First for accelerator-driven nuclear reactor

A first-of-a-kind reactor system has been set up in Belgium by coupling a subcritical assembly with a particle accelerator. The equipment, known as 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). The overall project is supported by 12 other European laboratories and the European Commission.

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. SCK-CEN said, “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, which should begin operation in 2023.

MYRRHA: Multi-purpose hybrid research reactor for high-tech applications project pages

MYRRHA, a flexible fast spectrum research reactor (50-100 MWth) 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. During the 2010-2014 period the following items will be accomplished:

* The Front End Engineering Design (FEED) and the associated R&D programme.
* The licensing process.
* The set-up of the international consortium.

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 is currently estimated at M€ 960 (€ 2009).

The total cost of Myrrha has been put at €960 million ($1.2 billion), with 40% of this coming from the Belgian government. SCK-CEN is looking to set up an international consortium to ensure additional financing and 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 on an industrial scale, transmutation could greatly simplify the permanent geologic disposal of radioactive waste. Myrrha can also be used to test the feasibility of lead fast reactor technology and is seen as complimentary to the Jules Horowitz Reactor, a thermal spectrum reactor under construction in Cadarache, France.

Types of Accelerators

The proton accelerator is the driver of the ADS (accelerator driven system). It provides the high energy protons that are used in the spallation target to create neutrons which in their turn feed the sub-critical core.

3 fundamental types of particle accelerators

Single gap accelerators: In the context of MYRRHA these are discarded because the energy they can reach is far too low.

Recirculating accelerators: The particles are repeatedly accelerated by successive passages through the same accelerating cavities. In order to achieve this, the particle beam has to be bent after each acceleration and brought back to the entry of the cavity. Bending is usually obtained by magnetic fields. Higher energy is translated into a larger diameter machine due to the larger bending radius. Typical examples of recirculating machines are synchrotrons (having a constant orbit) and cyclotrons (having constant bending field).

Linear accelerators: The particles are repeatedly accelerated by successive single passages through many subsequent accelerating cavities. There is no need for bending. The cavities are organized as a linear chain, hence the name Linear Accelerator or Linac. Higher energy is translated into a longer machine due to the need for more cavities.

2 types of accelerator for MYRRHA – ADS

For MYRRHA (or ADS in general) a high intensity CW beam is required. Practically 2 types of accelerator can provide this:

The isochronous cyclotron: Like any cyclotron it is a recirculating machine with constant magnetic field, but in which the magnetic field is shaped in such a way that the particle’s revolution time is constant with energy (even if relativistic effects appear). Thereby it is also a constant frequency machine, which makes it CW compatible.

The Continuous Wave linac: For technological and functional reasons, most linacs operate as pulsed machines. However, fundamentally the linac satisfies the double condition of fixed frequency and fixed fields, as a consequence of a frozen particle velocity profile along the accelerator. It is therefore a steady state machine, and CW compatible.

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