Commissariat À l'Energie Atomique Research Articles

An efficient approach for predicting the nonlinear vibrations of a beam system subjected to multipoint correlated random excitation

In linear vibration studies, the statistical description of multipoint random excitations is sufficient to calculate the vibration response of a structure. For nonlinear vibrations, it is necessary to model each excitation point separately, taking into account the correlation between each excitation point. The objective of this paper is to show how to reduce the number of excitation terms while remaining in a formalism compatible with nonlinear vibration studies.The reduction of the number of stochastic excitation terms can be achieved by Galerkin methods (such as the Karhunen–Loève decomposition). This paper presents an original method which consists of projecting the excitation terms on the eigenmodes of the structure. These two methods are illustrated in the concrete case of a benchmark structure developed by the Commissariat à l’Energie Atomique (CEA), i.e., the mechanical beam system called the CEA-beam benchmark structure, previously studied in Talik et al. (2022), restrained to its first vibration mode and seen as a Duffing oscillator. A random excitation, composed of a consequent number of points of excitation distributed spatially along the structure (more exactly 101 points) and partially correlated, is used to illustrate the effectiveness of the proposed methodology. The proposed method makes it possible to reduce the number of random excitation signals to a single modal excitation term.

Receiving inspection and tests of the ITER poloidal field #4 cryostat feed-through

The ITER Magnets are in the procurement and assembly phases. During these phases, critical components need to be tested and assembly procedures need to be developed and qualified on mockups. To this end, ITER Organization (IO) and the Commissariat à l’Energie Atomique (CEA) have built up a support structure with space, expertise and equipment: MIFI – Magnet Infrastructure Facilities for ITER. In this framework, IO and CEA perform Receiving Inspection and Tests (RIT) on components delivered to IO by the Domestic Agencies (DA). RIT is a critical stage in the life process of components. These tests are meant to ensure the good quality of components and occur between the Factory Acceptance Test (performed before delivery to IO) and the Site Acceptance Tests that are performed after installation in the tokamak pit. The paper explains the RIT sequence managed on the Poloidal Field #4 Cryostat Feedthrough (PF4 CFT), the first ITER Magnets component to be delivered by a Domestic Agency to ITER Organization, and details leak tests, metrology tests and High Voltage tests performed.

Critical heat flux model on a downward facing surface for application to the IVR conditions

A mechanistic model, which is widely applicable to the flow boiling regime, is developed to predict the critical heat flux (CHF) in the In-Vessel Retention (IVR) configuration. The model is based on five general equations describing the CHF mechanism and four primary CHF variables: vapor velocity, liquid velocity, microlayer thickness and the slug length. The CHF mechanism of liquid film dryout underneath the slug is considered. Velocities of vapor and liquid are determined by the Karman velocity distribution and the force balance between buoyancy and drag force. The microlayer thickness is defined by Cheung and Haddad (Cheung and Haddad, 1997) model, based on the Helmholtz instability for the vapor stem located in the microlayer. The slug length is postulated to be the critical Helmholtz wavelength. The solution is numerically obtained starting from seven scattered input parameters: mass flux, local quality, pressure, inclination angle, gap size, working fluid and heater material. In the model, four unknown constants are used for the premature CHF due to heater deformation, the minimum length of slug and the approximation of microlayer thickness and liquid velocity. To find the best-fitted values of the unknown constants, the URANIE code, developed by Commissariat à l'Energie Atomique (CEA), is used. The CHF predicted by the new model is compared with the integrated IVR-CHF database, including experimental data from KAIST, CEA (SULTAN experiments), UCSB (ULPU experiments) and MIT. Starting from 278 experimental data points, the CHF model is affected by a root-mean-square (RMS) error of 14.8%. The CHF predicted by the model is in good agreement with the experimental IVR-CHF database, except for the condition of high mass flux conditions (>500 kg/m2 s) and low inclination angle (<16°). Further improvement of the model is suggested to cover this range and reduce the RMS error based on future worldwide experimental series.

