CFD Topics at the US Nuclear Regulatory Commission. Christopher Boyd, Ghani Zigh Office of Nuclear Regulatory Research June 2008

Transcription

1 CFD Topics at the US Nuclear Regulatory Commission Christopher Boyd, Ghani Zigh Office of Nuclear Regulatory Research June 2008

2 Overview Computational Fluid Dynamics (CFD) is playing an ever increasing role in the Nuclear Community and this trend is mirrored at the Nuclear Regulatory Commission. There is a growing trend for applicants to submit safety evaluations or other supporting calculations that include CFD analyses. The NRC continues to develop its own capabilities in this area in order to evaluate these submissions and to support our safety analyses. 2/26

3 CFD tools at the NRC Commercial codes used for safety evaluations and in-house research. ANSYS/FLUENT STARCCM+ Research tools (typically contract work) used for selective developments such as two-phase modeling or other problem specific capabilities. OpenFoam used for HTGR work NPHASE used for two-phase research FDS used fire analysis 3/26

4 NRC applications for CFD Now confirmatory calculations scoping of technical issues development of regulatory guidance tuning/refining system level codes consideration of 3D effects support for experimental design Future possible role in an overall evaluation model HTGR systems 4/26

5 Examples of Current Applications In-House (Commercial Codes) induced failures of SG tubes prediction of flow and heat transfer during counter-current flow of superheated steam during a severe accident ESBWR review boron mixing during ATWS mixing in the chimney region HTGR code validation DOE MIR facility data fuel transportation and storage casks safety analyses confirmatory studies development of best practice guidelines 5/26

6 Examples of Recent CFD Research Funded Research NPHASE (two-phase tool) research at RPI and ARL/PSU OPENFOAM predictions and developments for HTGR work other HTGR related work on basic model developments using basic CFD tools 6/26

7 Verification and Validation Issues The NRC has participated in the development of best practice guidelines for CFD. Best Practice Guidelines for the use of CFD in Nuclear Reactor Safety Applications Assessment of Computational Fluid Dynamics (CFD) for Nuclear Reactor Safety Problems NEA/CSNI/R(2007)13 The NRC is following developments in the verification and validation areas and participates in these activities when practical. DOE work related to HTGR NEAMS project under GNEP broader CFD community work 7/26

8 V&V issues continued Clearly we are moving beyond the time when any CFD prediction is considered good. V&V issues are an issue that will need to be addressed. The NRC has helped support best practice guidelines for CFD and would like to see problem specific best practice guidelines. existing guidelines are typically generalizations problem specific guidelines are being worked on for the spent fuel storage and transportation casks data bases for CFD validation are needed under conditions that are relevant to specific safety issues 8/26

9 Some Examples of NRC CFD Work dry cask storage and transport induced SG tube failures spent fuel pool natural circulation boron mixing in RPV downcomer boron mixing in ESBWR core bypass upper plenum flows HTGR issues related to code validation various supporting calculations 9/26

10 Code Coupling The NRC is interested in the issue of coupling a CFD tool with a system level code but has not committed funding for this issue. system code developments still underway commercial Code vs. Research Code Issues specific safety issue for coupling has not been identified manual one-way coupling has been sufficient for the problems of interest 10/26

11 NRC review Concerns Best Practice Guidelines Were they considered? What was the model selection and development process? Is the model appropriate for the physics? Boundary Conditions and Assumptions What is the basis for assumptions? What is the sensitivity of results to assumptions? One prediction just below the limit limit is 10, prediction submitted with value of 9.8 What is the uncertainty? What if? Without proper verification and validation of CFD tools, a single solution is difficult to defend (and should not be accepted). 11/26

12 NRC Review Example Submitted result below the limiting value. NRC results show that reasonable variations in boundary conditions or modeling parameters will put the result over the limit. 12/26

13 Some Examples of NRC Analysis Induced tube ruptures during severe accidents in PWRs Boron mixing in the core bypass region of ESBWR during ATWS Analysis of spent fuel storage and transportation casks 13/26

14 Induced Tube Failures The NRC has been looking at induced tube failures during severe accidents in PWR s. The calculations are completed using system codes such as SCDAP/RELAP5 or MELCOR but there are some 3D issues that require additional analyses or testing to fully understand. 14/26

15 Natural Circulation Flow During Severe Accidents Predictions of flows during a severe accident extend limited experimental data base to accident conditions. CFD predictions reduce the need for experiments saving money for the NRC. 15/26

16 Various designs, detailed tube temperature predictions, numerous sensitivity studies Three different SG s considered 16/26

17 CFD Predictions for Two Steam Generators hot leg T Design A Steam Generator Design B Steam Generator secondary side T Inlet Plenum Geometry Impact on Mixing 17/26

18 ESBWR- Boron Mixing in Core Bypass One small part of NRC s ESBWR review includes a class of scenarios called Anticipated Transients without Scram (ATWS). The reactor is designed with a backup system to shutdown the reactor in the event that the primary shutdown mechanism (control rods) fail. The Standby Liquid Control System (SLCS) is used to shutdown the reactor during an ATWS. SLCS floods the reactor core bypass region with a borated solution which shuts down the reactor 18/26

19 Background One small part of NRC s review includes a class of scenarios called Anticipated Transients without Scram (ATWS). The reactor is designed with a backup system to shutdown the reactor in the event that the primary shutdown mechanism (control rods) fail. The Standby Liquid Control System (SLCS) is used to shutdown the reactor during an ATWS. 19/26

20 Modeling Overview bulk flow is from top to bottom SLCS injected into outer peripheral open region of bypass How effectively does boron penetrate into the inner regions of the core bypass? Horizontal Slice 20/26

21 Boron Penetration into Core Bypass normalized concentration contours red 1.0, blue /26

22 Fuel Storage and Transportation NRC uses CFD for confirmatory analyses in dry cask applications, and validation exercises for method validation. CFD is used to analyze dry cask for: Normal storage and transportation (long-term): Steady state analysis Accident analysis (short-term): Transient analysis, e.g. (blocked vents scenario, fire analysis, etc.) Vacuum Drying (Transient analysis) 22/26

23 Fuel Storage and Transportation Dry cask thermal performance should comply with 10CFR72 requirements. Thermal evaluation follows the guidelines of NUREG-1536 and Interim Staff Guidance Memorandum No.11 to demonstrate thermal compliance of the dry cask Specifically, some of the requirements are: The fuel cladding temperature for long term storage shall be limited to 400 C The fuel cladding temperature for short term scenarios (accidents) shall be limited to 552 C The system should be passively cooled 23/26

24 Example: Validation of CFD method using VSC17 Experiment ¼of a cask Top view at mid height of the cask 24/26

25 VSC-17 Results (Comparison) Fuel axial temperature using low Reynolds k-ε turbulence model Liner and MPC walls axial temperature using low Reynolds k-εturbulence model 25/26

26 Conclusion The NRC is actively involved in the application of CFD for nuclear reactor safety analysis confirmatory analysis independent analysis developmental analysis There is an interest in best practice guidelines and the whole area of verification and validation of CFD tools. Work still needs to be done in these areas to ensure the proper application of CFD tools. 26/26