hybrid nuclear systems for energy production and waste management

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This article was downloaded by: [the Bodleian Libraries of the University of Oxford] On: 21 November 2014, At: 08:21 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Nuclear Physics News Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gnpn20 Hybrid nuclear systems for energy production and waste management H. Nifenecker a a Institut des Sciences Nucleaires/UJF/IN2P3 53 Ave. , Des Martyrs F-38026, Grenoble, France Published online: 19 Aug 2006. To cite this article: H. Nifenecker (1994) Hybrid nuclear systems for energy production and waste management, Nuclear Physics News, 4:2, 21-23, DOI: 10.1080/10506899408222879 To link to this article: http://dx.doi.org/10.1080/10506899408222879 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions

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Page 1: Hybrid nuclear systems for energy production and waste management

This article was downloaded by: [the Bodleian Libraries of the University of Oxford]On: 21 November 2014, At: 08:21Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Nuclear Physics NewsPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/gnpn20

Hybrid nuclear systems for energy production andwaste managementH. Nifenecker aa Institut des Sciences Nucleaires/UJF/IN2P3 53 Ave. , Des Martyrs F-38026, Grenoble,FrancePublished online: 19 Aug 2006.

To cite this article: H. Nifenecker (1994) Hybrid nuclear systems for energy production and waste management, NuclearPhysics News, 4:2, 21-23, DOI: 10.1080/10506899408222879

To link to this article: http://dx.doi.org/10.1080/10506899408222879

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information. Taylor and Francis shall not be liable forany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Page 2: Hybrid nuclear systems for energy production and waste management

feature article

Hybrid nuclear systems for energy production and waste management

H. NIFENECKER

Institut des Sciences Nucleaires/UJF/IN2P3 53 Ave. Des Martyrs F-38026 Grenoble, France

Carlo Rubbia (1) recently made the headlines by pro- posing to use hybrid systems to produce energy. A hybrid system associates a high intensity proton or deuteron accelerator to a neutron multiplier assembly. Such systems might have a number of advantages when compared to standard reactors. If correctly designed, they would make critical accidents, such as that of Tchernobyl, impossible. However, it is true that loss of cooling accidents such as that of Three Mile Island might still be possible, unless we resort to systems with natural convection cooling proper- ties, such as those with molten salt fuels or liquid metal coolants. The number of neutrons available for breeding or waste transmutation is larger in hybrid systems than in reactors. Systems based on the Thorium-Uranium cycle, be they hybrids or traditional, would produce much less Plutonium and higher actinides than those based on the usual Uranium-Plutonium cycle. The interest in hybrid systems based on the Thoriud-Uranium cycle originates, to a large degree, from the wish to make nuclear power more socially acceptable by reducing operating hazards as well as waste production.

Hybrid systems Hybrid systems, generally aim at

Producing energy; Breeding new fissile isotopes; and, eventually, transmuting nuclear wastes.

The majority of proposed hybrid systems (1,2,3,4,5,6) foresee the use of proton beams of energy between 1 and 2 GeV. Depending upon the properties of the neutron multiplier assembly, each incident proton might produce between 300 and 1500 neutrons, with an energy released by the associated fissions between 25 and 120 GeV. The primary energy necessary to accelerate a proton to, say, 1.5 GeV is estimated to be about 10 GeV. One sees that the available energy will vary between 15 and 110 GeV per incident proton. Clearly, such an uncertainty requires extensive additional concept studies, experimental testing, as well as R&D work. The key factors are, here, the

maximum achievable value of the neutron multiplication factor and the number of primary neutrons per incident charged projectile.

For transmutation applications, the value of the neutron flux is, also, of paramount importance. Indeed, the lifetime of a nucleus in a neutron flux is inversely proportional to the magnitude of this flux. As an example, nuclei with absorption cross-section of one barn, rather typical for fission products, would have a half-life of 200 days in a neutron flux of 1014/cm2/sec. Such figures explain the high neutron flux values reaching 10%m2/sec involved in projects like that of C.D. Bowman et al. (2) which give priority to waste transmutation and incineration. It is interesting to note that very high fluxes also allow a more efficient incineration of some actinides like 237Np and 241Am.

