Coupled Thermal-hydraulic and Neutronic Model for the Ascó NPP using RELAP5-3D/NESTLE

1. Coupled Thermal-hydraulic and Neutronic Model for the Ascó NPP using RELAP5-3D/NESTLE L. Batet, R. Pericas, E. Morales and F. Reventós Technical University of Catalonia (UPC) Department of Physics and Nuclear Engineering

2. Contents Introduction Ascó NPP description Ascó model description • Thermal-hydraulic • Neutronics Model testing • Steady State • Load Rejection transient • Main Steam line Break transient • Sensitivities Conclusions

3. Introduction • The Dept. of Physics and Nuclear Engineering of the (UPC) holds a large background in the use of TH codes for the Safety Analysis of Nuclear Power Plants (NPP). The Thermal Hydraulic Studies Group has been cooperating for 15 years with the operators of the Catalan nuclear plants, Ascó (2 units) and Vandellós II (all of them 3 loop PWR Westinghouse design). • Ascó-1 NPP started commercial operation on December 1984. • RELAP5 model developed by its analysts. • Extensively qualified and validated. • The presentation represents a continuation of the presentation in 2006 seminar: L. Batet et al. “Status of the activities related to the use of RELAP5-3D at the Technical University of Catalonia”

4. Introduction • A 3D vessel component and neutronic data have been added to the existing model allow full 3D NKTH coupling. • Neutronic data taken from previous model RELAP5 (NRC)/PARCS. We have faced problems in adapting the data. • What we are presenting is a set of preliminary calculations. • The work has been basically done by undergraduate students (following a previous work presented in the 2006 seminar). • This is a first step done with the aim of achieving full neutronic-TH simulation capabilities in the UPC.

5. Ascó NPP description • Ascó-I is owned by ENDESA (100%). • Ascó-II is owned by ENDESA (85%) and IBERDORLA (15%). • Units are located close to Tarragona, in the north east of Spain, and they use the Ebro River as a final heat sink.

6. Ascó NPP description • The actual nominal power of each unit is 2952.3 MWt and 1028 MWe. • The reactor vessel is cold head type. • 3 Siemens (type SG 61 W/D3) steam generators. FW fed directly to the upper part of the downcomer via J-tubes. The circulation ratio on the secondary side of the steam generators is 3.65 at rated power. • The auxiliary feed water system is impulsed by one turbo pump and two motor pumps. In the plant there are, among others, control systems for the reactivity (rods and boron), primary pressure, pressurizer level, steam dump and steam generator level. The reactor protection system includes safety valves in the pressurizer and the steam generator.

7. Ascó NPP description • Summary of the plant main features

8. Plant thermal-hydraulic model • The RELAP5 model of Ascó NPP is prepared to simulate both units of the plant. • Only slight changes are needed, concerning mainly to the fuel load, to switch from one to another. • The model includes: • Hydrodynamic elements (primary, secondary, safety systems and auxiliary systems) • Heat structures, and • Control and protection systems. • The model has been prepared for RELAP5/MOD3.2 and has been subjected to a thoroughly validation and qualification process, which includes the simulation of transients occurred in the plant itself.

9. Plant thermal-hydraulic model • Summary of the model degree of detail

10. Plant thermal-hydraulic model

11. Plant thermal-hydraulic model AFW FW modeling MFW

12. Plant thermal-hydraulic model Logic diagram (partial) of the MFW control system

13. Vessel thermal-hydraulic model • In order to perform 3D NK calculations, in the Ascó model, the vessel has been converted in a set of RELAP5-3D multid components. • The 3D vessel model had been previously developed for Vandellòs-II by an undergraduate student (X. Sabaté). • A previous R5/Parcs model existed (used in the MSLB benchmark) Pseudo 3D Model RELAP5/PARCS 1D Vessel Model Relap5/3.2 3D Vessel Model Relap5-3D

14. Vessel thermal-hydraulic model One of the nodalization proposals: • RPV consists of 5 multid (cartesian + cylindrical) • 500 hydraulic nodi • Some of them are disabled (by applying small volume factors) in the corners 21 core channels

