Delayed Hydrogen Cracking Velocity and J-Integral Measurements on Irradiated BWR Cladding

Abstract

Propagation of an axial crack has been characterized by means of the Pin-Loading Tension (PLT) test. The same experimental arrangement is used for the Delayed Hydrogen Cracking (DHC) and for the J-integral measurements. The DHC tests are performed under variable temperature and/or load conditions simulating a variable PCMI. Tests have been performed in air on unirradiated SRA Zircaloy-4 tubing and irradiated BWR Zircaloy-2 cladding containing from 30–350 ppm of hydrogen. Variable loading facilitates DHC: in the experiment at 300°C a crack velocity as high as 9.3·10−7 m/s (80 mm/day) has been observed for irradiated cladding. V-K curves or diagrams of DHC crack velocity, V, as a function of stress-intensity factor, K, are suggested for different BWR claddings. Despite a limited amount of tests, the V-K curves appear to depend on the type of cladding. DHC does occur at much lower stress intensities compared to the Kc converted from the J0.2 values, confirming DHC to be the dominating secondary failure crack propagation mechanism in BWR cladding.

Delayed Hydride Cracking in Zircaloy Fuel Cladding - An Iaea Coordinated Research Programme

“…In the current study at 283 °C in one specimen no cracking was detected after 9000s at a KI of 15 MPa√m but once KI was increased by about 7% the crack progressed, but at a much reduced rate based on expectations from an extrapolation of Equation (2). In this study, as in others on Zircaloy [5,7,8] the fractographic features called striations were absent. These observations are in marked contrast to the fractography of Zr-2.5Nb where striations are easily observed.…”

“…The values of D and C used for this exercise are derived from Sawatzky (Zircaloy [11] and Zr-2.5Nb [12]) and Kearns [13] while the values of UTS are mostly provided in the papers on DHC. The current results fit into the broad band of data on several different materials in different metallurgical conditions, including irradiation [4,[7][8][9]14] indicating that the DHC behaviour of fuel cladding is as expected from the experience with pressure tube materials.…”

“…The current results are not suitable for direct application to the behaviour of fuel cladding because the material is unirradiated. The added strength generated by irradiation has two consequences: the crack velocity is increased by a factor between ten to fifty [7,8,19] easily agreeing with the rates observed in-reactor, and the high temperature decline in velocity is postponed to higher temperatures [17]. Other differences between cladding on an operating fuel element and these laboratory experiments include temperature gradients, which can affect crack velocity and response to temperature history [20].…”

“…The same principles are being applied for the testing of Zircaloy-4 fuel cladding in nine countries. Several methods are available for testing such thin-walled, small diameter material: a centre-cracked half-tube loaded in tension [5] a centre-cracked length of tube loaded by a wedge and mandrel (SPLIT test) [6] and the Pin-Loading Tension (PLT) technique [7]. The last method was chosen for this The rate of delayed hydride cracking (DHC), V, has been measured in cold-worked and stress-relieved Zircaloy-4 fuel cladding using the Pin-Loading Tension technique.…”

Stress-induced Hydride Reorientation and Cracking in Fuel Cladding Tube

“…Several methods are known for testing of thin-walled cladding tubes: centre-cracked half-tube loaded in tension [5], a centre-cracked length of tube loaded by a wedge and mandrel (SPLIT test) [14] and the Pin-Loading Tension (PLT) technique [15]. In some works [7] cladding tube is subjected to thermal cycling in an autoclave under a constant hoop stress by regulating the differential pressure between its internal and external pressures with a constant differential pressure control system.…”