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

18th Symposium on Zirconium in the Nuclear Industry

May 15-19, 2016

SRNL-STI-2015-00192

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

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

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 400o C at 10o C per minute

– Most hydrogen is absorbed between 300o and 350o C

• Radial Hydride Growth Treatment (RHGT)

– Pressurize tube to hoop stresses of • 90, 130 and 170 MPa (argon gas)

– Heat to 400o C at 10o C/min

– Hold for one hour

– Cool at 5o per hour to 200o C

4

Hydride Morphology After 170 MPa Hydride Reorientation Treatment

5

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

RCT Ring Compression Testing

7

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

Examples of RCT test results

• Ductility

increases with

test

temperature

• Ductility

decreases with

hoop stress

increase –

radial hydrides

9

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

TPB Three Point Bend Testing

11

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

Example of a TPB Test Profile

0

500

1000

1500

2000

2500

0 2 4 6 8 10 12 14

Load

(N

)

Deflection - mm

ZIRLO SRA

Zr-53 200 ppm/130MPa

Zr-30 400 ppm/170MPa

Zr-18 800 ppm/90Mpa

13

Effects of temperature and Hydrides on Three Point Bend Relative Ductility

14

FEA Finite Element Analysis

15

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

0

100

200

300

400

500

600

700

800

900

1000

0 2 4 6 8 10Tr

ue

Str

ess

(M

Pa)

Load-Point Displacement (mm)

True Stress vs Displacement

TPB S33 Axial

RCT S11 12/6O'clock (N-S)

RCT S22 3/9O'clock (E-W)

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

17

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

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.

•

0

0.05

0.1

0.15

0.2

0.25

0 0.5 1 1.5 2 2.5 3

Stra

in -

mm

/mm

RCT Diameter Deflection - mm

Comparison of Maximum Strain Levels for FEA and Diameter Deflection Calculations

Normalized Quasi-Plastic strain -(Total -Elastic deflection)/ODFEA -N/S Total Strainat ID

FEA-E/W Total Strainat OD

Normalized Quasi-Total Strain -Deflection/OD

contact flatening effect

Deflection point where E/W ( 3 and 9 o'clock) exceeds N/S (12 and 6 o'clock) strain

19

DBTT

Evaluations

20

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

Radial Hydride Related RCT DBTT

21

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.

Radial Hydride Related RCT DBTT

22

DBTT is estimated to be when the

material transitions from an area

of higher relative ductility ( >10%)

to a lower level ( <4%).

Radial Hydride Related RCT DBTT

23

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

RCT – TPB Comparison DBTT values

•

• 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

Ring Compression Tests (Quasi-)DBTT

RGHT Nominal Pressure (MPA)

RHGT Hydrogen

Level (PPM) 90 130 170

100 <RT <RT 1800C

200 <RT 750C 1800C

400 <RT 500C 1100C

800 <RT <RT 350C

Three Point Bend DBTT

RGHT Nominal Pressure (MPA)

RHGT Hydrogen

Level (PPM) 0 90 130 170

0 <RT

200 <RT

400 <RT

800 <175

25

RCT – TPB Comparison DBTT values

•

• 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

Ring Compression Tests (Quasi-)DBTT

RGHT Nominal Pressure (MPA)

RHGT Hydrogen

Level (PPM) 90 130 170

100 <RT <RT 1800C

200 <RT 750C 1800C

400 <RT 500C 1100C

800 <RT <RT 350C

Three Point Bend DBTT

RGHT Nominal Pressure (MPA)

RHGT Hydrogen

Level (PPM) 0 90 130 170

0 <RT

200 <RT

400 <RT

800 <175

26

27

Summary

and

Conclusions

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 (100 – 200 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.

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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

Thank You

For Your

Attention

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