Reactivity monitoring in ADS, application to the Myrrha ADS project

Monitoring of reactivity in an ADS should be performed on-line with a simple, accurate and robust technique. Within the range of experimental reactor techniques, no single technique can be selected which meet these requirements. Therefore a combination of different techniques has to be chosen in a way that various off-line techniques serve as a calibration method for the on-line measurement technique. As an on-line measurement technique, the current-to-flux reactivity indicator is the most simple and robust solution. The current-to-flux reactivity indicator is based on the fact that in a sub-critical multiplying medium with a driving source the flux level is proportional to the driving source intensity, hence the beam current, and the reactivity level. However, since the proportionality constant depends on a number of core-dependent parameters and detector characteristics, this current-to-flux indicator has to be calibrated on a regular basis. For this calibration, one could benefit from the occurrence of accelerator beam trips to determine the reactivity level in dollars by means of a prompt jump analysis of the flux level change. Hence, the prompt jump reactivity indicator could act as a first calibration tool of the current-to-flux indicator. Since the prompt jump indicator still relies on the value for the effective delayed neutron fraction to determine reactivity level, complementary techniques have to be used to obtain a more accurate determination of the reactivity. Techniques based on reactor noise methods such as the RAPJA-technique which is a combination of the Rossi-Alpha method and a Prompt Jump Analysis can be used in this respect. In the future the bi-spectral ratio from the Cf-source driven noise analysis could be used for this purpose.