Stochastic approach for radionuclides quantification

Gamma spectrometry is a passive non-destructive assay used to quantify radionuclides present in more or less complex objects. Basic methods using empirical calibration with a standard in order to quantify the activity of nuclear materials by determining the calibration coefficient are useless on non-reproducible, complex and single nuclear objects such as waste packages. Package specifications as composition or geometry change from one package to another and involve a high variability of objects. Current quantification process uses numerical modelling of the measured scene with few available data such as geometry or composition. These data are density, material, screen, geometric shape, matrix composition, matrix and source distribution. Some of them are strongly dependent on package data knowledge and operator backgrounds. The French Commissariat à l’Energie Atomique (CEA) is developing a new methodology to quantify nuclear materials in waste packages and waste drums without operator adjustment and internal package configuration knowledge. This method suggests combining a global stochastic approach which uses, among others, surrogate models available to simulate the gamma attenuation behaviour, a Bayesian approach which considers conditional probability densities of problem inputs, and Markov Chains Monte Carlo algorithms (MCMC) which solve inverse problems, with gamma ray emission radionuclide spectrum, and outside dimensions of interest objects. The methodology is testing to quantify actinide activity in different kind of matrix, composition, and configuration of sources standard in terms of actinide masses, locations and distributions. Activity uncertainties are taken into account by this adjustment methodology.

Phenomenological Model for Gold-Copper Electrodeposition: Application to Thick Coatings

Pure gold-copper alloys are known to be difficult to electrodeposit because of a strong variation in composition after a few microns have been deposited. Commissariat àl’Energie Atomique (CEA) studied the phenomenon and showed that the decrease in gold’s content is accompanied by an evolution of the microstructure that could be attributed to the free cyanide released near the cathode. During electrolysis, free cyanides provoke a decrease of the copper overpotential (until copper reduction is stopped) and promote the formation of Cu(CN)43− that conduct to an instantaneous three-dimensional nucleation of copper. This phenomenological model well explains why the growth mechanism changes and why only gold is deposited for thick deposits. On the basis of this model, CEA has developed a specific process using ultrasonic waves in order to remove the free cyanides from the cathode. This process allows CEA to perform thick gold-copper deposits with a constant concentration in copper through all the thickness. By controlling the applied potential, different thick alloys with a concentration of copper between 0 wt% up to 40 wt% can be deposited.

External radiation dose and cancer mortality among French nuclear workers: considering potential confounding by internal radiation exposure.

French nuclear workers have detailed records of their occupational exposure to external radiation that have been used to examine associations with subsequent cancer mortality. However, some workers were also exposed to internal contamination by radionuclides. This study aims to assess the potential for bias due to confounding by internal contamination of estimates of associations between external radiation exposure and cancer mortality. A cohort of 59,004 workers employed for at least 1year between 1950 and 1994 by CEA (Commissariat à l'Energie Atomique), AREVA NC, or EDF (Electricité de France) and badge-monitored for external radiation exposure were followed through 2004 to assess vital status and cause of death. A flag based on a workstation-exposure matrix defined four levels of potential for internal contamination. Standardized mortality ratios were assessed for each level of the internal contamination indicator. Poisson regression was used to quantify associations between external radiation exposure and cancer mortality, adjusting for potential internal contamination. For solid cancer, the mortality deficit tended to decrease as the levels of potential for internal contamination increased. For solid cancer and leukemia excluding chronic lymphocytic leukemia, adjusting the dose-response analysis on the internal contamination indicator did not markedly change the excess relative risk per Sievert of external radiation dose. This study suggests that in this cohort, neglecting information on internal dosimetry while studying the association between external dose and cancer mortality does not generate a substantial bias. To investigate more specifically the health effects of internal contamination, an effort is underway to estimate organ doses due to internal contamination.

Preliminary study of the decay heat removal strategy for the gas demonstrator allegro

