Accelerators for nuclear energy applications

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Radiat. Phys. Chem. Vol. 51, No. 4±6, pp. 645±651, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0969-806X(97)00229-6 0969-806X/98 $19.00 + 0.00

ACCELERATORS FOR NUCLEAR ENERGY APPLICATIONS R. K. BHANDARI* Variable Energy Cyclotron Centre, Department of Atomic Energy, I/AF Bidhan Nagar, Calcutta 700 064, India AbstractÐAccelerators are set to enter into the ®eld of nuclear energy production, o€ering several advantages over conventional nuclear reactors. The subcritical assemblies driven by high beam power accelerators are expected to generate safe, clean and highly ecient nuclear power utilizing the proven ®ssion route. In this paper the advantages of such devices will be discussed. Status of the development of relevant accelerators will be reviewed. A general introduction to the relevant accelerator physics, technology and engineering problems will be presented. # 1998 Elsevier Science Ltd. All rights reserved

INTRODUCTION

Two prominent routes in production of nuclear energy are nuclear ®ssion and thermonuclear fusion; the ®rst represents a well proven technology while the latter shows considerable promise. Accelerators have been playing an important role in the development of both of these routes. For example, right from inception, accelerators have helped to obtain crucial data like cross sections and other constants needed to design the relevant devices like reactors and tokamaks. Thermonuclear fusion will certainly generate very clean and globally acceptable nuclear power, but this option is still somewhat futuristic and involves rather complex technologies. The accelerators for inertial fusion must deliver extremely high power beams in short pulses of several nanoseconds duration for heating the targets to the desired temperatures (Bangerter and Faltens, 1994). Beam quality requirements on such accelerators are extremely stringent and these machines are largely speci®c to fusion applications. For all of these reasons and because of the complexities involved in the building of such accelerators an element of ``reluctance'' has been noted in the development of an accelerator-based thermonuclear fusion approach to nuclear power generation. Considerable amounts of research and development work, however, still continue in several laboratories in the world. The ®ssion route to nuclear power generation in conventional reactors has become somewhat controversial due to environmental and safety reasons. Intense development work is going on at several places to re-introduce con®dence in the ®ssion based nuclear power and to make it globally acceptable and economical. This includes development of new types of reactors. Major objectives are: *To whom all correspondence should be addressed. 645

minimum production of nuclear waste almost 100% inherent safety use of abundant fuels like thorium in an economic way, minimization/avoidance of proliferation of potentially dangerous substances such as plutonium. A very prominent role will be played by accelerators in achieving these objectives. Such a role by accelerators in the nuclear power programme was envisaged about half a century ago when the machines themselves were almost in their infancy stage (Lawrence, 1990). Conceptual designs of linear accelerators (linacs) with several megawatts of beam power were developed at Livermore National Laboratory, Chalk River National Laboratories and Brookhaven National Laboratory. In these cases the beam energy was 0.5 to 1.5 GeV, with beam current of several hundreds milliamperes (mA). The ions were deuterons or protons. Such beam currents have still not been achieved at those energies but feasibility has been established by several groups on the basis of simulations and prototype studies. The applications which were envisaged included transmutation of nuclear waste, fuel breeding and tritium production apart from energy production. The projects were either abandoned or assigned low priority for various reasons. Now these ideas are back in the arena with the current advantages of much better technology and understanding of the physical phenomena involved. Neutrons with extremely high ¯uxes, not easily achievable in reactors, are required for the applications mentioned above. Several accelerator-based spallation neutron sources are operational in the world for condensed matter and allied research. In a way the accelerator-based nuclear power installa-

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tions will be an extension of such facilities. The quest for high beam currents for Radioactive Ion Beam (RIB) facilities for advanced nuclear physics research is another motivation for increasing the accelerated beam currents in many accelerators. Protons are suitable particles for both applications. It may be mentioned here that use of high current deuteron linacs in the energy regime of about 100 MeV has also been investigated for transmutation applications (Blagovolin et al., 1990). Brief details of major applications of accelerators in the nuclear power programme will be described in the following sections. SUBCRITICAL ASSEMBLIES

