Accelerator Driven System

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An accelerator-driven system (ADS), accelerator -driven system or hybrid reactor is a power nuclear reactor that is operated subcritically and supplied with neutrons by a spallation source . Such reactors could serve to transmutate long-lived reactor waste at the same time as electricity is generated .

Neutron physical advantages

In order for a reactor to achieve criticality , components with a particularly small generation factor - such as the minor actinides (MA) neptunium , americium and curium - may only form a small admixture with the fuel. However, this limitation does not apply if the reactor works subcritically with an external neutron source, so that the chain reaction does not have to sustain itself. The other disadvantages of MA as nuclear fuel, e.g. B. the low proportion of delayed neutrons and a small Doppler coefficient , play no role in subcritical operation. Such a reactor could generate usable energy with any fissile nuclear fuel; some of this energy would be sufficient to operate the accelerator of the required spallation neutron source.

Due to their long half-lives, the MA present a special problem in waste from nuclear power plants . An accelerator-driven reactor could convert them into faster decaying nuclides ( fission products ) by fission and still supply useful energy.

Concepts

Two concepts have become particularly well known. Charles D. Bowman et al. M. ( LANL ) published a first proposal in 1992 and later an amended and expanded version. An alternative proposal, the energy amplifier (sometimes referred to as the rubbiatron or rubbiator ), comes from a group led by former director of CERN , Carlo Rubbia . The suggestions have served many other research groups as a basis for further considerations.

description

An ADS consists of three main components:

reactor

Bowman concept

Bowman's reactor concept provides for a thermal neutron spectrum because here the fission cross- sections of the easily fissile nuclides are much larger than in a fast spectrum; Nuclides with an even number of neutrons cannot be fissionable with thermal neutrons, but are converted into more easily fissile nuclides through neutron capture - with a likewise large effective cross section. The spallation target is surrounded by a graphite moderator that slows down the neutrons. The fuel flows through holes in the graphite, dissolved in a melt of the salt mixture NaF-ZrF 4 in the manner of a liquid salt reactor . The thermal output of the nuclear fission is thus generated directly in the heat transport medium. A very high heat output can be dissipated in this way. The neutron flux in the reactor is about 2 × 10 14 cm −2 s −1 , similar to that in today's power reactors .

New fuel and carrier salt can be added continuously ( on line ) to the molten salt in a bypass flow, and salt with burn-up products can be removed. According to Bowman, fuel production would be simple: crushed, spent fuel rods from nuclear power plants are converted into fluorine compounds , together with the Zircaloy cladding material, through a chemical process , which are soluble in the salt. The uranium that is still present is automatically separated so that the fuel does not contain any breeding material. No further partitioning is necessary, since fission products should also be transmuted. The continuously withdrawn salt with burn-up products is described as directly disposable.

Rubbia concept

The more conventional Rubbias concept reactor uses solid, metallic fuel in fuel rods. A fast neutron spectrum is used in which all transuranic elements are fissile. The coolant must therefore not have a moderating effect; Liquid lead or, because of the lower melting temperature, a eutectic lead-bismuth alloy is provided. The reactor core surrounds a fuel-free region filled with the coolant, which serves as a spallation target (see below). The heat exchangers are arranged in the same container well above the reactor core. This spatial structure with a large vertical extension enables heat to be dissipated without pumping through passive convection . This is an important safety advantage of the concept. Here too, the neutron flux in the reactor core is around 10 14 cm −2 s −1 .

The solid fuel must be produced by conventional reprocessing of the reactor waste and partitioning of the transuranic elements (plutonium with MA) and processed into fuel elements. It stays in the reactor for about two years and then - due to the limited durability of the fuel rod cladding tubes, built-up neutron poisons and pressure increase due to gaseous fission products - must be replaced and processed again. In order to dismantle the majority of employees, the material has to be recycled up to ten times in this way. To compensate for the gradual burn-off during operation, that is to say by the multiplication factor constant at z. B. 0.95, the fuel thorium is added to the fuel , from which the easily fissile uranium- 233 arises during operation . A risk of abuse in terms of proliferation can be seen in the production of this isotopically pure fission material.

Residual heat removal

Even a nuclear reactor operated subcritically with an external neutron source inevitably generates decay heat after the chain reaction has ended . An ADS reactor will also need facilities for emergency cooling so that this can be discharged non-destructively even in the event of an incident with failure of normal cooling .

Target

Flowing liquid lead is used as the target in both ADS concepts, since this way the high thermal output introduced by the proton beam can be dissipated. In Rubbia's design, the lead or lead-bismuth is also the coolant of the reactor. Protons with an energy of 1 GeV have a range of about 1 m in lead.

