In this article, we evaluate how the use of threshold fission chambers (FCs) affects the performance of the current-to-flux (CTF) technique being considered for accelerator-driven system (ADS) reactivity monitoring. These detectors are envisaged because of their longer service life in the lead-bismuth eutectic reactor of the MYRRHA ADS. For this purpose, we study an extract of the new CTF experiments carried out at the GUINEVERE facility, equipped with FCs featuring 235U or 238U deposits. The extract selected for its representativeness is about the measurement of the change in subcriticality of the VENUS-F reactor triggered by the removal of one of the control rods (CRs) from its core. The results show that raw CTF reactivity values depend on the composition of the FC used and on its relative position to the CR moved. The space-energy effects, which distort the reactivity values, are understood with the help of simulations computed with the Monte Carlo neutron transport code MCNP. Based on simulated fluxes and detector count rates at all assembly positions, we investigate the physics mechanisms of absorption and diffusion by the boron carbide filling the CRs, which wear away the flux around the CRs. The amplitude of the phenomenon depends on the neutron energy and the position relative to the CR and consequently affects differently the FCs with or without a fission threshold placed close to the neutron absorber.

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Large inconsistencies still exist in nuclear data libraries regarding the kinetic parameters of delayed neutron (DN) precursors. As an example, there is a 17 T_1/2 = (8.87 ± 0.10) s. Those results are consistent with the values recommended by the IAEA/CRP work and they come with reduced uncertainties compared with previously published results.

The potential use of a pulsed neutron source (PNS) to measure reactivity during nuclear fuel loading as a means to prevent core loading errors has been studied at the GUINEVERE facility. This facility couples the deuteron accelerator GENEPI-3C to the fast neutron subcritical reactor VENUS-F at the Belgian Nuclear Research Center SCKmiddotCEN. The 14-MeV neutrons are produced in the reactor core center via T(d; n)He-4 fusion reactions. PNS experiments were performed in five reactor configurations corresponding to the different loading steps of VENUS-F. The evolution of the neutron flux during these PNS experiments was measured by several( 235)U fission chambers in various positions in the inner and outer reflector and analyzed using the area-ratio method. The results show that, despite strong spatial effects, a strong correlation between the reactivity values given by the area-ratio method and some reference reactivity values remains throughout the reactor unloading. Monte Carlo simulations were first validated by comparison with the data and then used to investigate the sensitivity of the method to a core loading error. First results show that some loading errors could be experimentally detected using a PNS.

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The Source Jerk Integral (SJI) method has been extensively used to determine the subcriticality in VENUS-F zero-power experiments since 2012. The obtained results were in the range from -5 $ to about -20 $ and concerned the subcriticalities of accelerator-driven system MYRRHA mockup cores. Within the SALMON program, which is dedicated to the safe loading procedure of pressurized power reactors, five subcritical core configurations were assembled and studied in the VENUS-F reactor in 2019. These cores simulated the loading process in inverse mode: from more reactive to deep subcritical. The subcriticality of five variants of the SC11 VENUS-F core was changed in steps from -20 $ to about -100 $ by replacing the fuel assemblies with lead reflector assemblies. The subcriticality levels were determined with the pulsed neutron source (PNS) and SJI methods. The GENEPI-3C deuterium accelerator coupled with VENUS-F was used as an external neutron source. The results of the measurements obtained with the SJI method are presented in this paper. Time-dependent Monte Carlo calculations were performed to simulate the SJI experiments and to determine spatial-energy correction factors. Static Monte Carlo simulations were performed to calculate neutron spectra and reactivity. The results of the measurements (both SJI and PNS) are compared with the static MCNP calculations.

Three sub-critical (SC) core configurations were investigated in the VENUS-F zero power reactor coupled with the GENEPI-3C accelerator. The SC10 and SC12 were a mock-up of a MYRRHA start-up core and SC11 represented a more complex MYRRHA core loaded with various types of in-pile-sections. The sub-criticality of 11 variants of these VENUS-F cores was changed in several steps from -6$ down to -30$ using the safety and control rods. Their sub-criticalities were determined with the Source Jerk Integral (SJI) method using 11 fission chambers located all over the reactor. For the data analysis, the 8-group delayed neutron parameters from the JEFF-3.1.2 evaluated nuclear data library were used. Reliability and reproducibility of the experimental results were tested by repeating the measurements, swapping the detectors and varying the accelerator beam intensity, thus changing the detector count rates and verifying the validity of the dead time corrections. The obtained results are compared with MCNP calculations.

