FAST: The Merger of NRC s Fuel Performance Codes FRAPCON and FRAPTRAN for Scoping and Regulatory Decision Making

Transcription

1 FAST: The Merger of NRC s Fuel Performance Codes FRAPCON and FRAPTRAN for Scoping and Regulatory Decision Making Ian E. Porter, Ph.D. United States Nuclear Regulatory Commission (U.S.NRC) Washington, DC, 20555, USA Tel: Ian.Porter@nrc.gov WRFPM 2017 Jeju Island, South Korea September 10-14, 2017

2 What is FAST? Fuel Analysis under Steady-state and Transients (FAST) is the merger of FRAPCON-4.0 Patch 1 and FRAPTRAN-2.0. It can model the steady-state, AOO and DBA response of LWR fuel rods. FAST can run in steady-state mode with a FRAPCON-4.0 input file. FAST-1.0 was built off of the FRAPCON-4.0 architecture while incorporating the transientspecific functionality from FRAPTRAN. Some of the new functionality includes: Transient conduction solution Transient FGR and internal gas flow New clad-to-coolant heat transfer correlations, including reflood High temperature oxidation Ballooning Cladding failure criteria Some of the deleted functionality from FRAPCON-4.0: Removed all coolant temperature and HTC correlations, replaced with the correlations from FRAPTRAN, including a new transient solution to the Jens-Lottes correlation Replaced existing temperature drop correlations with FRAPTRAN s correlations FAST-1.0 is now largely written in SI units September 2017 TopFuel 2017 Jeju Island, South Korea 2

3 Why Merge? Simplified code development No longer maintaining two separate codes that have a significant amount of overlap Codes changes now need to be implemented only once (e.g., new material or physics) rather than implemented into FRAPCON and FRAPTRAN Faster turn around time for code changes No inconsistencies FRAPCON and FRAPTRAN were not identical with respect to internal variables, input variables and models. Predictions of burnup-dependent FRAPTRAN cases were not necessarily identical to the FRAPCON final state under the same conditions. Improved ease of modeling transients No longer need FRAPCON-to-FRAPTRAN restart file and two input files that contained different input variables (e.g., FRAPCON input file uses dco, FRAPTRAN uses RodDiameter) AOO timespace Maintains existing assessment database (176 assessment cases) Maintains knowledge of entire code architecture and how the code predicts relative to US fuel vendor codes (faster turn around time for Topical Report reviews) September 2017 TopFuel 2017 Jeju Island, South Korea 3

4 What s New that wasn t in FRAPCON or FRAPTRAN? The transient solution now accounts for the temperature drop across the ID oxide layer, OD oxide layer and OD crud layer (If existent) The structural thickness of the cladding can be reduced to account for clad thinning due to oxidation (Requirement in NRC Regulation ISG-11, Rev. 3 for Spent Fuel Cladding Hoop Stress Analysis) FAST-to-FAST restart capability. How this is beneficial: Modeling father rod s life in-reactor (single FAST input file) and then the series of test rods created from sections of the father rod (Many FAST input files). Ability to continue an analysis after a transient Can now input burnup timesteps rather than time Material Properties Library No existing functionality from FRAPCON-4.0 or FRAPTRAN-2.0 was removed September 2017 TopFuel 2017 Jeju Island, South Korea 4

5 MatLib: The NRC s Material Library Material Library (MatLib) is the new library of material properties used by FAST It is compiled as a separate library and linked to FAST (has also been used by TRACE for ATF scoping studies) The library consists of thermal-mechanical properties for fuel, cladding, gases, coolants, and structural materials Fuel Cladding Gases Misc / Structural Coolants Zirc-2 Helium ZrO Zirc-4 Argon 2 UO 2 + (Gd 2 O 3, ZrB 2 ) M5 TM Crud Krypton (U,Pu)O 2 ZIRLO TM Boron Nitride Water Xenon U 3 Si 2 Opt. ZIRLO TM Constantan Sodium Hydrogen Steels (SS304, SS316, Helium E110 Nitrogen SS347, Inconel 600, FeCrAl Air Inconel 718, CSA508) SiC Water Only the UO 2 -Zirc fuel and cladding material properties are recommended and assessed against. The ATF properties (U3Si2, FeCrAl, SiC) are currently included in the library for scoping purposes Future development of MatLib will focus on ability for user to define material properties via input file, to accommodate using proprietary correlations September 2017 TopFuel 2017 Jeju Island, South Korea 5

