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The Cherenkov power meter developed at the Jožef Stefan Institute (JSI) is an independent, reliable and cost-effective measurement system based on Cherenkov radiation detection, used to measure fast power transients during pulse operation at the JSI TRIGA research reactor. It is based on a closed tube in order to avoid interference from external light sources and radiation damage experienced by optical fibers. The tube is placed in the reactor core periphery, the bottom part of the tube is filled with water, serving as the source of Cherenkov light. The measurements obtained in the framework of an extensive experimental campaign in collaboration with the French Atomic and Alternative Energy Commission (CEA) focused on reactor pulse operation using the Cherenkov power meter show excellent agreement with existing nuclear instrumentation (TRIGA pulse recorder), especially for high peak power pulses. However, the Cherenkov power meter outperforms the TRIGA pulse recorder in accurately recording low peak power pulses. The power vs. time behavior measured with the Cherenkov power meter is in accordance with measurements using miniature fission and ionization chambers developed at the CEA. The possibility of reactor pulse characterization based on measurements with the Cherenkov power meter combined with neutron dosimetry measurements sets the basis for enabling irradiation during pulse operation at JSI TRIGA research reactor in the future.

This paper presents our experimental work to assess the capability to estimate the transient-induced power distribution in the CABRI experimental reactor using Cherenkov radiation. The CABRI reactor is designed to produce a power transient up to 21 GW within a time less than 100 ms in order to irradiate a test fuel pin in condition representative of a Reactivity insertion Accident in pressurized water reactors. The large response range and short response time required to follow the flux evolution during a complete transient makes classical means of detection, such as ionization or fission chamber, inoperative. For that purpose, we suggest to measure Cherenkov light produced within optical fibers. Indeed, Cherenkov light emission is linked to the local electron production, which is proportional to the local gamma flux through the Compton or pair production cross-section, the intensity of Cherenkov radiation is related to the photon flux intensity. The knowledge of the fission photons emitted by the reactor gives direct insight on the fission rate, hence a spatial power density distribution could be reconstructed thanks to the measure of the Cherenkov light at different point in the reactor.

In the normal operation of nuclear reactors, the kinetic behavior of the neutron population in the core is driven by the so-called delayed neutrons (DN). The DN yield per fission, their average lifetime and their groups’ abundances are the main physical parameters used to predict the kinetic behavior of the reactor and its reactivity. The overall uncertainty associated to reactivity predictions, which is linked directly to the nuclear reactor safety margins, is thus closely dependent on a few parameters associated with DN. Depending on the nuclear data library, DN kinetic parameters present significant discrepancies, even for major fissile isotopes such as 235U or 239Pu. In this context, CEA has long been working for improving DN nuclear data. In 2018, CEA launched the ALDEN project (Average Lifetime of DElayed Neutrons) in the framework of a collaboration between CEA/DES, CEA/DRF, CNRS/IN2P3 (LPSC, CENBG, LPC), ENSICAEN and Caen University. This program aims at measuring the kinetics of the delayed neutrons to derive the DN yield, their average lifetime and abundances. Two experimental campaigns focusing on the thermal fission of 235U were conducted in 2018 and 2019. They demonstrated the concept feasibility and gave high quality estimations of the DN multiplicity (1.631 ± 0.014 %) and average lifetime (8.82 ± 0.6 s) for 235U. More recently in March 2021, a more ambitious irradiation campaign was conducted on 239Pu and 233U. This paper details the new experimental setup, which was upgraded to meet ILL safety requirements for handling plutonium. The data analysis process is presented, with a focus on the problem of dead time correction. Finally, some preliminary delayed neutron decay curves are showed and discussed.

Constant improvements of the computational power and methods as well as demands of accurate and reliable measurements for reactor operation and safety require a continuous upgrade of the instrumentation. In particular, nuclear sensors used in nuclear fission reactors (research or power reactors) or in nuclear fusion facilities are operated under intense mixed neutron and gamma-ray fields, and need to be calibrated and modeled to provide selective and accurate neutron and gamma-ray measurements. The French Atomic Energy and Alternative Energies Commission (CEA) and the Jožef Stefan Institute (JSI) have started an experimental program dedicated to a detailed experimental benchmark with analysis using Monte Carlo particle transport calculations and a series of neutron and gamma-ray sensor types used in the JSI TRIGA Mark II reactor. CEA has setup a simplified TRIPOLI-4® modeling scheme of the JSI TRIGA reactor based on the information available in the IRPhEP benchmark in order to facilitate analysis of future neutron and gamma-ray measurements. These allow the CEA to perform a TRIPOLI-4 instrumentation calculation scheme benchmarked with the JSI MCNP model. This paper presents the main results of this CEA calculation scheme application and the analysis of their comparison to the JSI results obtained in 2012 with the MCNP5 & ENDF/B-VII.0 calculation scheme. This paper will conclude with some information about the new experimental program to be carried out in 2022 in the TRIGA reactor core.

