  Accelerator-Driven Systems (ADS) could be employed to incinerate minor
actinides and so partly contribute to answer the problem of nuclear waste
management. An ADS consists of the coupling of a subcritical fast reactor to a
particle accelerator via a heavy material spallation target. The on-line
reactivity monitoring of such an ADS is a serious issue regarding its safety.
In order to study the methodology of this monitoring, zero-power
experimentswere undertaken at the GUINEVERE facility within the framework of
the FP6-IP-EUROTRANS programme. Such experiments have been under completion
within the FREYA FP7 project. The GUINEVERE facility is hosted at the SCK-CEN
site in Mol (Belgium). It couples the VENUS-F subcritical fast core with the
GENEPI-3C accelerator. The latter delivers a beam of deuterons, which are
converted into 14-MeV neutrons via fusion reactions on a tritiated target. This
paper presents one of the investigated methods for ADS on-line reactivity
monitoring which has to be validated in the program of the FREYA project. It
describes the results obtained when Pulsed Neutron Source experiments are
analysed using the so called Area Method, in order to estimate the reactivity
of a few sub-critical configurations of the VENUS-F reactor, around keff= 0.96.
First the GUINEVERE facility is described. Then, following general
considerations on the Area method, the results of its application to the
neutron population time decrease spectra measured after a pulse by several
fission chambers spread out over the whole reactor are discussed. Finally the
reactivity values extracted are compared to the static reactivity values
obtained using the Modified Source Multiplication (MSM) method.
