Behavior of high burnup fuel during LOCA - Key observations and test plan at JAEA -

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1 Behavior of high burnup fuel during LOCA - Key observations and test plan at JAEA - Fumihisa Nagase Japan Atomic Energy Agency IAEA Technical Meeting on Fuel Behaviour and Modelling under Severe Transient and LOCA Conditions Ibaraki, Japan Oct ,

2 Contents Whole view of LOCA study at JAEA Main results on oxidation behavior Main results on thermal shock resistance New program with advanced cladding 1

3 Whole view of LOCA study at JAEA Sample Hydrided, unirradiated cladding (Zircaloy) GWd/t fuel cladding (Zircaloy) GWd/t cladding (Improved alloys) Test method Oxidation rate measurement Semi-integral quench test Mechanical test (Ring-compression, ring-tensile, axial tensile and bending) Separate test on secondary hydriding, restraint load, etc. 2

4 Oxidation kinetics evaluation in a wide range Weight gain ( mg/cm 2 ) K 1523K 1473K 1423K 1373K 1323K 1273K 1223K 1173K 1123K 1073K 973K 873K 1253K Results for low-tin Zircaloy-4 - Parabolic rate law < 3,600 s from 1,273 to 1,573 K < 900 s from 773 to 1,253 K - Cubic rate law > 900 s from 773 to 1,253 K - The Baker-Just equation not conservative below 1,073 K. 773K Breakaway Oxidation time ( s ) - Breakaway oxidation for longer periods at specific temperatures. 3

5 Effect of pre-oxidation Unirradiated cladding - Low-tin Zry-4 Weight gain is smaller in the preoxidized cladding than in the as-received one. Protective effect of the pre-formed oxide for high temperature oxidation 120 s 180 s 900 s 1800 s Pre-formed oxide metal HT oxide Oxidation initiated at cracking position Uniform growth 100µm 4

6 Effect of pre-hydriding - Unirradiated cladding - Unirradiated low-tin Zry-4 Weight gain (mg/cm 2 ) Influence of pre-hydriding varies depending on oxidation temperature and hydrogen concentration. The influence is estimated to be small under the postulated LOCA conditions (<800 ppm, <900 s). Oxidation time (s) 5

7 Oxidation test with high burnup fuel cladding Test specimens were prepared from high burnup fuel rods, irradiated at European power plants. Cladding Reactor Local Burnup (GWd/t) Corrosion layer thickness (µm) hydrogen content (ppm) M5 Ringhals ZIRLO Vandellos Zry-2(LK3) Leibstadt M5 ZIRLO Zry-2 6

8 Test Apparatus and Test Conditions Specimen Electrical resistance furnace Reaction tube Specimen T.C T.C Recorder Steam supply rate: 30 mg/s Test method: two-sided oxidation Temperature Control Steam Generator Temperature: an R-type Thermocouple at the specimen holder M5, Zry , 1100, 1200 o C ZIRLO 900, 1050, 1200 o C Time: 120 to 4000s Measurement: weight gain and oxide layer thickness 7

9 Weight Gain of irradiated ZIRLO Cladding - The weight gain of the irradiated specimens is smaller than that of the unirradiated samples. - The difference becomes smaller at higher temperatures. - The oxidation of the irradiated ZIRLO cladding approximately obeys a parabolic rate law. W 2 = K w. t W : weight gain K w : parabolic rate constant t : oxidation time 8

10 Metallography of irradiated ZIRLO Samples after oxidation test 1173 K, 4022s 1323 K, 1801s 1473 K, 601s HT oxide Magnified Corrosion layer OD HT oxide HT oxide HT oxide 50µm - The HT oxidation initiated at cracking positions of the corrosion layer on the cladding outer surface. The oxide layer uniformly grows at higher temperature or after longer oxidation times. - Oxide layer is uniform on the cladding inner surface. 9

11 Oxide Layer Growth in Irradiated ZIRLO Cladding Oxide layer thickness (mm) - The outer surface oxide is thinner in the irradiated Zry-2 specimens at temperatures <1200 o C. Protective effect of the corrosion layer - Growth of the inner surface oxide in the irradiated cladding is almost equivalent to that in the nonirradiated Zry-4. 10

12 Parabolic Rate Constants for Weight Gain Parabolic rate constant, K w (g 2 /m 4 /s) BJ eq. irradiated ZIRLO Zry-2 Temperature ( o C) CP eq. 900 non-irradiated M5 MDA Zry Reciprocal temperature (x10-4 /K) - Rate constants of the irradiated cladding are lower than those of the non-irradiated Zry-4 at 1100 o C and below, while difference is small at 1200 o C. - Oxidation rates of the examined alloy cladding are lower than that given by the Baker-Just equation. 11

