This article has been supplied by SCK•CEN.

Since 1998 SCK•CEN is developing the MYRRHA project as an accelerator driven system (ADS) based on the lead-bismuth eutectic (LBE) as a coolant of the reactor and a material for its spallation target. MYRRHA is a flexible fast-spectrum pool-type research irradiation facility, also serving since the FP5 EURATOM framework as the backbone of the Partitioning & Transmutation (P&T) strategy of the European Commission concerning the ADS development in the third pillar of this strategy.

MYRRHA is proposed to the international community of nuclear energy and nuclear physics as a pan-European large research infrastructure in ESFRI to serve as a multipurpose fast spectrum irradiation facility for various fields of research.

The subcritical core of the MYRRHA reactor (~100 MWth) has to be driven by a 600 MeV proton beam with a maximum intensity of 4 mA. The ADS application requires this beam to be delivered in a continuous regime — the resulting beam power of 2.4 MW classifies the driver machine as a High Power Proton Accelerator.

SCK CEN

Already in the early design phase of MYRRHA the choice for linac has been endorsed, motivated to a large extent by the unprecedented reliability requirements.

The design of the MYRRHA linac has been conducted   through an intense European collaborative effort and supported by several consecutive Euratom FP’s. Today the design effort is pursued under the H2020 MYRTE project complemented by several bilateral collaboration agreements.

The MYRRHA linac consists of 2 fundamental entities: (i) the injector and (ii) the main linac. The injector is fully normal conducting and brings the proton beam from the source through a 4-rod RFQ followed by a series of CH-type multigap cavities to 17 MeV.

A MEBT line matches the beam into the main linac, which is fully superconducting and operated at 2K.

2 families of spoke cavities prepare the beam for final acceleration in a sequence of 5-cell elliptical cavities. The 600 MeV proton beam is then transported through an achromatic line for vertical injection from above into the reactor. A beam window centered in the subcritical core closes the line.

MYRRHA

A specific requirement for ADS application is the high level of the proton beam reliability, in other words the absence of unwanted beam trips. In the case of MYRRHA it is defined as follows: during a 3-months operational period the number of beam trips longer than 3s should be limited to 10. Shorter beam trips, on the other hand, are tolerated in large numbers. It has been acknowledged from the early design stage that such a level of availability/reliability clearly requires a coherent approach to all accelerator components, but also that it compels to implement a global fault tolerant concept.

This has since been confirmed by extensive reliability modeling. The final design of the linac introduces the possibility of fault tolerance at the level of the superconducting cavities through conservative nominal conditions on beam dynamics and on cavity set points.

A similar fault tolerant concept is applied in the solid state RF amplifiers, which may therefore continue feeding the accelerating cavities even in case of failing components. However, such a scheme, based on redundancy from modularity, may not be applied in the injector. Fault tolerance is then recovered by a mere duplication of the injector: 1 active, 1 hot standby.

The phased approach of MYRRHA will primarily concentrate on its linac limited to 100 MeV (first spoke family), albeit with 1 injector only. This installation will be a relevantly sized test platform of various fault tolerance mechanisms, and thereby it will allow for a thorough investigation and extrapolation of the realistic capabilities of the full size 600 MeV linac.

www.sckcen.be/MYRRHA