Core-Wide Estimates of Fuel Dispersal During a LOCA

Core-Wide Estimates of Fuel Dispersal During a LOCA

U.S.NRC 2014 FRAPCON/FRAPTRAN Code Applications and Developments Patrick Raynaud, Ph.D. United States Nuclear Regulatory Commission (U.S.NRC) Washington, DC, 20555, USA Tel: +1-301-251-7542 Email: [email protected] FRAPCON/FRAPTRAN User Group Meeting September 18, 2014 U.S. NRC Research 2014 Applications and Developments FRAPCON IFA-716 fission gas release modeling Grain size and fission gas release models FRAPCON-DATING/SFMOD Extended spent fuel storage modeling Incorporation of DATING Spent Fuel MODification: additional gas moles and fuel swelling FRAPTRAN WGFS RIA benchmark phase 2 Convergence difficulties FEA model FRAPCON/FRAPTRAN Fuel dispersal calculations

Code modernization Gamma heating FRAPCON coolant temperatures and pressures FRAPCON deformation printouts and gap printout bug FRAPTRAN/TRACE coupling 18 September 2014 FRAPCON/FRAPTRAN User Group Meeting 2 Presented at EHPG September 11, 2014 IFA-716 Background and Objectives The IFA-716 test provided experimental data to evaluate fission gas release (FGR) models in fuel performance codes Test rods were instrumented with pressure gauges and linear variable differential transformers Fission gas release behavior can determined from known pressures and rod free volume 4 rods: Objectives: Rod 1: UO2 with 0.16% Cr2O3 pellets, 71 m grainsm grains Rod 2: UO2 pellets, 11 m grainsm grains (Halden data was poor quality, likely transducer failure)

Rod 5: UO2 pellets, 55 m grainsm grains Rod 6: UO2 with 0.1% Cr2O3 pellets, 59 m grainsm grains Calculate the rod free volume, gas temperature, and fission gas production Combine the above information from FRAPCON with Haldens pressure measurements Calculate the FGR for all four rods modelled Secondary Objective: Assess the accuracy of the Modified Forsberg-Massih FGR model in FRAPCON for chromia doped pellets NOTE: found problem with variable axial node lengths and axial zoning under investigation 18 September 2014 FRAPCON/FRAPTRAN User Group Meeting 3 Presented at EHPG September 11, 2014 IFA-716 Analytical Methods & Results Adjusted rod plenum length to match initial rod internal volume Test Rod IFA716 rod 1 IFA716 rod 2 IFA716 rod 5 IFA716 rod 6 Measured rod free volume (cm3) 5.80 5.95

5.90 6.00 Measured Plenum Length (mm) 40.80 40.80 41.10 41.40 FRAPCON calculated rod free volume for measured plenum length (cm3) 4.34 4.31 4.10 4.33 Adjusted plenum length (mm) 66.79 69.89 72.99 70.75 FRAPCON calculated rod free volume for adjusted plenum length (cm3) 5.80 5.95 5.90

6.00 18 September 2014 FRAPCON/FRAPTRAN User Group Meeting 4 Presented at EHPG September 11, 2014 IFA-716 Analytical Methods & Results Initial calculation used default FRAPCON-3.5 options, where a 10 m grainsm grain size is hard-wired in the code for the modified Forsberg Massih FGR model Poor agreement with Halden data Modified the code so that GRNSIZE was actually read and used by the subroutine Much better agreement with the Halden data 18 September 2014 FRAPCON/FRAPTRAN User Group Meeting 5 Pre-decisional work submitted to JNM for publication on September 10, 2014 Cladding Stress in Spent Fuel During

