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Accelerator-driven nuclear reactor and transmutation

Nuclear Monitor Issue: 
WISE Amsterdam

Guinevere, a first-of-a-kind reactor system has been set up in Belgium by coupling a subcritical assembly with a particle accelerator. The work is heralded as a major step in the program to research advanced radioactive waste management.

Transmutation of long-lived isotopes into short lived ones would simplify the permanent geologic disposal of radioactive waste.

The equipment, Guinevere, is a demonstration model that supports the project for a larger version that will be called Myrrha (Multipurpose Hybrid Research Reactor for High-tech Applications). It was assembled by France's National Centre for Scientific Research and is managed by the Belgian Nuclear Research Centre (SCK-CEN) at Mol, about 50 kilometers east of Antwerp. The overall project is supported by 12 other European laboratories and the European Commission. The research infrastructure for Guinevere was inaugurated on March 4, 2010, at the Belgian Nuclear Research center SCK-CEN in Mol.

Nuclear terminology classifies an item of equipment as in a critical state if the chain fission reaction is self-sustaining and each reaction leads on average to one more. The term supercritical means the number of fissions is increasing, while subcritical means it is decreasing and will therefore dwindle to nothing.

Guinevere is designed to be subcritical if it were not for an accelerator system that sends a constant stream of protons to a target that emits neutrons to trigger fission. According to a SCK-CEN statement, "This type of reactor is very safe because the reactor section relies on a particle accelerator: when it is turned off, the reactor will stop immediately."

As well as this kind of accelerator-driven operation, Guinevere is also capable of 'classic' criticality triggered by a neutron source in the reactor core and maintained by the reactor geometry and operation of its lead cooling system. This mode of operation was 'inaugurated' in February 2011.

Guinevere has "very limited power" and is being used to learn more about the operation and control of this kind of reactor arrangement. The knowledge will be put to use at Guinevere's larger relation, Myhrra.

Myrrha, a flexible fast spectrum research reactor (50-100 MW-th) is conceived as an accelerator driven system (ADS), able to operate in sub-critical and critical modes. It contains a proton accelerator of 600 MeV, a spallation target and a multiplying core with MOX fuel, cooled by liquid lead-bismuth (Pb-Bi).  Myrrha will be operational at full power around 2023. Until 2014 the Front End Engineering Design (FEED), the associated R&D program, the licensing process and the set-up of the international consortium will take place. Construction of the facility and assembly of the components is foreseen in the period 2015-2019. Three years (2020-2022) are foreseen for the full commissioning of the facility. The total investment cost was estimated in 2009 at 960 million euro.

The Belgian government will support 40 % (384 million euro) of the total budget (M€ 960) of which 60 million euro until construction phase in 2014. Almost three years into the project, SCK-CEN is still looking to set up an international consortium to ensure additional financing and, according to a World Nuclear News article, it has completed a memorandum of understanding with the Chinese Academy of Sciences focusing on Myrrha.

Myrrha will be able to produce radioisotopes and doped silicon, but its research functions would be particularly well suited to investigating transmutation. This is when certain radioactive isotopes with long half lives are made to 'catch' a neutron and thereby change into a different isotope that will decay more quickly to a stable form with no radioactivity. If achievable and on an industrial scale, transmutation could greatly simplify the permanent geologic disposal of radioactive waste.

Partitioning and Transmutation 
The purpose foreseen for ADS is the "burning" of transuranic elements, particularly the minor actinides (Neptunium, Americium and Curium) that place severe constrains on geological disposal of nuclear waste, more effectively and more safely than is achievable in critical reactors. The fraction of neutrons which are delayed in the fission of minor actinides is much smaller than for uranium, with the result that control of a critical core comprising mostly minor actinide fuel is expected to be difficult, yet using few largely-minor-actinide-fuelled reactors may prove more advantageous than distributing (transporting) minor actinide fuel throughout the whole reactor fleet. ADS can achieve the required control and safely burn the transuranics in a largely minor-actinide fuelled core.

Although the driver fuel proposed for Myrrha does not contain minor actinides, it would demonstrate the essential features of ADS for the first time, that is, the combination of a high-power proton-beam accelerator, spallation source and a subcritical core. "The scientific and technological value of this demonstration would therefore be very high is successfully achieved", is written in a 2009 'independent' evaluation of the Myrrha project. Well, independent…? The international team of experts only existed of experts from the nuclear sector, who all believe in the necessity of nuclear power and the possibility to fix technological problems with technological solutions.

And even these independent experts admit the technical problems to be overcome are not trivial. A high-power proton-beam accelerator that meets the project reliability requirements has yet to be developed, the techniques for precise positioning and controlled displacement of the proton beam need to be mastered; maintaining a stable, free surface of flowing lead-bismuth liquid target is also necessary: these are all formidable challenges.

It should also be pointed out that there may be a question of timing, of whether the demonstration of ADS will eventually prove necessary within the time frame proposed for Myrrha, as Belgian and/or European fuel cycle and waste management strategies evolve. It should also be kept in mind that the successful demonstration of ADS is only part of partitioning and transmutation (P&T); advanced fuel cycle technology in which substantial amounts of minor actinides are handled and incorporated into new fuel is also necessary. This is still in a very early stage of development.

Even the most elaborate transmutation schemes will leave behind substantial amounts of long-lived radionuclides requiring disposal, while generating large new volumes of operating and decommissioning wastes. Transmutation does not eliminate the need for a high-level waste repository. Waste from prior reprocessing operations, whether for commercial or military purposes, is highly unlikely to be transmuted since almost all of it will have been vitrified for safety reasons before a transmutation program can be put into place. This large amount of waste would have to be sent directly to the repository. In other words, there are fundamental and substantial limitations to the reduction in long-lived radioactivity that can be achieved even with an elaborate and very expensive transmutation program.

All transmutation schemes require reprocessing and separation of transuranic radionuclides. The current use of commercial reprocessing and MOX-fuel, the simpliest of schemes to transmutate a small fraction of existing plutonium, results in the separation of significant quantities of plutonium, which is undesirable from a proliferation standpoint. Transmutation would greatly increase separation of –weapons-usable material and/or the diffusion of technologies that would facilitate such separation. It will thereby considerably increase the risks of proliferation.

Reprocessing, which is required in all transmutation schemes, is one of the most damaging components of the fuel cycle. It results in the discharges of large volumes of waste and radioactive emissions to air and water.

Sources: Nuclear Alchemy, An assessment of Transmutation as a nuclear waste management strategy, Hisham Zerriffi & Annie Makhijani, May 2000 / Independent evaluation of the MYRRHA project, OECD Nuclear Energy Agency, 2009 / Press releases SCK-CEN, 4 & 12 March 2010 /
World Nuclear News, 11 January 2012
Contact: WISE Amsterdam