Analysis of Halden overpressure tests using the FALCON code

WIR SCHAFFEN WISSEN HEUTE FÜR MORGEN G. Khvostov, W. Wiesenack Analysis of Halden overpressure tests using the FALCON code Enlarged Halden Programme Group Meeting 216

Contents Scope and goals of the study Main data Models and methods Results Thermal response to overpressure in IFA-61.1 (BWR) Thermal response to overpressure in IFA-61.1 (/3/5) (PWR) Response of cladding elongation to overpressure Rod FGR and cladding failure Conclusions http://www.psi.ch/stars 216.5.8-13/STARS/KG41 - ( 2 / 13 )

Scope and goals of the study Four tests of the IFA-61 series, using uranium dioxide as fuel material, were analyzed with the FALCON code Sample origin Local parameters after BI Test Vendor/NPP Fuel type Segment type Burn-up, MWd/kgU IFA-61.1 WSE / KKL 1X1 SVEA-96+ BWR IFA-61.1 Siemens- KWU GE / KKG 14X14 PWR A segment cut from integral rod A segment from specially designed segmented rod A segment from specially designed segmented rod Fast fluence, 1 21 n/cm2 Oxide thickness, microns Overpressure sequence, bar 59 7 12 (5-1-15)- 15-175-2-25 59 8 3 5-1-2-15-25-3-15 IFA-61.3 61.8 1.3 2 1-5-2 IFA-61.5 Framatom / EDF Gravelines 5 17X17 PWR 63.2 1.45 2 25-1-175 General goals: verify the code s ability to treat lift-off find out governing processes and specific features of high burnup fuel behaviour explore critical conditions for rod failure caused by very high internal gas pressure http://www.psi.ch/stars 216.5.8-13/STARS/KG41 - ( 3 / 13 )

Cladding elongation, mm Temperature, o C Main data CL temperature in TC-section Normalized temperature 8 75 7 Data for fuel temperature in TC-section of IFA-61.1: Measured temperature. Normalized temperature at LHRTC=11 kw/m. T fn = T f T c LHR LHR + T c 65 Normalized temperature against calculated temperature-variation across the pellet radius 6 55 T fn = T f LHR LHR 5 1 2 3 4 5 6 7 8 Time, eph SST cladding elongation shows significant difference, likely depending on degree of bonding PWR: increasing BWR: decreasing Note: The data taken into account, but remaining beyond the scope of the paper, are: Cladding elongation during power ramps; Noise analysis for transient elongation; measurement of TH diameter 1.9.8.7.6.5.4.3.2.1 -.1 -.2 -.3 -.4 -.5 Raw data for cladding elongation in: IFA-61.1. IFA-61.1. IFA-61.3. IFA-61.5. 1 2 3 4 5 6 7 8 Time, eph http://www.psi.ch/stars 216.5.8-13/STARS/KG41 - ( 4 / 13 )

Models and methods Use of a research version of EPRI s FALCON code coupled with the mechanistic model, GRSW-A, for fission gas release and gaseous-bubble swelling RIP is treated as code input, as function of time Use of code restart to simulate re-fabrication after BI Rod overpressure, bar 35 Raw data for rod overpressure in: IFA-61.1. IFA-61.1. IFA-61.3. IFA-61.5. 3 25 2 15 1 5 1 2 3 4 5 6 7 8 Time, eph http://www.psi.ch/stars FE model is designed to simulate thermal response in TC section, in consideration of its details 216.5.8-13/STARS/KG41 - ( 5 / 13 )

Cladding elongation, mm Models and methods TM calculation for two limit cases: without bonding and with complete bonding 1.9.8.7.6.5.4.3.2.1 -.1 -.2 -.3 -.4 -.5 -.6 -.7 -.8 -.9-1 Cladding elongation: Measurement. Calculation without bonding. Calculation with bonding. AAFB=33% 2 3 4 5 6 7 8 Burn-up, MWd/kgU Use of the two calculation for decomposition of measured elongation dl = AFB dl db exp db w bonding + (1 AFB) dl db w/o bonding Proposed estimation of Apparent Fractional Bonding (AFB) in the sample http://www.psi.ch/stars 216.5.8-13/STARS/KG41 - ( 6 / 13 )

