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Controlling the reactivity of a nuclear reactor requires the knowledge of its kinetic parameters. Even though they can be computed by neutronic codes, they often have to be measured to finish the commissioning of a new installation. Amongst the list of kinetic parameters, we are specifically interested in the prompt-decay constant ($\alpha_p$). In practice, the presence of a reflector generates a decay constant, which will differ from core's $\alpha_p$ , and that might disturb the measurement of the whole set of kinetic parameters. Such a behaviour is a drawback for experimentalists since neutron detectors are usually located in non-fissile areas. The aim of the study is to give numerical estimates of the bias that can come out when measuring kinetic parameters of a light-water nuclear reactor. The work is based on on time-impulse techniques to derive kinetic parameters from the flux decay of the system. The impact of the detector position, as well as the reactivity level have been studied in a concurrent way. As an example, it was shown that $\alpha_p$ could be underestimated by a factor ranging from 8 to 12 % in a case of a sub-criticality of −3600 pcm, if the detector was located in the core or the reflector, respectively.

International audience; For some years now, CEA, namely the French Alternative Energies and Atomic Energy Commission, leads researches and developments to improve safety and availability of the GEN-IV Sodium-cooled Fast Reactors (SFR). One of these items of development concerns the core control and monitoring of the SFR Instrumentation and control system (IandC system). The Core Control and Monitoring System (CCMS) must be able to operate in all core conditions. The CCMS involves measurements, estimations and tracking of core control and monitoring parameters. It also commands the core control actions performed by the neutron absorber rods and primary system pumps. In this way, the CCMS uses all the means available to keep the operator informed of the state of the core components and core operation, while being able to detect events that could lead to the deterioration of core components. The measurements system of the CCMS comprises five main sub-systems devoted to take continuous measurements to insure a high level of both nuclear safety and power plant availability. Those five measurements sub-systems are the Neutronic Measurement System (NMS), the Thermal Measurement System (TMS), the Hydraulic Measurement System (HMS), the cladding Leaktightness Measurement System (LMS) and the core Geometry Measurement System (GMS). The five measurements sub-systems following features can be distinguished-NMS provides neutronic indications such as neutronic power and reactivity and relies on in-vessel high-temperature fission chambers. -TMS provides thermal indications such as inlet core or outlet subassembies coolant temperatures and relies on short response time thermal devices. -HMS provides hydraulic indications such as inlet core or outlet subassembies coolant flow rate and relies on miniaturized eddy current flowmeters. -LMS provides fuel cladding leaktightness defect indications (cladding rupture) and fuel cladding leaktightness defect location indications (ruptured sub-assembly). -GMS provides dynamic core geometry deformation indications (abnormal movements or vibrations of a fuel sub-assembly) and relies on high temperature ultrasonic transducers, high temperature strain gauges and accelerometer.This paper describes the definition and design of three of these five measurements sub-systems (TMS, HMS and GMS) which has been conducted in accordance to the methodology of systems engineering for the basic design of an innovative GEN-IV nuclear-fission power demonstrator called ASTRID. Moreover, in order to comply with their key operational requirements, the measurement sub-systems must be reliable and maintainable, that means capable respectively of achieving a continuous service and undergoing modifications and repairs. These dependability requirements completed with other operating safety requirements such as the inability of leading to damages to other vessel components (called innocuity in systems engineering). Then furthermore, this paper highlights how the measurements system development fulfils all the key GEN IV SFR requirements.

International audience; The neutron flux monitoring system of the French sodium-cooled fast reactor will rely on fission chambers that permit both reactivity control and power level monitoring from startup to full power. They are installed inside the reactor vessel, putting severe constraints on the detector design to ensure its dependability. In this paper, we present the Photonis high-temperature fission chambers (HTFC) featuring wide-range flux monitoring capability and justify their specifications with the use of simulation and experimental results. We show that the HTFC dependability is enhanced thanks to a robust physical design. In order to satisfy the requirement of wide-range capability, we propose to estimate the count rate of a HTFC using the third-order cumulant of its signal. The use of this cumulant can be seen as an extension of the so-called Campbelling mode, based on the variance, hence the name high order Campbelling method (HOC).

ISBN: 978-0-89448-726-2; International audience; Fission chambers are the detector of choice for online, in-vessel neutron flux measurements. One of their interesting features is their ability to be operated in different modes, depending on their counting rate. This open the possibility, provided that the acquisition chain is accordingly designed and that the chamber specifications are wisely chosen, to monitor the neutron flux over a wide dynamics of reactor power with the same detector.This paper studies the case of a fission chamber alongside with a preliminary design of the foreseen ASTRID demonstrator. The quantities of interest, namely the fission rate, saturation curve, mean current, pulse shape, spectral density, are computed with a combined usage of the various modelling tools our team has developed within a decade. It is shown that the fission chamber can deliver a signal that is easy to interpret over seven decades of reactor power.