Hydride Effects on Discharged Fuel Clad Related to Accident Conditions During Dry Storage and Handling

Transcription

1 Hydride Effects on Discharged Fuel Clad Related to Accident Conditions During Dry Storage and Handling R.L. Kesterson, R.L. Sindelar, P.S. Korinko, P-S. Lam SRNL-STI th Symposium on Zirconium in the Nuclear Industry May 15-19, 2016

2 Background Spent Fuel Pool storage is limited and dry storage of used fuel is required. Even with onsite storage some handling and transport is encountered along with the risk of accidents and mechanical damage. On road transportation stress cycles. Dry storage preparation begins with container loading and then drying. Temperatures near 400 C may be experienced with associated hoop stresses and hydride reorientation. Testing is best done with discharged fuel clad samples but that is an expensive and hard to obtain route so hydrogen charged but unirradiated samples have been used for scoping studies. Early work focused on ring compression samples which reproduced a diameter crush type failure mode. Most fuel clad zirconium alloys have non-isometric mechanical properties, this study focused on a comparison of factors affecting failure in a both the diameter crush and axial bend modes. Hydrogen levels and orientation within the clad are included as variables. 2

3 The hydrided clad may not be isotropic regarding some mechanical properties like ductility and DBTT. Diameter compression tests produce a circumferential stress in the clad which is representative of a pinch loading during accident conditions. An axial bend test produces axial stresses that are representative of fuel rod bending conditions. 3

4 Sample Preparation Tubing material ZIRLO tm - in SRA condition Charge with high purity hydrogen under internal tube pressure to achieve desired hydrogen levels of 100 to 800 ppm. Heat to 400 o C at 10 o C per minute Most hydrogen is absorbed between 300 o and 350 o C Radial Hydride Growth Treatment (RHGT) Pressurize tube to hoop stresses of 90, 130 and 170 MPa (argon gas) Heat to 400 o C at 10 o C/min Hold for one hour Cool at 5 o per hour to 200 o C 4

5 Hydride Morphology After 170 MPa Hydride Reorientation Treatment 5

6 Hydride Morphology Change With Hoop Stress Increase ZIRLO sample charged with 200 ppm H (a) RHGT 90 MPa, (b) RHGT 130 MPa and (c) RHGT 170 MPa 6

7 RCT Ring Compression Testing 7

8 DBTT Testing -RCT Ring Compression Tests 9.52 mm diameter samples 8 mm long 5 mm/sec crosshead speed Nominal 1.7 to 2.3 mm deflections Quasi plastic Strain % calculated by : Strain % = ( total elastic deflection)/ sample OD X 100 8

9 Examples of RCT test results Ductility increases with test temperature Ductility decreases with hoop stress increase radial hydrides 9

10 Relative Diameter Deflections for Failure Pellet contact is predicted before clad reaches failure strains from diameter deflection With pellet contact the resistance to further deformation increases significantly 10

11 TPB Three Point Bend Testing 11

12 DBTT Testing Three Point Bend -TPB Three Point Bend Tests 92 mm span 3 mm dia. lower roller 32 mm dia. upper roller 5 mm/ sec cross head speed 6.35 mm - 13 mm deflection Quartz pellets 8 mm dia. / 12 mm long were loaded into tube to prevent crimping and partially represent pellets 12

13 Load (N) Example of a TPB Test Profile ZIRLO SRA 1500 Zr ppm/130 MPa 1000 Zr ppm/170 MPa 500 Zr ppm/90 Mpa Deflection - mm 13

14 Effects of temperature and Hydrides on Three Point Bend Relative Ductility 14

15 FEA Finite Element Analysis 15

16 FEA Analysis using Abaqus element type C3D8R RCT / 18,954 nodes One- quarter model Frictionless contact with platens is assumed TPB / 20,979 nodes plus 1827 nodes for filler Assumed frictionless contact between filler and clad 16

17 True Stress (MPa) FEA True Stress Results RCT hoop stress varies significantly between 12/6 and 3/9 o clock positions Initially 12 o clock ID has max stress Flattens on the platens and ID stress peaks 3 o clock OD stress continues to increase with deflection and exceeds 12 o clock position TPB fast stress increase with initial deflection Nearly linear increase with deflection Similar stress failure levels as seen for RCT True Stress vs Displacement TPB S33 Axial RCT S11 12/6 O'clock (N-S) RCT S22 3/9 O'clock (E-W) Load-Point Displacement (mm) 17

18 FEA True Stain Results Similar to the stress profiles RCT strain at 12 / 6 peaks at about 1 mm deflection 12/6 o clock strain equals 3/9 o clock strain at about 2.4 mm deflection 18

