An interesting question. I doubt that they would, as you might flood Uranium Lava with a liquid gas and end up with a gas layer between the uranium and the liquid. Supposition only on my part, but it might retard the cooling down of the Uranium.Can they flood the core with liquid nitrogen?
Liquid Nitrogen might also not carry boron in it, thus removing boron from the presence of the Uranium Lava and that could invite another criticality transient hiccup within the Lava. Not good if you are trying to cool it down.