A subcritical assembly is a reactor-like assembly, but unlike a conventional nuclear reactor in which ®ssion goes on in a chain reaction without supply of external neutrons, the subcritical assembly depends on external neutrons for operation. These neutrons are produced by an accelerator beam when incident on a suitable target located within the assembly. They are mostly the spallation neutrons and as many as 50 neutrons are produced per incident proton of about 1 GeV energy. The neutrons produce ®ssion after slowing down to the suitable energy and are multiplied many fold in the process. They are then available for several purposes like waste transmutation, energy production by ®ssion, fuel breeding etc. Multiplication of the accelerator-produced neutrons by ®ssion in the subcritical assembly is a must for economic operation of the system (Takahashi and Rief, 1994). Subcritical systems have an inherent advantage in that safety is extremely high in their operation. In case of a dangerous situation, the accelerator beam can be quickly withdrawn cutting o€ the supply of external neutrons. As such, the operating power level is instantaneously brought down. This is the most important feature of the accelerator-driven subcritical systems in the context of present day safety demands. TRANSMUTATION OF NUCLEAR WASTE

Nuclear waste from reactors consists of minor actinides and ®ssion products. Minor actinides are long-lived and are generally the isotopes of elements like curium, neptunium, americium etc. The ®ssion products like 90Sr, 99Tc, 129I, 137Cs etc. consist of isotopes with both long as well as short half lives. In accelerator beam transmutation the beam is directed to the target/core assembly which consists of the waste to be transmuted. Depending upon the option of using fast or thermal neutrons for transmutation, the target/core assembly is suitably con®gured (Takahashi and Rief, 1994; Mizumoto, 1994). These assemblies operate in subcritical mode. The accelerator beam incident on a suitable target

in the middle of the assembly produces spallation neutrons. The fast neutron assemblies are more ecient but require a large inventory of the actinides. The advantage, however, is that relatively low accelerator beam current is needed. The disadvantage lies in the fact that tedious fuel reprocessing is involved. Thermal neutron systems require a small actinide inventory but much higher beam accelerator current. However, while the actinides are ®ssioned, the long-lived ®ssion products are transmuted into short-lived ones. In this option much less or no reprocessing is required. Moreover, the physics and technology involved in operation of the thermal neutron systems is rather well understood (Schriber, 1994; Lawrence, 1991; Kapchinskiy et al., 1993; Jameson et al., 1992). Thermal neutron systems are more ecient for transmuting the ®ssion products because the neutron capture cross sections of these isotopes at thermal energies are very high. They are thus eciently converted into stable or short-lived isotopes. At high neutron energies the capture cross sections are low and so the transmutation is not ecient. An important ``byproduct'' of operating these transmutation systems is the thermal energy released by ®ssion during the process. This energy can be converted into electrical energy and fed to the power grid. A fraction of this power is used to operate the accelerator complex. As we shall see later on, these accelerators consume a substantial amount of power, so that the ``byproduct'' energy makes the entire transmutation device economically viable. Several details of both types of waste transmutation and energy production systems, based on accelerators, are given by Takahashi and Rief (1994) and Mizumoto (1994) in their review articles.

ENERGY PRODUCTION

The primary application of accelerators in nuclear power programmes started out by looking at transmutation of nuclear waste and fuel breeding, but over the years it has became quite apparent that they might totally revolutionize the entire nuclear power programme. As is obvious from the relevant studies (Rubbia, 1994; Andriamonje et al., 1995) accelerator-driven subcritical assemblies will turn out to be far more ecient systems for energy production than conventional reactors. The advantages are:almost complete burn-up of the fuel utilization of much more abundant fuels, like 232 Th minimal production of nuclear waste, and that too being rather short-lived no expensive reprocessing requirements (Jameson et al., 1992) virtually no production of dangerous substances like plutonium