The technology of spallation targets made from liquid heavy metal is being developed in various research institutions around the world, including a facility in the Karlsruhe Institute of Technology .

accelerator

At Bowman, the accelerator is a linear accelerator . He could work with superconducting resonators to save energy and reduce the size of the building .

Rubbia provides a linear accelerator as an injector, the beam of which receives its final energy in a subsequent cyclotron . A comparable combination of accelerators has been implemented at the SINQ spallation source in Switzerland; there the proton beam is supplied by two cyclotrons connected in series.

An important requirement for the accelerator of an ADS is high operational reliability, because beam failures immediately lead to lower performance and thus lower temperature in the reactor, which has disadvantages for the durability of the fuel elements and the reactor structure. There are arguments in favor of the linear accelerator.

Test facilities

An ADS test facility is under construction at the J-PARC accelerator center in Japan. It should start operating with fuel containing MA around 2020.

In research Mol in Belgium to MYRRHA (multi-purpose hybrid Research Reactor for High-tech Applications), a planned European demonstration plant, to be built and about 2,030 become operational. Technologically, it is closer to today's reactor types and the Rubbia concept than the Bowman concept. MYRRHA is also intended to replace the older general-purpose BR-2 research reactor and would therefore initially run on U-Pu-MOX fuel, but later also be used for MA transmutation experiments.

literature

  • Ortwin Renn (Ed.): Partitioning and Transmutation - Research, Development, Social Implications. Herbert Utz Verlag, Munich 2014, ISBN 978-3-8316-4380-6 .
  • Ken Nakajima (Ed.): Nuclear Back-end and Transmutation Technology for Waste Disposal. Springer, 2014, ISBN 978-4-431-55110-2 .
  • H. Nifenecker, S. David, JM Loiseaux, A. Giorni: Hybrid nuclear reactors. Progress in Particle and Nuclear Physics. 43, 1999, pp. 683-827.
  • Mikhail K. Khankhasayev (Ed.): Nuclear Methods for Transmutation of Nuclear Waste: Problems, Perspectives, Cooperative Research. In: Proceedings of the International Workshop, Dubna, Russia, 29-31 May 1996. World Scientific, 1997.
  • Wolf Häfele : Accelerator-based nuclear reactors. The proposals of C. Rubbia and CD Bowman. Physik Journal 50, 10/1994, pp. 935-938, doi: 10.1002 / phbl.19940501007 .

Individual evidence

  1. ↑ WT Hering: Applied nuclear physics: introduction and overview. Teubner 1999, ISBN 978-3-519-03244-1 , p. 303.
  2. ^ CD Bowman et al .: Nuclear energy generation and waste transmutation using an accelerator-driven intense thermal neutron source. Nuclear Instruments and Methods A 320, 1992, pp. 336-367.
  3. ↑ a b c C. D. Bowman: Accelerator-driven systems for nuclear waste transmutation. Annual Review of Nuclear and Particle Science 48, 1998, pp. 505-556, staff.ustc.edu.cn (PDF).
  4. ^ F. Carminati, C. Rubbia et al .: An energy amplifier for cleaner and inexhaustible nuclear energy production by a particle beam accelerator. Report CERN / AT / 93-47 (ET) cds.cern.ch (PDF).
  5. ↑ C. Rubbia, JA Rubio, S. Buono et al. a .: Conceptual Design of a fast neutron operated High Power Energy Amplifier. Report CERN-AT-95-44 (ET), Geneva 1995.
  6. ^ Review of national accelerator driven system programs for partitioning and transmutation. Proceedings of an Advisory Group meeting, Taejon, Republic of Korea, November 1999. IAEA-TECDOC-1365, 2003, ISBN 92-0-106803-4 , pub.iaea.org (PDF).
  7. ↑ Renn (see list of literature) p. 107
  8. ↑ KALLA, KArlsruhe Liquid metal LAboratory
  9. ↑ a b A. Mueller, H. Abderrahim: Transmutation of radioactive waste. Physik Journal 11/2010, pp. 33-38.
  10. ^ T. Sasa: Status of J-PARC transmutation experimental facility. 2008, oecd-nea.org (PDF).
  11. ^ T. Sasa: Design of J-PARC transmutation experimental facility. (Status around 2014). In: Nakajima (see bibliography), pp. 73–79.
  12. ↑ MYRRHA Home page ( Memento from February 19, 2015 in the Internet Archive )