Zircaloy-4 cladding embrittlement due to inner surface oxidation under simulated loss-of-coolant condition.

Abstract

Embrittlement of Zircaloy-4 cladding by oxidation of the inner surface occurring in an L WR loss-of-coolant accident was studied using simulated fuel containing of Al.Oa pellets sheathed in Zircaloy-4 specimen cladding, filled with Ar gas, and sealed. This simulated fuel rod was heated from outside until the isothermal oxidation temperature between 880 and 1,167"C was obtained after the cladding burst. This exposed the inner surface of the cladding to the environmental atmosphere, provided by steam flowing at a constant rate in the range of 0.13-1.6g/cm 2 ·min. The embrittlement of the specimen due to inner surface oxidation is influenced primarily by the amount of hydrogen absorbed by the Zircaloy-4. Ring compression tests conducted at 100"C on test pieces constituted of sliced sections of oxidized specimen showed that Zircaloy containing more than 200-300 wt.ppm of absorbed hydrogen became brittle when oxidized at temperatures above 1,000"C. In the range of oxidation temperature 932 to 972"C, brittleness did not appear below 500-750 wt.ppm absorbed hydrogen. Hydrogen absorbed by the Zircaloy precipitated in the form of fine hydride crystals formed along previous ,9-phase grain boundaries. Peaks were found in the distribution of hydrogen absorbed on the inner surface, at a distance of 15-45 mm upward and downward of the rupture opening. Within this range, the distance was influenced by the oxidation temperature and steam flow rate.

Hydride behavior in Zircaloy cladding tube during high-temperature transients

“…6, a basket-wave structure is seen, and the microstructure of this specimen is apparently different from that in the specimens annealed at temperatures below 973 K. Therefore, the rapid annealing out of the hydride rim and hydride morphology change can be correlated with the phase transformation to the b phase. Although the hydride morphology is not clear in the figures, hydrides are considered to become very fine and precipitate on the boundary of needle-like a-grains in Zircaloy, which transformed to the b phase and rapidly cooled [15].…”

“…4). Hydrides are considered to finely distribute in the specimen [15], which probably resulted in the high hardness.…”

Overview of high-temperature fuel behaviour with relevance to CANDU fuel

“…Measurements have also been made on the loss of ductility and embrittlement of Zircaloy-4 cladding by oxidation and hydriding under LOCA conditions and with a water quench [149][150][151][152][153][154]. In particular, the LOFT FP-2 test was a relatively large inreactor experiment to determine the effect of reflood for a severely damaged core assembly [155,156].…”

Modeling of Zr alloy burst cladding internal oxidation and secondary hydriding under LOCA conditions

“…The inner surfaces of the top and bottom bands of such specimens, %30-mm away from the burst center, contained H up to %2200 wppm in [2] and %1800 wppm in [3]. The typical distributions of oxide layer thickness, hydrogen concentration, and ring deflection to failure measured in [3] are shown in Fig. 1.…”

“…The embrittlement of Zircaloy-4 (Zry-4) cladding due to oxidation and hydriding of the inner surface was examined for the first time at Argonne National Laboratory (ANL, USA) [2] and Japan Atomic Energy Research Institute (JAERI, Japan) [3], through the simulation of LOCA conditions by heating specimens of Zircaloy cladding containing Al 2 O 3 pellets and filled with Ar. The inner surfaces of the top and bottom bands of such specimens, %30-mm away from the burst center, contained H up to %2200 wppm in [2] and %1800 wppm in [3].…”