3.4 The Fracture Toughness Testing of Unirradiated and ...

JAERI-Conf 99-009 JP9950645

3.4 The Fracture Toughness Testing of Unirradiated and Irradiated

Zr-2.5Nb CANDU Pressure Tube

Sangbok AHN°, Dosik KIM0, Daeseo KOO", Sangchul KW0N2), Yongsuk KM2)

Korea Atomic Energy Research Institute

P.O. Box-105, Yusong, Daejon, Korea

ABSTRACT

The test techniques of fracture toughness test for irradiated Zr-2.5Nb CANDU pressure

tube materials were developed in hot cell. The curved compact specimens of 17mm in width

with a notch in the axial direction were made directly in the hot cell from the irradiated and

unirradiated Zr-2.5Nb pressure tubes using a specially designed electric discharge machine

(EDM). The crack growth was measured by reversing direct current potential drop method. J-

Integral was determined from the measured load and displacement value accordance with

ASTM E813, E1152 and 1737. The tests of the unirradiated and irradiated Zr-2.5Nb

specimen with fluence 8.9xl022n/m22 were conducted in hot cell. The dJ/da of the unirradiated

Zr-2.5Nb pressure tubes agreed well the measured values on the same tubes out of hot cell.

The toughness of the irradiated specimen was dropped drastically comparing to the

unirradiated. Further, the fractographies of the irradiated Zr-2.5Nb pressure tubes were

discussed to investigate the neutron effect on the fracture toughness of Zr-2.5Nb pressure

tubes.

INTRODUCTION

Since the Wolsung Unit 1 has started operation in 1983, 3 PHWRs (Pressurized Heavy

Water Reactors) currently are in operation and one more will start operation in the late half of

1) Irradiated Material Experimental Facility 2) Zirconium Development Team

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1999. Thin-walled pressure tubes of cold worked Zr-2.5Nb (nominally 6.3m long, 103mm

in diameter, and 4.2mm thick) is used as the primary containment for the uranium dioxide

fuel. Heavy water flows through the tubes to cool the fuel, under an internal pressure of

about 10 MPa and at a temperature range from about 260 to 320 °C. Over the expected

lifetime, the pressure tube is subjected to degradation due to exposure to high stresses,

temperature and neutron flux. One criterion for lifetime of a tube would be an inability to

defend leak-before-break (LBB). This condition can be met if LBB if a crack initiate,

penetrates the tube wall and leakage of heavy water is detected before the crack grows the

critical crack length(CCL) and become unstable. The critical crack length is governed by

fracture toughness [1]. Thus, it is necessary to characterize the fracture toughness of the Zr-

2.5Nb pressure tubes with neutron irradiation till the lifetime of 3x1026 n/m2. In the fast,

fracture toughness was characterized by slit burst tests which were very expensive, consumed

a lot of material, and could not be used on a burst tube.[2-4] Therefore, it was desirable to

develop small specimen test methods in hot cell.

The objective of this study is to evaluate the feasibility of a fracture testing procedure for

irradiated Zr-2.5Nb pressure tubes. By using the curved compact tension specimens cut

away from Zr-2.5Nb pressure tubes by a specially designed electric discharge machine, crack

growth resistance of unirradiated and irradiated Zr-2.5Nb pressure tubes was determined.

Their dJ/da values obtained in the hot cell were compared with the reported values or the

measured values out of the hot cell to identify the feasibility of the fractue toughness testing

procedure. The middle ring of Zr-2.5Nb pressure tubes were used in this study, which had

been operated in Wolsung Unit 1 for 10 years with the neutron fluence of 8.1x1025 n/m2.

DEVELOPMENT OF EXPERIMENTAL PROCEDURES IN HOT CELL

1.Material andspecimen preparation.

Non-irradiated and irradiated Zr-2.5Nb pressure tube materials were used in this study. The

tube are manufactured by extrusion, after p-quenching, of hollow forged billets at a

temperature of about 815-850°C, i.e., in the (a+p)-phase field.

The Specimen was cut from tube with retaining original curvature by EDM in hot cell. The

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cutting condition was 6-8Ampere in currents and 0.2jisec in on time. The inplane dimensions

were in the ratio to specimen width, w, as described in ASTM E 813, and the thickness and

curvature were identical to those of tube. The detailed dimensions of the 17mm curved

compact tension(CT) are as Figure 1. A simple analytical calculation estimates that the

maximum curvature-induced stress was less than 10% of the inplane tensile stress at the

crack tip[5]. By comparing results from the flat and curved specimens, the equation of flat-

plate J-integral can be applied to both specimen types to calculate the fracture toughness

parameters with little error.

