The accelerator driven system (ADS) demonstration for waste transmutation requires an approach involving many aspects such as advanced fuel and sub assembly options, new reprocessing capabilities and ADS operation and control. The Experimental ADS (XADS) now in its Preliminary Design Stage , will include different design aspects such as reactivity control and core instrumentation in order to demonstrate its operation and safety, and will then go into a transition to transmutation core demonstrator (XADT), which requires among other things its convertibility from conventional to advanced fuels based cores i.e. heavily loaded with minor actinides. Among the devices which require particular attention in an ADS instrumentation is the on-line reactivity measurement. It is critical to the design strategy of an ADS that the system remains subcritical in both normal operation and hypothetical accident conditions. Therefore an accurate on-line reactivity measurement is of prime importance in order to minimize the consequences of hypothetical serious accidents. The Source Jerk (SJ) method , Prompt Neutron Decay method (PND) , the Source Prompt Jump method (SPJ)  and the Harmonic Source (HS) method ,would provide absolute reactivity measurement while the complementary Multiplied Source (MS) method  provides a relative reactivity measurement. The PND and SPJ methods would require monitors situated in or near the core in order to limit spatial effects that distort the measurement.It must be noted that potential solutions to this problem depend on the techniques and the accuracy required. R&D would be required on these particular difficult tasks and in particular other programmes such as the European Union MUSE programme [7,8] or the future TRADE programme  should be able to answer some of the raised questions. Within the PDS-XADS project, which is sponsored by the European Commission, a protocol is being designed to ensure that the reactivity of the system never exceeds a limit which is set by design.