UNDERSTANDING AND USING THERMAL-HYDRAULICS CATHARE 2 CODE FOR SAFETY ASSESSMENTS

1. UNDERSTANDING AND USING THERMAL-HYDRAULICS CATHARE 2 CODE FOR SAFETY ASSESSMENTS R. FREITAS PSN/SEMIA/BAST

2. CONTENTS • General Information on termal-hydraulics codes • CATHARE Code • Development and methodology • Operating range • GUITHARE: Visualizationtool • Validation of the code CATHARE • The news steps • Multi-filières • Systems Codes

3. General Information on Thermal-Hydraulic Codes

4. General Information on Thermal-Hydraulic Codes • Objective of a thermal-hydraulic code: • Assess the overall response of the boiler and all its state parameters (power, pressure, temperature, flow or velocity, void fraction) during normal operation, incidental or accidental • System codes: APROS (VTT/FORTUM - Finlande) ATHLET (GRS - Allemagne) CESAR (IRSN - France) KORSAR (NITI - Russie) MARS (KAERI - Corée du Sud) SPACE (KAERI - Corée du Sud) RELAP-PWR (NRC - USA) TRAC-PWR (NRC - USA) TRACE (NRC - USA) CATHARE (IRSN – France) Nuclear Power Plant

5. EDF AREVA-NP IRSN Joint effort of Developed at CEA/Grenoble CATHARE CODE • Simulation of the thermal-hydraulics behavior of a Pressurized Water Reactor: • Normal transients, • Incidentals, • Accidentals CodeAvancédeTHermohydrauliquepour les AccidentsdesRéacteurs àEau Code forAnalysis ofThermal-hydraulicsduring an Accidentof ReactorsafetyEvaluation

6. Methodologyand Development A MODULAR CODE: • Able to model the separate effect tests, integral effect tests and all kind of nuclear power plants (PWR, BWR, VVER, …) MODULES: • 1D module • 1D module with tees • Volume module • 3D module • 1D pump module • Boundary conditions • Heat exchangers

7. Methodologyand Development THERMAL CALCULATIONS: • Radial heat conduction for • Multi-layer wall • Fuel rod • 2D conduction for rewetting • Multi-layer wall and fuel rod • Fuel thermo mechanics • Clad deformation • Clad rupture • Clad oxidation SUBMODULES: • Source • Sink • Break • SGTR • Valves • Safety valves • Point pump • Point accumulator • Point SG • Point kinetics • CCFL • CATHARE: capable of simulating all types of • hydraulics circuit (nuclear or not)

8. Methodologyand Development Methodology for development & validation of physical models • The code was designed to perform best-estimate calculations of accidents initially in pressurized water reactor. Then specific modules have also been implemented to allow modeling of other reactor types • Its range of application covers all Loss Of Coolant Accidents (LOCA) and all transients (SGTR) with degraded operating conditions in primary and secondary circuits of the NPP without core melt • CATHARE 2 is based on a two-fluid model (6 equation - mass, energy and momentum equations for each phase),with additional optional equations for non-condensible gases and radio-chemical components • CATHARE has a flexible modular structure for the thermal-hydraulic modeling in applications ranging from simple experimental test facilities to large and complex installations like Nuclear Power Plants. Several modules can be assembled to represent the primary and secondary circuits of any reactor and of any separate-effect test or integral test facility

9. Methodologyand Development Methodology for development & validation of physical models • Modules • The 1-D (or axial) module to describe pipe flow. A TEE sub-module can be added to the 1-D module to present a lateral branch, • The 0-D (or volume) module, a two-node module used to describe large size plena with several connections, such as the pressurizer, the accumulator, the steam generator dome, the lower and upper plenum of a PWR, for example. • The volume predicts swell level, total or partial fluid stratification and phase separation phenomena at the junctions,

10. Methodologyand Development Methodology for development & validation of physical models • Modules • The 3-D module to describe multidimensional effects in the reactor vessel or fuel assembly • The modules for boundary conditions (BC). • To complete the modeling of the circuits, sub-modules can be connected to the main modules: • The multi-layer wall module in which radial conduction is calculated, • The reflood model with 2-D heat conduction in the wall or fuel rod for predicting quench front progression, • The reactor point kinetic module

11. Methodologyand Development Methodology for development & validation of physical models • A single version for studies or training • Including all functional features and models for all applications • Independent of the plant type • Portable on different machines • with standard coupling tools • An unique set of physical models • Limited optional physical models for specific components are clearly explained in the user guidelines) • Advantage to the physical assessment process (validation effort reduced) • Advantage to the code reliability • Maximization of the number of users in a large domain • All users benefit by the reliability of the code • New functional features can be used by all users for others applications • Reducing the user’s effect

12. Methodologyand Development Methodology for development & validation of physical models • Version(ex. CATHARE V2.5_3 mod3.1) = set of modules + numerical scheme + solution procedure • Revision(ex. the revisions 6 and 6.1) = set of physical models (mass, momentum, and energy exchanges between phases and phase-wall energy exchanges (ex : interfacial friction or wall to fluid heat exchange) assessed on the validation matrix • On Separate Effect Tests (qualification step) • On Integral Effect Tests (verification step) Limited choice between physical models is given to the users • A validated version refers to a specific revision Ex : the CATHARE 2 V2.5_3 mod3.1 R6.1 (revision 6.1)

