MYRRHA accelerator: Reliability

The level of reliability required for an ADS accelerator is closely related to the number of permitted beam trips. Frequent occurring trips may damage fuel rods, the target and the core structure and reduce the overall availability of the machine. In general, short beam trips (<1 s) may be tolerated by the subcritical reactor due to its thermal inertia. The number of allowable interruptions longer than one second depends on the technological details of the target and the reactor but are of the order of 5-10 per year. In nuclear research accelerators, where reliability is not a high priority, these numbers are mostly exceeded by some orders of magnitude. Clearly, due care is required to improve on this situation.

In order to improve the reliability of the accelerator, some strategic principles are adopted:

Careful component tests and strict acceptance criteria
Clearly, each individual component must be tested very carefully and assured of the highest quality. Use of advanced control systems with fast electronics. These must ensure prompt detection of beam trips and start fast recovery actions for the components concerned. In this way a number of short and medium duration beam trips could be moved in the millisecond range.

Derating of critical components in the sense that they need to operate below their nominal specifications to increase their stability and "time between failure".

Component redundancy and spares on line
Redundant components must be included in the design which can take over the function of the failing components. The nature of the component connection strongly affects the system reliability and needs to be carefully investigated.

Fault tolerance
The accelerator design must be able to function with some faulty components while still meeting nominal operation criteria in terms of energy, current and stability. Automatic procedures need to certify instant detection of these components and the subsequent beam retune within a short period. The faulty components will only be replaced during planned breaks.