0

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 Modellingunder Severe Transient and LOCA Conditions

Ibaraki, JapanOct. 18-21, 2011

1

Contents

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

2

Whole view of LOCA study at JAEA

• Sample– Hydrided, unirradiated cladding (Zircaloy)– 39 − 44 GWd/t fuel cladding (Zircaloy)– 66 − 76 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.

3

Oxidation kinetics evaluation in a wide range

100

101

102

103

104

105

106

10710

-2

10-1

100

101

102

1573K1523K

1473K1423K1373K

1323K1273K

1253K

1223K

1173K1123K

1073K

973K

873K

773K

Wei

gh

t g

ain

( m

g/c

m2 )

Oxidation time ( s )

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.

- Breakaway oxidation for longer periods at specific temperatures .

Breakaway

Effect of pre -oxidation – Unirradiated cladding -

metal

120 s 180 s 900 s 1800 s

Uniform growth

Oxidation initiated at cracking position

Pre-formedoxide

HToxide

Weight gain is smaller in the pre-oxidized cladding than in the as-received one.

Protective effect of the pre-formed oxide for high temperature oxidation

Low-tin Zry-4

100µm

4

Oxidation time (s)

Wei

ght g

ain

(mg/

cm2 )

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).

Unirradiated low-tin Zry-4

Effect of pre -hydriding - Unirradiated cladding -

5

Oxidation test with high burnup fuel cladding

M5 ZIRLO Zry-2

Test specimens were prepared from high burnup fuel rods, irradiated at European power plants.

Cladding ReactorLocal Burnup

（GWd/t）Corrosion layerthickness （µm）

hydrogencontent （ppm）

M5 Ringhals 68 6-7 60-80

ZIRLO Vandellos 79 84-98 900-990

Zry-2（LK3） Leibstadt 73 20-33 170-180

6

Test Apparatus and Test Conditions

Steam supply rate: 30 mg/s Test method: two-sided oxidationTemperature: an R-type Thermocouple at the specimen holder

M5, Zry-2 1000, 1100, 1200oCZIRLO 900, 1050, 1200oC

Time: 120 to 4000sMeasurement: weight gain and oxide layer thickness

T.C

Electrical resistance furnace

T.C

Specimen

Steam GeneratorTemperatureControl

Recorder

Reaction tubeSpecimen

7

- 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 = Kw .t ∆W : weight gain

Kw : parabolic rate constantt : oxidation time

Weight Gain of irradiated ZIRLO Cladding

8

Metallography of irradiated ZIRLO Samples after oxi dation test

- 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.

1173 K, 4022s

HT oxideHT oxide

1323 K, 1801s 1473 K, 601s

HT oxide

Corrosionlayer

OD

50µm

Magnified

HT oxide

9

- The outer surface oxide is thinner in the irradiated Zry-2 specimens at temperatures <1200oC.

- Growth of the inner surface oxide in the irradiated cladding is almost equivalent to that in the non-irradiated Zry-4.

Oxide Layer Growth in Irradiated ZIRLO Cladding

Protective effect of the corrosion layer

Oxi

de la

yer

thic

knes

s (m

m)

10

11

BJ eq.

CP eq.

Parabolic Rate Constants for Weight Gain

6.5 7 7.5 8 8.5Reciprocal temperature (x10-4/K)

Par

abol

ic r

ate

cons

tant

, Kw

(g2 /

m4 /

s)

100

101

102

10-1

Temperature (oC)

ZIRLO

non-irradiated M5MDAZry-4 - Oxidation rates of the examined

alloy cladding are lower than that given by the Baker-Just equation.

Zry-2

irradiated

900100011001200- Rate constants of the irradiated

cladding are lower than those of the non-irradiated Zry-4 at 1100oC and below, while difference is small at 1200oC.

11

12

JapanBased 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 countriesBased on Zero-ductility criteria determined by the ring-compression test of oxidized cladding.

Base of ECCS acceptance criteria

13

Semi-integral quench test

• 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.

Quartz tube

Grabbing device

Steam outlet

Steam inlet Flooding water inlet

Infrared furnace

Test rod

Restraint load control system

Equipped on tensile testing machine

Load cell

Cla

ddin

g le

ngth

:19

0 to

600

mm

TC

Wel

ded

Wel

ded

Cla

ddin

g le

ngth

:19

0 to

600

mm

TC

Wel

ded

Wel

ded

0

400

800

1200

1600

2000

0

200

400

600

800

1000

0 400 800 1200

No.4No.4

No.2

No.3

No.1 TC

Tem

pera

ture

(K

)

Axi

al L

oad

(N)

Time (s)

Rupture

Temperature

Load

14

Main results from experimentswith 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.

15

High burnup fuel cladding samplesand test conditions

• Original fuels: PWR (Zirlo, M5, MDA, NDA), BWR (Zry-2)< 76 GWd/t (local)

• Oxidation temperature: 1463−1480 K (1190−1207°C)• Oxidation time: 131−719 s• Oxidation amount: 18.3−38.0% (16.4−35.6%) 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.

