content: The article has an incomplete definition of supercritical fluids - Why it's important is that it's in neither state, but has the properties of both. Essentially, it can both dissolve substances inside of it, and effuse through solids like a gas (to create a solution, which is why they keep mentioning molten salts, and then keep the pressure up by making the hole that it needs to pass through really small). As it cools, it would precipitate said molten salt, which should create a drop in the specific heat capacity required to pressurize the CO2 again.    TLDR: If I had to guess, the molten salt is powering the system.    EDIT: Well, I was partially right. Supercritical water reactors use uranium.