Subcriticality-Level Evaluation in Accelerator-Driven Systems by Harmonic Modulation of the External Source

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 sub-criticality margin during the ADS operation life and the related comparison between 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 break-off, 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 kinetic approximation is used to test the method by numerical simulations. To take into account spatial effects, a multi-mode 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.