Fission chambers are widely used in nuclear reactors, either occasionally to probe the neutron flux maps in nuclear power plants or as part of the nuclear instrumentation used in research reactors to characterize neutron flux levels in irradiation locations. Wide dynamic range acquisition systems are very limited in the nuclear instrumentation market. Thus, since 2012, CEA is developing the MONACO system, which allows measurement in pulse, fluctuation, and current mode running simultaneously on the same channel. The simultaneous use of three measurement modes places high requirements on the amplification stage. Indeed, the current mode exploits a bandwidth between 0 and a few kHz, while the fluctuation mode is based on the frequency range of 100 kHz 1MHz. Finally, for the pulse mode, it is necessary to identify current pulses of a few uA in amplitude and with a duration of about ten nanoseconds, which implies a bandwidth that extends up to 10 or 20 MHz. In the frame of the industrialization of the MONACO system, a new preamplifier was designed to meet those requirements. With a first prototype, LDCI and I-TECH performed very promising bench tests in October 2020. Tests with synthetic signals corresponding to pulse and fluctuation mode showed very good performance of the electronics. The positive and negative current pulses are correctly amplified and the measurement noise was estimated at 0.1 µApp thanks to proper electronic shielding. The DC measurement is also satisfactory in the tested range with an accuracy of less than nA and a bandwidth of 1kHz. Preamplifier qualifications are planned within realistic experiments using a fission chamber placed in a neutron field.

The availability of neutron fields with a high neutron flux, suitable for irradiation testing of nuclear instrumentation detectors relevant for applications in nuclear facilities such as material testing reactors (MTRs), nuclear power reactors and future fusion reactors is becoming increasingly limited. Over the last several years there has been increased interest in the experimental capabilities of the 250 kW Jožef Stefan Institute (JSI) TRIGA research reactor for such applications, however, the maximal achievable neutron flux in steady-state operation mode falls short of MTR-relevant conditions. The JSI TRIGA reactor can also operate in pulse mode, with a maximal achievable peak power of approximately 1 GW, for a duration of a few ms. A collaboration project between the JSI and the French Atomic and Alternative Energy Commission (CEA) was initiated to investigate absolute neutron flux measurements at very high neutron flux levels in reactor pulse operation. Such measurements will be made possible by special CEA-developed miniature fission chambers and modern data acquisition systems, supported by the JSI TRIGA instrumentation and activation dosimetry. Additionally, measurements of the intensity of Cherenkov light are proposed and being investigated as an alternative experimental method. This paper presents the preparatory activities for an exhaustive experimental campaign, which were carried out in 2019-2020, consisting of test measurements with not fully appropriate fission chambers, activation dosimetry and silicon photomultipliers (SiPMs) The presented results provide useful and promising experimental indications relevant for the design of the experimental campaign.

Nuclear instrumentation is a complex topic since it involves a wide range of physics phenomena like nuclear reactions, heavy ion interactions with matter, electrostatic, charge creation etc. Understanding and modelling fission chambers is a difficult task usually performed with Monte-Carlo and finite element simulations. Since a few years, analytical and simplified Monte Carlo models were introduced at the French Atomic Energy Commission to easily design detectors. It is proposed here to present the derivation of such model, called COSICAF, for academic purposes; this numerical model provided with this article, will help students and researchers to understand and design fission chambers. To demonstrate the interest and the limitation of proposed work in research field, the model is applied to simulate two real miniature fission chamber designs.

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 235U 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.

The CEA Sensor, Dosimetry and Instrumentation Laboratory (LDCI) has been working since 2011 on an integrated acquisition system called MONACO, ’Multichannel Online Neutron Acquisition in Campbell mOde’, for fission chamber measurements. Taking into account the feedback acquired up to the CEA TRL7 version, the LDCI Lab and the Instrumentation Technologies company (Solkan – Slovenia) initiated in 2020 a two years collaboration agreement to build the Libera MONACO 3 industrialized version required for reactor use and mass production. After a final qualification performed in the Slovenian TRIGA Mark II reactor in March 2023, the system will be available on the nuclear instrumentation market by the end of 2023.

In this paper, the UV spectrum of the OH radical (transition A 2Σ → X 2Π) is used to find easy and reliable methods to measure both rotational and vibrational temperatures. A numerical simulation is developed to compute synthetic OH spectra between 300 and 330 nm with a normalization on a lines group called Gref , located at 309 nm. A large number of spectra are computed with different temperatures and global apparatus functions, a quantity which contains all the line broadening sources. Their study allows us to show that amplitude of some 0-0 band's line groups and global apparatus functions or Gref widths are usable to carry out the rotational temperature. Moreover, it is proved that vibrational temperature is accessible through the mean spectrum value of a part of the 1-1 band. To overcome the difficulty of finding the part where the spectrum mean value is evaluated, the authors propose to work with multiples of 'on spectrum accessible lengths'. Abacuses are presented to connect each quantity of interest to temperatures. Finally, proposed methods are checked first experimentally on an oxy-acetylene torch and then on spectra produced with the LIFBASE software. [ABSTRACT FROM AUTHOR]