15. Vessel thermal-hydraulic model • Vessel inlets and downcomers. • 3 azimuthal sectors . • 1 radial sector. • 5 axial levels. • Primary circuit to vessel inlets (120º). • Bypass. • 8 azimuthal sectors . • 1 radial sector. • 1 axial level. • Core. • 21 thermal-hydraulic nodes • 9 axial levels including: Core entrance, Core exit, Core auxiliary exit and Core. • 5x5 Cartesian matrix. • Model adopted for the vessel • Lower plenum. • 8 azimuthal sectors. • 2 radial sectors. • 1 axial level. • Upper head. • 8 azimuthal sectors . • 2 radial sectors. • 2 axial levels.

16. Vessel thermal-hydraulic model Core bypass Downcomer Core Core Upper plenum and Upper head Lower plenum

17. Vessel neutronic model • Neutronic data taken from previous model R5/PARCS. • At that time (2001) Cross Section libraries were taken from SEANAP (Univ. Polit. Madrid) • Additional work has been done to transfer from PARCS format values to NESTLE format values. • We have faced problems in adapting the data

18. Vessel neutronic model • 17x17 Matrix; 157 elements/level. • 13 axial levels. • 2 reflector levels. • 11 power levels. • 190 different compositions (XS). • Mapping from 13 levels (NK)  8 levels (TH). Don’t ask me why

19. Vessel neutronic model 157 neutronic nodes

20. Control Rod core distribution ControlRod group A Control Rod group B Control Rod group C Control Rod group D

21. Model Testing • Steady state • Load rejection transient. • Load rejection (50%) corresponding to the testing after fuel reload (Ascó I, 1999, cycle 13) • MSLB • UPC participated in the OECD-CSNI PWR MSLB benchmark with RELAP5/PARCS. As a culmination, a MSLB was calculated for the Ascó NPP (A. Cuadra, JL Gago, F. Reventós, Analysis of a Main-Steam-Line Break in Ascó NPP, ANS Technical Paper - Thermal Hydraulics, Volume 146, Number 1, April 2004, Pages 41-48)

22. Steady State • Steady state calculation at Beginning Of Life conditions, for the Ascó-1 NPP cycle 13.

23. Steady State Something to be improved in the kinetics data or in mapping

24. Steady State

25. Load rejection • Transient from 100% to 50% steady state power in 12 seconds. • Control Rod group D insertion. • Intervention of kinetics, TH, control systems; the actuation of one system affecting the others • We usually perform this test whenever we make an improvement to the model. • We performed this test with the 1D TH (point kinetics) model using R5-3D in 2006

26. Load rejection

27. Load rejection Control Rod group D

28. Load Rejection Power axial profiles

29. Load rejection Axial top level temperature evolution Power evolution

30. Main Steam Line Break • Objective: Try to get the good results obtained after the MSLB benchmark using R5/PARCS • Loop 2 pipe break at 10,060 seconds (loop 2) inside containment (double-guillotine between nodes 750-752). • Stop AFW Turbopumps, 3 min after the break. • Flow control to 15% of AFW motorpumps, 4 min after the break. • Base case + 1 sensitivity: no safety injection (no credit for boron injection)

31. MSLB Base Case

32. MSLB Base Case Isn’t the primary a closed system?

33. MSLB Base Case Axial top level temperature evolution Power evolution

34. MSLB Sens. No safety injection

35. MSLB Sens. No safety injection

36. Ongoing work and conclusions • The well validated Ascó model for R5 has been converted into a 3D model by substituting the vessel volumes by multid components. • 3D NK using Nestle • XS library adapted from previous Parcs model (facing problems) • On going: try to refine the conversion process and use the >600 compositions in the original model • At present we depend on borrowed XS • On going: initiating a research line in neutronics (try to use SCALE to produce 2 group XS)

37. Ongoing work and conclusions • The goal is to have fully 3D NKTH computational capabilities at the THSG. • Plant data available to (partially) validate the future model (e.g. temperature and power maps for the 50% load rejection transient) • Comments and suggestions are welcome.