13 Base of ECCS acceptance criteria Japan Based on fracture/no-fracture boundary determined by the semi-integral quench test that simulates the LOCA sequences such as ballooning, rupture, oxidation, thermal shock by reflooding and mechanical loading. U.S. and European countries Based on Zero-ductility criteria determined by the ring-compression test of oxidized cladding. 12

14 Semi-integral quench test Steam outlet Steam inlet Load cell Infrared furnace Restraint load control system Grabbing device Quartz tube Test rod Equipped on tensile testing machine Flooding water inlet Cladding length: 190 to 600 mm Welded TC Welded Temperature (K) Rupture Temperature No Time (s) No.4 Load No.1 TC No Axial Load (N) Test rod experiences the whole process during a LOCA, ballooning and rupture, oxidation and hydrogen absorption at high temperature, and thermal shock at quenching. The rod is quenched with axial constraint to represent a possible condition of fuel rods between grid positions. 13

15 Main results from experiments with unirradiated cladding Decreases of rupture temperature and circumferential burst strain were observed with an increase in the initial hydrogen concentration, which might be associated with the reduction of cladding strength and the shift of phase transformation temperatures. The cladding fracture during quench was primarily dependent on the amount of oxidation. The fracture threshold for the amount of oxidation was reduced by an increase in the initial hydrogen concentration and the axial restraint load. An increase in hydrogen concentration increases the precipitation of fine hydrides and the oxygen concentration in the prior phase region, which causes of enhanced embrittlement of oxidized claddings with a higher initial hydrogen concentration. 14

16 High burnup fuel cladding samples and test conditions Original fuels: PWR (Zirlo, M5, MDA, NDA), BWR (Zry-2) < 76 GWd/t (local) Oxidation temperature: K ( C) Oxidation time: s Oxidation amount: % ( %) ECR Calculated with the Baker-Just equation with oxidation temperature and time, for reduced metallic thickness after ballooning. The value in parenthesis is ECR for the initial metallic thickness before ballooning. 15

17 High burnup fuel cladding for se-mi-integral tests Segment ID Burnup (GWd/t) Cladding Corrosion layer (mm) Initial hydrogen (wppm) MDA1R MDA MDA2R MFI M5 MFI NDA1 69 NDA ZIR2R ZIRLO ZIR3R ZRT Zry-2 ZRT MDA: Zr-0.8Sn-0.2Fe-0.1Cr-0.5Nb NDA: Zr-1.0Sn-0.27Fe-0.16Cr-0.1Nb-0.01Ni 16

18 Post-test appearances and cross section MDA-1R Rupture opening ZIR-2R Rupture opening MFI-2 Rupture opening NDA-1 Rupture opening MDA-2R Fracture position Radial cross section at rupture position Rupture 17

19 Summary of test results Segment ID Burst Temp. ( C) Burst Strain (%) Oxidation Temperature ( C) Oxidation Time (s) Oxidation amount (%ERC) Fracture/ nofracture Restraint load (N) MDA1R N.F. <350 MDA2R F. 530 MFI N.F. <400 MFI N.F. ~0 NDA N.F. 518 ZIR2R N.F. 518 ZIR3R N.F. 519 ZRT N.F. 519 ZRT N.F. 0 ECR is calculated with the Baker-Just equation with oxidation temperature and time, for reduced metallic thickness after ballooning. The value in parenthesis is ECR for the initial metallic thickness before ballooning. 18

20 Fracture map Calculated ECR (%) Fracture during quench N o-fracture Japanese safety criteria: <1473 K <15% ECR Oxidation temperature (K) ECR was calculated with the Baker- Just equation with oxidation temperature and time, for reduced metallic thickness after ballooning. 38.0% 18.3 to 27.3% Oxidation (%ECR) The fracture boundary of the high burnup cladding is higher than the 15% limit in the Japanese ECCS acceptance criteria The fracture boundary is not reduced significantly by high burnup and use of new alloys in the examined burnup level, though it may be somewhat reduced with pre-hydriding as observed in unirradiated Zry-4 cladding GWd/t Fracture MFI-2 (0N) ZRT-2 MFI-1 (0N) (<400N) No-fracture ZRT-1 ZIR-2R MDA-1R (<350N) MDA-2R Fracture boundary of unirradiated Zry Initial hydrogen concentration (ppm) ZIR-3R Safety limit (15%ECR) 19

21 Ring compression tests after semi-integral quench test Objective To obtaining a wide range of database for the safety of high burnup fuel and compare the two test methodologies, which are used for the regulatory judgments Example of sampling 20

