UNIVERSITÀ DI PISA DIPARTIMENTO DI INGEGNERIA MECCANICA, NUCLEARE E DELLA PRODUZIONE VIA DIOTISALVI 2, PISA Gruppo Ricerca Nucleare S. Piero a Grado Atucha-2 PHWR three dimensional neutron kinetics coupled thermal- hydraulics modelling and analyses by the RELAP5-3D© C. Parisi, A. Del Nevo, O. Mazzantini, F. D’Auria, K. Ivanov RELAP5-3D User Seminar Idaho University Campus, Idaho Falls, USA November 2008

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Outline  Introduction  Atucha II PHWR Features  280 channels TH model  HELIOS Cross-Sections Libraries  3D NK - TH model  Sample results  Conclusions

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Introduction  Argentinean electric utility “Nucleoelectrica Argentina – Societad Anonima” (NA-SA) signed an agreement with the San Piero a Grado Nuclear Research Group (GRNSPG) of the University of Pisa (UNIPI) in 2007 for the development of advanced simulation models for the Atucha-II NPP  GRNSPG/UNIPI developed Thermal-hydraulics, Neutronics, CFD and Structural Mechanics models for the safety analysis of the plant  Currently GRNSPG/UNIPI is also assisting NA-SA in the development of the Chapter 15 of the FSAR  Best-Estimate Plus Uncertainty (BEPU) method pursued for licensing calculation  development of a BE RELAP5-3D model for licensing analyses

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Atucha II PHWR Features  Atucha-II is a 692 MWe Siemens designed PHWR under construction in Lima, Argentina  Constructions started in 1981, suspended in 1994  Constructions resumed in 2005, first criticality scheduled for 2010  Heavy water cooled, heavy water moderated PWR  Unique features:  Primary circuit based on the Konvoi-PWR design  Circuit for moderator cooling / FW pre-heating  Natural Uranium fuel  Vertical Fuel Channels  Large RPV (7.3 meter internal diam.)

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Atucha II PHWR Features  Primary circuit characteristics & configuration  2 U-Tubes SG, 2 MCP  Primary side pressure: 11.5 MPa  Primary side temperatures: 278 °C at RPV inlet, 313.3°C at RPV outlet  Total thermal power transferred to the steam water cycle : 2174 MW  Average Moderator Temperature: 170 °C  4 U-Tubes HX for Moderator cooling / FW pre-heating Moderator Cooling Circuit Primary Circuit

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Atucha II PHWR Features  Fuel placed in 451 vertical fuel channel (FC)  37 Natural Uranium Fuel rods per each FC  On-line refueling  Active Core Height: 5.3 m  Oblique CRs CR layout Fuel Element RPV Layout

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Atucha II PHWR Features  Emergency Boron Injection System designed to act during RIA (e.g., LBLOCA)  To counteract positive reactivity excursion due to the reactor positive void coefficient  4 Lances inject high pressure solution of boric acid into the moderator tank  System reaction time reduced by NA-SA to roughly 0.5 secs. Boron Injection Lance (1 of 4)

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Rationale for the Nodalization Structure  Nodalization is the result of a brainstorming process where the computational resources available and the experience of the user play a major role consistently with the objectives of the analyses  Selected code was RELAP5-3D© developed for industrial applications in US, independent from the code(s) used by the regulator  Key role of:  Maximum allowed number of nodes,  Maximum allocable computer-code memory  Typicality of the accident scenarios to be analyzed  DEGB LBLOCA – Initial Reactivity Excursion and Recovery  LBLOCA and other transients  User in supplying unavailable information  Providing suitable qualification

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Rationale for the Nodalization Structure  “280 channels nodalization” consistent with the typicality of the accident scenarios to be analyzed (e.g. DEGB LBLOCA – Initial Reactivity Excursion and Recovery)  Computer resources not sufficient to implement all CNA-2 ECCS and Logics  “60 channels nodalization” consistent with the typicality of the accident scenario (e.g. LBLOCA and other transients)  Code computer resources saturated by implementation of suitable ECCS and Logics, so:  “280 channels nodalization”  for 3D NK – TH analyses  “60 channels nodalization”  for 0D NK – TH analyses