2. Equipment Apparatus

For the test the Instron 8502 machine with furnace and 1 ton load cell was used. The test

fixture consisted of pull rods and grips to be electrically insulated from the test machine to

prevent short circuiting from DCPD currents. The insulated grip shape is in Photo 1. The pins

were made from hardened steel to minimize pin deflection and had tapered to 1.5° for

producing straighter fatigue precrack in Figure 2. These pins distribute more loads to the

outside surface of the specimen to compensate for the bending stress caused by the curvature

of the specimen. To measure load-point displacement, an alternate method to measure the

movement of grips was adopted with LVDT pick up device in Photo 2. The displacements

were corrected with values obtained from the measurement of deflection in a rigid specimen,

over the full load ranges. The measurement method of crack extension was by the reversing

DCPD described in reference. [6] The power supply lines were attached to the upper and

lower sides using brass cap. The lead wires to measure dropped voltages were dia. 0.6mm

Nickel-Copper electric lines and were attached to the crack mouth of the specimen by spot

welder. The welding condition was 3000A in current and 0.2sec in pressuring time.

ITestPnxedures

The specimen was carefully inserted in the grips and connected with pins so as not to

disturb the potential leads. The DCPD system were switched on and allowed to be stabilized

until the potential drop indicated that there were no more voltage changes for at least 20 min.

The fatigue precracking was carried out in the spirit of ASTM E 399 section A22. To

initiate the fatigue crack evenly, an initial stress intensity factor range (AK) about 15 MPaVm.

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with the load ratio(R) of 0.2. Once the fatigue crack was started, AK was decreased. The

loading cycles were 5Hz. The AK used for the last 0.5mm of fatigue crack was on the order

of 11 MPaVm. The final fatigue crack length was enough to have an a/w value of about 0.5.

To the end of fatigue, the crack growth is monitored by the potential drop signal using an

estimated value from the unirradiated specimen calibration constant.

The J-integral test was carried between 2.5-4 mm of crack extension. The displacement

increasing speed is 0.25mm/min. Throughout the test, the values of load, load point

displacement, dropped voltage and specimen temperature were to be continuously monitored.

The sampling rate for the data acquisition system was 150 points for each unloading

procedure. At the end of test the final crack length was marked by heat tinting for 20 min. at

280~290°C. During the tinting procedure a small load (approximately 50% of the final load)

was applied to the specimen to prevent crack closure. The initial and final crack lengths were

measured by the nine point average method with highscope system. From the total dropped

voltages and crack growth, the calibration constant was derived to calculate the crack

increments during the test.

Using the data from the test, the J-integral was calculated according to ASTM E-1737 to

generate the J-resistance(J-R) curve. The J value was calculated from the next equations.

J,=J*+J* (1)

Where JeI and JpI are the elastic and plastic component of J-integral, respectively

Jd=K,2(l-v2)/E

(2)

Where B is the specimen thickness, v is the Poisson's ratio, E is the Young's modulus

and;

f(at/w)=-0.886 + 4.64(a,. / w) - 13.32(a, / w)

(3)•14.72(fl,./w)3-5.6(a,./w)4

The value of Jpl was calculated using the equation derived by Ernst et al.[7] and Clark and

Landes[8]

•'/>'(<) " ~J (4)

Where b is the ligament (w-a^, T], = 2.0+0.552(b/w) and y{ = 1.0+0.76(b/w). The value of

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Apl(i)- ApKJ.,) is the increment of plastic area under the load and load point displacement

record between lines of constant displacement at point (i) and (i-1)

AP«» = AP,O-» + i(pi + Pi-i X<5 pun - * P((M>) / 2] (5)

Where 5pli is the plastic part of the total load line displacement, 8.

tpm^ti-PiC, (6)

Where Ci is the specimen compliance and given by

• 12.219(a; / w) - 20.065(a,. Iw)2- 0.9925^ / w)3'

E*B [ w - at4 99314(a / w)5+ 20.069(a,. / w)4 - 9.9314(a, / w)

Where E* (=E/(l-v)) is the effective Young's modulus.

(6)

RESULTS AND DISCUSSION

Figure 3a, b show load vs. displacement curves with the ratio(a/w) of about 0.53 for the

unirradiated and irradiated Zr-2.5Nb tubes at room temperature. The unirradiated Zr-2.5Nb

tube had a continuous change in load and voltage with displacement. In contrast, the

irradiated tube had a discontinuous increment in load and voltage with displacement due to a

series of crack jump occurring during fracture at room temperature. It is worth noting that the

irradiated Zr-2.5Nb tube has the lower maximum load and earlier crack initiation than the

unirradiated Zr-2.5Nb tube. It was reported that the lower temperature, the longer jump[9].