16

SegmentID

Burnup(GWd/t)

CladdingCorrosion

layer(mm)

Initialhydrogen(wppm)

MDA1R76 MDA

48 720

MDA2R 60 838

MFI166 M5

6 73

MFI2 7 69

NDA1 69 NDA 32 214

ZIR2R 71ZIRLO

50 496

ZIR3R 73 79 764

ZRT1 66Zry-2

30 297

ZRT2 73 24 182

High burnup fuel cladding for se-mi-integral tests

MDA: Zr-0.8Sn-0.2Fe-0.1Cr-0.5NbNDA: Zr-1.0Sn-0.27Fe-0.16Cr-0.1Nb-0.01Ni

17

Post-test appearances and cross section

Radial cross sectionat rupture position Rupture

MDA-1R Rupture openingRupture opening

MFI-2 Rupture openingRupture opening

ZIR-2R Rupture openingRupture opening

NDA-1 Rupture openingRupture opening

Fracture positionFracture positionMDA-2R

18

SegmentID

BurstTemp.(°C)

BurstStrain(%)

OxidationTemperature

(°C)

OxidationTime(s)

Oxidationamount(%ERC)

Fracture/no-

fracture

Restraintload(N)

MDA1R 715 9.9 1207 131 18.3 N.F. <350

MDA2R - 6.0 1190 719 38.0 F. 530

MFI1 780 20.1 1197 151 19.5 N.F. <400

MFI2 762 19.2 1196 229 23.6 N.F. ～0

NDA1 715 8.9 1194 280 22.5 N.F. 518

ZIR2R 672 28.0 1200 228 27.3 N.F. 518

ZIR3R 676 20.6 1186 153 20.2 N.F. 519

ZRT1 763 17.7 1195 222 21.2 N.F. 519

ZRT2 778 20.6 1194 232 22.0 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.

Summary of test results

19

Fracture map

ECR was calculated with the Baker-Just equation with oxidation temperature and time, for reduced metallic thickness after ballooning.

1400 1450 15000

10

20

30

40

50

Oxidation temperature (K)

Cal

cula

ted

EC

R (

%)

Japanese safety criteria:<1473 K<15% ECR

Fracture during quenchNo-Fracture

18.3 to 27.3%

38.0%

0

10

20

30

40

50

0 200 400 600 800 1000

Oxi

datio

n (%

EC

R)

Initial hydrogen concentration (ppm)

MFI-1(<400N)

MFI-2(0N)

MDA-1R(<350N)

ZIR-3R

MDA-2R

ZIR-2R

ZRT-1ZRT-2(0N)

Fracture boundary of unirradiated Zry-4

Safety limit(15%ECR)

Fracture No-fracture66-77GWd/t

0

10

20

30

40

50

0 200 400 600 800 1000

Oxi

datio

n (%

EC

R)

Initial hydrogen concentration (ppm)

MFI-1(<400N)

MFI-2(0N)

MDA-1R(<350N)

ZIR-3R

MDA-2R

ZIR-2R

ZRT-1ZRT-2(0N)

Fracture boundary of unirradiated Zry-4

Safety limit(15%ECR)

Fracture No-fracture66-77GWd/t

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.

20

Ring compression testsafter 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

0

50

100

150

200

-0.5 0 0.5 1

Load

(N

)

Displacement (mm)

LMDA1R

RC1

RC2

Load -displacement curves and post-test appearances of MDA cladding

Cladding Specimen Hydrogen Ox. Temp. ECR (B-J) ECR (C-P)

MDA-1RRC1 870 ppm 1158ºC 13.4% 10.6%

RC2 750 pp 1137ºC 11.8% 9.5% 21

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

23

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

24

Claddingmaterial

Burnup Reactor

PWR fuel cladding

M-MDA 73GWd/tVandellos-2

(Spain)

M5 78GWd/tGravelines-5

(France)

Low-tin ZIRLO 73GWd/tVandellos-2

(Spain)

BWR fuel cladding

Zry-2/LK3 92 GWd/tLeibstadt

(Switzerland)

Sample and test conditions in new program

25

Test conditions

• Oxidation test• Oxidation temperature: 1223−1473 K (950−1200°C)• Oxidation time: 120−3600 s• Post-test analysis: Weight gain measurement,

Microstructure observation and Oxide layer thickness measurement

• Integral thermal shock test• Oxidation temperature: 1423−1473 K (1150−1200°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)

26

LOCA test schedule

Jp FY(Apr-Mar)

2009 2010 2011 2012 2013 2014 2015

Shipment

Oxidation test

Integral thermal shock test

Inside EuropeTo Japan

Unirradiated cladding

High burnup cladding

Unirradiated cladding

High burnup cladding

27

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.

28

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).

Acknowledgment

29

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) 213-219.

• 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.163-170.

• Y. UDAGAWA, F. NAGASE and T. FUKETA, “Effect of Cooling History on Cladding Ductility under LOCA Conditions,” J. Nucl. Sci. Technol., 43[8] (2006) 844-850.

• 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) 723-730.

• F. NAGASE and T. FUKETA, “Behavior of Pre-hydrided Zircaloy-4 Cladding under Simulated LOCA conditions,” J. Nucl. Sci. Technol., 42[2] (2005) 209-218.

• F. NAGASE and T. FUKETA, “Fracture Behavior of Irradiated Zircaloy-4 Cladding under Simulated LOCA Conditions,” J. Nucl. Sci. Technol., 43[9] (2006) 1114-1119.

• F. NAGASE, T. CHUTO, and T. FUKETA, “Behavior of High Burn-up Fuel Cladding under LOCA Conditions,” J. Nucl. Sci. Technol., 46[7] (2009) 763−769.

• 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) 1369–1376.