13. CATHARE Verificationand Validation CATHARE version management Assessment based on Separate Effect Tests • Purposes • To check the validity of the involved physical models • To obtain the validity domain • To estimate the uncertainties of the physical models • To define the best nodalization of each component • To define the best meshing and time step for converged solutions • Also used for physical model development • Qualification matrix: • 45 experiments and more than 1000 tests • Basic phenomena or component tests • Determination of model uncertainties • Quantify uncertainties associated to closure laws parameters CATHARE CODE R. FREITAS - IRSN 12-05-2014

14. CATHARE Verificationand Validation CATHARE version management Assessment on Integral Tests Facilities • Purposes • To assess the consistency of the physical model package • To test overall code performance • To check the code capability to represent system effects • To draw attention to points which need further physical investigations • To give guidelines to users (modeling, meshing etc.) • Validation matrix • 19 Transients on 5 integral test facilities

24. CATHARE CODE CATHARE distributions worldwide/23 countries and 39 organisations JRC,DELFT INSC UCL,BEL V VTT CGNPC, NPIC, CIAE, BINE INR KI CCFE NCBJ, WUT, ITC CATHARE Map - Europe CEA, IRSN, AREVA, EDF SNECMA CCHK BARC NRC JNRC VARANS, VAEC INRNE ENSI, PSI NRI-UJV BELGRADE AEKI IVS, VUJE IAEC ENEA, PISA, ROME

25. CATHARE CODE Applications of the CATHARE code: • Realistic studies on the behavior of: REP 900, 1300, 1400, EPR, BWR, VVER, NP, RJH, GEN IV • Optimization of the procedures in accidental situation • Study of safety reactors LB LOCA, IB LOCA, SGTR, MSLB, LOFA, SBO • Determination of the basic uncertainties of the physical models • Representation of a reactor in simulators Physical behavior in system facility BETHSY(PWR 900 MWe) Boucled’ÉtudeTHermohydrauliqueSYstème

26. CATHARE CODE • Physical behavior in system facility  Integral Test • BETHSY : Boucled’EtudeTHermohydraulqueSYstème • Mock up PWR 900 MWe • 1/100 volume • 1/1 elevations • Commissioning to the CEA/Grenoble: CEA-EDF-AREVA-IRSN (1980) • Integral test (more than 80) : IB/LB LOCA – SGTR – Naturelle Circulation – Scale factor

27. CATHARE CODE Main features of CATHARE 6 equation model (2 fluid) in all modules (0D, 1D, 3D) • mass, energy and momentum equations • Mechanical non-equilibrium • Phase separation • Stratification • Co-current and counter current flows • Counter-current flows limitation (CCFL) • Thermal non-equilibrium • Critical flow • Cold water injection • Reflooding • All flow regime and all heat transfe regimes

28. Mass equation: Energy equation: Momentum equation: General Description Of The CATHARE Code

29. General Description Of The CATHARE Code The computation of any CATHARE code consists of three parts, corresponding to three separate executable: The CATHARE pre-processing where data acquisition is done. This involves a user input deck. These data contain descriptions of the hydraulic circuit(s) to be simulated, the events occurring during the simulation and how calculation is managed. The CATHARE calculation process where the execution of the simulation described in the input deck, i.e. basically the thermal hydraulic computation is done. The CATHARE post-processing results where users may process CATHARE binary output files to output useful information.

30. Modeling A modular structure : Use a set of objects to represent any kind of hydraulic circuit Types of objects : 5 kinds of main modules (physical and numerical different models) : AXIAL : pipe (1D) VOLUME : large size plenum with several connections (0D) THREED : 3D (vessel) BCONDIT : boundary condition RG : double ended break Sub Modules Reflood, multi-layer wall, point neutronics (core), fuel pin thermo-mechanics, exchangers… Gadgets connected to one point (scalar or vector) TEE, source, sink, accumulator, break, SGTR, valves, pump, CCFL, point SG...

31. Examples Axial Volume Boundary condition 3D is an extension of the 1D meshing Modeling Zc

32. Assembly of modules Modeling junction AXIAL AXIAL BCONDIT VOLUME

33. The junction Connection, link between two modules Weight notion (for example: volume and three-D modules) Continuity of the section, gravity, hydraulic  and weight Weight of an object Input deck simplification (loss of dissymmetry effects) "Heritage" notion for sub modules and gadgets Modeling

34. Weight notion – example: Modeling 4 2 2 2 4 2 2 VOL3 VOL5 VOL4 VOL3 VOL4 1 1 1 1 2 2 1 VOL2 VOL2 VOL2 VOL1 VOL1 VOL1 1

