Phase diagram of the UO2–FeO1+x system

New modelling of the U–O–Zr phase diagram in the hyper-stoichiometric region and consequences for the fuel rod liquefaction in oxidising conditions

“…The melting temperature was around 2840 ± 20 K. They concluded that the influence of minor constituents (mainly stainless steel oxides representing 7-8 wt.%) on the decrease in the melting temperature of UO 2 -ZrO 2 compositions could be considered as negligible. According to the phase diagrams, FeO-ZrO 2 and FeO-UO 2 , experimentally determined by Bechta et al [32,49], stainless steel oxides could strongly reduce the solidus temperatures for UO 2 -ZrO 2 compositions. Nevertheless, the amount of stainless steel and zirconium oxides measured inside the FPT1 fuel pellets ( Table 6) seems too low to, alone, significantly decrease the solidus temperature.…”

Fuel and fission product behaviour in early phases of a severe accident. Part I: Experimental results of the PHEBUS FPT2 test

“…The minimum melting point of the (UZr)O 2 solid solution is 2800 K for a composition around 50 mol%-UO 2 . This temperature was not reached at 823 mm but stainless steel oxides are known to possibly form with ZrO 2 and UO 2 low melting temperature mixtures [22][23][24]. They could have played a role in a possible partial ''remelting'' of the oxidic phase during the heat-up phase (from 14,000 s).…”

“…Iron oxides can thus potentially attack urania at low temperatures. A rough evaluation of this degradation can be performed on the basis of the UO 2 -FeO x phase diagram [23] and some exploratory separate-effect tests [29]. These interaction tests aiming to investigate the UO 2 /FeO interaction carried out at 1673 K under inert atmosphere with a FeO charge inside a UO 2 crucible demonstrated that iron oxide quickly penetrated the UO 2 fuel grain boundaries.…”

“…16, bottom) if a significant amount of gaseous fission products was present in the intergranular locations (this point is discussed in [8]). According to [23], the solubility of UO 2 in liquid FeO at 1900 K (maximum temperature at 900 mm) would be around 30 wt.%. It means a potential dissolution 7 of 30 g of UO 2 at 1900 K, in other words no more than 0.3% of the initial fuel mass inventory (10 kg UO 2 ).…”

Phase equilibria in the FeO1+x–UO2–ZrO2 system in the FeO1+x-enriched domain

“…The publications of Bechta et al [5,6] have presented a more complete version of the phase diagram of the UO 2 -FeO 1+x system in which the eutectic point (1335 ± 5°C, 4.0 ± 0.1 mol.% UO 2 ) slightly differs from the data in [4]. This seems to arise from differences between the FeO 1+x stoichiometry in this work, because the onset melt temperature of the FeO 1+x -based phase and the liquidus line position is known to change with the Fe:O ratio according to Darken and Gurry' phase diagram [7].…”