Results 5 4 No thermal feedback up to overpressure of 15 bar. Thermal feedback starts at overpressure of 175 bar. Gap opening at overpressure of 25 bar. Rod pressure, MPa Thermal response to overpressure in IFA61.1 (BWR) Rod pressure 3 2 Coolant pressure 1 2 3 4 5 6 7 8 Burnup, MWd/kgU Normalized temperature in TC-section: Measurement. Running average for measured temperature. Calculation for normalized centreline temperature. Calculation for normalized temperature-variation accross pellet radius (Tc-Ts). 35 4 69 68 34 33 3 67 2 1 32 66 31 65 3 64 29 63 28 62 27 61 26 2 3 4 5 Burnup, MWd/kgU http://www.psi.ch/stars 6 Calculated diametral gap size. Pellet-cladding contact pressure. 5 7 8 2 Radial gap size, microns Contact pressure, MPa Normalized temperature, oc 7 25 36 Normalized temperature variation, oc 71 15 1 5 2 3 4 5 6 7 8 Burnup, MWd/kgU 216.5.8-13/STARS/KG41 - ( 7 / 13 )

Normalized temperature in TC-section: Measurement. Calculation w/o bonding. Calculation assuming bonding. 12 Thermal response to overpressure in IFA-61.1 (/3/5) (PWR) The model, assuming pellet-cladding bonding, inhibits gap opening. For the periods with overpressure maintained at 2 bar, 25 bar and 3 bar, a perfect agreement is obtained when bonding is assumed. Normalized temperature, oc 11 1 9 1 8 9 7 8 6 7 5 6 4 5 3 Over-pressure, bar Results Overpressure 4 2 3 1 2 5 1 15 2 25 3 35 4 45 Time, hours Splashes in temperature at power fluctuations A hysteresis: rearrangement of the fuel fragments resulting in circumferential crack opening. 75 15 LHGR in TC-section: 74 Raw data. FALCON input (condesed). 73 Temperature, oc LHRTC, kw/m 14.5 14 72 71 7 69 68 38 P = 2 bar Normalized temperature, oc 77 76 37 75 36 74 35 Normalized temperature in TC-section: Measurement. Linear fit to measurement. Running average for selected data-set. Calculation for normalized temperature variation accross pellet radius (Tc-Ts). 73 13.5 67 66 13 72 65 11 12 13 14 15 16 17 18 19 Time, h http://www.psi.ch/stars 34 12 13 14 LHRTC, kw/m 15 16 33 1 1.2 1.4 1.6 1.8 2 Burnup, MWd/kgU 216.5.8-13/STARS/KG41 - ( 8 / 13 ) Normalized temperature variation, oc

Cladding elongation, mm Cladding elongation, mm Results Response of cladding elongation to overpressure Assumption of bonding in calculation maximizes impact of fuel swelling on cladding elongation under over-pressure The degree of bonding is quantified with the AFB number (current and test average), viz.: an effective fraction (%) of pellet surface bonded with cladding Test Bu dl_db-exp dl_db-nb dl_db-b AAFB CAFB STD,% 61.1 59 -.125447688 -.22192487.66566 33.4424484 5 5 3 4 61.1 59.17167367 -.268958969.277463 8.6396548 7 12 88 17 61.3 61.8.177894837 -.124427858.487333 49.4184171 65 3 61.5 63.2.68629857 -.7975472.234377 83.83157881-16.18521248 -.23821279.43423 61.33642642-13 The degree of pellet-cladding bonding in the BWR fuel segment is lower than in the PWR fuel segments (ca. 3 % vs 7 %) Bonding was not broken in the PWR tests throughout experiments, even though the applied maximum overpressure was very high (3 bar in IFA-61.1) 2 3 4 5 6 7 8 Burn-up, MWd/kgU http://www.psi.ch/stars 216.5.8-13/STARS/KG41 - ( 9 / 13 ) 1.9.8.7.6.5.4.3.2.1 -.1 -.2 -.3 -.4 -.5 -.6 -.7 -.8 -.9-1.8.6.4.2 -.2 Cladding elongation: Measurement. Calculation without bonding. Calculation with bonding. Cladding elongation: Measurement. Calculation without bonding. Calculation with bonding. T c =36 o C; AAFB=83 16 % P=175 bar AAFB=33% IFA-61.1 T c =316 o C; AAFB=61 13 % IFA-61.5.4.5.6.7.8.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 Burnup, MWd/kgU

Temperature, K FGR, % Fission gas release, % Results Rod FGR and cladding failure The calculated FGR is a smooth continuation of the FGR at the end of BI No effect of thermally induced feedback is predicted. 16 14 Base Irradiation 1 9 7.5 7 Overpressure tests 12 8 7 6.5 6 5.5 1 8 6 4 2 Centreline temperature during BI in segments used for: IFA-61.1. IFA-61.1. IFA-61.3. Empirical FGR threshold. Relative FGR during BI in a segment used for: IFA-61.1. IFA-61.1. IFA-61.3. 6 5 4 3 2 1 5 4.5 4 3.5 3 2.5 2 1.5 1.5 Calculated relative FGR during: IFA-61.1. IFA-61.1. IFA-61.3. IFA-61.5. 5 1 15 2 25 3 35 4 45 5 55 6 65 Burn-up, MWd/kgU 58 6 62 64 66 68 Pellet burnup, MWd/kgU According to the modelling, no considerable deterioration of rod reliability resulting from applied test conditions of overpressure can be expected, in spite of rather severe overpressure utilized in the tests. none of the test rods failed. This is consistent with the results of the calculations. Need of additional single effect experiments resulting in cladding failure due to the predicated lift-off phenomenon http://www.psi.ch/stars 216.5.8-13/STARS/KG41 - ( 1 / 13 )