Extended Dry Storage NRC has performed a study of cladding stress in spent nuclear fuel (SNF) for a 300 year period of dry storage This work is part of the ongoing research effort for Extended Storage and Transportation (EST) GOAL: to assess the potential for low temperature creep (LTC) and delayed hydride cracking (DHC) failures FRAPCON developments to support this work: FRAPCON-DATING Cladding creep in storage at the end of irradiation Calculation based on rod internal pressure or specified cladding stress, and temperature decay curve for helium cooling, nitrogen cooling, or user-specified FRAPCON-SFMOD FRAPCON-3.5 with option to specify additional gas moles and additional fuel pellet swelling as a function of time 18 September 2014 FRAPCON/FRAPTRAN User Group Meeting 6 Pre-decisional work submitted to JNM for publication on September 10, 2014 Dry Storage Cladding Stress: Scope and Methodology 7 fuel designs with many different power histories (243 cases modeled) Sequence modeled: Reactor irradiation to 60 GWd/MTU 5 years in wet storage 300 years in dry storage

Potential sources of stress: Decay gas production and release (from ORIGEN calculation) Fuel pellet swelling during storage (from literature survey: best-estimate and upper-limit correlations developed) FRAPCON-SFMOD Stresses as a result of decay gas production/release and pellet swelling Did not produce accurate creep strain predictions FRAPCON-DATING Cladding creep strain as a result of the stresses calculated with FRAPCON-SFMOD 18 September 2014 FRAPCON/FRAPTRAN User Group Meeting 7 Pre-decisional work submitted to JNM for publication on September 10, 2014 Dry Storage Cladding Stress: Decay Gas and Fuel Swelling Stress calculated with FRAPCON-SFMOD Initial drop in stress due to rapid cladding creep (red lines: upper limit swelling, green lines: bestestimate swelling) Pellet swelling saturates at 50 to 150 years

Stress increase after initial drop is primarily due to decay gas production Stress reaches levels that could potentially result in hydride reorientation, but at times when hydrogen solubility and temperature are low Resulting strains calculated with DATING code: BWR 10x10 PWR 17x17 0.44% - 0.50% for BWR 0.72% - 0.95% for PWR 18 September 2014 FRAPCON/FRAPTRAN User Group Meeting 8 Pre-decisional work submitted to JNM for publication on September 10, 2014 Dry Storage Cladding Stress: Critical Crack for DHC DHC critical flaw size calculated assuming KIH=5 MPam (conservative value) Assuming existing flaw of 120m grainsm (conservative) Critical flaw size becomes smaller than the existing flaw size after 292 and 264 years of storage for BWR9x9 and BWR 10x10 cladding, respectively For other designs (BWR8x8, PWR14x14, PWR15x15, PWR16x16, PWR17x17), the critical flaw size exceeds the size of any expected flaw for the 300 year period of dry storage Fuel Design Normalized flaw size at 120 mm

Time to reach 120 mm critical flaw size (years) Temperature at 120 mm critical flaw size (K) Stress at 120 mm critical flaw size (MPa) BWR 8x8 BWR 9x9 BWR 10x10 PWR 14x14 PWR 15x15 PWR 16x16 PWR 17x17 0.1476 0.1688 0.1818 0.1628 0.1688 0.1890 0.1967 >300 292 264 >300 >300 >300 >300 <370 375 385 <370 <370 <370

<370 >205 200 195 >175 >155 >165 >155 18 September 2014 FRAPCON/FRAPTRAN User Group Meeting 9 To be discussed at OECD/NEA WGFS meeting September 2014 OECD/NEA RIA Benchmark Phase 2 FRAPTRAN without FRAPCON 8 stylized RIA cases 3 coolant conditions: NSRR capsule, PWR hot zero power, BWR cold zero power Gap vs. no gap, sliding vs. no sliding contact 2 different power pulses 2 different rod pressures No FRAPCON initialization Most cases ran well (6/8) FEA model was used for sliding contact case only Problems encountered

Cannot specify no spring (non-zero but negligible values were used) BWR cold zero power did not converge easily Problem with specifying zero flow was identified (very low flow specified instead) For higher powered pulse, ballooning predicted, as well as boiling crisis Injected energy continued to increase after pulse: could be error or due to metal water reaction Very high temperatures predicted Code-to-code comparison results will be discussed later this month at OECD/NEA WGFS meeting 18 September 2014 FRAPCON/FRAPTRAN User Group Meeting 10 Presented at WRFPM 2014 Fuel Dispersal Predictions with FRAPCON/TRACE/FRAPTRAN Experimental Research Fuel rod rupture conditions Fuel relocation conditions Burnup dependent particle size distribution ANL, Halden and Studsvik LOCA research, documented elsewhere 18 September 2014 Fuel dispersal predictions

