The oxidation behavior of Zircaloy‐4 in steam between 600 and 1600°C

Abstract

With respect to the behavior of Nuclear Pressurized Water Reactor fuel cladding during accidents the oxidation kinetics of Zircaloy‐4 tubing in steam and the related changes in the mechanical properties have been investigated. Short tube sections were exposed to steam between 600 and 1600°C under isothermal and temperature transient conditions. Tube capsules were creep‐rupture tested in steam under isothermal/isobaric (600–1300°C, 7–150 bar) and various temperature/pressure transient conditions. The oxidation kinetics of Zircaloy‐4 is described in case of shortterm reaction by simple rate laws. The long‐term behavior is proved to be highly influenced by the breakaway transition and finally by total consumption of the tube wall. On the basis of the isothermal behavior the short‐term temperature transient case can be understood and also modelled. Deviations from predictable behavior are the consequences of Zr‐ and ZrO2‐phase transformations. The protective character of preexistent scales is lost above 1100°C for the same reason. The consequences of oxidation on the mechanical properties are increased strength and decreased ductility, whereas the response to creep deformation is an overall increase in oxidation due to oxide cracking.

Cladding embrittlement during postulated loss-of-coolant accidents.

“…Belt polishing removes surface impurities in the range of 0.1-0.3 µm and may result in some additional cold working of the near-surface metal. In terms of hydrogen content or hydrogen pickup, the HBR-type Zry-4 (1980s) has a much higher breakaway-oxidation time (≈3800 s) than the time (≈1800 s) determined by Leistikow and Schanz [19,20] for 1970s Zry-4. The breakaway-oxidation time (≈5000 s) for modern AREVA 15×15 Zry-4 tested by ANL is comparable to the breakaway-oxidation time (≈5400 s) determined by Mardon et al [21] for modern AREVA 17×17 Zry-4.…”

“…Breakaway oxidation has been investigated by Leistikow and Schanz [19,20] for Zry-4 and Mardon et al [21] for Zry-4 and M5. Although the breakaway oxidation time is generally defined as the time corresponding to an increase in oxidation rate (i.e., weight gain rate), it is the associated hydrogen pickup that causes embrittlement.…”

“…Conversely, irregularities in surface geometry can result in alternating regions of compressive and tensile stresses. According to Leistikow and Schanz [19,20], such irregularities along the metal-oxide surface are a precursor to tetragonal-to-monoclinic transformation and breakaway oxidation.…”

Pressure effects on high temperature steam oxidation of zircaloy-4

“…The oxide thickness in the Arsteam mixture (steam partial pressure: 0.5 MPa) is quite close to that made at 1 atm steam. The solid line indicates the Leistikow's isothermal oxidation data of Zry-4 [6,7]. Based on this observation, it can be concluded that steam pressure is a more important factor affecting the high temperature oxidation of Zry-4 than is the total gas pressure.…”

“…The kinetics of Zircaloy oxidation in atmospheric steam below 1500 o C can be divided into two regions: the cubic rate law (below 950 o C) and the parabolic (above 1070 o C) [6]. Between 950 and 1070 o C, a mixed behavior of cubic and parabolic rates appears [7].…”