22 Load-displacement curves and post-test appearances of MDA cladding 200 LMDA1R Load (N) RC2 50 RC Displacement (mm) Cladding Specimen Hydrogen Ox. Temp. ECR (B-J) ECR (C-P) MDA-1R RC1 870 ppm 1158ºC 13.4% 10.6% RC2 750 pp 1137ºC 11.8% 9.5% 21

23 Plastic strain at failure as function of oxidation amount and hydrogen concentration The plastic strain to failure decreases with the oxidation and the hydrogen concentration. Better correlation is seen in the hydrogen dependence. Ductility obviously decreases when the hydrogen concentration is above 500ppm. Most the examined specimens fractured without plastic strain, though the high burnup cladding did not fracture on quenching in the integral thermal shock tests. 22

24 Subjects to be studied Behavior and fracture under LOCA conditions of high burnup cladding with advanced alloys M-MDA, Opt. Zirlo and Zr-Nb binary alloys including M5 Loading conditions including restraint during quench Secondary hydriding High burnup effect Influence of temperature transient Tests for long-term cooling e.g. Mechanical tests considering possible loading during in-core and out-core storage, handling, etc. Tests for beyond DBA-LOCA in consideration with the accidents at Fukushima-Daiichi 23

25 Sample and test conditions in new program Cladding material Burnup Reactor M-MDA 73GWd/t Vandellos-2 (Spain) PWR fuel cladding M5 78GWd/t Gravelines-5 (France) Low-tin ZIRLO 73GWd/t Vandellos-2 (Spain) BWR fuel cladding Zry-2/LK3 92 GWd/t Leibstadt (Switzerland) 24

26 Test conditions Oxidation test Oxidation temperature: K ( C) Oxidation time: s Post-test analysis: Weight gain measurement, Microstructure observation and Oxide layer thickness measurement Integral thermal shock test Oxidation temperature: K ( C) Oxidation amount: 15 30% B-J ECR Restraint condition: to be determined (Basically 540 N, but may be reduced depending on results of the evaluation in future) 25

27 LOCA test schedule Jp FY (Apr-Mar) Shipment Inside Europe To Japan Oxidation test Unirradiated cladding High burnup cladding Integral thermal shock test Unirradiated cladding High burnup cladding 26

28 Conclusion JAEA has been conducting LOCA studies with unirradiated and irradiated fuel cladding. As a result, various information has been obtained on cladding oxidation, ballooning and rupture behavior, cladding embrittlement and condition of fracture on quench of high burnup fuel under LOCA conditions. JAEA is proceeding with the second phase of the experimental program with high burnup fuel cladding irradiated European power reactors. It is expected that the experiments provide information for future regulation of high burnup fuels which use advanced cladding alloys. More investigations on secondary hydriding, restraint loading and long term cooling are required for better understanding of the high burnup fuel behavior under LOCA conditions. Tests for beyond DBA-LOCA would be necessary in consideration with the accidents at Fukushima-Daiichi power plant. 27

29 Acknowledgment The experimental program with the European high burnup fuel is conducted as part of a program sponsored and organized by Nuclear and Industrial Safety Agency (NISA), Ministry of Economy, Trade and Industry (METI). 28

30 References F. NAGASE, T. OTOMO and H. UETSUKA, Oxidation Kinetics of Low-Sn Zircaloy-4 at the Temperature Range from 773 to 1573 K, J. Nucl. Sci. Technol., 40[4] (2003) T. CHUTO, F. NAGASE and T. FUKETA, High Temperature Oxidation of Nb-Containing Zr Alloy Cladding in LOCA Conditions, Nuclear Engineering and Technol., 41[2], 2009, p Y. UDAGAWA, F. NAGASE and T. FUKETA, Effect of Cooling History on Cladding Ductility under LOCA Conditions, J. Nucl. Sci. Technol., 43[8] (2006) F. NAGASE and T. FUKETA, Effect of Pre-Hydriding on Thermal Shock Resistance of Zircaloy-4 Cladding under Simulated Loss-of-Coolant Accident Conditions, J. Nucl. Sci. Technol., 41[7] (2004) F. NAGASE and T. FUKETA, Behavior of Pre-hydrided Zircaloy-4 Cladding under Simulated LOCA conditions, J. Nucl. Sci. Technol., 42[2] (2005) F. NAGASE and T. FUKETA, Fracture Behavior of Irradiated Zircaloy-4 Cladding under Simulated LOCA Conditions, J. Nucl. Sci. Technol., 43[9] (2006) F. NAGASE, T. CHUTO, and T. FUKETA, Behavior of High Burn-up Fuel Cladding under LOCA Conditions, J. Nucl. Sci. Technol., 46[7] (2009) F. NAGASE, T. CHUTO, T. FUKEATA Ring-compression ductility of high burn-up fuel cladding after exposure to simulated LOCA conditions, J. Nucl. Sci. & Tech., 48[11] (2011)