Fuel economy considerations Most currently operating reactors are light water mod-

erated and cooled. They belong to the Pressurized Water (PWR) or to the Boiling Water (BWR) types. They princi- pally use 235U as fuel, and to a lesser extent 239Pu, which they produce from 238U. The only naturally occurring fissile isotope 235U is only present in proportion of 0.7% in natural Uranium. The corresponding energy reserves are of the same order of magnitude as those corresponding to oil. It is this relative scarcity of 235U which led the early promoters of nuclear energy to consider the development of breeder systems. The most popular breeders are of the Liquid Metal Cooled Fast Neutron type (LMFBR). Here, the 238U fertile isotope is transformed into the fissile 239Pu one, in such a way that each fissioning 239Pu is replaced, on the average, by approximately 1.2 same nucleus. After about four years, the initial amount of Plutonium is, thus, doubled. The famed Super-Phoenix is an archetype of this concept.

It is, in principle, possible to resort to another breed- ing cycle, namely to the Thorium-Uranium cycle. Here, the fissile isotope is 233U, which does not exist in nature, while the fertile isotope is 232Th which is about five times more abundant in nature than natural Uranium. A small thermal reactor based on this cycle has worked in Oak Ridge for about ten years, with great reliability (7). Breeding was, indeed, demonstrated, but with a doubling time of around 20 years. This long doubling time was one of the reasons that led to the dropping out of this scheme.

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It is’ significant that the production of higher actinides would he reduced by several orders of magnitude in systems based upon the Thorium-Uranium cycle, when compared to the present day reactors. At the time when the choice was made in favor of the Fast Plutonium Breeding technology, considerations concerning higher actinide wastes were not considered to be a priority. Nowadays, these considerations appear to be important enough to justify the ambitious programme CAPRA, developed by the French CEA. This programme aims at incinerating plutonium a d higher actinides by fission in fast reactors. If successful, such a programme would lead to the con- struction of one fast reactor for every two or three PWR reactors. It seems clear, therefore, that resorting to the Thorium-Uranium cycle would have great advantages in reducing the needs for actinide incineration.

This is where hybrid systems have their strongest point. Indeed, due to their favorable neutron economy, such systems lead to doubling time of a few months, to be compared to the previously mentioned 20 years. It is also important that the breeding process at work with hybrid systems is of an electrical nature. An energy-producing hybrid system, where the concentration of the fissile 233U had reached its equilibrium value, could operate several similar systems with fresh 232Th fuel. After a few months, these daughter systems would have built up enough 233U concentration to become energy self-sufficient, and they would reach the final equilibrium 233U value after around two years. Thus, such electro-breeding does not require the costly and potentially hazardous fuel reprocessing which is necessary for reactor breeding. With hybrid systems, if found necessary, it would be possible to rapidly switch to a Thorium-Uranium economy from the current Uranium- Plutonium one.

Rubbia’s project Rubbia’s project obeys, precisely, such a logic. At the

present stage, the accelerator would be a 1 GeV, 10 milliamperes cyclotron. Such a cyclotron represents a quantitative, rather than qualitative, step as compared to the PSI (Zurich) cyclotron where intensities of more than 1 mA should be reached soon. The neutron multiplier assembly would look like a PWR, with cooling and slow- ing down by light water at a pressure of 150 bars, or, alternatively, like a High Temperature Reactor (HTR), where slowing down is assured by graphite and cooling by helium at high pressure. The window through which the proton beam would penetrate into the multiplier would be subject to intense radiation damage, although it should resist to high pressures. It is clear that designing such a window is a very serious technological chal- lenge. One might think of considering other types of multiplying assemblies working at low pressure, such

as liquid metal cooled or molten salt fueled systems ( 2 3 ) .

Theneutron flux in Rubbia’s approach would be limited to a maximum value of around 1014/cm2/sec. This limit is due to the production process of 233U. This process in- cludes a neutron capture followed by two beta decays:

-1 22 min. 27 days

232Th + n + 233Th j 233Pa 2331J

At high fluxes a significant proportion of the 233Pa nuclei may capture a neutron before they decay. This reduces the final equilibrium amount of 233U and makes it difficult to obtain a high multiplication factor.

