Switching Off the Accelerator Beam

If the accelerator beam is switched off, the external spallation source will turn off and the reactor will go sub-critical with the power at decay heat levels. In a critical reactor, shutdown is achieved via the mechanical insertion of control rods. In both cases there is a delay from the trip signals for shutting off the beam or for activating the control rod release, which might be of the order of 0.5 s. However, overall, the time to switch off the current to the accelerator would be much faster than the control insertion time in a critical reactor, e. g. 1.5-3 s in a PWR (a little faster for a fast reactor with a smaller core).

A number of different beam shut-off systems are being considered. Diverse trip signals are necessary that result in beam shut-off. Since shut-off is important in cooling failure accidents, the current could be coupled to that driving the coolant pumps on the various cooling loops or on the feedwater pumps. Other passive means involve dropping the spallation target. This could be achieved by supporting with a low melting point metallic structure, which would melt in the event of sufficient temperature increase. Another could be via a magnetic structure, which would drop the target once the Curie temperature is reached. Other methods include deflecting the proton beam or, as in the ADS Rubbia design, by interrupting the beam by the rising lead level in the event of a cooling failure.

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