The helium cooled Gas Fast Reactor (GFR) is one of the six reactor concepts selected in the frame of the Generation IV International Forum. Since no gas cooled fast reactor has ever been built, a medium power demonstrator reactor – named ALLEGRO – is necessary on the road towards the 2400 MWth GFR power reactor. The French Commissariat à l’Energie Atomique (CEA) completed a wide range of studies during the early stage of development of ALLEGRO, and later the ALLEGRO reactor concept was developed in several European Union projects in parallel with the GFR2400. The 75MW thermal power ALLEGRO is currently developed in the frame of the European ALLIANCE project. As a result of the collaboration between CEA and the Hungarian Academy of Sciences Centre for Energy Research (MTA EK) new improvements were done in the safety approach of ALLEGRO. A complete Decay Heat Removal (DHR) strategy was devised, relying on the primary circuits as a first way to remove decay heat using pony-motors to drive the primary blowers, and on the secondary and tertiary circuits being able to work in forced or natural circulation. Three identical dedicated loops circulating in forced convection are used as a second way to remove decay heat, and these loops can circulate in natural convection for pressurized transients, providing a third way to remove decay heat in case of accidents when the primary circuit is still under pressure. The possibility to use nitrogen to enhance both forced and natural circulation is discussed. This DHR strategy is supported by a wide range of accident transient simulations performed using the CATHARE2 code. This paper presents the DHR strategy selected for ALLEGRO and the CATHARE2 simulations are supporting this strategy.

Mortality associated with chronic external radiation exposure in the French combined cohort of nuclear workers

ObjectiveThe long-term effects of protracted low level ionising radiation exposure are investigated in a combined analysis of French nuclear workers employed by the Commissariat à l'Energie Atomique (CEA), AREVA Nuclear...

Analysis of MERCI Decay Heat Measurement for PWR UO2 Fuel Rod

Decay heat measurements, called the MERCI experiment, were conducted at Commissariat à l'Energie Atomique (CEA)/Saclay to characterize accurately residual power at short cooling time and verify its prediction by decay code and nuclear data. The MOSAÏC calorimeter, developed and patented by CEA/Grenoble (DTN/SE2T), enables measurement of the decay heat released by a pressurized water reactor (PWR) fuel rod sample between 200 and 4 W within a precision of 1%. The MERCI experiment included three phases. At first, a UO2 fuel rod sample was irradiated in the CEA/Saclay experimental reactor OSIRIS. The burnup achieved at the end of irradiation was [approximately]3.5 GWd/tonne. The second phase was the transfer of the fuel rod sample from its irradiation location to a hot cell, to be inserted inside the MOSAÏC calorimeter. It took 26 min to carry out the transfer. Finally, decay heat released by the PWR sample was measured from 27 min to 42 days after shutdown. Postirradiation examinations were performed to measure concentrations of some heavy nuclei (U, Pu) and fission products (Cs, Nd). The decay heat was predicted using a calculation scheme based on the PEPIN2 depletion code, the TRIPOLI-4 Monte Carlo code, and the JEFF3.1.1 nuclear data file. The MERCI experiment analysis shows that the discrepancy between the calculated and the experimental decay heat values is included between -10% at 27 min and +6% at 12 h, 30 min after shutdown. From 4 up to 42 days of cooling time, the difference between calculation and measurement is about ±1%, i.e., experimental uncertainty. The MERCI experiment represents a significant contribution for code validation; the time range above 105 s has not been validated previously.

CATHARE-3: A new system code for thermal-hydraulics in the context of the NEPTUNE project

After a thorough analysis of the industrial needs and of the limitations of current simulation tools, EDF and CEA (Commissariat à l’Energie Atomique) launched the NEPTUNE Project in 2001 (see Guelfi et al., 2007) with the support of AREVA-NP and IRSN. The NEPTUNE activities include software development, research in physical modeling and numerical methods, development of advanced instrumentation techniques and new experimental programs. Four different simulation scales were addressed including DNS (Direct Numerical Simulation), CFD in open medium (Computational Fluid Dynamics), component (subchannel-type analysis) and system (reactor modeling) scales. In 2006 CEA, EDF, AREVA-NP and IRSN defined the strategy for the system scale of NEPTUNE and the CATHARE-3 development was launched. The main objectives are: • advanced physical modeling of two-phases flows, mainly by using multi-field and turbulence models, • improved 3D modeling by the use of fine and non conforming structured meshes, • generalized coupling capabilities with other thermal-hydraulic scales and with other disciplines (core physics, structural mechanics, …), • extension of the applicability to new Gen IV reactors (Sodium Cooled Fast Breeder Reactors, Gas Cooled Reactors, Supercritical Light Water Reactors), • a true object-oriented code architecture. At the same time CATHARE-3 is in continuity with the CATHARE-2 code which is the current industrial version of CATHARE and internationally used for nuclear power plant safety analysis, in simulators and in coupled simulation tools. The road map of these two codes will allow a smooth transition from CATHARE-2 to CATHARE-3 for all users. This paper gives an overview of the choices made for the development of CATHARE-3 including new physical models, validation strategy and experimental programs, numerical improvements, enhanced coupling capability and software architecture evolution. The current status of the project as well as the overall schedule will be presented.

