Investigations of the melting behaviour of the U–Zr–Fe–O system

Abstract

During a severe nuclear accident, the UO 2 fuel rods, Zircaloy cladding, guide tubes, absorber and steel structural components inside the reactor pressure vessel overheat and a series of interactions between these elements and the steam atmosphere occur. These produce more heat in addition to the decay heat and result in a liquid corium of oxidic and metallic phases depending on the exact conditions and processes. A major systems resulting from this is the U-Zr-Fe-O system. High-temperature data for this system is important in order to be able to model these interactions. The Joint Research Centre, Institute for Transuranium Elements (JRC-ITU) has been examining the melting ranges for this system over the whole FeO range by means of a specialized laser flash technique that achieves very high temperatures and avoids crucible contamination. The melted zones were examined for their structure, composition and for estimation of the liquidus and solidus temperatures. The results showed that with FeO contents of over 20mol% there was a very large melting range that would permit long liquid cooling times and extend the relocation of fuel material within the reactor pressure vessel. Based on these results, the main phase regimes expected under severe accident conditions could be identified.

Mass spectrometric study of the laser‐evaporated Fe–Zr–O system up to 3300 K

“…17 In fact, it turned out that the thermal arrest at 2810 K is close to the liquidus temperature of ZrO 2 -30 mol% FeO, as derived from the ZrO 2 -FeO phase diagram from Beshta et al 4 and Bechta et al 5 and in other laser-melting experiments with ZrO 2 -FeO-UO 2 . 6 The latter fact may additionally confirm the credibility of the performed experiments both in regard to the temperature measurements and in keeping the original composition of the ZrO 2 -FeO samples even at high temperatures. However, in this particular case, such a good agreement should be taken with caution.…”

“…It is likely that the ratio p(ZrO)/p(ZrO 2 ) is strongly affected by the presence of more volatile FeO oxide. The ratio of Zr and Fe atoms (Figure 9) in vapor was calculated from the relative pressures using an Equation like (6). Due to the strong release of Fe in the first shot the Zr/Fe ratio is rather small.…”

“…Previous work on the FeO-ZrO 2 -UO 2 system indicates that the system reacts largely as a pseudo-binary (U,Zr)O 2 -FeO system, with a large liquids ranging from 1800 to 2800 K, and is sensitive to the atmosphere, particularly at higher temperatures. [4][5][6] The collection of accurate vapor pressure data for (Fe-Zr-O) and (Zr-O) systems at high temperatures and MS data in the vicinity of the melting temperature will be vital to the improved accuracy of the thermodynamic databases for the U-Zr-Fe-O system which represents the in-vessel system in a severe accident.…”

Corium Experimental Thermodynamics: A Review and Some Perspectives

“…Since this work, a number of key oxide systems were investigated, e.g., FeO x -UO 2 [134,135], FeO x -ZrO 2 [136,137], FeO x -UO 2+x -ZrO 2 [138][139][140][141], and Fe 2 O 3 -SiO 2 [142] within the framework of the ISTC CORPHAD and PRECOS Projects and other international Projects [10,11].…”