19 Strain - mm/mm RCT and TPB Ductility Comparisons RCT diameter deflection and TPB calculated strains work well for making relative ductility comparisons. If actual strains are needed then analysis like FEA provides true strain levels Comparison of Maximum Strain Levels for FEA and Diameter Deflection Calculations contact flatening effect RCT Diameter Deflection - mm Normalized Quasi- Plastic strain -(Total - Elastic deflection)/od FEA -N/S Total Strain at ID FEA-E/W Total Strain at OD Normalized Quasi- Total Strain - Deflection/OD Deflection point where E/W ( 3 and 9 o'clock) exceeds N/S (12 and 6 o'clock) strain 19

20 DBTT Evaluations 20

21 Radial Hydride Related RCT DBTT No significant radial hydrides observed at 90 MPa All samples from 100 ppm to 800 ppm H have relatively good ductility even at room temperature 21

22 Radial Hydride Related RCT DBTT As radial hydrides increase due to higher hoop stress at reorientation treatment the DBTT temperature increases. The samples with lower total hydrogen are more affected by the radial hydride formation than the higher (800 ppm ) samples. 22

23 Radial Hydride Related RCT DBTT DBTT is estimated to be when the material transitions from an area of higher relative ductility ( >10%) to a lower level ( <4%). 23

24 RCT -DBTT as a Function of the Radial Hydride Ratio Using a simple overlay intercept method to estimate radial and circumferential hydride densities, the observed trend was a direct relationship between RCT - DBTT and radial hydrides. Hydride levels alone do not seem to be a major factor for RCT-DBTT 24

25 RCT TPB Comparison DBTT values RHGT Hydrogen Level (PPM) RHGT Hydrogen Level (PPM) Ring Compression Tests (Quasi-)DBTT RGHT Nominal Pressure (MPA) <RT <RT C 200 <RT 75 0 C C 400 <RT 50 0 C C 800 <RT <RT 35 0 C 0 <RT Three Point Bend DBTT RGHT Nominal Pressure (MPA) <RT 400 <RT 800 <175 Few to no radial hydrides 800 ppm RCT = <RT 800 ppm TPB = <175 C Some radial hydrides 200 ppm /130 MPa RCT DBTT = 75 C TPB DBTT = < RT 400 ppm / 170 MPa RCT DBTT = 110 C TPB DBTT = <RT 25

26 RCT TPB Comparison DBTT values RHGT Hydrogen Level (PPM) RHGT Hydrogen Level (PPM) Ring Compression Tests (Quasi-)DBTT RGHT Nominal Pressure (MPA) <RT <RT C 200 <RT 75 0 C C 400 <RT 50 0 C C 800 <RT <RT 35 0 C 0 <RT Three Point Bend DBTT RGHT Nominal Pressure (MPA) <RT 400 <RT 800 <175 For lower DBTT thresholds axial bend TPB Low hydrogen levels ( maybe radial hydrides are an advantage) Diameter pinch RCT High hydrogen levels are a mild benefit Radial hydrides are a negative 26

27 Summary and Conclusions 27

28 CONCLUSIONS FROM THE RCT TESTS A. There was no significant effect of the 90 MPa RHGT in producing significant levels of radial hydrides nor a high RCT-DBTT temperature. B. At 170 MPa RGHT the RCT-DBTT does show significant temperature increases due to the resulting radial hydride structure. C. The RCT sample with lower hydrogen levels ( PPM) shows more sensitivity to the RHGT stress than the samples with high hydrogen content. This is due to the large relative inventory of hydrogen that goes into solution and then re-precipitates. D. The FEA results are consistent with general knowledge in that there is a difference in the stress and strain generation characteristics for the two prime directions; the ID surface in the 12 / 6 o clock direction experiences the highest strain initially and that the OD surface in the 3 / 9 o clock direction experiences high strain which are initially lower than the 12 o clock position but exceed it after large diameter deflections. 28

29 CONCLUSIONS FROM THE TPB TESTS A. The DBTT generated using RCT does not represent the DBTT associated with axial bend TPB tests. B. High hydrogen levels rather than high radial hydrides are detrimental to TPB DBTT. C. The FEA results regarding failure strains are consistent with failure strains observed in the FEA results from the RCT tests. D. If axial bend strain conditions are to be evaluated for fuel performance then areas of high hydrogen need to be considered. For axial bending at high hydrogen locations in the fuel clad, such as at pellet interfaces, the axial bending DBTT may be much higher than predicted by RCT data. E. Radial hydrides may not have a significant effect on axial bend failures. (More data needed to fully support conclusion.) 29

30 Thank You For Your Attention 30