Conclusions The FALCON code has been shown to be applicable to best-estimate predictive analysis of the overpressure tests using fuel without, or weak pellet-cladding bonding, and scoping analysis of the tests with fuels where stronger pellet-cladding bonding occurs. Significant effects of bonding and fuel cracking/relocation on the thermal and mechanical behaviour of highly over-pressurized rods, with a fuel burnup of ca. 6 MWd/kgU, have been identified. The effect of bonding is particularly pronounced in the tests with the fuel segments preirradiated in PWRs. The identified effects are beyond the current traditional fuel-rod licensing analysis methods. Derivation of appropriate acceptance criteria and model parameterization, which is important for the corresponding licensing analysis, would need additional single effect experiments resulting in cladding failure due to lift-off phenomenon. http://www.psi.ch/stars 216.5.8-13/STARS/KG41 - ( 11 / 13 )

Wir schaffen Wissen heute für morgen The main goals of the study are basically reached: - All the tests, as planned, successfully analyzed. - Considerable effects of bonding and fuel cracking/relocation identified. - The identified effects are beyond current traditional licensing analysis. The paper and presentation outline the main results, while the whole activity is described in PSI Report TM-41-16-1

Wir schaffen Wissen heute für morgen My thanks go to ESB OECD HRP

Back-up slides http://www.psi.ch/stars 216.5.8-13/STARS/KG41 - ( 14 / 13 )

Contact pressure, MPa Radial gap, m Normalized temperature, o C IFA-61.1 (BWR) 7 Thermal response to overpressure in IFA-61.1 (BWR) Calculated and measured normalized temperature (top) in TC-section against contact pressure and gap size (bottom) at the BOC4 in IFA-61.1 Complex interplay of effects of temporary irradiation with lowpower and overpressure and use of He at EOC3, followed by gap opening after overpressure-ramp to 25 bar Signs of secondary gap closure after power/overpressure dips and rises, presumably due to fuel fragment relocation into the open gap space and reduced cladding creep-out rate 35 3 Cycle 3 Cycle 4 69 68 67 66 65 64 63 62 5E-6 4.5E-6 4E-6 Cycle 3 Ar He Ar Cycle 4 Calculation assuming Ar as filler gas. Calculation assuming He as filler gas. Measurement. Running average for measurement. 56 57 58 59 6 61 62 63 64 65 Cycle 3 Cycle 4 25 3.5E-6 3E-6 2 2.5E-6 15 2E-6 1 1.5E-6 1E-6 5 5E-7 56 57 58 59 6 61 62 63 64 65 56 57 58 59 6 61 62 63 64 65 Time, h Time, h http://www.psi.ch/stars 216.5.8-13/STARS/KG41 - ( 15 / 13 )

Normalized temperature, o C Rod pressure, MPa Gap size, micron Rod pressure, bar LHR, W/cm IFA-61.1 (BWR) Thermal response to overpressure in IFA-61.1 (BWR) 4 3 Assumed idealized BCs for simplified analysis of IFA-61.1 test 2 1 1 2 3 4 5 6 7 8 Time, hours Calculated normalized temperature in IFA- 61.1 assuming hypothesized BCs and initial open-gap condition 76 75 74 73 72 Calculation with idealized BC.: Normalized temperature. Daimetral gap size. LHR = 11 kw/m +5 bar 14 13 12 11 1 Effects of primary- and secondary creep. Effect of irradiation at zero-overpressure that results in temperature decrease due to gap reduction. 71 7 69 68 67 66 P= 15 bar +25 bar +25 bar 9 8 7 6 5 4 65 Rod peressure 3 64 2 63 Coolant peressure 1 62 1 2 3 4 5 6 7 8 9 Time, hours http://www.psi.ch/stars 216.5.8-13/STARS/KG41 - ( 16 / 13 )

