1. Application of RELAP5 to Safety Analysis of Czech NPPs
Assessment of U.S. Thermal Hydraulic Computer Code RELAP5 against Experimental Facilities of VVER and PWR Design.
Application of RELAP5 to Safety Analysis of Czech NPPs
Pavel Kral, Jelena Krhounkova, Frantisek Lahovsky, Marek Bencik, Jan Hadek
Nuclear Research Institute (UJV) Rez
International Conference TRANSATLANTIC NUCLEAR DAYS
Prague, October 6-7, 2011

2. Contents
Brief History of RELAP5 Development and Its Implementation in UJV Rez
Assessment of RELAP5 in UJV Rez
Application of RELAP5 to Analyses of Czech NPP’s
New TH Phenomena and Regimes Revealed in Process of Assessment and Application of RELAP5 to VVER Analysis
Summary

3. 1. Brief History of RELAP5 Development and Its Implementation in UJV Rez
In the first decades of Pressurized Water Reactors (PWR) era – starting in at Shippingport - nearly all the attention in field of nuclear safety was focused on large break loss-of-coolant accident (LB LOCA) as the design basis accident (DBA) for PWR.
The thermal hydraulic (TH) computer codes developed in late 60-ies and used for LOCA safety analysis in next decades were based on homogeneous equilibrium model (HEM) of water-steam mixture.
Codes like FLASH (Westinghouse) and RELAP4 (US AEC) utilizing HEM approach and built-in conservative correlations created so called “evaluation model” (EM) category of computer codes. They respected the requirements of NRC 10 CFR Appendix K to Part 50.
Substantial change in the approach to PWR nuclear safety happened after the Three Mile Island accident (TMI-2, 1979), that showed possible severe consequences of small break LOCA and plant transients.
Evaluation of TMI-2 showed urgent need of advanced TH computer codes enabling more precise best-estimate (realistic) analysis of NPP behavior in wide spectrum of accidents. Conservatism of the analysis with best-estimate code could be ensured by conservative initial & boundary conditions or by uncertainty evaluation.

4. 1. Brief History of RELAP5 … (cont’d)
The TMI-2 accident accelerates development of so-called “best estimate” system TH codes enabling better prediction of system behavior under diverse off-normal plant conditions. In the decade following TMI-2 several advanced TH computer codes of “best-estimate” (BE) category were introduced:
TRAC (Los Alamos - USNRC, USA)
RELAP5 (INEL - USNRC, USA)
RETRAN (EPRI, USA)
CATHARE (CEA - EdF - Framatom, France)
ATHLET (GRS, Germany)
The development of advanced TH computer codes was strongly supported by experimental data from separate-effect tests (SET) and integral test facilities (ITF) like Semiscale, LOFT, LOBI, LSTF etc.

5. 1. Brief History of RELAP5 … (cont’d)
Fig.1 LOFT facility (operated , 1:60, Idaho, nuclear core)

6. 1. Brief History of RELAP5 … (cont’d)
Basic features of best-estimate TH computer codes are as follow:
Two-fluid 6 equation hydrodynamic model with mass, momentum and energy balance equation for liquid and for vapor phase. Resulting in nonequilibrium and nonhomogeneous model (enabling different temperatures and velocities of liquid and vapor).
Additional field equations for transport of non-condensable gases and for non-volative solute field (boron).
One- or multi-dimensional interpretation of flow in the system (3D capabilities are present in TRAC, RELAP-3D, TRACE, CATHARE etc.).
Extensive set of closure equations to solve large number of unknowns in basic field equations (wall and interphase heat transfer, interphase drag, wall friction etc.).
Latest trends in system TH codes development: 3D hydraulics combined with 3D neutronics model, basic model with 9 eqs (droplet field), dissolved non-condenable gases in liquid field, crystalization of boric acid, coupling with containment code, coupling with CFD, etc.

