Assessing the fracture toughness of Zircaloy-4 fuel rod cladding tubes: impact of delayed hydride cracking

Delayed hydride cracking (DHC) is a hydrogen embrittlement phenomenon that may potentially occur in Zircaloy-4 fuel claddings during dry storage conditions. An experimental procedure has been developed to measure the toughness of this material in the presence of DHC by allowing crack propagation through the thickness of a fuel cladding. Notched C-ring specimens, charged with 100 wppm of hydrogen, were used and pre-cracked by brittle fracture of a hydrided zone at the notch root at room temperature. The length of the pre-crack was measured on the fracture surface or cross-sections. Additionally, a finite element model was developed to determine the stress intensity factor as a function of the crack length for a given loading. Two types of tests were conducted independently to determine the fracture toughness with and without DHC, \(K_{I_\text {DHC}}\) and \(K_{I_\text {C}}\), respectively: (i) constant load tests at 150 \(^{\circ }\)C, 200 \(^{\circ }\)C, and 250 \(^{\circ }\)C; (ii) monotonic tests at 25 \(^{\circ }\)C, 200 \(^{\circ }\)C, and 250 \(^{\circ }\)C. The results indicate the following: (1) there is no temperature influence on the DHC toughness of Zircaloy-4 between 150 and 250 \(^{\circ }\)C (\(K_{I_\text {DHC}} \in \left[ 7.2;9.2\right] \) MPa\(\sqrt{\text {m}}\)), (2) within this temperature range, the fracture toughness of Zircaloy-4 is halved by DHC (\(K_{I_\text {C}} \in \left[ 16.9;19.7 \right] \) MPa\(\sqrt{\text {m}}\)), (3) the crack propagation rate decreases with decreasing temperature and (4) the time before crack propagation increases as the temperature and loading decrease.