Solid Cancer Mortality Associated with Chronic External Radiation Exposure at the French Atomic Energy Commission and Nuclear Fuel Company

Studies of nuclear workers make it possible to directly quantify the risks associated with ionizing radiation exposure at low doses and low dose rates. Studies of the CEA (Commissariat à l'Energie Atomique) and AREVA Nuclear Cycle (AREVA NC) cohort, currently the most informative such group in France, describe the long-term risk to nuclear workers associated with external exposure. Our aim is to assess the risk of mortality from solid cancers among CEA and AREVA NC nuclear workers and its association with external radiation exposure. Standardized mortality ratios (SMRs) were calculated and internal Poisson regressions were conducted, controlling for the main confounding factors [sex, attained age, calendar period, company and socioeconomic status (SES)]. During the period 1968-2004, there were 2,035 solid cancers among the 36,769 CEA-AREVA NC workers. Cumulative external radiation exposure was assessed for the period 1950-2004, and the mean cumulative dose was 12.1 mSv. Mortality rates for all causes and all solid cancers were both significantly lower in this cohort than in the general population. A significant excess of deaths from pleural cancer, not associated with cumulative external dose, was observed, probably due to past asbestos exposure. We observed a significant excess of melanoma, also unassociated with dose. Although cumulative external dose was not associated with mortality from all solid cancers, the central estimated excess relative risk (ERR) per Sv of 0.46 for solid cancer mortality was higher than the 0.26 calculated for male Hiroshima and Nagasaki A-bomb survivors 50 years or older and exposed at the age of 30 years or older. The modification of our results after stratification for SES demonstrates the importance of this characteristic in occupational studies, because it makes it possible to take class-based lifestyle differences into account, at least partly. These results show the great potential of a further joint international study of nuclear workers, which should improve knowledge about the risks associated with chronic low doses and provide useful risk estimates for radiation protection.

Projection-based curve clustering

This paper focuses on unsupervised curve classification in the context of nuclear industry. At the Commissariat à l'Energie Atomique (CEA), Cadarache (France), the thermal-hydraulic computer code CATHARE is used to study the reliability of reactor vessels. The code inputs are physical parameters and the outputs are time evolution curves of a few other physical quantities. As the CATHARE code is quite complex and CPU time-consuming, it has to be approximated by a regression model. This regression process involves a clustering step. In the present paper, the CATHARE output curves are clustered using a k-means scheme, with a projection onto a lower dimensional space. We study the properties of the empirically optimal cluster centres found by the clustering method based on projections, compared with the ‘true’ ones. The choice of the projection basis is discussed, and an algorithm is implemented to select the best projection basis among a library of orthonormal bases. The approach is illustrated on a simulated example and then applied to the industrial problem.

Obituary: Jacques Labeyrie (1920–2011)

After more than 50 years dedicated to scientific research and to the dissemination of knowledge, Dr Jacques Labeyrie recently passed away at the age of 91. Upon graduating from the Ecole Supérieure de Physique et Chimie Industrielle de la Ville de Paris (1940–43), he became the assistant to Frédéric Joliot-Curie at the Collège de France. With Joliot's recommendation, he was recruited in 1946 to the Commissariat à l'Energie Atomique (CEA), where he stayed until his retirement in 1985. He was named Head of the Section d'Electronique Physique (later known as the Service d'Electronique Physique) of the CEA in 1955, which he managed until 1982.