7. 1. Brief History of RELAP5 … (cont’d)
1980: RELAP5/MOD-0 (Reactor Excursion and Leak Analysis Program)
1981: RELAP5/MOD1
1984: RELAP5/MOD2
1985: ICAP (International Code Assessment Program), MOD-2/cycle36
1989: RELAP5/MOD3
1990: Implementation of RELAP5/MOD2/RMA in UJV Rez (IAEA regional project)
1991: CAMP (Code Assessment and Maintenance Program)
1994: Signature of CAMP agreement btwn Czech regulatory body and US NRC
1996: RELAP5/MOD3.2
1997: RELAP5-3D Ver:1.0 (DOE) (branching of code development to NRC 1D version and DOE 3D version)
2000: IRUG (1st meeting of International RELAP5-3D User Group)
2000: UJV agreement with INEL on RELAP5-3D Installation
2001: RELAP5/MOD3.3
2006: RELAP5-3D Ver:2.4
2010: RELAP5/MOD3.3patch04 (so far the last NRC version)
2010: RELAP5-3D/Ver:3.0.0 Beta (so far the last DOE version)

8. 1. Brief History of RELAP5 … (cont’d)
Present status of RELAP5 development:
At present, 2 major versions or branches of RELAP5 code can be distinguished:
NRC version RELAP5/MOD (maintenance mode only, ISL)
INL version RELAP5-3D sponsored by DOE and consortium of sponsors (continuing development at INL Idaho – plans for RELAP6 and RELAP7)
Besides of the two major developmental branches listed above, a lot of further versions and local code modifications have been developed, assessed and applied (including various plant analyzers and simulators – e.g. NPP Temelin simulator).
As a special and very important developmental branch, the RELAP5-SCDAP with BDBA and severe accident analysis capabilities should be mentioned.
After more than 30 years of development and 25 years of international cooperation, the system TH code RELAP5 has become the most widespread and utilized system TH code world-wide.

9. 1. Brief History of RELAP5 … (cont’d)
Current status of RELAP5 installations in UJV Rez:
At present, the RELAP5 is utilized in 3 sections of UJV Rez and Research Centre Rez:
Department of Safety Analysis Head of department: Dr. Jiri Macek RELAP5 team with 4-6 members Installation of both RELAP5/MOD3.3 and RELAP5-3D Whole spectrum of assessment and application analyses for NPP’s
Research Reactor Section (Research Centre of UJV Group, RC) Support analyses for Rez research reactors and loops with R5/M3.3
Nuclear Safety Regulation Support Section (RC) Support safety analyses for Czech regulatory body with R5/M3.3

10. 2. Assessment of RELAP5 in UJV Rez
The world-wide assessment of RELAP5 gives to users very good knowledge about code characteristics and capabilities. The assessment in UJV Rez has been naturaly oriented to VVER (and lately also to PWR) design.
The following list shows some example references from more than 30 technical reports and papers documenting RELAP5 assessment in UJV Rez:
Ref.1/ Kral P.: Introductory Calculation with RELAP5/MOD2 Computer Code. Analysis of Primary-to-Secondary-Leak at PMK-NHV Facility. UJV June * Hungarian exp. facility PMK-NVH (1:2070 model of VVER-440/213) * Primary to secondary leak 11.9% through SG hot collector head * Post-test analysis with RELAP5/MOD2/RMA code version
Ref.2/ Krhounkova J.: Analysis of Pressurizer Leak at High-Pressure Experimental Loop RVS-3. UJV April * Czech exp. facility RVS-3 (experimental water loop for VVER-440 and VVER-1000) * Leak from pressurizer steam line * Post-test analysis with RELAP5/MOD2/RMA code version

11. 2. Assessment of RELAP5 in UJV Rez (cont’d)
Fig.2 Nodalization of PMK experimental facility for RELAP5

12. 2. Assessment of RELAP5 in UJV Rez (cont’d)
Fig.3 Analysis of PMK LOCA test T3.1 (pressure, DC temp, R level, clad temp.)