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, ° CL axis HL axis CL1HL1 CL2 HL2 DC Nodalization & RPV Symmetry Due to the legs geometry not all the coolant coming from a loop exits from the same loop. A mixing (between inlet and outlet) has to be taken into account

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, DC Nodalization (Hydraulics) 6 vertical (downward oriented) pipes axially subdivided in 25 volumes joined by cross-flow (multiple) junctions (component no. 7 in the left figure) 6 branches represent the upper part, two of them linked to the CL DC-UP bypass connection

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, LP Nodalization Three layers: 1) Bottom part – horizontally oriented (red): Level-1 (L1) 2) Bottom part up to the channel inlet – vertically oriented (green): Level-2 (L2) 3) Channel inlet up to the lower face of core plate – vertically oriented (blue): Level-3 (L3)

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, LP Nodalization LP grid nodalization approach Five rings are identified containing an ‘entire’ number of boxes Without channel inlets! CL1 HL1 CL2 HL2

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, LP Nodalization LP grid nodalization approach 36 sectors (amplitude of each sector is 10° = “maximum common divisor” for 90°, 40° and 50°) divide the boxes into 200 parts (sub-box) represented by 200 ‘branches’

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, LP Nodalization Bottom part 5 radial rings (according to LP plate subdivision) 6 azimuthal parts (according to DC subdivision) 25 horizontal ‘branches’ constitute the bottom LP The external ring is connected to DC pipes (green circles) Each hor. branch is connected to the vert. branches forming the LP grid (red circles)

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Core channels Nodalization Five throttle types Green zone 1 Blue zone 2 Red zone 3 Black zone 4 Light blue zone 5 Considering 65 Boxes 36 Radial sector 5 Rings 1 Max power channel 280 equivalent channels

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, UP Nodalization Four layers: 1)First horizontal part connected to the coolant channel outlets (blue) 2)First vertical part up to the HL axis (orange) 3)Second horizontal part along the HL (green) 4)Second vertical part to account for the nearly semispherical shape (yellow)

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, UP Nodalization The three different layers modelled by several branches E.g., the first layer – horizontal 25 ‘branches’ connected to the 280 equivalent core channel (red circles)

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, RPV Nodalization Not all channels represented!

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, LOOP Nodalization (including SG) ‘Standard’ nodalization technique General rules followed: 1 junction only connected to a pipe Ratio length of two consecutive nodes stays within 0.5 – 2.0 Junction at inlet and outlet of nodes only SG modeled till the isolation valves Pump homologous curves taken from NA-SA nodalization 2 loops modeled 1 loop represented

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Moderator system 4 loops modeled – 1 loop represented Heat exchangers Primary side of moderator cooler Safety injection Moderator pump Moderator downcomer Safety injection port Pump homologous curves given by NA-SA

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Neutron XSec Libraries model - Generation ENDF/B-VINJOY MultiG XSec (47 Groups) HELIOS v. 1.9 XSecs Libraries (2 Groups) 2D Transport Calculations

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Neutron XSec Libraries Code HELIOS  Advanced lattice physics code HELIOS used to calculate the cross section sets  HELIOS  HELIOS is a neutron and gamma transport code for lattice burnup, in general two- dimensional geometry STUDSVIK ® Scandpower  Developed by STUDSVIK ® Scandpower  One of the best features of this code is the complete geometric flexibility  Cartesian, Hexagonal, Cylindrical, …  HELIOS  HELIOS can calculate almost any two-dimensional geometry, it can generate the cross sections for most of the current nuclear reactor applications PWR, BWR, WWER, CANDU, AGR, RBMK  Successfully applied to PWR, BWR, WWER, CANDU, AGR, RBMK

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Neutron XSec Libraries model  Channel-by-channel burnup distribution supplied by NA-SA  4510 burnup values  reduced to 780 by using 1/6th core pseudo-symmetry

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Neutron XSec Libraries model  For every composition a cross section set is generated  Each Cross Section set contains tables for:  Fast and Thermal Diffusion Coefficients  Fast and Thermal Capture Cross Sections  Fast and Thermal Fission Cross Sections  Fast and Thermal Nu-Fission Cross Sections  Removal Cross Sections (Group 1  2)  Inverse Neutron Velocities  Assembly Discontinuity Factors (ADFs)  Photoneutron effect (supplied by NA-SA) to be taken into account directly in NESTLE input

