Degraded Fuel Cladding Fractography and Fracture Behavior

Abstract

An increase in the frequency of fuel degradation following a primary breach in boiling water reactor (BWR) rods was observed during the late-1980s and early 1990s; these rods were often found to contain axial splits. Several failed rods containing splits have been returned to the Vallecitos Nuclear Center (VNC) for post-irradiation examination (PIE). This paper describes in detail the characteristics of fracture in these rods and relates the observations to postulated mechanisms of split formation. Crack initiation invariably occurred at localized hydrides well away from the primary defect. Crack propagation was almost entirely brittle except for fracture of the zirconium liner, which was typically ductile. Fractography of the matrix exhibited nonsymmetrical chevron features with the crack front generally leading near the outside of the cladding. Detailed metallography demonstrated that cracks propagated through regions of both high and low (i.e., 150 ppm) hydride concentrations with the same brittle features. A modified Charpy test was developed to elucidate the effects of metallurgical and stress state variables on the crack phenomena and provide relevant fracture toughness measurements as a function of hydride level for thin-walled tube specimens. A parallel series of tests examined the role of the environment on cracking behavior. The sum of the information provides valuable insight into the mechanism of split formation in degrading fuel rods.

The first step for delayed hydride cracking in zirconium alloys

“…A time-dependent mechanism of fracture that requires cracking of zirconium hydride has been responsible for several failures of zirconium alloy components in both nuclear reactors [1][2][3] and chemical plants [4]. The mechanism is called delayed hydride cracking or DHC.…”

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

“…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.…”

Performance of Pressure Tubes in Candu Reactors

“…DHC is a stable crack-growth mechanism that has led to failures in Zr-2.5Nb fuel cladding [16], pressure tubes [17], and possibly in Zircaloy fuel cladding [18][19][20]. The conventional view is that the DHC process consists of several steps [16,17,21,22]:…”