Accelerators for nuclear energy applications

Until recently accelerator-based subcritical assemblies had been designed with the primary purpose of waste transmutation, while energy was produced as a ``byproduct''. The entire device is extremely complex as handling of highly radioactive wastes is involved. There exists an added complication of placing the target for production of spallation neutrons in the middle of the assembly. About half a decade ago a group at CERN (Rubbia et al., 1994; Fietier and Mandrillon, 1995a) developed the concept of an accelerator-based device whose primary function is to produce energy, and eventually electricity. In this device fuel breeding and waste transmutation goes on as energy is produced. The accelerator is a complex of cyclotrons with ®nal proton beam energy of 1 GeV. It could alternatively be a linac. The important point is that the average accelerated beam current will be in the range of 10 mA. The entire device, consisting of accelerator and the subcritical assembly, has been named ``Energy Ampli®er'' (EA) because the energy produced by ®ssion is many folds higher than that required to run the accelerator complex. Both thermal as well as fast neutron versions have been investigated (Rubbia, 1994) in extensive studies and experiments. Energy ampli®cation of 40 to 50 for thermal neutron and 100 to 120 for fast neutron versions are expected to be realistically achievable. The phenomenon of energy ampli®cation in such devices has been experimentally veri®ed (Andriamonje et al., 1995). The subcriticality level, ke€ or multiplication factor, is the designers choice depending upon the application. The safety of operation demands that ke€ should be as low as possible, below one. But a very low value of ke€ a€ects the neutron economy which is also very important. A higher value of ke€ leads to higher energy and neutron production in the system. Moreover, higher ke€ relaxes the beam energy and current requirements and thus results in lower cost (Ado et al., 1994). ISOTOPE PRODUCTION

Subcritical assemblies driven by accelerators can also be used to produce neutron-rich radioisotopes for which reactors are used at present. As an instance, 99mTc is a very widely used radioisotope in nuclear medicine. It is supplied in the form of a 99 Mo/99mTc generator from reactor installations. In view of general opposition to nuclear power at present in several countries, production of this isotope may run into serious problems. The accelerator-driven subcritical assemblies o€er an acceptable solution to this problem (Jongen et al., 1995; Clark, 1995). The beam energy need not be as high as that needed for transmutation and energy production applications. The advantage of this system lies in the fact that it can be switched on or o€, conveniently, at will in a very short time. A 150 to

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200 MeV, 1.5 mA cyclotron delivering a proton beam on Pb±Bi target in a subcritical assembly will generate enough neutrons to economically operate the entire complex for 99mTc production. The cost of the facility will be signi®cantly less than a 10 MW isotope production reactor (Jongen et al., 1995). TRITIUM PRODUCTION

This application of accelerators is not of such direct interest to the average person as the applications mentioned above. We discuss it here because reactors are used to produce tritium. The accelerator beam produces spallation neutrons inside a specially designed assembly, as described earlier. Suitable targets like Li or 3He are placed in special enclosures in the assembly. Tritium is produced by the 6Li(n,a)T or 3He(n,p)T reaction and extracted by chemical processing (Lawrence, 1990; Lawrence, 1991). At present tritium production is carried out for strategic applications. Acceleratorbased tritium production programmes are seriously being pursued in the USA. ACCELERATOR CONSIDERATIONS

High current proton accelerators in the energy range 0.8 to 1.6 GeV are suitable for most of the applications under discussion here. The important feature is their capability to deliver cw (continuous wave) beams with several to several hundreds of milliamperes beam current. As discussed, the accelerator complexes are essentially high power spallation neutron sources. Only cyclotrons and linacs have the capability to deliver such high current beams. As a general rule, cyclotrons are less expensive machines as compared to linacs but there is an upper limit on the achievable beam current, say about 10 mA. However, concepts of multistage cyclotrons (parallel) have been developed where beams from three di€erent cyclotrons are stacked in time and are accelerated with the help of a ®nal stage separated sector cyclotron (Takahashi and Rief, 1994). The ®nal beam current may still not exceed 15 to 20 mA. On the other hand, linacs are more expensive machines, primarily due to their radiofrequency (RF) dominated hardware, but conceptual designs exist which can deliver up to 300 mA average beam current. It may be mentioned here that the highest beam power cyclotron presently operational is the PSI separated sector cyclotron at Zurich. It accelerates protons to 0.59 GeV energy with routinely available beam current of 1.5 mA (Schryber et al., 1995). On the linac front, the highest beam power is obtained from the LAMPF accelerator at Los Alamos National Laboratory, accelerating protons to 0.8 GeV energy with 1 mA routinely available average beam cur-

