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ESNII+ summer school, KTH, Stockholm, May 20, 2014 Swedish contributions to the safety of ASTRID reactor: multi-grant project at KTH Diana Caraghiaur 1, Augusto Hernandez Solis 1, Ranjan Kumar 1, Sebastian Raub 1, Pavel Kudinov 1, Weimin Ma, Nathalie Marie 2, Christophe Journeau 2, Laurent Trotignon 2, Frédéric Bertrand 2 and Sevostian Bechta Division of Nuclear Power Safety at Royal Institute of Technology (KTH) 2 - French Alternative Energies and Atomic Energy Commission - Commissariat à l'énergie atomique et aux énergies alternatives (CEA), Cadarache

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VR framework grants to support first long term direct collaboration between France and Sweden in nuclear energy The purpose is to strengthen nuclear research and to stimulate the development of Jules Horowitz Research Reactor (JHR) and sodium-cooled prototype reactor (ASTRID): Multi-grant projects: 60 MKr, in two calls with deadlines in 2011 and 2013 PhD students/Postdocs: - Work at CEA, Cadarache - Enrolled in Sweden and do a Swedish PhD/Postdoctoral project - Both Swedish and French supervisors

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VR Multi-Project Grants in Nuclear Energy Research -DEMO-JHR (coordinator: Prof. Christophe Demazière, Chalmers): 3 PhD projects including 1 at KTH: DEPTHS, Development of Procedures of Thermal-Hydraulic Simulations for JHR -ASTRID core physics and diagnostics (coordinator; Prof. Imre Pázsit, Chalmers): 4 PhD projects including 1 at KTH: ALDESA, Acoustic Leak DEtection in Sodium Applications -ASTRID safety (coordinator: Prof. Sevostian Bechta, KTH): 1 PhD + 3 post-doc projects. 3 multi-grant projects funded by the Swedish Research Council in the spring of 2012 (projects in collaboration with CEA, France – French Alternative Energies and Atomic Energy Commission):

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VR Multi-Project Grants in Nuclear Energy Research (2) 2 nd VR call project of 2014: Dr Staffan Jacobsson Svärd, Uppsala universitet: Assessing fuel behavior in the sodium-cooled fast reactor ASTRID, 2.5 MSEK Prof. Janne Walenius, KTH: two PhD projects on Thermodynamic assessments of relevance for fuel- cladding interaction and on Modelling of fission product transport in MOX fuel, 6 MSEK Prof. Imre Pazsit, Chalmers: Neutronic modeling of control rod withdrawal, 2.5 MSEK

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ASTRID-Safety multi-grant project at KTH Aimed at safety improvement of ASTRID SFB including severe accident prevention and mitigation WP1 – Corium retention 1 PhD student, 4 years WP2 – Simulation of an early phase of a severe accident 1 post-doc, 2 years WP3 – Probabilistic safety analysis 1 post-doc, 2 years WP4 – Analysis of severe accident scenarios with simplified models 1 post-doc, 2 years

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WP1: Corium retention PhD student: Sebastian Raub Supervisors: Pavel Kudinov (KTH) and Christophe Journeau (CEA) WP is aimed at integration of CEA and KTH experience in LWR and SFB safety and improvement of severe accident management (SAM) of ASTRID reactor

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7 Contribution into ASTRID core catcher with the studies of: - melt fragmentation in sodium coolant and corium debris bed formation, - long term coolability of corium debris by sodium natural convection Application of models and tools developed in LWR safety for analysis of to usage for Sodium The current approach is building on KTH-Expertise in Severe Accidents in a typical Swedish Light Water Reactor. The mathematical underpinning of existing Software Tools, developed at KTH is analyzed for assumptions and deductive steps are no longer valid or need adjustment due to the shift from water to liquid sodium and their respective sets of physical properties. Incorporate the adjustments into the existing code structure Evaluate modified Code Find under which range of parameters and boundary conditions the code will perform according to expectations ( debris mass flow into coolant pool, debris temperature, etc) Add Improvements to existing Code Package Possibilities include enhanced heat transfer and vapor production capabilities, as well as debris heap settling and Research tasks

