ESNII+ summer school, KTH, Stockholm, May 20, 2014

Swedish contributions to the safety of ASTRID reactor:

multi-grant project at KTH

Diana Caraghiaur1, Augusto Hernandez Solis1, Ranjan Kumar1, Sebastian Raub1, Pavel Kudinov1, Weimin Ma, Nathalie Marie2, Christophe Journeau2, Laurent Trotignon2, Frédéric Bertrand2 and Sevostian Bechta1

1 - 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

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

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):

VR Multi-Project Grants in Nuclear Energy Research (2)

2nd 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

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

WP1: Corium retention

PhD student: Sebastian Raub

Supervisors: Pavel Kudinov (KTH)

and Christophe Journeau (CEA)

WP is aimed at integration of CEA and KTHexperience in LWR and SFB safety andimprovement of severe accident management(SAM) of ASTRID reactor

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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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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 flow fields for both phases• Provides drag force on particles• Debris Bed From by particle depositon

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 developmentof SIMMER-III code for the initial phase of SA with core melting and relocation

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

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Motivation

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)

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

| PAGE 13

Solution scheme

SAS one-pin h = point-kineticCurrent 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 assemblySIMMER-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 stageIrradiation

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

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SIMMER-III code

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

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Detailed PIN-I model of the SIMMER-III code

DPIN-1 (<11 grids)

Grid (<11)Cp(Ti), ri, li,Porosities ei Local concentrations of FPs

Pcav

Cavity = Pcav (PU02+PPF), Tcav.

Thermal-hydraulics of UO2 are not modeled in the cavity

Simplified mechanical model of the fuel

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)

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Objectives of the project

cavi

ty

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)

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Sensitivity analysis in SAS-SFR

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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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Some sensitivity analysis results obtained with SAS-SFR (2)

TOP

Porosity (1-Sigma = 1%), Normal PDF

Maximum axial clad. temp. after 100 calculations

| 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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Future plans

SAS-SFR SIMMER-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

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

Probabilistic Safety Assessment of ASTRID DHR Systems

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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.

2009-11-18

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.

2009-11-18 Probabilistic Safety Assessment of ASTRID DHR Systems 24

Schematic Diagram of ASTRID DHR System

2009-11-18 Probabilistic Safety Assessment of ASTRID DHR Systems 25

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%)

Grace Period for Components Recovery

2009-11-18 Probabilistic Safety Assessment of ASTRID DHR Systems 26

• 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/S3 S4

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)

2009-11-18 Probabilistic Safety Assessment of ASTRID DHR Systems 27

C1-C5 are either OK or NOT OK based on users defined decoupling criteria

A Repairable Fault Tree in Proposed PSA level I

2009-11-18 Probabilistic Safety Assessment of ASTRID DHR Systems 28

  

   

  

Top Event

  

  

  

 

   

FDEP

G1

G2 G3

G4

 RFT (λ, μ) FT(λ, μ=0)

Event probability at G1:

  

Top event probability

(PG1)

0.06 0.11

« Recovery of failed items can reduce up to 54% the chance of top event failure »

(without recovery)(with recovery)

G1 & G3 : Series gatesG2 & G3: Parallel gatesλ: failure rate μ: recovery/repair rate

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.

2009-11-18 Probabilistic Safety Assessment of ASTRID DHR Systems 29

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

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

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

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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 𝐸𝑀=∫𝑃 d𝑉

Δ𝑄=h𝐴 Δ𝑇

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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

𝑇 𝑖=1004℃

𝑇 𝑖=1266℃

…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

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

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Model results for different sizes of fuel droplets

𝐸𝑀=∫𝑃 d𝑉Geometry used for calculations, applicable to simulate CORECT 2 experiments. Z(t) represents the interface between the interaction zone and the cold liquid sodium column

𝐸𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒=𝑚𝑁𝑎𝐶𝑝 , 𝑁𝑎 (𝑇 𝑓 −𝑇𝑁𝑎 )

η=𝐸𝑀

𝐸𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒

∙100 %

𝑅 𝑓=143 μ𝑚→𝐸𝑀=125 kJ→𝜂=3.5 %

𝑅 𝑓=300 μ𝑚→𝐸𝑀=74 kJ→𝜂=2.1 %𝑅 𝑓=500 μ𝑚→𝐸𝑀=13 kJ→𝜂=0.4 %

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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

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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

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