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Neutron XSec Libraries model HELIOS  Several HELIOS input decks developed:  Fuel Channel (hexagonal lattice)  Fuel Channel + Moderator  CR absorbers (black & grey CR, upper & lower part)  Reflector: Radial, Bottom, and Top Fuel Channel Fuel Channel & Reflector Fuel Channel & CR

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Neutron XSec Libraries model Interpolation in Five-Dimensional Tables Five  For CNA-II core model Five Independent Parameters will be used: 1)Fuel Temperature (Doppler Feedback) 2)Coolant Density (Void Effect) 3)Coolant Temperature 4)Moderator Temperature 5)Moderator Boron Concentration (Emergency System)

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Cross Section Modelling ATUCHACROSS.exe (FORTRAN 90 program) Reference Cross Section Values CROSS SECTION LIBRARIES (nemtab, nemtabr_1, nemtabr_2, nemtabr_3, nemtabr_4) Cross-Section Variation Coefficients 5D LINEAR INTERPOLATION ROUTINE (LINT5D) LEAST SQUARE METHOD (MC) Σ ref

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20,  Final Version of the ATUCHACROSS program  5000 lines Fortran program  Automatically perform  ad-hoc interpolations, for several zones of the reactor core  Variation coefficient calculation  NESTLE input writing Cross Section Modelling Σ ref CR information (Type, angles, number) NESTLE INPUT

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, D NK – TH coupled model  3D NK nodes obtaining “weighted” feedback from TH model  NESTLE sending back power distribution to FA heat structures PowerNESTLE Fuel Channel TH model Moderator TH model Coolant Density Coolant Temperature Moderator Temperature Boron Concentration Fuel TemperatureRELAP5-3D

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, D Neutron Kinetic model  NESTLE 3D NK model characteristics:  Hexagonal lattice  cm pitch  10 axial layers for active core: cm  1 layer for the Bottom Reflector: 48.2 cm  1 layer for the Top Reflector: 34.4 cm  535 X 12 nodes = 6420 NK nodes  Feedback from RELAP5-3D(c) TH model  Hydraulic zones Fuel Channels (coolant density, coolant temperature) Moderator Zones (moderator temperature, boron conc.)  Heat structure zones Fuel Channels (fuel temperature)  All CR type simulated by an ad-hoc representation  Upper & Lower absorber

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, CR modeling  18 Control Rods inserted diagonally (17-25°)  Black CR = Hafnium absorber  Grey CR = Steel absorber  Each CR has upper and lower section with different materials/sections  CR arranged in 4 groups  G10, G20, G30, S10 = Safety & regulation  Shut-off = Shutdown CR

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, CR modelling  Four Cross Sections Libraries calculated for CR modelling  CR XSec corrections due to the 3D geometry effects introduced using 3D MCNP5 Monte Carlo Simulations CR and Fuel Channels Section NESTLE modelling

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, D NK – TH coupled model – Fuel Channels 280 channel  Atucha II NPP 280 channel RELAP5-3D TH model used  # of TH channels modeled & coupled according to:  Hydraulic characteristics (throttled type/un-throttled)  Romboidal sub-plena belonging  Power distribution  Transient type  Code resources 451 FA simulated by 3D NK NESTLE code 280 FA simulated by RELAP5

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, D NK – TH coupled model – Fuel Channels  280 RELAP5 Fuel Channels coupled with 6420 NESTLE neutronic nodes  Feedbacks for Radial, Top and Bottom Reflector coming also from neighbouring fuel channels RELAP5-3D / NESTLE coupling Map – Fuel Channels

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, D NK – TH coupled model – 3D Moderator tank  Moderator Model using RELAP5-3D - 3D components  possibility to simulate with high degree of realism the boron clouds calculated by CFD  simulation of asymmetric transients  Very sophisticated mapping scheme resulted from the use of Cylindrical 3D TH Components coupled with Hexagonal NK cells 6 radial sectors = 12 axial layers for Bottom Reflector, Core Active zone, Top reflector = 12 axial layers for Bottom Reflector, Core Active zone, Top reflector 16 azimuthal sectors 16 azimuthal sectors 3D Moderator Tank – NESTLE mesh overlay