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rent. This is seen as the predecessor of future high beam power linacs. The choice of accelerator will be decided by the type of application, economics and the available technology at the installation. It is quite obvious that in either case several accelerator physics and technology issues need to be addressed very carefully before such accelerators come into existence. Most of the issues are common in nature. Several established groups in the world are working to identify and solve the critical problems. Some of the critical issues will be brie¯y discussed here. SPACE CHARGE EFFECTS

Space charge forces become very important and become the determining factor in the design of very high beam power accelerators. The beam currents are unusually high. Moreover, since the beam is bunched at the operating frequency of the accelerator system, the peak current in the bunch is even higher. The problem of space charge is particularly severe at the low energy end operation of the accelerator. Several space charge models are being investigated depending upon the energy regime the particles are passing through during acceleration. Further, it is necessary to apply non-linear formulations since dealing with linear theory is insuciently accurate. The space charge problem is rather complex to deal with in the ion source extraction region where the electrons are also present (Fietier and Mandrillon, 1995b). It is necessary to use 3D codes to accurately estimate the e€ects. Whether the accelerator ultimately chosen is a cyclotron or a linac, the current trend shows that a Hÿ multicusp source will be used. These sources reliably operate with several milliamperes of Hÿ beam current and with very good beam quality. Linacs are the most suitable accelerators for high intensity applications because it is easier to incorporate sucient external focussing forces to counter the space charge e€ects. Quadrupole magnets can be conveniently placed at several places along the accelerator according to requirement, in a ¯exible manner. The proven con®guration of the linac option consists of the ion source followed by a low energy beam transport system (LEBT), and the sequence of accelerators is radiofrequency quadrupole (RFQ), drift tube linac (DTL) and coupled cavity linac (CCL), also called side-coupled linac (SCL). A more optimized con®guration developed at Los Alamos National Laboratory also uses a new type of linac called the bridge-coupled drift tube linac (BCDTL) between the DTL and the CCL (Lawrence, 1994). There is ``sucient'' space available for placing focussing quadrupole magnets between the accelerating cavities. In order to minimize the space charge e€ects, it is recommended to use high frequency accelerating structures so that for a given average beam current the peak current

in a bunch is reduced (Wangler et al., 1990). Moreover, the cw mode of operation must be followed for the same reason. At the low energy end where space charge e€ects are more severe, the Los Alamos con®guration has two sets of ion source, LEBT, RFQ and DTL. Each set accelerates half of the desired beam current. Beams, upon exiting from the DTLs, are ``funnelled'' into the BCDTL for further acceleration (Lawrence, 1994). Separated sector cyclotrons, as opposed to compact pole cyclotrons, are used for accelerating high power beams due to the inherently strong focussing forces acting in them. Focussing forces in the cyclotrons arise from azimuthal variation in the magnetic ®eld and radial ®eld components generated by introducing spirals in the sectors. This focussing action cannot be increased beyond a limit for several technical reasons. Therefore, an upper limit on the accelerated beam current is imposed (Fietier and Mandrillon, 1995b; Joho, 1981). This limit is well below the limit which is attainable in linacs because much stronger focussing can be provided in the latter case. Longitudinal space charge forces introduce another limiting factor on the accelerated beam current in both cyclotrons and linacs. In view of various ¯exibilities available for shaping and handling the beams, linacs are far superior to cyclotrons for producing high power beams. However, as mentioned earlier, economics may govern the choice of accelerator for a particular type of application. BEAM HALOS