13. 2. Assessment of RELAP5 in UJV Rez (cont’d)
Ref.3/  Kral P.: ISP-33 PACTEL Natural Circulation + Secondary Side Depressurization Test Analyses with RELAP5/MOD2.5 and RELAP5/MOD3. Presentation at final session of OECD ISP-33. May * Finnish experimental facility PACTEL (1:2070 model of VVER-440/213) * OECD Internation Standard Problem No * Stepwise reduction of primary inventory up to core uncovery and following EOP’s * Pre- and post-test analysis with RELAP5/MOD2.5, MOD3.0, and MOD3.1 code versions
Ref.4/  Kral P.: Optimization of Pressure in Hydroaccumulators of NPP Dukovany – Summary Report. UJV April * Measurement from Czech NPP Dukovany (VVER-440/213) * Hydroaccumulator blowdown tests * Post-test analysis with RELAP5/MOD2.5 (report in Czech)
Ref.5/  Krhounkova J.: Analysis of RCP Trip at NPP Zaporozska. Comparison with Experiment. UJV December * Measurement from Ukraine NPP Zaporozskaya (VVER-1000/320) * RCP trip transient * Post-test analysis with RELAP5/MOD3.1

14. 2. Assessment of RELAP5 in UJV Rez (cont’d)
Fig.4 Results of analysis of HA blowdown test at NPP Dukovany

15. 2. Assessment of RELAP5 in UJV Rez (cont’d)
Ref.6/  Krhounkova J.: Post-Test Analysis of SSP-2. UJV Rez * Russian exp. facility ISB (1:3000 model of VVER-1000, originally with vertical SG) * Standard Russian Problem No % LOCA (without HA and HPIS) * Post-test analysis with RELAP5/MOD3.1
Ref.7/  Sommer J.: Benchmarking of TH Codes Critical Flow against Analytical Solution. Presentation at 16th CAMP meeting. Terrasini * Benchmark against analytical solution * Flow of dry air from volume with 0.3 MPa to volume with 0.1 MPa * Analysis with RELAP5/MOD3.1 and MOD3.2 (plus variant with “pv” term)
Ref.8/  Krhounkova J.: Trip of 6/6 RCP – Comparison against Measurement at NPP Dukovany. UJV June * Measurement from Czech NPP Dukovany (VVER-440/213) * Trip of 6/6 RCP at full power * Post-test analysis with RELAP5/MOD3 (report in Czech)
Ref.9/  Hádek J., Kral P.: Results of the Sixth Three-Dimensional AER Dynamic Benchmark Problem Calculation. Solution of Problem with DYN3D and RELAP5-3D Codes. May * Benchmark on MSLB in VVER-440/ * Analysis with RELAP5-3D

16. 2. Assessment of RELAP5 in UJV Rez (cont’d)
Fig.5 Results from RELAP5-3D calculation of MSLB presented at AER Meeting

17. 2. Assessment of RELAP5 in UJV Rez (cont’d)
Ref.10/  Sommer J.: Safety Analyses of Loss-of-Flow Events. UJV Z 1199 T. Rez, June * Measurement from Czech NPP Dukovany (VVER-440/213) * Various trips of RCP * Post-test analysis with RELAP5/MOD3 (report in Czech)
Ref.11/  F. Reventós,, J. Freixa, L. Batet, C. Pretel, D. Luebbesmeyer, D. Spaziani, J. Macek, F.Lahovsky, F. Kasahara, K. Umminger: An analytical comparative exercise on the OECD-SETH PKL E2.2 experiment. Nuclear Engineering and Design * SETH-OECD project E2.2 test on German PKL exp. facility (ITF modeling PWR) * RELAP5 and ATHLET post-tests analysis
Ref.12/  Kral P.: RELAP5/MOD3.3 Assessment Against New PMK Experiments. CAMP In-Kind Contribution. UJV T. Srpen * Hungarian exp. facility PMK2 (1:2070 model of VVER-440/213) * Tests T2.1, T2.2, and T2.3 – MBLOCA with various initial and boundary conditions * Post-test analysis with RELAP5/MOD3.3

18. 2. Assessment of RELAP5 in UJV Rez (cont’d)
Fig.6 Results of RELAP5 post-test calculation of PKLIII E2.2 Test