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Modelling the boron emergency injection system  Boron Injection Clouds calculated by a previous CFD CFX TM code calculation  reconstructed by ad-hoc TMDPJUN components in the 3D Moderator Tank Injection Zones Comparison CFD – RELAP5-3D injected mass of boron RELAP5-3D Mass of boron distribution in different layers

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, “280 Channels” Nodalization Resources  Number of Hydraulic Nodes: 5,744  Number of Meshes for Heat Conduction: 67,640  Number of Junctions: 6,987  Number of Materials: 9  Number of Control Variables: 950  Number of Trips: 200  Number of TMDPVOL: 13  Number of NK nodes: 6420  Total Number of input deck lines: 117,000  Typical CPU for running 1 sec of SS (max time step 0.02 s, on Pentium- IV 3.6 GHz): 850 s

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, D NK Coupled TH SS Results  Core SS conditions:  Hot Full Power: GW  Nominal values for Fuel, Coolant and Moderator Temperatures distribution  CR in (NA-SA configuration)  G10: 94% in  G20: 67% in  G30:24.5% in  S10: 4.29% in  Shut-Off (S20 & S30): All Rods Out  Boron Concentration: 0.05 ppm

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, SS – Radial Power  Keff=  Fxy = 1.38 Maximum Min= 1.01 MW Max = 6.65 MW Minimum

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, SS – Radial Power  Radial Power : Spatial Form Function

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, SS Normalized Reactor Axial Power Axial Power Distribution

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Fuel Channels Mass Flow  Mass Flow per Assembly [Kg/s]

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, SS Coupled Codes TH Parameters  All the main TH parameters converged to SS reference values  E.g., moderator Tank temperature distribution  Bottom & Top Reflector  Active Core

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20,  LBLOCA 0.1 A in CL2 (reference DBA for Atucha-2)  Actuation of  Scram by all CRs : s  Emergency Boron Injection System : s  MCP1 & 2 rundown : s  CR completely inserted : s  End of Boron Injection in moderator tank : s  No Safety threshold exceeded LBLOCA 0.1A in CL2 – Sample Results

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, LBLOCA 0.1A in CL2 – Sample Results Reactor Power Pressure Trends in UP and PRZ Fuel Clad Temp. in Central and Hot Channel Void Fraction in Central and Hot Channel

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, MCP Shaft seizure – Sample Results  MCP2 Shaft seizure in 1 s  MCP1 continues operation  CRs scram: s  CRs completely inserted: s  No Safety threshold exceeded Reactor Power Void Fraction in Central & Hot Channel Fuel Clad Temp. in Central and Hot Channel

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, FC Blockage (BDBA) – Sample Results  Hot channel (6.63 MW) total blockage in 0.2 s  No scram signal actuated  Severe Fuel Assembly damage Fuel Clad Temp. in Blocked Channel Blocked Channel Power Blocked Channel Mass Flow

Gruppo Ricerca Nucleare S. Piero a Grado RELAP5-3D User Seminar – Idaho University Campus, Idaho Falls, USA – November 18-20, Conclusions  GRNSPG/UNIPI developed for Argentinean electric utility NA-SA sophisticated RELAP5-3D models for Atucha II NPP  3D NK TH “280 channel nodalization” was presented  3D NK TH model set-up required strong interactions with other technology fields (e.g., CFD, neutron XSecs generation)  RELAP5-3D models calculations will constitute the Chapter 15 of the FSAR for the Atucha II licensing  RELAP5-3D demonstrated to be a very sophisticated tool, allowing a detailed modelling of all relevant phenomena & peculiarities of the Atucha II design  Key points for the code improvement were identified and suggested to INL. E.g.:  Increase of the allowable Trip cards  Increase the number of 3D Volumes per Multi-D component  On-line Cross Section libraries interpolation  Automatic Calculation of Reactivity components (e.g., Doppler reactivity, coolant temp. reactivity,..)  Further works are ongoing at GRNSPG/UNIPI in order to:  Complete models qualification  Extend the model to a full RPV 3D TH representation, including the simulation of all 451 FC  Complete simulation of Logics