Particles which stray away from the core of the beam during acceleration or transport form halos. These actually represent a very small fraction of the main beam. They travel along with the core beam but do not follow the ``desired'' dynamics. Since very large beam currents are being handled, the halos may contain a substantial number of particles and can seriously activate the accelerator components. The accelerator forms a major part of the commercial/industrial complex, and so regular maintenance is a must. A high level of activation will hinder hands-on maintenance, which is preferably done without the use of sophisticated manipulators. Moreover, the radioactivity may be spread out and be distributed over long distances introducing shielding problems. In order to minimize this problem it is essential to minimize the halos or to eliminate them altogether, if at all possible. The physics of halo formation is still not well understood. Unless the mechanism of halo formation is known in detail, their e€ective control is dicult. Extensive research work has been done by Los Alamos groups in this area (Jameson et al., 1992; Lawrence, 1994). This work particularly relates to linacs and very useful experimental data has been provided by the existing LAMPF linac for the studies. Experience shows

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that halo formation is particularly prominent when transitions during the acceleration process are rather abrupt or sudden. These transitions may be in the RF frequency, structural geometry, transverse focussing, accelerating gradient, accelerating phase etc. (Gluckstern, 1994). Even if the halos are removed by using collimators or other such devices, they almost invariably reappear. The mechanisms of halo formation and space charge forces are intimately coupled and make the problem highly complex. Both have direct impact on the cost of the accelerator as large apertures must be provided to contain the beam to minimize activation of the components. Beam loss must be limited to 0.1 nA/GeV/m to 1 nA/GeV/m depending upon the energy in the particular part of the accelerator (Lawrence, 1994). Larger apertures for the accelerating cavities, focussing elements and diagnostic devices also have serious e€ects on operational costs. Gluckstern (1994) and Jameson (1993) have proposed excellent models for the mechanisms of halo formation and beam loss, being in general, for linacs. It is also very important to control the misalignment of the accelerator components. The beam must also be matched throughout the accelerator to minimize the losses.

tion. In order to minimize activation of the components, the ratio of physical aperture to the beam size must be a safe value. For example, for the Los Alamos design the aperture to rms beam size varies from 13 to 26 in the CCL. In view of these considerations, extensive studies of space charge e€ects, halo formation and general beam dynamics is required for these accelerators. Choice of operating frequency for di€erent accelerators i.e. RFQ, DTL and CCL, is also crucial from the point of view of optimizing the accelerator operation. Maximizing the overall accelerator eciency is also the most important technological issue for the cyclotron option. In this case too the RF system is crucial and consumes most of the input AC power. Unlike in the linacs, the accelerating waveform is not sinusoidal but ¯at-topped to improve the extraction eciency and reduce energy spread. There is a substantial amount of power dissipation also in the magnets of the cyclotron. The cyclotron complex proposed for the ``Energy Ampli®er'' consists of three cyclotrons at di€erent energy stages in tandem to accelerate a 1 GeV proton beam with 10 to 12.5 mA beam current. An overall eciency of about 40% seems possible (Rubbia et al., 1994; Fietier and Mandrillon, 1995c).

TECHNOLOGICAL ISSUES

SUPERCONDUCTING RF

Considerable technological advances are needed to realize high power beam accelerators. These challenges lie in all aspects of the accelerator, whether it is a linac or a cyclotron. Accelerator scientists and technologists have already made considerable progress in many if not all of the areas. The challenges involved are an interesting mix of cost e€ectiveness, reliability and accelerator physics. In order to bring down operational cost of the entire system, it is necessary to maximize the eciency of the accelerator. This is to say that the ratio of available beam power to the input AC power required to run the accelerator must be maximum. In the case of linacs almost all of the e€ort is concentrated in making the RF system more ecient. Generally, the klystrons operating at megawatt power level have an eciency of DC to RF conversion of about 70%. By proper choice of the accelerating structure it is possible to achieve an accelerator eciency of 80 to 90% for RF to beam power conversion. Thus, the overall eciency of the total accelerator system may be about 50% (Lawrence, 1990; Lawrence, 1994). The accelerator eciency, which is also the RF eciency for linacs, strongly depends upon the choice of accelerating gradient and the physical aperture of the accelerating cavities of the CCL. As mentioned earlier, the CCL provides most of the acceleration, e.g. 100 MeV to 1 GeV, to the beam. A large physical aperture results in a large amount of power dissipa-

