A crack initiation and growth mechanism known as delayed hydride cracking (DHC) is a concern for Zr-2.5Nb alloy pressure tubes of CANada Deuterium Uranium or CANDU (CANDU is a trademark of the Atomic Energy of Canada Limited, Ontario, Canada) nuclear reactors. DHC is a repetitive process that involves hydrogen diffusion, hydride precipitation, formation, and fracture of a hydrided region at a flaw tip. An overload occurs when the flaw-tip hydrided region is loaded to a stress, higher than that at which this region is formed. For the fitness-for-service assessment of the pressure tubes, it is required to demonstrate that the overload from the normal reactor operating and transient loading conditions will not fracture the hydrided region, and will not initiate DHC. In this work, several series of systematically designed, monotonically increasing load experiments are performed on specimens, prepared from an unirradiated pressure tube with hydrided region, formed at flaws with a root radius of 0.1 mm or 0.3 mm, under different hydride formation stresses and thermal histories. Crack initiation in the overload tests is detected by the acoustic emission technique. Test results indicate that the resistance to overload fracture is dependent on a variety of parameters including hydride formation stress, thermal history, hydrogen concentration, and flaw geometry.

1.
Dutton
,
R.
,
Nuttall
,
K.
,
Puls
,
M. P.
, and
Simpson
,
L. A.
, 1977, “
Mechanisms of Hydrogen Induced Delayed Cracking in Hydride Forming Materials
,”
Metall. Trans. A
0360-2133,
8
, pp.
1553
1562
.
2.
Cheadle
,
B. A.
,
Coleman
,
C. E.
, and
Ambler
,
J. F. R.
, 1987, “
Prevention of Delayed Hydride Cracking in Zirconium Alloys
,”
ASTM Spec. Tech. Publ.
0066-0558,
939
, pp.
224
240
.
3.
Sagat
,
S.
,
Shi
,
S. Q.
, and
Puls
,
M. P.
, 1994, “
Crack Initiation Criterion at Notches in Zr-2.5Nb Alloys
,”
Mater. Sci. Eng., A
0921-5093,
176
, pp.
237
247
.
4.
Sagat
,
S.
,
Lee
,
W. K.
, and
Bowden
,
J. W.
, 1994, Chalk River Laboratories of Atomic Energy of Canada Ltd. and Ontario Hydro Technologies.
5.
Sagat
,
S.
,
Newman
,
G. W.
, and
Scarth
,
D. A.
, 2002, “
Crack Initiation by Delayed Hydride Cracking at Sharp Notches in Zr-2.5Nb Alloys
,”
Proceedings of the International Conference for Hydrogen Effects on Material Behaviour and Corrosion Deformation Interactions
, Jackson Lake Lodge, Moran, WY, Sept. 22–26.
6.
Cui
,
J.
,
Shek
,
G. K.
,
Scarth
,
D. A.
, and
Lee
,
W. K.
, 2004, “
Delayed Hydride Cracking Initiation at Simulated Secondary Flaws in Zr-2.5Nb Pressure Tube Material
,”
Proceedings of the 2004 ASME Pressure Vessels and Piping Conference
, San Diego, CA, Jul. 25–29, PVP-Vol.
474
, pp.
53
65
.
7.
Scarth
,
D. A.
, and
Smith
,
E.
, 1999, “
Developments in Flaw Evaluation for CANDU Reactor Zr–Nb Pressure Tubes
,”
Proceedings of the 1999 ASME Pressure Vessels and Piping Conference
, Boston, MA, Aug. 1–5, PVP-Vol.
391
, pp.
35
45
.
8.
Scarth
,
D. A.
, and
Smith
,
E.
, 2000, “
The Use of Failure Assessment Diagrams to Describe DHC Initiation at a Blunt Flaw
,”
Proceedings of the 2000 ASME Pressure Vessels and Piping Conference
, Seattle, WA, Jul. 23–27, PVP-Vol.
412
, pp.
63
73
.
9.
Scarth
,
D. A.
, and
Smith
,
E.
