Severe Accident Analysis Of BWR Core Fueled With UO₂/FeCrAl with Updated Materials and Melt Properties From Experiments

Modifications to MELCOR 1.8.6 for modeling Accident Tolerant Fuel in Boiling Water Reactors

“…The material properties used in the example input are those for FeCrAl and its oxide summarized previously in [3], but modified in the following ways to demonstrate the multi-region oxidation capability. First, the region 1 Arrhenius law pre-factor has been changed to 230.0 kg 2 -metal/m 4 -s in accordance with the values given in [4]: (3) This reflects the proper conversion of previous measured FeCrAl oxidation rates, which in fact had units of kg 2 -oxide/m 4 -s, to the proper metal basis (kg 2 -metal/m 4 -s) as required by MELCOR. When using oxidation rate correlations from the literature, similar care should be exercised to ascertain whether these are based on the mass of oxygen consumed, mass of oxide generated, or mass of metal consumed, and to convert these to the latter when necessary.…”

“…Secondly, a different Arrhenius law is specified in temperature range 2, based on the assumption [4] that beyond some temperature at which the FeCrAl oxide layer (predominantly Al2O3) fails, oxidation proceeds as for stainless steel, dominated by the oxidation of iron. The correlation used in this temperature range is the MELCOR default for oxidation of stainless steel: (4) The first correlation applies up to 1763 K, and the second above 1783 K. In between these temperatures, the oxidation rate constant is determined by interpolating between the two correlations, as described above. Note that the nominal transition temperature, 1773 K [4], is not necessarily associated with failure of the FeCrAl oxide layer as predicted by MELCOR, which likely does not occur until the melting temperature of the oxide (1901 K in this case) is reached, as described above.…”

“…The correlation used in this temperature range is the MELCOR default for oxidation of stainless steel: (4) The first correlation applies up to 1763 K, and the second above 1783 K. In between these temperatures, the oxidation rate constant is determined by interpolating between the two correlations, as described above. Note that the nominal transition temperature, 1773 K [4], is not necessarily associated with failure of the FeCrAl oxide layer as predicted by MELCOR, which likely does not occur until the melting temperature of the oxide (1901 K in this case) is reached, as described above. Equation 4implies a jump in the oxidation rate constant of almost seven orders of magnitude at the transition point ( Figure 2); the implications of such an assumption are briefly explored in Section 4.…”

Steam Oxidation Testing in the Severe Accident Test Station

“…In order to collect more accurate physical property data on FeCrAl alloys, a series of experiments were conducted that also have been reported elsewhere [22]. First, the solidus temperature was determined by using a differential scanning calorimeter/thermogravimetric analysis (DSC-TGA) instrument.…”

“…Similar results were found with other oxide specimens. Steam oxidation testing of tube specimens was conducted at 1400°-1700°C to compare the behavior of a Zr-based alloy, Zircaloy-4 (Zr-4), and FeCrAl, particularly to see how the FeCrAl tubing would perform above its melting temperature (Zircaloy tubing is known to undergo cracking and loss of any notable strength, though it retains its shape after oxidizing to ZrO 2 [22]). The initial testing was conducted with the tube in a crucible and an alumina rod inside the tube to simulate the fuel, Figures 7a and 7b.…”