In containment simulations, thermal radiation has typically been ignored due to the moderate temperatures expected in the atmosphere. However, some recent small-scale experiments have indicated that thermal radiation can be significant whenever an infrared absorbing gas such as vapor is present, even in minute concentrations. This motivated the H2P2 series of tests performed in the large-scale PANDA facility at the Paul-Scherrer-Institut. The tests involved compressing an air-helium-steam mixture with helium injection
from the top of the vessel, thus causing a hot bubble to form. The initial vapor volume fraction varied between 0.1 % and 60%. CFD simulations were conducted for the five H2P2 tests. The k- SST turbulence model was used, while thermal radiation was considered using the P1 model as a default, in conjunction with the Weighted Sum of Gray Gases approach to specify the mixture absorptivity. CFD Best Practice Guidelines were employed, in particular through preliminary grid and time-step sensitivity studies. For the sake of conciseness, we report simulation results of three tests, namely H2P2_2 and H2P2_3, featuring 2% vol. initial steam content, and respectively room and high initial temperatures, as well as H2P2_5 with a steam content of 60% vol.

The predicted temperature and helium concentration as well as pressure time histories matched the experimental data quite accurately. Two major conclusions can be drawn from this study: 1) ignoring thermal radiation can lead to large over-predictions of temperature, even when the steam content is low (order of 1%) and 2) the simple P1 model provides sufficient accuracy. Use of the more sophisticated Monte-Carlo model yielded similar results, albeit at a considerably larger CPU expenditure (5-7 times).