During proton irradiation, the equilibrium concentra- tion of 233U is the result of two competing processes:

Production by neutron capture by 232Th; Destruction by neutron absorption by 233U.

It is found that, for thermal neutrons, an equilibrium is reached when the proportion of 233U nuclei approaches 1.3% of that of the 232Th nuclei. During irradiation, a stock of 233Panuclei is, also, built up. When the irradiation stops, neutron absorption by the 233U also stops, but additional 233U nuclei are formed by disintegration of 233Pa. Thus, the concentration of 233U increases, qnd therefore the multi- plying factor of the assembly also.increases. If the stock of 233Pa is large enough, criticality might be eventually reached. Since the stock of 233Pa is proportional to the neutron flux, this should be limited in order to prevent critical behavior after stopping the irradiation.

It appears that both earlier-mentioned effects can be managed if the maximum neutron flux is kept below cm2/sec.

Conclusion The greenhouse effect has recently been the subject of

many discussions and debates. Nobody doubts that a strong increase in the carbon dioxide content of the atmo- sphere has been correlated with the surge of industrial use of fossil fuels. What will be the buffering role of the ocean, to what extent will this increase of the CO, concentration lead to an increase in the temperature of the planet and, even, to what extent might such a temperature increase be detrimental to mankind are questions subject to a hot and lively debate. Huge industrial, environmental and eco- nomic interests are at stake.

Pending this debate, it seems reasonable to take the conservative view that when in doubt, one should reduce as much as possible the production of greenhouse gases. Certainly, energy saving measures will be very useful, especially in the more energy-greedy economies. How- ever, they will have restricted impact when one considers the fast development of huge potential energy consumers

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like China, and, probably, India. It appears that the only possiblemassive and economically competitive substitute for fossil fuels is nuclear power, at least for the foreseeable future. A large scale use of fusion energy seems, at best, to be remote.

We are left, therefore, with the need for an increased use of fission power. However, it seems that a large increase in the number of fission reactors will face strong social opposition, due to the fear of major accidents (Tchernobyl effect) as well as a deep concern about the treatment of nuclear wastes. In this context, it appears that the develop- ment of safer and less polluting means of using fission energy may be the key to the dilemna. Hybrid systems based on the Thorium Uranium cycle are quite promising here. Although these concepts are not new (3,4,5,6), it is true that they did not lead to large scale realizations. This was not a consequence of technical impossibilities, but of the lack of economic incentive.

Today, the situation is different, characterized by a reexamination of well-established nuclear policies like fuel recycling and fast neutron reactor breeding. Even if liquid metal fast breeder reactors face a difficult situation, it is clear that a large increase in the use of nuclear energy requires breeding systems. In this respect, hybrid systems

are unbeatable. They would allow a fast move from a Uranium-Plutonium economy to a much less polluting Thorium-Uranium one. They might allow an advanced combustion of Thorium fuel without recycling. They could, also, allow the realization of intrinsically safer systems than those of current reactors.

Curiously enough, nuclear institutions seem to have a rather skeptical stand on hybrid systems. These have been encouraged by physicists like C.D. Bowman at Los Alamos and C . Rubbia at CERN. Is it possible that physicists are again needed, as at the beginning of the nuclear energy era, to develop new concepts, while engineers should be en- couraged to explore new territories?

References 1. F. Carminati et al., Rapport CERN-AT-93-47(ET). 2. C.D. Bowman et al. NIM A320 (1992) 336 and F. Venneri

et al., Los Alamos Report LA-UR-93-752. 3. K. Furukawa et al., Journ. of Nucl. Sci. and Tech. 27

(1990) 1157. 4. A.A. Harms et al., Annuls ofNuc1. Energy 8 (1981) 431. 5. S.O. Schriber, “ZEBRA, the First Stage of an Accelerator

Breeder Program,” AECL report (Sept. 1983). 6. See, for example, M. Mizumoto, 9th Symposium on Accel-

erator Science and Technology, Tsukuba, Japan, p. 1. 7. H.G. MacPherson, Nucl. Sci. and Tech., 90 (1985) 374.

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