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8 3 Major Modules: Flow in Porous Media/Evaporation: Solves Ergun’s Filtration equation for flow in a packed bed Solves Phase continuity with flow Does not attempt heat transfer calculations Heat Release goes completely in evaporation Flow in the coolant pool: Solves Equation of continuity and Turbulent Equation of Momentum, using the k – ε turbulence model Melt Particle Motion and Depositon: Lagrangian model for each particle Particles do not interact Velocity vector of particles consists of 2 parts 1.Velocity field for liquid phase with corrections for buoyancy force 2.Random vector with Gaussian probability distribution, magnitude of variance tied to turbulence First ideas about modeling Provides corrections to the flow fields due to: pressure differentials density changes Phase changes Provides flow fields for both phases Provides drag force on particles Debris Bed From by particle depositon

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WP2: Simulation of an early phase of a severe accident Postdoctoral researcher: Augusto Hernandez-Soliz Supervisors: Weimin Ma (KTH) Laurent Trotignon, Pierre Gubernatis (CEA) WP is focused on further development of SIMMER-III code for the initial phase of SA with core melting and relocation

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The core design of ASTRID had posed many challenges for the modeling of neutronic and TH phenomena | PAGE 10 Motivation

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ASTRID fuel pins Unprotected Transient of Power (UTOP) We are focused on simulating what happens at the fuel pin at the begining of an accident State-of-the art safety analysis relies on computer codes (e.g. SAS-SFR, SIMMER) in order to understand what happens during an unexpected reactor transient Fuel assambly(271 rods) : 95 kg of fuel + 53 kg of steel SNa 1/3 Sth 8,5 mm Fissile zone of fuel rods : 0,4 kg of fuel + 0,1 kg of steel Relatively high internal pressure ( 100b)

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The power transient (initial or induced) causes a rapid heating of the fuel, creating a cavity inside the pin. Degradation is driven by the melting of UO2, and due to the dilatation and rapid pressurization of the cavity up to mechanical failure On such a sequence, we look to represent the so called early stage of the primary phase of the transient, i.e.: - At the pin level: Fuel heating and meelting, evolution and pressure of the cavity up to cladding rupture and ejection of molten fuel to Na Transient of Power | PAGE 12 Fuel behaviour during a TOP

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| PAGE 13 Solution scheme SAS one-pin = point-kinetic Current scheme New scheme SIMMER III or IV : 1 or 2 mesh per assembly Drawbacks Point kinetics Limitations on handling CFV core geometry Cannot handle multi-pin modeling per channel, (i.e. one average pin/channel) Neutronics = PARIS Point-kinetic OR spatial SIMMER III or IV : 1 or 2 mesh per assembly SIMMER-III* Multi-classes in // and Multi-pins + DPIN GERMINAL Cathare : TH of the reactor loop Transition (1° rupture of can wall) Initiating event Secondary stageInitiating (primary) stage Transition stage Irradiation

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The code that will be used in this project corresponds to the SIMMER-III (V. 3E) code Developed by KIT (Germany), JNC (Japan) and CEA Cadarache to study the consequence of core disruptive accidents in SFRs | PAGE 14 SIMMER-III code

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The best possible way to model a fuel pin can be found in DPIN-1: Can handle annular pellets A mesh of 11 nodes can be defined for the pellet in order to model accurately temperature profiles | PAGE 15 Detailed PIN-I model of the SIMMER-III code DPIN-1 (<11 grids) Grid (<11) Cp(T i ), i,l i, Porosities i Local concentrations of FPs P cav Cavity = Pcav (P U02 +P PF ), Tcav. Thermal-hydraulics of UO2 are not modeled in the cavity Simplified mechanical model of the fuel