35. GUITHARE Visualization Tools • GUITHARE:GraphicUser Interface for CATHARE • Pre-processing To import, to read, to visualize and to modify existing input data decks To create and to visualize a new input deck (creation from scratch) To run Cathare calculations either in interactive or in batch mode • Post-processing To import, to read files resulting from other calculation plateforms To visualize variables of a circuit, of one or several elements To plot different variables (variable evolution vs time/abcissae) To generate views in order to create a video clip To save a study (input deck + result files + printouts)

36. GUITHARE Visualization Tools

37. GUITHARE Visualization Tools

38. CANON Experiments A vertical pipe initially full of under saturated water under a strong pressure (generally about 15 MPa). The test section consists of a stainless steel pipe of 0.1 m internal diameter and 4.483 m length. The pipe is vertical, and the wall thickness is equal to 12.5 mm. It is insulated to prevent the heat exchange in the environment . The blow-down was the consequence of the opening of a break through the top of the pipe. The bubbles then go up because of the Archimedes force and are slowed down by the interfacial friction. Under the swell level, the void fraction is all the higher since the interfacial friction is high (the bubbles are indeed more strongly slowed down).

39. CANON Experiments

40. PERICLES Reflooding Experiments The experiment consists of three different assemblies which contained in a vertical housing with a rectangular section, each assembly containing 7* 17 = 119 full-length heater rods. Thus, the total number of heater rods is 357. The rods are heated by two independent electrical power sources (in order to bring the cladding temperatures to some initial values), giving the possibility to heat more the central assembly than the two lateral. The length of the rods is equal to the length of the channel (3656 mm), and their diameter is equal to 9.5 mm. The three assemblies are initially empty of water.

41. During a first stage, the rods are electrically heated in order to bring the cladding temperatures to some initial values. When these values are reached, water is injected at the bottom of the test section in order to reflood the three assemblies, the rod powers being kept constant. During the reflooding stage, a quench front goes up in each assembly. The reflooding stage is finished when the three quench fronts reach the top of the assemblies (the velocities of the three quench fronts may be slightly different because of the different rod power between the hot and the cold assemblies) PERICLES Reflooding Experiments

42. The reflooding sub module contains a two-dimensional fine meshing to calculate the heat conduction in the wall in the very vicinity of the quench front. However, this two-dimensional meshing only concerns the heat conduction through the wall and is connected to the coarser hydraulic meshing through the heat exchange laws between the wall and the fluid. PERICLES Reflooding Experiments

43. PERICLES Reflooding Experiments TITRE DE LA PRESENTATION - DATE DE LA PRESENTATION

44. PERICLES Reflooding Experiments

45. 900 MWe PWR SB LOCA This section focuses on the observation of a few of the highly-ranked phenomena of typical small break (SB-LOCA transients postulated to occur in the cold leg of a standard 3-loop 900 MWe pressurized water reactor (PWR).

46. 900 MWe PWR SB LOCA

47. Blow-down: Reactor trip is initiated by a low pressurizer pressure set point (approximately 12.95 MPa). A safety injection (SI) signal occurs when the primary pressure decreases below the set point (approximately 11.75 MPa), and SI begins after some delay time. During the blow down the break flow is single-phase liquid. The entire RCS saturates, the rapid depressurization ends, and the RCS reaches a pressure just above the SG secondary side pressure. 900 MWe PWR SB LOCA

48. Natural circulation: At the end of the blow down period, the RCS pressure reaches a quasi-equilibrium condition that can last for several hundred seconds, during which the SG secondary side acts as a heat sink. The system drains from the top down, with voids beginning to form at the top of the SG tubes and continuing to form in the upper head and top of the upper plenum regions. There is still adequate liquid to allow significant natural circulation two-phase flow in the loops; decay heat is removed through condensation in the SGs during this time. Significant coolant mass depletion continues from the RCS, and vapour generated within the core is trapped within the upper regions by liquid plugs in the loop seals (the U-shaped section in the cold leg between the steam generator outlet plenum and the reactor coolant pump inlet), while a low-quality flow still exits the break. Critical flow at the break remains a highly-ranked phenomenon during this phase, as it influences the rate of inventory depletion. 900 MWe PWR SB LOCA

49. Loop seal clearance: The break flow, previously a low-quality mixture, transitions to primarily steam. Prior to loop seal venting, the static head balances within the RCS can cause the vessel collapsed mixture level (the level of liquid excluding vapour) to depress into the core, potentially allowing the cladding to heat up. Following the venting, the vessel level recovers to about the cold leg elevation, as the imbalances throughout the RCS are relieved. Some phenomena ranked ‘H’ during this phase include the prediction of horizontal stratification and interfacial drag in the loop seal, since they influence the timing of the loop seal clearance. 900 MWe PWR SB LOCA

50. Boil-off: The vessel mixture level continues to decrease due to the boil-off of the remaining liquid inventory, since the RCS pressure is generally still too high to allow sufficient emergency core cooling system (ECCS) injection by the high-pressure SI pumps. The mixture level will reach a minimum, in some cases resulting in core uncovery, before the RCS has depressurized to the point where the break flow becomes lower than the rate of ECCS water delivery. The prediction of voids and mixture level in the core is important during this phase for reasonable prediction of cladding temperature excursions related to core uncovery. 900 MWe PWR SB LOCA