Quantitative evaluation of the embrittlement of Zr-2.Nb pressure tube by neutron irradiation

can be made by plotting J-R curves of pressure tubes. First, the crack distance of the

unirradiated and irradiated Zr-2.5Nb specimen was determined by using the 9 point method

on the fractured surfaces shown in Photo 3a, b. Through the measured crack distance and J-

integral calculated from the load-displacement curve, the J-R curves were plotted for the

unirradiated and irradiated Zr-2.5Nb as shown in Figure 4a, b. The J-R curve for the

unirradiated Zr-2.5Nb had a continuously decreasing slope. However, the J-R curve for the

irradiated one consists of three stages; the stage I where the crack starts to propagate

followed by a decreasing slope of the J-R curve, the stage II where the slope of the resistance

curve steeply increases, and finally the stage lU where the slope decrease and approaches a

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constant value. The results shown in Figure 4b confirm that the irradiated Zr-2.5Nb has much

decreased low fracture toughness compared with that of the unirradiated one. From the J-R

curves, we determined the dJ/da as a fracture toughness value, the linear slope of the J-R

curve between the 0.15 mm and 1.5 mm exclusion lines as defined by ASTM E 1152 and E

1737. It is because the crack initiation value, JIC defined in ASTM E813 standard could not

be reproduced consistently on this curved compact tension Zr-2.5Nb specimen due to the

geometric limitation [10]. Table 1 shows the measured dJ/da values for the unirradiated and

irradiated Zr-2.5Nb tubes.

Table 1. Fracture toughness results for the Zr-2.5Nb materials from CANDU pressure tube.

Specimen

Identity

UR-01

UR-02

IR-01

IR-02

Test

Temp.

(°C)

25

25

25

25

dJ/da

(MPa)

Meas

329.7

339.5

84.5

58.5

Ref.

342.1°

25.92)

J-Integral Value

(kJ/m2)

0.2mm

155.8

165.5

33.7

22.5

1.5m

m

575.7

580.5

134.2

102.3

Max load

221.6

230.0

99.12

73.5

Reference

Unirradiated

Unirradiated

Irradiated

Irradiated

+ 1) tested out of hot cell 2) tested at AECL

The measured dJ/da values of the unirradiated Zr-2.5Nb pressure tube in the hot cell were

quite in good agreement with those conducted on the similar specimen out of the hot cell.

However, the determined dJ/da values at room temperature for the two curved compact

tension specimens were found two times as much as dJ/da values reported by Han[l 1]. The

uncertainty of dJ/da values for the irradiated Zr-2.5Nb pressure tubes is under scrutiny.

Photo 3-a,b show the fractographs of curved compact specimen from unirradiated Zr-2.5Nb

pressure tube. The unirradiated ductile specimen had the crack with a thumbnail shape which

is quite symmetric with respect to the thickness of the compact tension specimen. The

material removal sometimes was observed in the middle of the fractured surface even though

its cause is yet to be investigated. Photo 4-a,b,c,d,e shows the SEM microstructures taken

from various parts of the fractured surface of the unirradiated Zr-2.5Nb tube. The fractured

surface just in front of the fatigue shows very fine dimples along with little fissures (Photo 4-

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a). However, at the middle of the fractured surface, some fissures appeared along with some

small voids (Photo 4-b and c). Some clear fissures, however, appeared just before the ductile

shear region (Photo 4-d). One thing to note is a considerable through-thickness yielding for

this unirradiated specimen with very narrow shear lips near the surface (Photo 4-e). In

contrast, the irradiated Zr-2.5Nb specimen had little through-thickness yielding and,

furthermore, well-developed flat fractured surface along with a slant fracture developing at

the surface as shown in Photo 5-a and b. Some long fissures, lying in the tube axial direction,

were observed in the middle section of the flat fractured surface as shown in Photo 5-c.