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Attempts to model the motion of the molten fuel inside the cavity have been carried out in DPIN-2 Altough not very succesfully Therefore, the main idea of the project is to improve the DPIN-1 model of SIMMER-III By trying to implement a time-dependent and axial in-fuel model within the cavity, where the thermal-hydraulics effects are taken into account Retro-engineering can be performed based on the in-fuel motion models of other codes (such as SAS-SFR) | PAGE 16 Objectives of the project cavity

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In order to do retro-engineering from SAS-SFR into SIMMER-III, the most important model parameters should be known in advance in order to simplify the work The aim is to rank the importance of the different input parameters towards a certain output Statistical methods can be used for such sensitivity analysis (SA) | PAGE 17 Sensitivity analysis in SAS-SFR

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| PAGE 18 Some sensitivity analysis results obtained with SAS-SFR TOP Fuel conductivity (1-Sigma = 1%), Normal PDF Maximum axial clad. temp. after 100 calculations

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| PAGE 19 Some sensitivity analysis results obtained with SAS-SFR (2) TOP Porosity (1-Sigma = 1%), Normal PDF Maximum axial clad. temp. after 100 calculations

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| PAGE 20 Some sensitivity analysis results obtained with SAS-SFR (3) TOP Gap Heat Transfer Coeff. (1-Sigma = 1%), Normal PDF Maximum axial clad. temp. after 100 calculations MOST DOMINANT PARAMETER

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| PAGE 21 Future plans SAS-SFRSIMMER-III Understand the SIMMER-III approach of fuel pin degradation modeling Build a general vision of the SAS-SFR approach + phenomena In-fuel motion model Thermo-mechanical cavity model Pin Model EJECT DEFORM Pin Model SPIN DPIN Compute selected CABRI tests (TOP) E5/E7 LT2/PF2 Improved DPIN-I In-fuel motion model

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WP3: PSA of ASTRID system design Postdoctoral researcher: Ranjan Kumar Supervisors: Pavel Kudinov (KTH) Frederic Bertrand (CEA) WP is aimed at Dynamic PSA of ASTRID Decay Heat Removal System taking into account failed component recovery

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ASTRID DHR Systems ASTRID has a significant boiling margin in normal operation (more than 300°C) together with a high thermal inertia of the primary system (advantage over PWR). Decay heat removal systems mainly use air as a cold source and they are based on forced and natural convection, which allows passive mode of systems operation. DHR system must be practically free from the loss of the decay heat removal function Probabilistic Safety Assessment of ASTRID DHR Systems23

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PSA Research Objectives To evaluate Core Damage Frenquency for current ASTRID DHR systems design and analyse potential accident scenarios with consideration of recovery of failed items. To perform PSA of current design of ASTRID DHR systems taking into account both probabilistic and deterministic approaches. To demonstrate and improve the design for practically failure free DHR function Probabilistic Safety Assessment of ASTRID DHR Systems24

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Schematic Diagram of ASTRID DHR System Probabilistic Safety Assessment of ASTRID DHR Systems25 ASTRID DHR systems consist of four types of DHR systems: 4 loops of S1 used in normal and accident conditions (100% for first 3 days) 2 active loops of S2 (2 x 100%) 2 passive loops of S3 (2x 100%) 2 loops in the bottom of the reactor vessel S4(2x 50%)

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Grace Period for Components Recovery Probabilistic Safety Assessment of ASTRID DHR Systems26 Figure shows the long term sodium temperature calculations taking into account the DHR system operation. Grace period depends on the sequences of failure and their effects on the sodium temperature rise. S2/S3S4

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CEA-KTH aproach: A PSA Level-1 Method with Repairable Components A PSA Level-1 Method with Repairable Components is proposed and implemented by exploiting the grace period using partial dynamic Event Tree and Fault Tree (To be presented in ESREL2014 conference) Probabilistic Safety Assessment of ASTRID DHR Systems27 C1-C5 are either OK or NOT OK based on users defined decoupling criteria