Hoop stress, MPa IFA-61.1 (BWR) Mechanical behaviour of IFA-61.1 (BWR) 2 19 18 Calculated cladding hoop stress as function of LHR in TCsection for three power-ramps in IFA-61.1 test Mitigation of the contact and gap opening (lift-off) would start, according to the calculation, at LHR of 8.6 kw/m and 1.4 kw/m at overpressure of 15 bar and 2 bar, respectively 17 16 15 14 13 12 11 Calculated hoop stress at different overpressure levels: at 15 bar (2726 eph). at 2 bar (463 eph). at 25 bar (6577 eph). 1 4 5 6 7 8 9 1 11 12 LHR, kw/m Calculated and measured cladding diameter during IFA-61.1 test Calculation shows good agreement with post-test measurement http://www.psi.ch/stars 216.5.8-13/STARS/KG41 - ( 17 / 13 )

Effective bonding fraction B, % Effective bonding fraction B, % Relative linear elongation rate, % per 1 MWd/kgU IFA-61.1 (BWR) Mechanical behaviour of IFA-61.1 (BWR) Measure and calculated elongation rate in IFA-61.1 test using two FALCON models 1.9.8.7.6.5.4.3.2.1 -.1 -.2 -.3 -.4 -.5 -.6 -.7 -.8 -.9-1 Estimated SST elongation rate at P=2 bar: based on measurement. based on the model with bonding. based on the model without bonding. 4.5 4.6 4.7 4.8 4.9 5 Burn-up, MWd/kgU Estimated current values of apparent fractional bonding (CAFB) in IFA-61.1 1 1 8 P=175 bar 8 P=2bar 6 5 6 4 4 3 2 2 3.7 3.8 3.9 4 Burn-up, MWd/kgU 4.5 4.6 4.7 4.8 4.9 5 Burn-up, MWd/kgU http://www.psi.ch/stars 216.5.8-13/STARS/KG41 - ( 18 / 13 )

Diametral gap size, microns Over-pressure, bar Temperature, o C IFA-61.1 (PWR) Thermal response to overpressure in IFA-61.1 (PWR) 82 81 8 79 78 77 Centreline temperature in TC-section: Measured. Calculated assuming bonding. Calculated after assumed de-bonding. 2 bar 25 bar 3 bar 76 Measured response of temperature in TCsection to overpressure ramps in IFA-61.1 test vs. calculation using two FALCON models Calculation assuming breach of bonding predicts the geometric gap opening immediately after the overpressure ramp to 25 bar, followed by the fast increase due to primary- and secondary- creep of the cladding Consequently, the calculated normalized temperature grows up drastically, significantly exceeding the measurement 75 74 73 72 71 7 69 4 35 3 25 2 15 Calculation Restart 19 2 21 22 23 Time, hours Rod overpressure. Calculated gap-size assuming bonding. Calculated gap-size after assumed de-bonding. Calculation Restart Overpressure Gap size 4 375 35 325 3 275 1 25 5 225 19 2 21 22 23 Time, hours 2 http://www.psi.ch/stars 216.5.8-13/STARS/KG41 - ( 19 / 13 )

Normalized temperature, o C Over-pressure, bar IFA-61.5 (PWR) Thermal response to overpressure in IFA-61.5 (PWR) Calculated normalized temperature in TCsection against rod overpressure during IFA- 61.5 test, using two FALCON models 66 65 64 63 62 61 Normalized temperature in TC-section: Measurement. Calculation w/o bonding. Calculation assuming bonding. 5 45 4 35 6 59 3 The model assuming pellet-cladding bonding shows good agreement with the measurement, whereas the model with the bonding switched off overpredicts temperature 58 57 56 55 54 53 T c = 36 o C Overpressure T c = 316 o C 25 2 15 1 52 51 5 5 5 1 15 2 25 3 Time, hours http://www.psi.ch/stars 216.5.8-13/STARS/KG41 - ( 2 / 13 )

Effective bonding fraction B, % Effective bonding fraction B, % Relative linear elongation rate, % per 1 MWd/kgU IFA-61.1 (PWR) Mechanical behaviour of IFA-61.1 (PWR) Measure and calculated elongation rate in IFA-61.1 test using two FALCON models 1.9.8.7.6 P = 2 bar Estimated SST elongation rate: based on measurement. based on the model with bonding. based on the model without bonding..5.4.3.2.1 -.1 -.2 -.3 -.4 -.5 -.6 -.7 -.8 -.9-1 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 Burn-up, MWd/kgU Estimated current values of apparent fractional bonding (CAFB) in IFA-61.1 1 15 9 8 7 7 % P = 2 bar 14 13 12 11 P = 15 bar 6 1 9 ca. 88 % 5 8 7 4 6 3 5 4 2 3 1 2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 Burn-up, MWd/kgU 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 Burn-up, MWd/kgU http://www.psi.ch/stars 216.5.8-13/STARS/KG41 - ( 21 / 13 ) 1