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Spallation target
Reliable nuclear data and codes for the intermediate energy region are required for the design
of an ADS. At present, most of the data and codes are available for an approximate evaluation of
conceptual designs and for feasibility studies. Detailed designs will need much higher accuracy. If a
±2% accuracy on the system’s energy balance is required, the spallation neutron yield should be
calculated with an accuracy of ±2%. Uncertainties still seem large in predicting the spallation product
yields and the high energy component of the neutron spectrum to evaluate activation and damage in
materials.
Injection of the intense proton beam into the target causes high fluxes of protons and fast
neutrons in the beam window, target, and wall material surrounding the target. These, particularly for
the beam window, suffer irradiation damage and are degraded in mechanical properties and dimensional
stability. The exposure of the materials to high fluxes and energies would be more severe than in normal
reactors. Research on the interaction between high-energy proton and neutron beam and window as well
as structural materials is required.
An intense proton beam deposits heat in the target. Heat removal requirements for the target
are essentially identical to those for the fuel.
Subcritical core
A subcritical core can be very similar in principle to a critical core except that the effective
neutron multiplication factor is less than unity. A subcritical core cooled by liquid metal can fully utilise
existing LMFR technologies.
Subcritical operation provides great freedom in design and operation. Criticality in a
conventional reactor imposes tight constraints on the fuel specifications and cycle length.
Accelerator-driven systems can accept fuels that would be impossible or difficult to use in critical
reactors, and can extend their cycle length if necessary.
Trips and fluctuations of the incident proton beam are inevitable, causing thermal shocks in
the core components. The design must take this into consideration; power distribution, effective neutron
multiplication factor, the neutron flux shape transient response and the size of the system.
Safety features [98]
The subcriticality of an ADS has clear safety advantages for severe reactivity accidents. It can
cope with fast ramp rate accidents which could occur too rapidly for scram systems in critical reactors.
A margin to accommodate fast reactivity insertions is important to avoid super-criticality accidents.
The consequences of cooling failure for ADSs are similar to critical reactors. A reliable beam
shut-off system is, therefore, required for an ADS, just as a reliable scram system is required for a
critical reactor. A reliable emergency decay heat removal system is required for both.
Performance assessment of accelerator-driven systems
Over the past few years a number of different ADS concepts have been developed. For the
purpose of illustration, the performance of an ADS described in References 102 and 103 is discussed
here. The objective is to incinerate Np, Am and Cm and transmute 99 Tc and 129 I in spent LWR fuel.
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