Studies on Delayed Hydride Cracking of Zircaloy Cladding

An assessment of delayed hydride cracking in zirconium alloy cladding tubes under stress transients

“…The calculated K IH values are in agreement with the lower bounds of the experimental data for Zr-2?5Nb 13 and Zircaloy-2. 38,44,50 A substantial number of the experimental K IH measurements, however, exceed the lower bound and the model calculations based on the crack tip shielding and phase transformation toughening theories. The results indicate that crack tip shielding by hydride formation at the crack tip is significant and is partly responsible for the experimental observation that the K IH is higher than K h IC .…”

“…The integrity of zirconium alloy cladding in nuclear fuel rods during service can be degraded by DHC, a time dependent crack growth process. 2,5,[8][9][10][11] Although it has been observed in Zircaloy-2, 37,38,[44][45][46] Zircaloy-4 42,46 and Zr-2?5Nb, 2,5,[8][9][10][11][12][13][14][15]44 most of the work on DHC has been conducted on Zr--2?5Nb. The physical processes involved in the DHC failure mechanism are fairly well understood.…”

“…1/2 ) with a 95% confidence limit for a single observation and a mean of 8?2 MPa m 1/2 (7?46 ksi in 1/2 ) for all tests in the database compiled by Shi and Puls. 13 The K IH value for Zircaloy-2 tubes 38,40,44 ranges from 5?2 to 14?2 MPa m 1/2 (4?73-8?29 ksi in. 1/2 ).…”

“…The reasons for the high K IH values observed in the Zircaloy-4 studies by Huang and Ho 42 are not known; possible reasons include low hydrogen contents, texture effect and incomplete hydride coverage and ligament toughening by Zr grain ligaments at the crack wake. All the K IH measurements for Zr-2?5Nb, 9,12 Zircaloy-2 37,38,40,[44][45][46] and Zircaloy-4 42,46 have been obtained using fracture mechanics specimens with a large crack. For large crack fracture mechanics specimens, the K IH for zirconium alloys is always larger than the fracture toughness of the hydrides, indicating that the limiting process associated with the DHC threshold is more involved than just the fracture of hydrides near the crack tip or notch tip region.…”

“…The onset of DHC in zirconium alloys is controlled by the growth threshold K IH , which is the stress intensity factor that a crack must exceed for DHC growth to proceed. 9,[12][13][14] As a result, the cladding stress and the size of cracks or defects in the cladding tubes, if present, are important contributing factors, among other factors such as temperature, 12,37,38 texture, 39,40 hydrogen content, 12,38,41,42 neutron irradiation fluence 38,[43][44][45] and cooling rate that affect the DHC growth kinetics in zirconium alloy cladding. Alloy composition, 46 microstructure 44 and heat treatment can also affect the susceptibility of an alloy towards delayed hydrides cracking because they influence the oxidation of zirconium to form zirconium oxide, hydrogen and the absorption of hydrogen into zirconium.…”