Uncertainty propagation through polynomial fitting and integration procedures with GUM and Monte Carlo applied to x-ray streak camera yield measurements

On the Laser Integration Line (LIL) facility at Commissariat à l'Energie Atomique (CEA) near Bordeaux (France), streak cameras (SCs) are extensively used as x-ray detectors by plasma physicists for the recording of transient phenomena as short as 100 ps. For proper interpretation of the experimental results measurements of the SC yield are carried out before each experimental campaign. The yield is actually an indirect measurement that depends on several other measured quantities. The uncertainty associated with the yield is evaluated using the analytical GUM approach and the Monte Carlo (MC) approach using GUM Supplement 1. It is shown in particular that in the assessment process the standard uncertainty and the probability density function of some input quantities propagate through polynomial fitting functions and integrals. A sensitivity analysis is also performed to identify the critical input quantities. The study shows that after correction of the manufacturer's fact sheet the GUM evaluation converges to MC evaluation when the uncertainties associated with non-linear input quantities become small. When propagating uncertainties through integrals it is observed that the uncertainty of the output quantity is mainly dominated by strong correlation effects with the covariance components adding up to produce large uncertainties. Finally, by comparing the experimental yield with a theoretical model good agreement is found.

Aerosol content survey by mini N2-Raman lidar: Application to local and long-range transport aerosols

This study shows an aerosol content survey in the low and middle troposphere over Paris with a compact and light Nitrogen-Raman lidar which has been recently developed by the Commissariat à l’Energie Atomique (CEA) and LEOSPHERE company. This eye-safe and wide field-of-view system (full overlap between 150 and 200 m) is particularly well-adapted to air pollution survey in the vicinity of Megalopolis. Extinction-to-backscatter coefficient (so-called Lidar Ratio LR) profiles obtained with a Tikhonov regularization scheme are presented for long-range transport events of aerosols (volcanic ash plume LR = 48 ± 10 sr, and desert dust, LR = 45 ± 8 sr) which may contribute to the local load of aerosols emitted by traffic and industries in Megalopolis. Due to an insufficient signal to noise ratio (SNR < 30), a new dichotomous algorithm has been developed to perform daytime inversions every hour which is in accordance with the typical time evolution of aerosols within the planetary boundary layer. This inversion scheme is based on the constraint of the elastic channel with the aerosol optical depth (between typically 0.2 and 0.7 km) determined with the N2-Raman channel and thus only gives access to an equivalent LR between 0.2 and 0.7 km with a relative uncertainty lower than 15%. This approach has been applied to retrieve diurnal cycle of LR for polluted continental aerosols over Paris and is compared with Tikhonov regularization applied during the night. We found a mean value of 85 ± 18 sr for polluted continental aerosols which is in agreement with other studies performed around the Paris urban area. Results for aerosol optical properties are presented and the error sources are discussed for each approach.

Recent Advances in the Development of X-Ray Cameras Inserted Inside a Pressurized Box for LMJ Plasma Diagnostics

For the first plasma diagnostics to be deployed at the Laser Megajoule (LMJ), the Commissariat à l'Energie Atomique (CEA) is developing ultrafast x-ray imaging cameras to analyze spatial and temporal evolution of laser-plasma interaction parameters. These cameras, which operate in the target chamber vacuum, are composed of a one-dimensional (1-D) streak or a two-dimensional (2-D) gated detector, power supply units, control electronic modules, and a charge-coupled device (CCD) readout system. In order to optimize their definition and to ensure easy maintenance, these cameras are packaged inside a hermetically sealed chamber comprised of a pressurized box (air box), which also enable the electronics to operate at atmospheric pressure and provide protection of the camera against the harsh operating environment induced by fluxes of neutrons, x-rays and gamma rays, and electromagnetic radiations. This paper introduces the work led by CEA concerning the development of the air box which integrates the x-ray streak and framing cameras for the first LMJ plasma diagnostics experiments, with regard to the integration and operational constraints of the facility.

LMJ on its way to fusion

The Laser Megajoule (LMJ), is under construction by the French Commissariat à l'Energie Atomique (CEA) at CESTA laboratory near Bordeaux. The LMJ is an important part of the French "Simulation Program". The LMJ is dedicated to high energy density physics experiments, and it is designed to obtain ignition and fusion of a DT fuel capsule. The laser and target systems have been designed to meet the specifications needed for these experiments, in an overall optimization including the fusion target. The current status of the LMJ project is presented.

