Insertion reliability studies for the RBC-type control rods in ASTRID

International audienceThis paper reports on preliminary studies performed regarding the insertion reliability of theRBC-type Control Rods designed for the ASTRID Sodium-cooled Fast Reactor. At this stage, the primary aim of the analysis is to evaluate the mechanical behavior of RBC Control Rods under Emergency Shutdown conditions, for which reactor core structures are subjected to significant misalignments (including earthquakeimposed displacements). Using a Finite Element Model based on the Cast3M solver and developed specifically for these studies, computations are performed that allow assessing contact reactions (and the associated friction forces and contact pressures), deformations and stresses (mostly due to bending-induced deformations) which are considered for design. Based on these preliminary results, some optimization of the Control Rod design is proposed that ensures some stable behavior all along the rod drop, with substantial design margin

Corium-sodium interaction the development of the SCONE software

International audienceIn the frame of the safety studies for the ASTRID reactor, CEA has started the development of the SCONE software. SCONE aims at being a mechanistic code dedicated to the simulation of the molten corium-sodium interaction that could occur in a sodium-cooled reactor. In particular, SCONE must allow studying the configuration of an interaction occurring in the lower plenum of ASTRID in case of massive relocation of corium discharged from the core by dedicated transfer tubes.In this paper, after a recall of the phenomenology of the interaction, the physics to represent with SCONE is described as well as the available modeling approaches. The main gaps in knowledge have been identified and are explained along with the foreseen experimental programs aiming at filling these gaps. The numerical aspects of the SCONE development are also discussed before concluding by giving the status of the SCONE software

PIRAT - An analytical tool for reliability assessment of reactivity control systems - Progress and applications to Sodium Fast Reactors

International audienceReactivity Control Systems (RCS) are essential safety components for nuclear reactors. One of the most dicult situations these systems experience is seismic motion due to the large deformation and crucial responsibility to shutdown the reactor.This paper introduces a new method for dynamic modeling that accounts for the contact and free motion of the mobile part of the RCS with the guiding sleeve. The new method introduces springs with a gap to account for material interaction. This method is implemented into the DEBSE solver within PIRAT, a toolbox currently in development for RCS design. The work presented in this paper explores the dynamic eects on a model (tentatively representative) system to investigate the assumptions made in some previous works using a quasi-static simulation. Based on the frequency of excitation, various responses are seen that are not captured in the quasi-static simulations. This work gives evidence tothe importance of the dynamic modeling of RCS

Implementation and Comparison of Contact Models within PIRAT for Nuclear Reactivity Control Systems

International audienceThis work shows the results for a stepped-sine excitation of a Reactivity Control System for a nuclear reactor. These simulations use the PIRAT toolbox, which is currently in development at CEA to study insertion reliability, to predict the maximum displacement of the response location compared to the static estimate used in previous studies. Within the PIRAT toolbox, the dynamic solver (DEBSE) has two contact models implemented to study the interaction between the subsystems of the RCS. One of these is a user-inputted, mass-less, two-stage spring to mimic the fluid compression in the first stage and material contact in the second, while the other model only uses material contact via the Lankarni and Nikravesh model with limited user-inputted parameters. The system is excited based on a stepped-sine excitation with a maximum amplitude to mimic the previous work using a static approach. This sweep shows that each contact model (as well as the same model with varied parameters) can produce significantly different responses. In addition to the varied responses, the different models had difficulty near resonance effects that manifest as numerical ill conditioning. This work shows the need for experimental calibration of these models due to the variability as well as some of the possible issues when performing these calculations. While this work does not present any novel contact modeling, it does show a new implementation for a nuclear energy system and some of the unique issues that arise from these unique systems

Model Validation for the RBC-type Control Rods in ASTRID during Seismic Activities

International audienceThe purpose of this paper is to compare methods for analyzing the interaction of a rod with a stiff sleeve. This is shown via the application to the RBC-type control rods for the ASTRID fast reactor. Current motivations are to replace experimental testing that was performed on previously designed reactors with a computationalmodel to allow for appropriate estimations in a timely manor to be used during the design phase of the development. The methods used include the use of a finite element model and a newly developed tool that uses an analytical representation of the beam. This tool currently contains a static solver using the Breese'sformulations and some preliminary work for a dynamic solver using the Euler-Bernoulli representation. The static solver is compared to the finite element model used in previous work on this example system, while the dynamic solver compares the modal information to the Eigen-solution via finite elements. Results from thesecomparisons show very good agreement between the methods. This work signifies the current progress of an ongoing project to produce an accurate, analytically based tool for design engineers to predict mechanical coupling during vibration events such as seismic activities

Threshold condition for nonlinear tearing modes in tokamaks

Low-mode-number tearing, mode nonlinear evolution is analyzed emphasizing the need for a threshold condition, to account for observations in tokamaks. The discussion is illustrated by two models recently introduced in the literature. The models can be compared with the available data and/or serve as a basis for planning some experiments in order to either test theory (by means of beta-limit scaling laws, as proposed in this paper) or attempt to control undesirable tearing modes. Introducing a threshold condition in the tearing mode stability analysis is found to reveal some bifurcation points and thus domains of intrinsic stability in the island dynamics operational space

The Farahat sodium natural convection film boiling experiment revisited

International audienceWith the renewal of interest for sodium-cooled fast reactors, looking at what is known from the past, it appears – in the frame of severe accident studies in general and FCI in particular, that very few is known about sodium film boiling around hot fuel droplets which is a condition allowing premixing. The past Farahat experiment (Reynolds et al., 1976), performed in 1971, in which hot solid spheres were transferred into sodium has been revisited. Looking in the detailed results, it appears that a phenomena has been ignored, i.e. the existence of two film boiling regimes as observed in similar experiments with water (Honda et al., 1995; Bradfield, 1966). These two film boiling regimes have been analyzed since the transition between these regimes could lead to the onset of spontaneous explosions if the melt is still liquid at this transition. A simple model has been built for the estimation of this transition point. As this model is able to describe the transition observed in Sn-H$_2$O experiments (Sher, 2012), it has been used for UO$_2$-Na systems