Number, timing, and location of fuel rod ruptures during a LOCA Fuel rod characteristics at and near rupture Estimation of dispersed fuel based on experimentally determined thresholds FRAPCON/FRAPTRAN User Group Meeting Consequence analysis Based on quantification and characterization of dispersed fuel Prediction of thermal hydraulic and radiological consequences Future analytical work 11 Presented at WRFPM 2014 Fuel Dispersal Modeling Strategy Steady-state fuel performance: FRAPCON PWR coolant temperatures from TRACE

Initialization parameters for TRACE steady-state calculation Initialization parameters for FRAPTRAN transient calculation TRACE Transient systems thermal hydraulics: TRACE TRACE steady-state initialization Initialization parameters for FRAPTRAN transient calculation FRAPCON Steady-state systems thermal hydraulics: TRACE FRAPCON steady-state initialization Initialization parameters for TRACE transient calculation Steady-State TRACE Transient fuel performance: FRAPTRAN FRAPCON steady-state and TRACE transient initialization FRAPTRAN Transient 18 September 2014 FRAPCON/FRAPTRAN User Group Meeting 12 Fuel Dispersal Code Problems and Fixes Gamma heating was not taken into account in FRAPCON and was a constant in FRAPTRAN Gamma heating for FRAPCON: user-input OR default values for PWR and BWR, based on

neutronic calculations performed by Ian Porter (journal article under review) Will be in next version of FRAPCON Updated gamma heating for FRATRAN: user-input OR correlation based on coolant density (based on 17x17 PWR fuel, to be verified for other designs) Will be in next version of FRAPTRAN Core coolant temperatures are relative uniform for PWR, but single rod cod predicts cladding temperature assuming no mixing and no cross flow, and no P taken into accountP taken into account Over prediction of temperatures, corrosion, FGR, etc. for high power rods, and under prediction for low power rods New ability to specify coolant temperature and pressure Noticed some problems with FRAPCON gap printout Gap calculation for printout based on OD cladding displacement, instead of ID displacement Will be fixed in next version of FRAPCON Printouts for fuel strain: thermal, densification, swelling, and relocation New printouts for cladding : ID and OD displacement 18 September 2014 FRAPCON/FRAPTRAN User Group Meeting 13 Fuel Dispersal FRAPCON/FRAPTRAN Developments Code modernization performed to date FRAPCON: fully upgraded to F90 files, no more common blocks, fully dynamic arrays FRAPTRAN: fully upgraded to F90 files, no more common blocks Code modernization in progress Removal of unused code Migration to FORTRAN 95: Full conversion to modules, no implicit variables, no include statements, use of PARAMETER statements,

120 character lines, etc Reduce the need for compiler options to make codes more portable Logic improvements when practical (CASE SELECT, GOTO, etc) Dynamic dimensioning in FRAPTRAN Develop guidelines for future programming Coupling of FRATRAN and TRACE Tested for 1 fuel rod and 1 heat structure: seems to work well Developed but not yet fully tested for larger TRACE model with more than 1 heat structure 1 TRACE run in parallel with many FRAPTRAN runs Coupling based on dynamic pointers 18 September 2014 FRAPCON/FRAPTRAN User Group Meeting 14 Fuel Dispersal Items for Future Investigation Fuel rod ruptures predicted during quench for LBLOCA Burst strain and balloon length have a large impact on fuel dispersal predictions, but relatively large uncertainty on these predictions, particularly balloon length Look into modeling potential effects of grid spacers on thermal and mechanical response Look into pros and cons of modeling grid spacers in FRAPCON 18 September 2014 FRAPCON/FRAPTRAN User Group Meeting 15

Others Items of Interest New tools: DOXYGEN for documentation GIT for version control Possible interface for users in the future, but not yet decided Both tools also used for BISON development Beta versions Future code documentation Back-compatibility Old versions for download Users suggestions? 18 September 2014 FRAPCON/FRAPTRAN User Group Meeting 16

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