Cancer mortality among French nuclear contract workers

Nuclear workers from French contracting companies have received higher doses than workers from Electricité de France (EDF) or Commissariat à l'Energie Atomique (CEA). A cohort study of 9,815 workers in 11 contracting companies, monitored for exposure to ionizing radiation between 1967 and 2000 were followed up for a median duration of 12.5 years. Standardized mortality ratios (SMRs) were computed. Between 1968 and 2002, 250 deaths occurred. Our study demonstrated a clear healthy worker effect (HWE) with mortality attaining half that expected from national mortality statistics (SMR = 0.54, 95% CI = [0.47-0.61]). The HWE was lower for all cancers (SMR = 0.65) than for non-cancer deaths (SMR = 0.46). The analysis by cancer site showed no excess compared with the general population. Significant trends were observed according to the level of exposure to ionizing radiation for deaths from cancer, deaths from digestive cancer and deaths from respiratory cancer. The mortality of nuclear workers from contracting companies is very low compared to French national mortality.

Photofission of 235U and 237Np with Bremsstrahlung Photons below 20 MeV: Measurements of Delayed Neutron Yields and Time Spectra

Recently, new applications based on detection of delayed neutrons (DNs) or delayed photons produced during photofission have shown the need for new basic nuclear data. Indeed, available data for DNs are scarce, incomplete, and sometimes contradictory. An experimental campaign has been dedicated to DN measurements for some important actinides with a bremsstrahlung end-point energy spectrum up to 20 MeV. The experiments were carried out at the electron accelerator facility ELSA installed at the Commissariat à l’Energie Atomique (CEA) center of Bruyères-le-Châtel, France. Delayed neutrons were measured by 12 3He counters embedded in an annular polyethylene cylinder. Delayed neutron absolute yields were determined. Different irradiation-decay time combinations allowed the extraction of the averaged six-group parameters of DNs. Similarly to the nuclear reactor physics, these groups are formed by lumping DN precursors according to their half-lives. The DN time dependence is then expressed by the sum of group contributions.In this paper, DN absolute yields and six-group parameters for 235U at 15 and 18 MeV and 237Np at 15 MeV are presented. The 235U and 237Np absolute DN yields are found slightly lower than those published in the literature. For 235U, the six-group parameters show no dependence on incident electron energy from 10 to 18 MeV. In addition, the group parameters are very close to those of 237Np. This work helped to resolve the discrepancies of the photofission DN parameters for 235U while the six-group parameters for 237Np are reported for the first time.

Behaviour of fission products under severe PWR accident conditions. The VERCORS experimental programme—Part 3: Release of low-volatile fission products and actinides

The VERCORS analytical programme consisted of a series of tests carried out on irradiated PWR fuel samples. The tests – funded jointly by EDF and IRSN – were carried out by the Commissariat à l’Energie Atomique (CEA) at their Grenoble site. They were performed in a hot cell belonging to the Active Materials Analysis Laboratory (LAMA). The general outline of the programme was set out in a first article (of a series of 3), which described the different levels of fission products (FP) volatility and their characteristics. This led to a classification into five main categories of volatility and/or behaviour: (1) Volatile FP including fission gases, iodine, caesium, antimony, tellurium, cadmium, rubidium and silver; (2) Semi-volatile FP, a category made up of molybdenum, rhodium, barium, palladium and technetium; (3) Low-volatile FP comprising ruthenium, cerium, strontium, yttrium, europium, niobium and lanthanum with generally low but significant release; (4) Non-volatile FP including zirconium, neodymium and praseodymium; and lastly (5) Actinides which group together uranium, plutonium, neptunium, americium and curium. The specific behaviour of fission gases and volatile FP is dealt with in the second article, which also includes the specific characteristics of volatile FP regarding transport. The main variables (i.e. temperature, which is the main variable at least until loss of sample geometry, oxidising–reducing conditions, burn-up, interactions with the cladding and/or the structural components, the nature of the fuel, and finally the state of the fuel) affecting the kinetics and/or the released fraction of these same FP could also be identified. This final article represents the Third Part of the series. It concerns the release of actinides and less volatile FP, in keeping with the classification by categories previously identified, which are as follows: (1) semi-volatile FP, comprising of Mo, Ba, Rh, Pd, Tc, (2) low-volatile FP, comprising of Sr, Y, Nb, Ru, La, Ce, Eu, (3) non-volatile FP, comprising of Zr, Nd and Pr and lastly (4) actinides. The main parameters favouring their release are also highlighted.