Abstract : Accelerator-driven systems (ADS) may allow the transmutation of the
most radiotoxic nuclear waste. They consist of the coupling of an intense
high-energy proton beam, hitting a high atomic number target, and a
sub-critical reactor core. For safety reasons, an on-line accurate and robust
core reactivity monitoring is mandatory. The beam current delivered by the
accelerator and the power level, or neutron flux, of the reactor core are
strongly correlated through a proportionality relationship which has to be
investigated, since, among different techniques, it could give access to any
reactivity change.
To demonstrate the feasibility of such an on-line reactivity monitoring, an
experimental program is planned at the YALINA facility, in Byelorussia, in
the framework of the EUROTRANS Integrated Project (6th FP). At this
sub-critical installation, the incident 14 MeV neutron flux is produced by a
deuteron beam impinging on a 3H target. Due to the consumption of the 3H
target, the deuteron beam current will not remain proportional to the neutron
production-rate over time. Therefore, in order to monitor the neutron
production rate, we developed a new detector device. It is composed of a
thin CH2 foil, followed by three Si detectors. This telescope will be installed
a few meters downstream the 3H target, at 0°. The detection method is based
on the conversion of neutrons into recoiling protons, which are then
detected by the three Si. Their thicknesses and thresholds have been chosen
so that the most energetic protons, associated with the 14 MeV neutrons, are
stopped in the last stage of the telescope. Requiring triple coincidences in
the telescope enable then to select events originating only from neutrons
produced in the d+3H reactions. Doing so, the correlation between a change
of the source intensity and the flux will be kept and the proportionality
constant can be investigated and determined (via a calibration) and therefore
used to detect any reactivity change of the sub-critical medium.
In this contribution, we will present the performances of this new neutron
beam monitoring device. They were studied during a test experiment done at
the GENEPI neutron source of the PEREN installation, in France.
Comparisons with simulations will also be presented.
