The sub-critical core

The sub-critical core of an ADS is constructed in such a way that it does not have enough fissile material to reach criticality. Neutrons are needed as an input. They are obtained from an energetic proton beam impinging on a heavy target (Pb-Bi), through a spallation reaction. The sub-critical core of an ADS acts as an amplifier: it needs a relatively small input in order to generate a much larger output. On average ~ 15 neutrons are produced for each proton.

The energy spectrum of the sub-critical core depends on the application of the core. Nevertheless, a fast neutron spectrum presents various advantages among them a larger excess of neutrons which can be used for minor actinide transmutation, a reduced amount of minor actinide production in the core itself, a better energetic yield for eventual energy production.

The nuclear fuel

The fuel composition and pin geometry can only be varied in a limited range as we want to use existing technology. The design and licensing of new fuels does not comply with MYRRHA's time frame. The reference fuel option is fast reactor MOX fuel.

Core engineering

In the design of the geometry of the core, there are quite some degrees of freedom although certain constraints must be kept in mind. The core must stay sub-critical at all times and ample cooling has to be guaranteed in all conditions in order to prevent damage to the system.

The design assures a large experimental flexibility of irradiation devices with a high degree of core and in-pile management possibilities. The structure must be sufficiently resistant against corrosion and erosion in the liquid heavy metal Pb-Bi environment.

Fuel assembly

MYRRHA core configuration

The sub-criticality level

To achieve inherent safety, the sub-criticality level of the core is set around 0.95. In this way, we have a margin with respect to criticality in normal and accidental conditions.

The power produced

The prime goal of MYRRHA is research and not energy production. This means that the total power should be kept as low as possible since it has large implications on the size and cost of all components and thus on the system as a whole. In order to achieve high flux levels with a limited power, a high core power density is required.

The MYRRHA core

For MYRRHA, it is envisaged to couple a fast spectrum zone where minor actinide transmutation studies, structural material research and ADS fuel studies can be performed, with a thermal spectrum island where radioisotope production, long-lived fission product transmutation research as well as LWR fuel safety studies can be conducted.

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