XX International Linac Conference, Monterey, California Why Accelerator-Driven Transmutation of Wastes Enables Future Nuclear Power? W. Gudowski, Royal Institute of Technology, 100 44 Stockholm, Sweden Abstract. resulted in today’s opposition against nuclear power, some of them can be effectively addressed by a Criticality concerns, decay heat management and successful combination of nuclear and accelerator radioactive waste handling are perceived as the primary, technologies. These hybrid systems, commonly called unsatisfactorily resolved technological problems of Accelerator-Driven Systems (ADS) or Accelerator- nuclear reactors. They all originate from very specific Driven Transmutation of Wastes (ATW), integrate a features of a fission phenomenon: self-sustained chain subcritical reactor core, i.e. a fissile material assembly reaction in fissile materials, very strong radioactivity of unable to support a self-sustained chain reaction, with fission products and very long half-life of some of the an intense spallation neutron source driven by a radioactive fission and activation products powerful particle accelerator. This intense neutron Accelerator-driven transmutation systems , which source supports the desired fission reaction rate in a operate in a subcritical mode and stay subcritical, fis sile assembly taking advantage of the finite neutron regardless of the beam being on or off, can in principle multiplication capabilities of this assembly. address the safety issues associated with criticality, The basic goal of ATW is reduction of hazards particularly for advanced fuel containing a high fraction related to handling and management of radioactive of minor actinides. Subcriticality can also improve the wastes through nuclear transmutation and improvement controllability of this nuclear system through a simple of operational safety of nuclear power facilities. electronic control of the accelerator. Subcriticality When coupled with the spallation process, high provides also substantial flexibility in fuel processing power accelerators can be used for an effective and managing. Accelerator-driven transmutation transmutation. Typically, several tens of neutrons will systems can accept such fuels that would be impossible be produced from each proton colliding with the target. or difficult to use in critical reactors, and can extend This means that a reasonable beam of protons (for their cycle length ensuring good transmutation example 5-10 mA at 1 GeV of proton energy) can performance. Moreover, an advanced subcritical core produce a large number of neutrons per unit of time - design can also address some concerns of decay heat see Fig. 1 [2]. The typical spectrum of neutrons management. emerged in spallation processes is presented on Fig. 2. However, a significant development of accelerator This neutron spectrum is not very different from a technology has to be achieved before a construction of typical fission neutron spectrum, having the most the first industrial ATW facility can be realized. The neutrons emerging with energies between 1 and 2 MeV. high-intensity accelerator with a beam power in the However, one can observe a very distinct difference – a range of 10-100 MW has to be available with the tail of high-energy neutrons (with a yield of about 10 stability, efficiency, reliability, operability and %) over 20 MeV reaching the maximum energy equal maintainability features never demanded before from to energy of the incident protons. the accelerator technology. 2 TRANSMUTATION PROCESSES 1 INTRODUCTION Nuclear transmutation can be practically induced by This review article due to the scope of the any particles or quanta enabled to penetrate nuclei and LINAC2000 conference does not cover all of the to interact with nucleons. However, charged particles important aspects of Accelerator-driven Transmutation have to pass through a Coulomb barrier, which requires of Waste (omitting for example reprocessing chemistry) high energies and it is an energetically costly and and has limited reference list. For more complete ineffective process. g - quanta on the other hand, have references look into [1] relatively small cross sections for transmutation Nuclear reactors based on self-sustained fission reactions - like (g,n) reactions – and moreover there are reactions, or so called “critical” reactors - after a no monoenergetic g-sources, making g-transmutation spectacular development in fifties and sixties of the 20th energetically very ineffective. The most effective century, that resulted in deployment of over 400 nuclear nuclear process that can be used for transmutation of power reactors - are today wrestling more with public radiotoxic isotopes is doubtless neutron absorption. acceptance than with irresolvable