An obvious solution to reduce power dissipation in the accelerator structures for the linacs is to use superconducting RF cavities. Such cavities for b values corresponding to several hundreds of MeV protons, the beams of interest here, have not received much attention so far with accelerator technologists. However, in view of the importance of the issue, development work has been taken up by some groups (Wangler et al., 1993; Delayen et al., 1993). However, there do not seem to be any show-stoppers. Use of the superconducting RF is highly advantageous as it allows much larger apertures to minimize the beam loss and activation problems. Further, higher accelerating gradients are possible without running into excessive power dissipation complexities. An added advantage is the reduced length of the accelerator which relaxes tolerance on the alignment and o€ers much better stability during operation. The major drawback of the superconducting RF is the technological complexity while very rich experience in construction and operation of conventional linacs exists at several laboratories. Superconducting technology has not yet made much of a mark on cyclotron RF systems. Further, the con®dence in fabricating separated sector type superconducting magnets for cyclotrons has not yet built up. In view of these facts, the cyclotron option to achieve high power beams largely depends upon conventional systems only. However, a superconducting separated orbit type cyclotron for the

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10 MeV injector stage has been considered by one designer group (Fietier and Mandrillon, 1995c). In this accelerator, which has recently become operational (Trinks, 1995), both the magnet and RF systems are superconducting. GENERAL ENGINEERING ISSUES

In their role as key devices in commercial power generating and material production systems, accelerators must follow the general industrial ``discipline'' of reliability, availability, maintainability and inspectability (RAMI) (Lawrence, 1994). To date they have primarily, been the tools of research and development, and so RAMI conditions have been relatively relaxed for operation. The RAMI techniques as applicable to commercial plants demand identi®cation of crucial components where reliability is critical, and sucient redundancy and back ups are needed. A linac, for example, may require ready RF stations for immediate switching in case of failures. Cost considerations may also demand working out rugged designs and developing such operating parameters which make the machine rather insensitive to minor failures. Further, in spite of all out e€orts to minimize the beam loss and activation, the maintenance of accelerator parts may still need special care. The Los Alamos group prefers modular designs, use of pre-aligned units like quadrupoles etc. for easy replacement and provision for remote maintenance in the tunnel housing the accelerator. Special e€orts are also being put to develop radiation resistant materials and components (Liska et al., 1993). SPALLATION TARGETS

The technology of targets for production of spallation neutrons is extremely complex, particularly in view of several tens or hundreds of megawatts of power being dissipated by the beam. Moreover, the target is located inside the subcritical assembly. It is made of heavy metal or alloy. Liquid lead-bismuth (Pb-Bi) alloy is a generally preferred target material. The subcritical assemblies proposed for energy production (Fietier and Mandrillon, 1995a) and isotope production (Jongen et al., 1995) will use this alloy. Solid tungsten targets cooled by heavy water are proposed for the Los Alamos nuclear waste transmutation system using thermal neutrons. The interface region between the beam transport system of the accelerator and the target also requires considerable attention. The beam tube in this region is ®lled with vapours of the target material. The pressure is high in this region and requires di€erential pumping. The beam must be defocussed to minimize power density over the beam spot and to spread out neutron production.

CONCLUSIONS

A signi®cant amount of research and development work has been carried out by several laboratories in connection with high beam power accelerators for nuclear energy applications. The general conclusion is that such accelerators will be possible to build in the near future even at beam powers of one to two orders of magnitudes higher than presently operating machines. However, intense engineering e€ort will be required during their development and construction. Both linac as well as cyclotron options have been investigated by di€erent groups and conceptual designs have evolved. Associated accelerator physics problems have been addressed. Signi®cant amounts of accelerator physics research still needs to be pursued to understand the phenomena of halo formation and the role of space charge e€ects. This research will lead to ecient engineering designs. The advantages of clean and safe nuclear energy, waste transmutation, ecient use of nuclear fuels and isotope production will more than o€set the higher costs involved in building such accelerators. The spin-o€ from their development in the ®elds of materials research and medicine will be enormous. AcknowledgementsÐThe author would like to thank Dr. Bikash Sinha for his encouragement in carrying out this study. REFERENCES

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