A significant problem with using any sort of water as a coolant is that the water tends to dissolve the fuel and other components and ends up becoming highly radioactive. This is mitigated by using particular alloys for the tubes and processing the fuel into a ceramic form. While effective at reducing the rate of dissolution, this adds to the cost of processing the fuel while also requiring materials that are susceptible to neutron embrittlement. More of an issue is the fact that water has a low boiling point, limiting the operating temperatures. A material with a higher boiling point can be run at higher temperatures, increasing the efficiency of the power extraction and allowing the core to be smaller.