Accelerator-driven systems (ADS) for nuclear waste transmutation will periodically need absolutely calibrated measurements of the subcriticality level. These measurements will allow both the check of the subcriticality margin during the ADS operation life and the related comparison between the experimental and foreseen fuel cycle reactivity effect. Until now, the possibility of absolute (source-jerk-type) reactivity measurements, to be made just after external source breakoff, has been mentioned in ADS proposals.

Complementary reactivity measurement techniques are investigated to analyze the ADS response to harmonic modulation of the external neutron source. The point-kinetics approximation is used to test the method by numerical simulations. To take into account spatial effects, a multimode case is finally investigated. Results show that in the case of ADS subcritical systems, the theory of an absolute "reactimeter" may be formulated. The need for explicit transport calculations, before any possible experimental validation, is also mentioned.