technological Neutrons are not repelled by nuclei and interaction cross problems. In a whole spectrum of reasons, which FR202 1038 XX International Linac Conference, Monterey, California 50 000 years), 242Pu, 237Np and long-lived fission products 129I, 135Cs and 99Tc. Plutonium and other actinides have 40 a very low mobility in geological environment, so they do not easily enter the biosphere. On the contrary Iodine, Cesium and Technetium, being much more 30 mobile can leak the geological repository. Proliferation concern is another strong argument for transmutation of 20 actinides, particularly Plutonium. Increasing a Pb worldwide stockpile of Plutonium in spent reactor fuel 10 W must be of concern, above all in few hundred-year 238U perspective when a “protective” barrier of radioactivity of short-lived fission product will decay out. 0 0 1 2 3 4 5 Fortunately, most of these isotopes of concern can be Secondary neutrons per incident proton/ GeV PROTON ENERGY, GeV effectively transmu ted. Virtually every transuranic elements, Np, Pu, Am and Figure 1. Number of neutrons per incident proton and Cm can be fissioned by one or few successive neutron its energy (GeV) produced in a spallation processes in absorptions with, in many cases, energy surplus, different thick targets. neutron gain and transmutation of transuranic elements into fission products. All transuranic isotopes are net sections for many transmutation reactions are neutron producers in fissions induced by fast neutrons sufficiently large. (so called fast spectrum fissions). In thermal neutron A final measure of transmutation efficiency, or its spectrum corresponding to Light Water Reactors, only “figure of merit” is not a trivial issue as long as a final 239Pu, 241Pu and Cm-isotopes are “unconditional” criterion for hazards related to radioactive waste is not neutron producers, other transuranic isotopes like 237Np, well defined. Therefore, reduction of source 238Pu and 241Am may become neutron producers in a radiotoxicity of the nuclear wastes seems to be the very high neutron flux. In these cases beta-decay of the least controversial reference goal for transmutation of intermediate neutron capture products competes with a radioactive wastes. fission probability. In a short time perspective, like 100 years, fission Transmutation of fission products through neutron products 90Sr and 137Cs, and Pu – isotopes, dominate absorption is also possible for the long-lived radiotoxic radiotoxicity of spent fuel. 90Sr and 137Cs belong to isotopes like 99Tc and 129I, converting them into stable short-lived fission products and are of big concern in a Ru and Xe, respectively. However, transmutation of case of nuclear accident. They can however, be readily fission products, in contrary to transmutation of retained in storage facilities for reasonable periods to transuranic isotopes, is a purely neutron consuming minimize their threat to the human environment. process and requires excess of neutrons. This surplus of Neither 90Sr nor 137Cs can be effectively transmuted by neutrons can be obtained in different ways: neutron absorption. In critical reactors, which can be designed as In the long time, comparable with life-time of “burners”, in order to use all available neutrons for containers in geological repositories, the radiotoxicity is transmutation processes. Only reactors with an excellent determined by transuranic elements: 239Pu (up to 100 neutron economy can be burners, which limits the choice to fast reactors with the hardest possible neutron energy spectrum, revival of heavy water moderated reactors or use of highly enriched fuel in standard 3 on 2 t LWRs. Neither of these choices is very probable today. o r p -4 t 10 Moreover, criticality conditions, dependence of safe n e d i c reactor operation on a delayed neutron fraction and n i 3 negative temperature feedbacks put severe constraints nd 2 a 2 -5 m 10 on the possible use of critical reactors; c r e In subcritical systems driven by an intense external p s 3 on r source of neutrons – in ATW. An external neutron t 2 u e N source and subcritical operation open new possibilities 10-6 for transmutation. 10-4 10-3 10-2 10-1 100 101 102 Energy (MeV) 3 ACCELERATOR-DRIVEN Figure 2. Energy spectrum of spallation neutrons TRANSMUTATION produced by a 1 GeV proton beam. The main components of ADS are a high-intensity FR202 1039 XX International Linac Conference, Monterey, California accelerator delivering a particle beam of 5 to 40 MW moderators in ATW, mainly due to the large gradients power, a transmuter - a sub-critical reactor with in power density. In subcritical systems power density spallation source, and chemical reprocessing – see Fig. varies in space as exponential function not as cosine or 3. Bessel functions