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A Repairable Fault Tree in Proposed PSA level I Probabilistic Safety Assessment of ASTRID DHR Systems28 Top Event FDEP G1 G2G3 G4 RFT (λ, μ)FT(λ, μ=0) Event probability at G1: Top event probability (P G1 ) « Recovery of failed items can reduce up to 54% the chance of top event failure » (without recovery)(with recovery) G1 & G3 : Series gates G2 & G3: Parallel gates λ: failure rate μ: recovery/repair rate

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Dynamic Safety Assessment of ASTRID DHR system PyCATSHOO (Pythonic Object Oriented Hybrid Stochastic Automata) method models a system with interacting probabilistic and deterministic variables explicitly. The modeling and analysis using PyCATSHOO is currently undergoing on ASTRID DHR system Probabilistic Safety Assessment of ASTRID DHR Systems29 PyCATSHOO model of ASTRID DHR systems in which the hybrid automata D1, D2, D3, and POOL communicate real-timely among themselves through message box (coloured) about their state such as random failure (probabilistic) of DHR systems and sodiulm temperature rise (deterministic) in pool

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WP4: Analysis of severe accident scenarios Postdoctoral researcher: Diana Caraghiaur Supervisors: Pavel Kudinov (KTH) Nathalie Marie and Frédéric Bertrand (CEA) WP aimed at assessment of expansion phase of a SFR FCI with simplified models

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Motivation Core melting fuel-coolant interaction => coolant vaporisation power excursion => fuel vaporisation mechanical energy release due to vapour expansion simplified modelling mechanistic modelling understanding the phenomena limitation of vessel loadings 31

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Phenomena of fuel-coolant interaction Molten fuel-sodium interaction creates favourable conditions for fine fuel fragmentation This leads to drastic increase of heat transfer surface area, and thus, the amount of heat rapidly transferred from the fuel to the (more volatile) sodium In consequence, a large amount of sodium vapour is produced in a short time The specific volume of sodium gas is about 3000 times larger than the specific volume of liquid sodium The increase in volume produces pressure in the enclosed volume of fuel assembly, reactor core or primary vessel The mechanical energy is released, which can endanger the surrounding structures 32

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Fine fragmentation – a prerequisite for energetic FCI! A large subcooling of sodium The interfacial temperature between molten fuel and sodium can be below the melting temperature of fuel Fragmentation can be due to formation of a solid crust on the surface of fuel droplet. The crust is ruptured due to an internal pressure build up Illustration of parameters which influence the molten fuel droplet fragmentation due to solidification Illustration of fragments obtained from interaction of a single molten metal droplet penetrating a sodium pool, Zhang et al, 2009 …the largest fragment shows that the inside of the lower hemisphere is empty. The diameter of the hemisphere is almost equal to the initial droplet. The appearance of lower part clearly shows to be the solid crust produced upon contact with sodium.. 33

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Simplified model Complement to mechanictic tools. The mechanistic tool is not yet available for SFR FCI, thus the simplified model can be the only tool for the design of ASTRID Fast calculation of parameters of interest – mechanical energy release and pressure evolution Possibility of conducting large parametric studies Easy adaptation to various FCI configuration (various scales, design evolution, etc.) Better treatment of epistemic uncertainties Consists of: Heat transfer from fuel to sodium (associated with fragmented fuel) Energy conservation in the sodium (one- or two-phase) Equation of state (one- or two-phase) Constraints (acoustic and inertial) Example of schematic representation of the system at t=0 34

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Model results for different sizes of fuel droplets Geometry used for calculations, applicable to simulate CORECT 2 experiments. Z(t) represents the interface between the interaction zone and the cold liquid sodium column 35

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Future plans Sensitivity studies on fragmentation using CORECT 2 tests Reactor case application for the ASTRID project at different scales for various typical bounding configurations Implementation of the tool in the PROCOR CEA severe accident platform 36

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Concluding remarks The ASTRID – safety projects are in progress but it is already visible that this collaboration is quite successful It is not only example of international collaboration - between France and Sweden, but also interdisciplinary one - between safety of LWRs and SFRs 37

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