However, fissure density looks quite low in comparison to that on Zr-2.5Nb pressure tubes of

low fracture toughness [5], implying that the irradiated Zr-2.5Nb tubes under investigation

seem to be of high toughness. On the other hand, near the end of the flat fracture region, the

fracture proceeded at 45 degrees to the transverse-axial plane which seems to correspond

with the plane of the maximum shear stress. There was no evidence of fissure formation but

of equiaxed and tearing dimples near that region as shown in Photo 5-d. These results lead to

a conclusion that neutron irradiation embrittles Zr-2.5Nb pressure tubes qualitatively

CONCLUSION

The test technique for fracture toughness was developed for the curved 17mm CT specimen

from CANDU pressure tube in hot cell. The specimen was cut by EDM retaining original

tube shape. The crack increments were measured by DCPD system. The data from test were

analyzed based on ASTM E813, El 152 and E1737. The conclusions we have drawn from this

study are summarized as in the following:

1. J-R curves of irradiated and unirradiated Zr-2.5Nb pressure tubes were successfully

measuured in the hot cell by using the curved compact tension specimens. The irradiated

Zr-2.5Nb pressure tubes had lower dJ/da values than that of the unirradiated Zr-2.5Nb.

The dJ/da of the unirradiated Zr-2.5Nb pressure tubes agrees well with the measured values

on the same tubes out of the hot cell. However, compared to the reported values of the

similar irradiated Zr-2.5Nb pressure tubes, the measured dJ/da values of the irradiated Zr-

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2.5Nb pressure tubes were found higher, the cause of which has to be elucidated further.

2. The fractured surface of irradiated Zr-2.5Nb pressure tubes consists of flat fracture region

and slant fracure developing at the surface while the unirradiated Zr-2.5Nb pressure tubes

had considerable through-wall thickness yielding.

REFERENCES

1. Hosbons, R. R., Davies, P. H., Griffiths, M., Sagat, S. and Colmann, C. E.: ASTM 12th

Symposium on Zirconium in the Nuclear Industry Abstract Paper, 1998

2. Langford, W. J. and Mooder, L.E.J., International Journal of Pressure Vessels and Piping,

Vol. 6, 1978,pp275-310.

3. Cowan, A. and Langford, W. J., Journal of Nuclear Materials, Vol.30, 1969, pp. 271-

281.Wilkins, B. J. S., Barrie, J. R , and Zink, R. J., Report AECL-6195, AECL, 1978

4. Wilkins, B. J. S., Barrie, J. N., and Zink, R. J., Report AECL-6195, AECL, 1978

5. Chow, C. K. and Simpson, Leonard A., Fracture Mechanics: Eight Symposim, ASTM STP

945, 1988, pp. 419-439

6. Dosik, KIM et al. KAERI-JAERI Joint Seminar, 1999.

7. Ernst, H. A., Paris, P. C. and Landes, J. D., Fracture Mechanics: Thirteenth Conference,

ASTM STP 743, 1981, pp. 476-502

8. Clark, G A. and Landes, J. D., Journal of Testing and Evaluation, vol.7, No.5, 1979, pp.

643-662

9. Chow, C. K., Colman, C. E., Hosbons, R. R., Davies, P. H., Griffiths, M., and Choubey, R.,

ASTM STP 1132, 1991, pp. 246-275.

10. Candu Owner Group, Instructions to Round Robin Participants, COG-98-161-1, RC-2069

1,1998, pp 21-28.

11. Private Communication with B. S. Han, KEPCO

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•I

Figure 1. Dimensions in 17mm compact tension specimen

Figure 2. Tapered loading pin for precrcking

"-1

A- "i. r

Photo 1. Electric shielding device for DCPD System Photo 2. LVDT pick up device

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1 2

Displacement [mm]

Figure 3-a. Typical load displacement curves Figure 3-b. Typical load displacement curves

for the unirradiated specimen for the irradiated specimen

1000

IS" 500

Comtructton o.15mUn* Exdm

/ • ' / •/ .'•• •

/ •'••' •

/ • ' • *

/ • '• : •/ :7 "

r/ • ji 0.2mm

£ jl*- OHs* lint

m ;' Jrkn Urw • mir

/j

U^_i5mm

1 2 3

Crack Increment, Aa [mm]

Figure 4-a. Typical J-R curve

for the unirradiated specimen

Corulniction Una

(MSmmExdmion Unt

Crack Increment, 4a[mm]

Figure 4-b. Typical J-R curvee

for the irradiated specimen

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t

Photo 3-a. The Facture surface

of the unirradiated specimen

Photo 3-b. The fracture surface

of the irradiated specimen

(Photo 4-a)

(Photo 4-b) (Photo 4-c)

'ij vyif''" i

(Photo 4-d) (Photo 4-e)

Photo 4. SEM images from the unirradiated Zr-2.5Nb fractured surface

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(Photo 3 a)

I V

(Photo 5-b)

(Photo 5-c) (Photo 5-d)

Photo 5. SEM images from the irradiated Zr-2.5Nb fractured surface

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