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My dad is a nuclear engineer, and he likes fluid dynamics. :) Last night he explained to me how the specific heat of a supercritical fluid sharply peaks at a certain pressure range (this relates, in some way too complicated for me, to the delta function). Okay, I didn’t understand all of it, but this is the idea behind supercritical water reactors (SCWRs) currently under investigation in 13 countries around the world. If the pressure of the coolant can be maintained within that range, we could get more efficient, simpler, and generally safer reactors because there would be no worry of such things as steam bubbles (different density = problems), pumps, or condensers.

In light water reactors (most reactors in the world are LWRs), water enters the reactor core at 290°C and exits at around 315°C—only a 35°C difference due to water’s high specific heat, but this could be improved by using supercritical fluids.

Wikipedia has better writers:

Advantages

• Supercritical water-cooled reactors promise to have thermal efficiencies of approximately 45% versus the current 33% of light water reactors.
• The supercritical coolant has a high specific enthalpy.
• The SCWR design is far simpler than current designs, eliminating circulation pumps, pressurizers, steam generators, steam separators and dryers.

Disadvantages

• Many of the materials needed for the SCWR are either expensive, rare, or do not exist.
• Specific start-up procedures required to avoid instability.
• Unknown chemistry.