request further re-processing options or an ultimate solution by vitrification of the entire inventory. With respect to the spatial solution options, there are on the one hand near soil storage solutions but requiring as drawback permanent access control and confinement integrity and on the other hand deep underground disposals with or without an access option demanding also an analysis for a long term safe confinement.
3.7 Transmutation and Generation-IV
In the view of the nuclear waste generation and their interrelated issues, the utilization solely of light water reactors will lead to an accumulation of the minor actinides (MA) such as Americium, Curium, Neptunium and also Plutonium. The energy released by the fission of Plutonium can be recovered by means of a fast spectrum reactor allowing for a sustainable use of uranium resources. This potential has been identified quite early several decades ago. In May 2001 under the lead of the United States Department of Energy, the Generation IV international forum (GIF) has been founded. The top level requirements postulated by GIF for the Gen-IV reactors are: sustainability (meaning transmutation capability), enhanced econom-ics (lower life cycle costs), improved safety (low probability or even absence of any off-site emergency measures) and non-proliferation. This GIF initiative currently consists of 12 coun-tries. The EU is involved by cooperations within international frameworks such as the IAEA's International Project on Innovative Nuclear Reactors and Fuel Cycles (INPRO) or within the Europe the Sustainable Nuclear Energy Technology Platform (SNETP).
One of the major aspects for the fast spectrum reactors is their transmutation capability. Transmutation hereby describes the transfer of radionuclides by neutron induced fission or neutron capture into another element as illustrated in figure 16. By dedicated design measures, fast spectrum reactor systems are therefore able either to breed fissile material or to destruct fissile minor actinides. One of the drawbacks of the fast reactor systems is that they require a
Stieglitz,
16
Figur(FP=ftation)
dedicateplant, war logist
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tial fueand tranfigure causes presencthat a fvented. option wand addand also Omany taconductonly tecand the only natergy utient of thone centrum rea
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ntly erected sectional cudeveloped a
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cooled fast viable reacton gained inis considereed to as equty) or even by dedicatefission pro
dia shows thin the fuel rast to so-ca
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reactors besafety perfbasis accidets of this sa
n through allgovernmengeneration c
& Tromm
17
in India
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Stieglitz, Knebel & Tromm
18
Hereby, the waste management, the processing and the logistics play an essential role, de-manding a continuous monitoring and a sensible long-term oriented technological planning complemented by public acceptance. The amount of nuclear waste and its volume is small compared to the ones of conventional fossil based energies. In this context, partitioning and transmutation in fast spectrum reactors offers a credible option to minimize the burden on future generation either by national efforts or integrated in a regional context. Independent of the societal decision on the future use of nuclear fission for energy production, the develop-ment of education in nuclear engineering must persist of vital interest to an industrialized country like Germany to assure not only a credible nuclear safety assessment capability but also further investigations to tackle the technical and scientific challenges related to the final disposal of nuclear waste, which is still far ahead of us. Acknowledgement
The authors thank for the support of numerous colleagues of the program nuclear safety of the Karls-ruhe Institute of Technology for providing material.
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19
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