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LOCA 규제검증해석에서 연소도 및 유동막힘의 고려 방향

LOCA 규제검증해석에서 연소도 및 유동막힘의 고려 방향. Young Seok Bang k164bys@kins.re.kr Korea Institute of Nuclear Safety Kwahak-ro 62, Yuseong , Daejeon, 34142. 2019 안전해석심포지엄 , 2019. 7.4~7.5, 파로스콘도 , 대천. Contents. Introduction Modeling Results and Discussion Concluding remarks. 1.Introduction.

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LOCA 규제검증해석에서 연소도 및 유동막힘의 고려 방향

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  1. LOCA 규제검증해석에서 연소도 및 유동막힘의고려 방향 Young Seok Bang k164bys@kins.re.kr Korea Institute of Nuclear Safety Kwahak-ro 62, Yuseong, Daejeon, 34142 2019 안전해석심포지엄, 2019. 7.4~7.5, 파로스콘도, 대천

  2. Contents • Introduction • Modeling • Results and Discussion • Concluding remarks

  3. 1.Introduction • Proposed ECCS rule revision • In USA since 2014, and in Korea since 2017 • Requires consideration of fuel burnup • Acceptance criteria regarding Peak Cladding Temperature (PCT) and Equivalent Cladding Reacted (ECR) as functions of Burnup • Swell & Rupture and Flow blockage • Important issue in the existing requirements on both Conservative EM and BE EM • Not so much attention • Extensively discussed in the course of review on Topical Report for LBLOCA methodology using SPACE code • Effect of burnup on S&R and flow blockage needs to evaluate

  4. 1.Introduction • Appendix K to 10 CFR 50 • A.5 Metal–Water Reaction • For rods whose cladding is calculated to rupture during the LOCA, the inside of the cladding shall be assumed to react after the rupture. The calculation of the reaction rate on the inside of the cladding shall also follow the Baker-Just equation, starting at the time when the cladding is calculated to rupture, and extending around the cladding inner circumference and axially no less that 1.5 inches each way from the location of the rupture, with the reaction assumed not to be steam limited. • B. Swelling & Rupture of the Cladding and Fuel Rod Thermal Parameters • Each evaluation model shall include a provision for predicting cladding swelling and rupture from consideration of the axial temperature distribution of the cladding and from the difference in pressure between the inside and outside of the cladding, both as functions of time.

  5. 1.Introduction • To be acceptable the swelling and rupture calculations shall be based on applicable data in such a way that the degree of swelling and incidence of rupture are not underestimated. • The degree of swelling and rupture shall be taken into account in calculations of gap conductance, cladding oxidation and embrittlement, and hydrogen generation. • C.7. Core Flow Distribution During Blowdown • Calculations of average flow and flow in the hot region shall take into account cross flow between regions and any flow blockage calculated to occur during blowdown as a result of cladding swelling or rupture. • D.5.b Refill and Reflood Heat Transfer for PWR • During refill and during reflood ~, and shall take into account any flow blockage calculated to occur as a result of cladding swelling or rupture as such blockage might affect both local steam flow and heat transfer.

  6. 1.Introduction • Regulatory Guide 1.157 • 3.2.5 Metal-Water Reaction Rate • For rods calculated to rupture their cladding during the loss-of-coolant accident, the oxidation of the inside of the cladding should be calculated in a best estimate manner. • 3.3.1 Thermal Parameters for Swelling and Rupture of the Cladding and Fuel Rods • A calculation of the swelling and rupture of the cladding resulting from the temperature distribution in the cladding and from the pressure difference between the inside and outside of the cladding, both as a function of time, should be included in the analysis and should be performed in a best-estimate manner. • The degree of swelling and rupture should be taken into account in the calculation of gap conductance, cladding oxidation and embrittlement, hydrogen generation, and heat transfer and fluid flow outside of the cladding.

  7. 1.Introduction • The calculation of fuel and cladding temperatures as a function of time should use values of gap conductance and other thermal parameters as functions of temperature and time. • Best-estimate methods to calculate the swelling of the cladding should take into account spatially varying cladding temperatures, heating rates, anisotropic material properties, asymmetric deformation of cladding, and fuel rod thermal and mechanical parameters. • 3.11 Core Flow Distribution During Blowdown • Calculations of the flow in the hot region should take into account any cross-flow between regions and any flow blockage calculated to occur during the blowdown as a result of cladding swelling or rupture. • 3.12.4 Post-Blowdown Heat Transfer for PWR • The calculations should also include the effects of any flow blockage calculated to occur as a result of cladding swelling or rupture.