19. 2. Assessment of RELAP5 in UJV Rez (cont’d)
Ref.13/  Bencik M.: Pre-test Calculation of LBLOCA with DEGBR of Cold Leg in Experimental Faciltiy PSB-VVER. UJV Z 1998 T. September * Russian exp. Facility PSB (1:300 model of VVER-1000) * LBLOCA test * Pre- and post-test analysis with RELAP5-3D (report in Czech)
Ref.14/  Kral P.: RELAP5/MOD3.3 Assessment Against PMK Test T LOCA with Nitrogen in PRZ. Prezentation at Autumn 2008 CAMP Meeting. Rockville. November * Hungarien exp. facility PMK2 (1:2070 model of VVER-440/213) * LOCA with break 30% in CL starting from low parameters (HZP) with nitrogen in PRZ * Post-test analysis with RELAP5/MOD3.2 and MOD3.3
Ref.15/  Kral P.: Introductory Calculations with RELAP5 Model of PWR PACTEL Facility with Vertical Steam Generators. CAMP Meeting. April * Finnish exp. facility PWR-PACTEL newly in PWR configuration with vertical SGs * Introductory tests with RELAP5/MOD3.3

20. 2. Assessment of RELAP5 in UJV Rez (cont’d)
Fig.7 PSB experimental facility and nodalization for RELAP5-3D

21. 2. Assessment of RELAP5 in UJV Rez (cont’d)
Some of the assessment works were supplied to US NRC as the “in-kind” contribution in frame of the CAMP agreement and consequently issued in the NUREG/IA series.
However, not only the assessment of RELAP5 has been performed in UJV Rez. The Czech side participated partially also in code development – both in discussing and prioritizing of further code development (during the CAMP and TPC meetings) and in the code development itself. Some examples of RELAP5 modifications prepared in UJV Rez:
Implementation of various critical heat flux (CHF) correlations
Extension of inner memory for large 6-loop input deck of VVER-440
Special valve model “end-valve” (included into standard R5/M3.3)
Coupling with CFD code (on-going work, FLUENT)

22. 3. Application of RELAP5 to NPP Analysis
During 20 years of RELAP5 implementation and utilization in UJV Rez, we have applied this advanced computational tool and the knowledge based on RELAP5 analysis to various areas of nuclear safety:
Licensing analyses for Chapter 15 of RSAR of VVER-440 (new fuel, new I&C, power uprate) and VVER-1000 (new fuel, power uprate).
Support analysis for new symptom-oriented Emergency Operating Procedures (EOP) of Westinghouse type developed for NPP Dukovany and Temelin.
Best-estimate and conservative analyses for NPP modifications (optimization of HA pressure at VVER-440, modified ESFAS signals etc.).
Support analyses for Russian VVER-440s (NPP Kola etc.) during DOE sponsored development of Emergency Operating Instructions.
Evaluation of Pressurized Thermal Shock (PTS) for both NPP Dukovany (VVER-440) and NPP Temelin (VVER-1000).

23. 3. Application of RELAP5 to NPP Analysis (cont’d)
TH analyses supporting Probabilistic Safety Assessment (PSA).
Development ot Best-Estimate Plus Uncertainty (BEPU) Methodology for licensing analysis of VVER-1000 (based on GRS method).
TH analyses covering issues identified in Periodical Safety Review of NPP Dukovany (shutdown analysis, BDBA, etc.)
TH analyses for VVER-440/213 Long Term Operation (LTO) project.
Water hammer analysis for VVER-440/213 (starting with SG overfilling transient potentially resulting in WAHA in MSL).
TH analyses of issues identified in the stress-tests of Czech NPPs (starting).