The Modified Source Multiplication method is used to determine an unknown reactivity level of a reactor from a known one if one has access to the detector counting for both levels when the reactor is fed by a constant neutron source like an Am-Be source. When available, an accelerator driven source, in continuous mode, can be useful as its intensity can be tunable and then adapted to the experimental conditions. However, in that case, the MSM technique must be extended to account for an external source whose intensity, energy and angular distributions can vary from one measurement to another. In this paper, this Modified Multi-Source Multiplication (MMSM) method is applied to measurements done during the FREYA project in the GUINEVERE facility, operated with the GENEPI-3C accelerator providing a mixture of (D,T) and (D,D) neutrons. The monitoring of these sources through the detection of the associated charged particles allows the calculation of the MMSM factors and the estimate of the reactivity values. The results are compared in different configurations with the reactivity obtained with an Am-Be source or in dynamic measurements performed with GENEPI-3C. Their excellent agreement shows the possibility of using such accelerator-based neutron sources for MSM measurements when they are correctly monitored. This is of great interest for deep sub-critical level characterization for which detector count rates per source neutrons might be low.

Delayed-neutron (DN) data is essential in inherent reactor safety and reactor control since it is needed for the estimation of the reactivity. Nowadays, discrepancies among the data in various international databases (JEFF, ENDF, JENDL) are large and bring excessive conservatism in the safety margins. The ALDEN (Average Lifetime of DElayed Neutrons) experiment, built in a collaboration between CEA and CNRS, aimed at re-measuring the DN data associated with several fissioning systems (average delayed-neutron yield and kinetic parameters). The first experimental campaign consisted in the integral measurement of the DN activity after the irradiation of an 235 U target. It took place under the cold neutron flux of ILL (Institut Laue-Langevin) at the beginning of September 2018, in the PF1b experimental zone (doi: 10.1016/j.nima.2006.03.020 ). The data analysis gave an average DN yield of 1.631E-02(2) DN/fiss and a mean precursors’ half-life of 8.93(9) s. The results are consistent with the literature, but they are affected by one third of the uncertainty.

In this article, we evaluate the quality and robustness of a method envisaged for the on-line monitoring of the subcriticality of an ADS, called the "current-to flux" (CTF) method. For this evaluation, we performed a dedicated experiment at the GUINEVERE facility. It is hosted at the SCK-CEN and consists of the subcritical VENUS-F reactor coupled to a continuous external neutron source provided by the GENEPI-3C accelerator. During this experiment, the reactor control rods were moved in various patterns, and the subsequent dynamical evolutions of the reactor reactivity were monitored using nine fission chambers (FCs). The space-energy effects that bias the reactivity values are corrected using a procedure based on simulations computed with the Monte Carlo neutron transport code MCNP. We investigate the precision of this correction procedure by comparison with a reactivity value extracted with the beam interruption technique and we demonstrate its insensitivity to the simplifications made on the VENUS-F reactor modeling and to a simulation key parameter such as the boron carbide density filling the control rods. (C) 2018 Elsevier Ltd. All rights reserved.