, 2002, “
The Effect of Plasticity on Process-Zone Predictions of DHC Initiation at a Flaw in CANDU Reactors Zr–Nb Pressure Tubes
,”
Proceedings of 2002 ASME Pressure Vessels and Piping Conference
, Vancouver, Canada, Aug. 4–8, PVP-Vol.
437
, pp.
19
30
.
10.
Canadian Standards Association
, 2005, “
Technical Requirements for In-Service Evaluation of Zirconium Alloy Pressure Tubes in CANDU Reactors
,” CSA N285.8-05.
11.
Cui
,
J.
, 2008, “
Initiation of Delayed Hydride Cracking in Zr-2.5Nb Pressure Tube Materials Under Constant Load and Overload Conditions
,” Ph.D. thesis, University of Toronto, Toronto, Ontario, Canada.
12.
Shek
,
G. K.
, and
Cui
,
J.
, 2003, “
Effects of Hydrided Region Overload on Delayed Hydride Cracking Initiation From Flaws in Zr-2.5Nb Pressure Tubes
,”
Proceedings of the 2003 ASME Pressure Vessels and Piping Conference
, Cleveland, OH, Jul. 20–24, PVP-Vol.
462
, pp.
117
122
.
13.
Shek
,
G. K.
,
Cui
,
J.
, and
Perovic
,
V.
, 2004, “
Overload Fracture of Flaw Tip Hydrides in Zr-2.5Nb Pressure Tubes
,”
Zirconium in the Nuclear Industry: 14th International Symposium, ASTM STP 1467
, Stockholm, Sweden.
14.
Eadie
,
R. L.
,
Metzger
,
D. R.
, and
Léger
,
M.
, 1993, “
The Thermal Rachetting of Hydrogen in Zirconium-Niobium—An Illustration Using Finite Element Modelling
,”
Scr. Metall. Mater.
0956-716X,
29
, pp.
335
340
.
15.
Sagat
,
S.
,
Coleman
,
C. E.
,
Griffiths
,
M.
, and
Wilkins
,
B. J. S.
, 1994, “
The Effect of Fluence and Irradiation Temperature on Delayed Hydride Cracking in Zr-2.5 Nb
,”
ASTM Spec. Tech. Publ.
0066-0558,
1245
, pp.
62
79
.
16.
Northwood
,
D. O.
, and
Kosasih
,
U.
, 1983, “
Hydrides and Delayed Hydrogen Cracking in Zirconium and Its Alloys
,”
Int. Met. Rev.
0308-4590,
28
(
2
), pp.
92
121
.
17.
Lee
,
W. K.
,
Metzger
,
D. R.
, and
Luo
,
L.
, 2000, Ontario Hydro Technologies.
18.
ASTM
, 1999, “
Standard Practice for Preparation and Use of Bent-Beam Stress-Corrosion Test Specimens
,” G39-99.
19.
Scarth
,
D. A.
, and
Smith
,
E.
, 2003, “
Improved Failure Assessment Diagrams to Describe Delayed Hydride Cracking Initiation at a Blunt Flaw
,”
Proceedings of the 2003 ASME Pressure Vessels and Piping Conference
, Cleveland, OH, Jul. 21–24, PVP-Vol.
462
, pp.
103
116
.
20.
Creager
,
M.
, and
Paris
,
P. C.
, 1967, “
Elastic Field Equations for Blunt Cracks With Reference to Stress Corrosion Cracking
,”
Int. J. Fract. Mech.
0020-7268,
3
(
4
), pp.
247
252
.
21.
Tada
,
H.
,
Paris
,
P. C.
, and
Irwin
,
G. R.
, 2000,
The Stress Analysis of Cracks Handbook
,
3rd ed.
,
ASME
,
New York
.
22.
Metzger
,
D. R.
, and
Sauvé
,
R. G.
, 1996, “
A Self-Induced Stress Model for Simulating Hydride Formation at Flaws
,”
Proceedings of 1996 ASME Pressure Vessels and Piping Conference
, Montreal, Canada, Jul. 21–26, PVP-Vol.
326
, pp.
137
144
.
You do not currently have access to this content.