24. 3. Application of RELAP5 to NPP Analysis (cont’d)
One example of RELAP5 application to VVER analyses:
TH analyses for Optimization of Hydroaccumulator Pressure in VVER-440
The initiative to optimize VVER-440 hydroaccumulator parameters – mainly reduce the HA pressure - arose in early 90ties at Dukovany NPP during preparation of symptom oriented EOP (WESE). The two primary objectives of this effort were as follows:
1. To improve boundary conditions for operator during mitigation of primary-to-secondary leak accidents (to enable the operator decreasing RCS pressure below SG SV set-points without disturbances caused by HA injection).
To get better efficiency of HA injection in LOCA (mainly in intermediate and large break LOCA):
Reduce “ECCS bypass”
Longer time margin for operator in case of failure of all active ECCS
Improve system behavior in case of failure of HPSI or LPSI
Reduction of time between HA injection end and LPIS injection start

25. 3. Application of RELAP5 to NPP Analysis (cont’d)
Unit
VVER-440/213
VVER-1000
Sizewell-B (Westinghouse)
KWU-1300 Konvoi
EPR-1600
Basic pressure characteristics of NPP: - primary pressure - secondary pressure - SG safety valves opening/closing press.
MPa MPa
MPa
/4.9
/8.0
/8.3
/8.3
/9.4
HPIS (MPIS) pumps: - number - shutoff head - normal flow rate
- MPa kg/s
LPIS pumps: - number - shutoff head - normal flow rate
Hydroaccumulators: - number - pressure (orig.) - water volume - gas volume - ratio gas/total volume
- MPa m3 m3 -
Points of ECCS injection -
CL, HL, UP, DC CL
CL, HL

26. 3. Application of RELAP5 to NPP Analysis (cont’d)
The analytical work performed by NRI Rez in 90-ies resulting in reduction of HA pressure at NPP Dukovany in The TH analyses with RELAP5 could be divided into the following groups:
Analysis of spectrum of LOCA with no active Safety Injection (SI) and 4 HA’s with various pressures.
Analysis of spectrum of LOCA with no active SI and 4 HA’s with various levels.
Analysis of spectrum of LOCA with no active SI and 4 HA’s with various temperatures.
Analysis of spectrum of LBLOCA with no LPIS, 1/3 HPIS and 2 HA’s at various pressures.
Analysis of SBLOCA with no HPIS, 1/3 LPIS and 2 HA’s at various pressures.
Safety analyses with final modification of HA pressure.
The work and expert discussion between UJV Rez, NPP Dukovany and also the Czech regulatory body (SUJB) resulted in 1995 in the final proposal of HA pressure reduction from 6.0 MPa to 3.5 MPa. This design modification was applied at NPP Dukovany in 1998.

27. 3. Application of RELAP5 to NPP Analysis (cont’d)
Example of a matrix of LOCA analyses – scenarios without active ECCS:

28. 3. Application of RELAP5 to NPP Analysis (cont’d)
Fig.8 RELAP5 calculations of SBLOCA D50 mm with various HA pressure (without active ECCS)

29. 3. Application of RELAP5 to NPP Analysis (cont’d)
Fig.9 SNAP animation of LBLOCA analysis with RELAP5 (VVER-440/213, HA injection phase, 50 s)

30. 3. Application of RELAP5 to NPP Analysis (cont’d)
The example of NPP Dukovany in optimizing of HA parameters was followed by some other VVER-440/213 in Europe:
1. NPP Dukovany, The Czech Republic, 1998: Reduction of HA pressure 6.03.5 MPa
2. NPP Loviisa, Finland, 2000: Reduction of HA pressure (5.53.5 MPa) Increase of HA level (equivalent volume increase 4050 m3)
3. NPP Paks, Hungary, 2006: Reduction of HA pressure (6.0  3.5 MPa) Increase of HA level (6.07.1 m)
4. NPP Jaslovske Bohunice, Slovakia, 2006: Reduction of HA pressure 6.03.5 MPa

31. 6. SUMMARY
In the 3 decades of its existence and development, the RELAP5 computer code has proved to be advanced and very flexible computational tool for best-estimate and conservative TH analyses of not only western PWR, but also VVER reactors.
Thanks to its world-wide spreading, assessment and application to various types of reactor systems, the code characteristics and capabilities are very well known. So the qualified users have very good conditions for correct usage of the code.
The application of RELAP5 to VVER analysis in last 20 years has substantially contributed to safe and efficient operation of VVER reactors.
The advanced system TH codes like RELAP5 enable improvements of very good and robust VVER design (which originates in 70ties).

32. Thank you for your attention