As part of the design studies conducted at CEA for future power and research nuclear reactors, the validation of neutron and photon calculation schemes related to nuclear heating prediction are strongly dependent on the implementation of nuclear heating measurements. Such measurements are usually performed in low-power reactors, whose core dimensions are accurately known and where irradiation conditions (power, flux and temperature) are entirely controlled. Due to the very low operating power of such reactors (of the order of 100 W), nuclear heating is assessed by using dosimetry techniques such as thermoluminescent dosimeters (TLDs). However, although they are highly sensitive to gamma radiation, such dosimeters are also, to a lesser extent, sensitive to neutrons. The neutron dose depends strongly on the TLD composition, typically contributing to 10-30% of the total measured dose in a mixed neutron/gamma field. The experimental determination of the neutron correction appears therefore to be crucial to a better interpretation of doses measured in reactor with reduced uncertainties. A promising approach based on the use of two types of LiF TLDs respectively enriched with lithium-6 and lithium-7, precalibrated both in photon and neutron fields, has been recently developed at INFN (Milan, Italy) for medical purposes. The CANDELLE experiment is dedicated to the implementation of a pure neutron field “calibration” of TLDs by using the GENEPI-2 neutron source of LPSC (Grenoble, France). Those irradiation conditions allowed providing an early assessment of the neutron components of doses measured in EOLE reactor at CEA Cadarache with 10% uncertainty at 1σ.

Energy and the environment are at the heart of current debates: the threat of climate change and dwindling fossil resources are causing global problems to which physicists cannot remain indifferent.The French Physical Society (La Société Française de Physique or SFP) and the science community in general have a responsibility to shed light on these debates.SFP's "Energy and Environment" group has historically been a forum for the sharing and dissemination of information; they provide frequent and scientifically-rigorous briefings for societal and political decision-makers.The SFP and the French Chemical Society have recently called on the French government and Parliament to set up a body for the scientific assessment of energy policy.They stated their position in an online stakeholders' consultation [1] in the context of the public debate on the revision of the Multiannual Energy Program.Nuclear power cannot be excluded from this debate into energy sources and their future.Whilst each source of energy has its own unique features, it is nuclear technology that has undoubtedly been one of the most controversial scientific and technical fields throughout the second half of the last century and which remains so today.The highly animated and contradictory nature of the debate, even within the SFP, highlights the need to take into account a variety of data in order to draw as accurate and objective a conclusion as possible in what is an extremely complex area.The aim of this special edition of Reflets de la Physique is to adopt a calm editorial approach, presenting the arguments of a contradictory debate in a highly factual manner of a large (albeit limited) number of viewpoints, so that the reader can form his or her own opinion.This editorial approach is typical of the SFP's magazine, which has in the past published several very detailed articles about nuclear power and its future.The physics community, and scientists in general, are increasingly aware of the political, societal and moral implications of their professional work.This is why the preparation of a reference document intended to be accessible to a very broad audience also needs to include contributions from non-scientists.We have therefore given them the opportunity to bring fresh insight that can sometimes be overlooked by scientists, even when it clearly influences public perception.A further issue of Reflets de la Physique, currently being prepared with the "Energy and Environment" group, will soon be devoted to energy in general to extend and enrich the debate.In the 1990s, under the presidency of René Turlay, the SFP had already published a study on nuclear waste [2], but this is the first time that we enter the debate on nuclear energy with such an extensive piece of work, including a comprehensive bibliography, and aspiring to reach a wider readership than just science enthusiasts.With this special issue, which reflects our determination to ensure an informed debate, we hope to provide an explanation of the controversies and the knowledge that the public often lacks in order to form an opinion.

The GUINEVERE project was launched in 2006 in order to study the feasibility of transmutation in Accelerator Driven subcritical Systems (ADS). This facility, hosted at the Belgian Nuclear Research Center SCK.CEN, couples the fast subcritical lead reactor VENUS-F with an external neutron source provided by the GENEPI-3C accelerator.In order to test on-line subcriticality monitoring techniques, an independent measurement of the reactivity of VENUS-F was achieved using the Modified Source Multiplication Method (MSM). The unknown reactivity of the subcritical configuration of interest was determined by comparing detector count rates driven by an external neutron source in this configuration with those obtained in a slightly subcritical configuration whose reactivity was determined by rod drop experiments. To account for spatial effects, MSM correction factors were calculated for all the detectors using the Monte Carlo neutron transport code MCNP. The corrected reactivity values of all the detectors were found to be consistent. This led to a final estimate of -5.28 +/- 0.13 $ for the reactivity of VENUS-F.The MSM factors calculated with MCNP were found to be insensitive to the assumptions made for modeling the reactor and the detectors. Thus the MSM method appears to be a robust technique for measuring large subcriticality values with good accuracy. (C) 2015 Elsevier Ltd. All rights reserved.