  8. 1.Introduction • Fuel behavior following LOCA • Based on simulated tests and analysis of LOCA • Ballooning of cladding, Fuel fragmentation  Swell and Rupture  Blockage Relocation  Dispersal Ruptured claddings from tests Fresh fuel before loading 2-f flow after blockage

  9. 1. Introduction • Objectives • Improve understanding of the fuel response following LBLOCA under the actual fuel burnup state, especially on effect of fuel burnup and effect of flow blockage • Develop modelling schemes as needed • Multiple channels and fuel rods simulating the actual burnup condition, swell/rupture, flow blockage * within the current capabilities of TH code and Fuel performance code • Ongoing research plan • Track 1: Previous Capabilities  Sensitivity study  Determine modelling schemes Uncertainty calculation • Track 2: Development of New codes  repeat the Track 1 flow

  10. 2. Modeling • Overall nodalization for APR1400

  11. 2. Modeling • Multiple hydraulic channels and fuel rods Average Channel 2 Average Channel 1 Average channel Hot Channel 2 Average rods Hot rods crossflow 1 Hot rod Hot channel Hot Channel 1 Conventional Lumped system model Multiple fuel rods model with multiple channels Fuel Assemblies in ¼ Core

  12. 2. Modeling • Powers of fuel rods • heat source of multiple fuel rods k-th hot channel k-th average channel nq,krods nF-nq,k rods np,krods mA,knF-np,k rods m=mF Hm m=1 Rod power q0*fHH,k Rod power q0*wH,k Rod power q0*a*wA,k Rod power q0*fAH,k

  13. 2. Modeling • Flow Blockage by swell & rupture • Change of Flow Area at i-th channel • Change of Flow Area at k-th fuel assembly:

  14. 2. Modeling • Flow Blockage by swell & rupture (cont) • Because of large amount of fuel rods calculation, Grouping needed  Grouping all fuel rod data by burnup (m) and peaking factor (n) • , , • Flow area change simulated by valve component • Additional form loss factors imposed at rupture

  15. 2. Modeling • Other models • Thermal conductivity of fuel pellet at burnup state (NFI model) • Effective thermal conductivity of the cladding including the oxide layer . • Initial Oxide thickness with burnup (from results of fuel code) oxide cladding gap r1 pellet r2 toxide r3

  16. 3. Results & Discussion • Cases tested in hydraulic viewpoint • 1 average channel and 1 hot channel (1A1H) • 1 average channel and 2 hot channels (1A2H) • 2 average channel and 2 hot channels (2A2H) • 2A2H with Crossflow between average channels (2A2H+c)

  17. 3. Results & Discussion • Sensitivity study of Multiple hydraulic channels • 2A2H case: most conservative • 2A2H+c case higher reflood PCT than 2A2H case

  18. 3. Results & Discussion • Calculation at the actual burnup (from Nuclear design data) • Burnup and Radial peaking factor at End-of-Cycle for Cycle 1, 2 and 5 (~68,000 fuel rods) • 241 Fuel Assemblies and nF=236 rod/FA • Fuel rod length~12ft and Fuel rod outer dia.~0.9cm

  19. 3. Results & Discussion • Grouping and Discretization • 13intervals for burnup by 5 GWD/MTU • 32intervals for radial peaking factor by 0.05 • Number of fuel rods at each interval  Two groups (Bu*=30GWD/MTU) • Selection of hot assembly and hot rods • Highest peaking factor applied • Rods of low peaking factor & burnup: lumped into average rods Group 1 Group 2

  20. 3. Results & Discussion • Calculation condition • LBLOCA at cold leg under EOC of Cycle 5 pf APR1400 • Power • Axially ~ 1.62 Top-skewed profile • Max. Radial peaking:=1.47 • Postulated limiting hottest rod added (=1.54) from the bounding curve • Maximum linear heat =14 kW/ft • 32 fuel rods simulated (rod of low power & low burnup lumped) • ECCS • 2 out of 4 ECCS trains operable

  21. 3. Results & Discussion • Cladding temperature behavior (2A2H+c) • 4 different categories with Radial peaking factor

  22. 3. Results & Discussion • Blockage fraction of Hot channel 1 • Base case: 33% flow area reduction (z=1.0) • Maximum blockage ~72%(z=2.0) • First significant blockage by swelling & rupture at time of ECCS water reaching the hot spot (47 sec) • Subsequent increases of blockage by 1 or 2 times

  23. 3. Results & Discussion • RefloodPCT for the hottest rod by changingz (blockage multiplier)

  24. 3. Results & Discussion • RefloodPCT for all the rods by changingz (blockage multiplier) • -9~38 K over the range of blockage from 0~ 72% and the range of radial peaking factor from 1.2 to 1.54.

  25. Concluding remarks • Use of Multiple fuel rods modeling with multiple channels can improve understanding of LBLOCA fuel thermal response with burnup effect • Grouping of fuel rods & allocation ~ important factors for blockage prediction (currently intervals of 0.05 for Fxy, 5 GWD/MTU for Bu) • Safety parameters (PCT and PLO) not significantly affected by the currently predicted level of flow blockage, but needs to further evaluation • limitation of current modeling (not all the effects of flow blockage, i.e, volume reduction) • Future works • Validation of the model with available/applicable experiments • Further improvement of the model

  26. Thank you for Attention

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