6 Temperature Verification During a power ramp, it takes ~3 thermal time constants to achieve convergence to the new steady-state value (typically 6-8 seconds for commercial rods, ~3-5 seconds if a smaller gap is assumed). Note: Thermal time constant as defined by NRC s operatinglicensing exam is the amount of time for the fuel cladding temperature to reach 63% of its total change. To verify the transient temperature solution, a 50% ramp and hold was performed using the FRAPCON steady-state temperature solution and the FAST transient temperature solution The thermal time constant was calculated as 3.7 seconds, which is in the expected range for the case analyzed. The fuel centerline predictions (right) show that the two solutions reached convergence, as expected September 2017 TopFuel 2017 Jeju Island, South Korea 6

7 Example AOO An example AOO was run that examines a short-duration 3x overpower event. A 3x overpower event would have resulted in a fuel melt with FRAPCON due to the assumption that it is instantaneous (steady-state). The 1s overpower results in a 0.2% permanent transient cladding strain whereas the 5s overpower results in a 0.8% strain, which is reflected in the increase in fuel centerline temperature after the transient. We will continue to work on further assessing power ramps and other AOOs September 2017 TopFuel 2017 Jeju Island, South Korea 7

8 Ongoing Work Updating existing Integral Assessment Before it s release, FAST will be run through the entire integral assessment database that currently exists for FRAPCON and FRAPTRAN. The FRAPTRAN input files will be re-written to work with FAST and all of the existing input files for FRAPCON are being re-examined to ensure they utilize the latest code features (some input files are from very early versions of FRAPCON). Like FRAPCON and FRAPTRAN, FAST undergoes extensive assessment against proprietary and non-publicly available data from test programs. As this data becomes publicly available, it will be added to the official assessment database. New integral assessment cases are being added. These are currently focused on cladding creep, FGR, fuel centerline temperature, void volume, and RIA/LOCA integral effects Development of an updated Fission Gas Release model. There currently exists two main (non-ans) models: the Massih and the FRAPFGR model. Each model has its strength (Massih produces best-estimate of FGR to the void, FRAPFGR produces best-estimate of locations of gases within the pellet). These models are being combined by PNNL to produce a single model. Next, the focus of the FGR model development will be to analyze impacts of grain size on FGR and the modeling of FGR under transient conditions (expanding upon existing burst release model) September 2017 TopFuel 2017 Jeju Island, South Korea 8

9 Ongoing Work (cont d) FAST is being upgraded to work with a 1-D, 2-D and 3-D finite volume solver using a set of parallel Fortran libraries built on the open source NEMO package. Currently for licensing applications, the 1.5D finite different solution is appropriate The reason for pursuing this option is in preparation for ATF and/or advanced reactors, where the potential exists for non-cylindrical geometries and physics that may need to be solved without the embedded assumptions used with the UO2/Zirc system (e.g., the rigid pellet assumption). This new solver will NOT replace the current finite-difference solver. Instead, it will be an option similar to the existing cladding FEA model. FAST is being upgraded to have a new plot file using the Exodus-II format which can be read using ParaView or other open-source tools Coupling with SCALE, TRACE and PARCS Development of a SNAP plug-in, including animation support and the ability to have interactive variables, as well as a APT-Plot support Increased unit testing (Currently use cmake/ctest with intel and gfortran) September 2017 TopFuel 2017 Jeju Island, South Korea 9

10 Future Work Continued implementation of new material properties (Including dopants, new Zirc-based claddings and (as appropriate) ATF and Advanced Reactor material properties) Incorporation of TRACE s pipe model to replace FAST s bulk coolant model, including using TRACE s water properties package in lieu of the sth2xt package Support in SNAP to run coupled FAST/TRACE, FAST/PARCS and FAST/TRACE/PARCS calculations. It is not yet clear if we will run coupled FAST/SCALE calculations through SNAP. Improvements in LOCA modeling, including looking at FGR and axial fuel relocation Update the way FAST performs spent fuel calculations (DATING does not utilize FAST s ability to calculate changes in RIP due to cladding creep and fuel swelling), as well as assess the creep correlations with newer data Developing a cladding ID oxide formation model (Driven by proposed 50.46c that requires fuel with a burnup > 30 GWd/mtU to assume id oxide is present) Developing a hydride rim formation model (rather than assuming a radially-average hydrogen concentration as is currently implemented) Work with code users to implement new features and material properties September 2017 TopFuel 2017 Jeju Island, South Korea 10

11 Summary and Conclusions FAST, the merger of FRAPCON and FRAPTRAN, is the NRC s new fuel performance code Plans are to release it in FY 2018, pending the completion of the integral assessment and associated NUREG documents (Code Description, Integral Assessment and MatLib) Significant effort is being placed on Code Assessment and continued expansion of unit testing Undergoing major enhancements in terms of functionality 3D Finite Volume solver not restricted to a cylinder New Exodus II output file for better images New FGR model If NRC starts work on ATF-related activities, MatLib will be updated with new properties and correlations September 2017 TopFuel 2017 Jeju Island, South Korea 11