When I was in the Navy, we were told of the immense importance of maintaining pressure inside the reactor (all the reactors in the Navy use a hydrogen moderator via water). My teacher discussed that one of the results of loss of pressure could be the formation of steam bubbles collecting on the fuel rods. The reason why this is dangerous is because the molecules on the surface of the rods had fewer H2O molecules to collide with and would therefore radiate heat via infra-red waves. But I don't recall learning why it was that molecules should act this way. Quantum physics wasn't really stressed that much in the Navy's curriculum. What is happening on the quantum level that makes molecules radiate infra-red energy when that energy is not able to be transferred via molecular collision?