A pile noise measurement campaign has been conducted by the CEA in the VENUS-F reactor (SCK-CEN, Mol Belgium) in April 2011 in the reference critical configuration of the GUINEVERE experimental program. The experimental setup made it possible to estimate the core kinetic parameters: the prompt neutron decay constant, the delayed neutron fraction and the generation time. A precise assessment of these constants is of prime importance. In particular, the effective delayed neutron fraction is used to normalize and compare calculated reactivities of different subcritical configurations, obtained by modifying either the core layout or the control rods position, with experimental ones deduced from the analysis of measurements. This paper presents results obtained with a CEA-developed time stamping acquisition system. Data were analyzed using Rossi-a and Feynman-a methods. Results were normalized to reactor power using a calibrated fission chamber with a deposit of Np-237. Calculated factors were necessary to the analysis: the Diven factor was computed by the ENEA (Italy) and the power calibration factor by the CNRS/IN2P3/LPC Caen. Results deduced with both methods are consistent with respect to calculated quantities. Recommended values are given by the Rossi-a estimator, that was found to be the most robust. The neutron generation time was found equal to 0.448 ± 0.012 μs and the effective delayed neutron fraction is 770.3 ± 19 pcm. Discrepancies with the calculated value (722 pcm, calculation from ENEA) are satisfactory: -6.3% for the Rossi-a estimate and -2.7% for the Feynman-a estimate.

This paper present the measurements of VENUS-F kinetic parameters using the Rossi-Alpha methods. The VENUS-F reactor is a zero-power reactor based in Mol, Belgium at SCK-CEN [1]: its fuel is made of metallic enriched uranium with pure lead in order to simulate the behavior of a lead fast reactor. The reactor can be operated in a sub-critical state when it is coupled with the GENEPI-3C neutron source [2]. At the beginning of 2014, a measurement campaign was performed in the critical state in order to estimate the kinetic parameters of the reactor. In this paper, two measurements are analyzed at two different powers (approximately 2W and 30W) with 7 different fission chambers (with a 235-U deposit that varies from 1g to 10mg). All the correlation functions needed for the Rossi-Alpha method have been built for each possible set of two detectors for the two power levels and values of the effective delayed neutron fraction obtained are then compared. Experimental results show the importance to operate at a very low power. The final value for the effective delayed neutron fraction is finally estimated to be (730 ±11) pcm and the prompt neutron generation time is estimated to be equal to (0.41 ± 0.04) μsec.

GENEPI2 (GEnerateur de NEutrons Pulse Intense) is an accelerator-based neutron source operating at LPSC laboratory in Grenoble (France). The neutrons are produced at 2.5MeV or 14.2MeV trough fusion reactions. GENEPI2 specifications allow performing efficiently accelerated irradiation tests of integrated circuits. This facility can also be operated to test and calibrate different types of detectors. This paper will describe the facility and its performances. Then, measurements of the neutron production will be presented as well as different types of experiments and irradiations. Finally, we describe upgrades undertaken to increase the neutron flux and optimize the facility for multiple applications.

The GUINEVERE project was launched in 2006, within the 6th Euratom Framework Program IP-EUROTRANS, in order to study the feasibility of transmutation in Accelerator Driven subcritical Systems (ADS). This zero-power facility hosted at the SCK·CEN site in Mol (Belgium) couples the fast subcritical lead reactor VENUS-F with an external neutron source provided by interaction of deuterons delivered by the GENEPI-3C accelerator and a tritiated target located at the reactor core center. In order to test on-line subcriticality monitoring techniques, the reactivity of all the VENUS-F configurations used must be known beforehand to serve as benchmark values. That is why the Modified Source Multiplication Method (MSM) is under consideration to estimate the reactivity worth of the control rods when the reactor is largely subcritical as well as near-critical. The MSM method appears to be a technique well adapted to measure control rod worth over a large range of subcriticality levels. The MSM factors which are required to account for spatial effects in the reactor can be successfully calculated using a Monte Carlo neutron transport code.