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MYRRHA: A Flexible Fast-Spectrum Irradiation Facility

MYRRHA (Multi-purpose hYbrid Research Reactor for High-tech Applications) is the flexible experimental accelerator-driven system (ADS) in development at SCK•CEN. MYRRHA is able to work both in subcritical (ADS) and in critical mode. In this way, MYRRHA targets the following applications catalogue:

• To demonstrate the ADS full concept by coupling the three components (accelerator, spallation target, and subcritical reactor) at reasonable power level (50– 100 MWth) to allow operation feedback, scalable to an industrial demonstrator;

• To allow the study of the efficient technological transmutation of high-level nuclear waste, in particular, minor actinides that would require high fast flux intensity (Φ>0.75MeV ¼ 1015 n/cm2 s);

• To be operated as a flexible fast-spectrum irradiation facility allowing for

– Fuel developments for innovative reactor systems, which need irradiation rigs with a representative flux spectrum, a representative irradiation temperature, and high total flux levels (Φtot ¼ 5• 1014 to 1015 n/cm2 s); the main target will be fast-spectrum GEN IV systems, which require fast-spectrum conditions;

– Material developments for GEN IV systems, which need large irradiation volumes with high uniform fast flux level (Φ>1 MeV ¼ 1~5• 1014 n/cm2 s) in

various irradiation positions, representative irradiation temperature, and representative neutron spectrum conditions; the main target will be fastspectrum GEN IV systems;

– Material developments for fusion reactors, which need also large irradiation volumes with high constant fast flux level (Φ>1 MeV ¼ 1~ 5• 1014 n/cm2 s), a representative irradiation temperature, and a representative ratio appm He/dpa(Fe) ¼ 10;

– Radioisotope production for medical and industrial applications by

• Holding a backup role for classical medical radioisotopes;

• Focusing on R&D and production of radioisotopes requiring very high thermal flux levels (Φthermal ¼ 2 to 3 • 1015 n/cm2 s) because of doublecapture reactions;

– Industrial applications, such as Si-doping, need a thermal flux level depending on the desired irradiation time: for a flux level Φthermal ¼ 1013 n/ cm2 s, an irradiation time in the order of days is needed, and for a flux level of Φthermal ¼ 1014 n/cm2 s, an irradiation time in the order of hours is needed to obtain the required specifications.

Further in this section, we discuss some basic characteristics of the accelerator and of the core and primary system design.

The MYRRHA Accelerator

The accelerator is the driver of MYRRHA because it provides the high-energy protons that are used in the spallation target to create neutrons, which in turn feed the core. In the current design of MYRRHA, the machine must be able to provide a proton beam with energy of 600 MeV and an average beam current of 3.2 mA. The beam is delivered to the core in continuous wave (CW) mode. Once per second, the beam is shut off for 200 μs so that accurate on-line measurements and monitoring of the subcriticality of the reactor can take place. The beam is delivered to the core from above through a beam window.

Accelerator availability is a crucial issue for the operation of the ADS. A high availability is expressed by a long mean time between failure (MTBF), which is commonly obtained by a combination of overdesign and redundancy. In addition to these two strategies, fault tolerance must be implemented to obtain the required MTBF. Fault tolerance will allow the accelerator to recover the beam within a beam trip duration tolerance after failure of a single component. In the MYRRHA case, the beam trip duration tolerance is 3 s. Within an operational period of MYRRHA, the number of allowed beam trips exceeding 3 s must remain under 10. Shorter beam trips are allowed without limitations. The combination of redundancy and fault tolerance should allow obtaining a MTBF value in excess of 250 h.

At present, proton accelerators with megawatt-level beam power in CW mode only exist in two basic concepts: sector-focused cyclotrons and linear accelerators (linacs). Cyclotrons are an attractive option with respect to construction costs, but they do not have any modularity, which means that a fault tolerance scheme cannot be implemented. Also, an upgrade of its beam energy and intensity for industrial application presently is not a realistic option. A linear accelerator, especially if made superconducting, has the potential for implementing a fault tolerance scheme and offers a high modularity, resulting in the possibility to recover the beam within a short time and increasing the beam energy and intensity toward industrial application of ADS technology.

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