Pressure effects on high temperature steam oxidation of zircaloy-4

Abstract

The pressure effects on the oxidation of Zircaloy-4 (Zry-4) cladding in high temperature steam have been analyzed. A double layer autoclave was made for the high pressure/high temperature oxidation tests. The experimental test temperature range was 700-900 o C, and pressures were 0.1-15 MPa. Steam partial pressure was found to be a more important factor than total pressure. Steam pressure enhances the oxidation rate of Zry-4 exponentially and depends on the temperature, the maximum being between 750-800 o C. A preexisting oxide layer decreases the enhancement by about 40-60%. The acceleration of the oxidation rate caused by high pressure steam seems to originate from the formation of cracks due to an abrupt transformation of the tetragonal phase in the oxide, where the instability of the tetragonal phase comes from the reduction of surface energy due to steam.

Effect of a pre-oxide on the high temperature steam oxidation of Zircaloy-4 and M5Framatome alloys

“…With the exception of a few specific conditions (post-breakaway oxidation at temperatures typically lower than 1050°C [4] [15][16] [20] [21], oxidation under high steam pressures [22] [23] or secondary hydriding due to steam starvation inside the cladding after ballooning and burst [24] [25]), as-received zirconium alloys do not absorb a significant amount of hydrogen when oxidized in steam at HT [26]. Hydrogen pickup during oxidation in steam at HT is not provided in most papers dealing with the oxidation of pre-oxidized zirconium alloys.…”

Parallel mechanism of growth of the oxide and α-Zr(O) layers on Zircaloy-4 oxidized in steam at high temperatures

“…The mechanisms causing the breakaway oxidation are still unclear. Several explanations have been suggested in the literature such as the destabilizing effect of the tetragonal-to-monoclinic phase transformation in the oxide layer [11,16,17]. A recent study using HT XRD analysis demonstrated that this phase transformation is not the main cause of the breakaway oxidation [18].…”

“…According to [7,9], when an adequate supply of steam is provided, the steam and hydrogen partial pressures have no effect on the weight gain. Moreover, when studying the oxidation of Zircaloy-4 samples in steam/argon mixtures with total pressures of 7.5 and 10 MPa, it was found that the oxidation rate is sensitive to the steam partial pressure and not to the total gas pressure [16,23].…”

Breakaway characterization of Zircaloy-4 oxidized in steam and in oxygen at high temperatures using HT- XRD analysis

“…However, today the mechanisms at the root of this behavior are still unclear. One of the main possible explanations proposed in the literature is the tetragonal-to-monoclinic phase transition of zirconia [8,[13][14][15][16]. Other explanations found in the literature are the stress relaxation in the oxide when it reaches a critical thickness [13,17] and the evolution of the oxide morphology during oxidation leading to the formation of a short circuit of oxygen via micro-pores [18,19].…”