<Dd> These use fuel molded into ceramic balls, and then circulate gas through the balls . The result is an efficient, low - maintenance, very safe reactor with inexpensive, standardized fuel . The prototype was the AVR . </Dd> <Dt> Molten salt reactors </Dt> <Dd> These dissolve the fuels in fluoride salts, or use fluoride salts for coolant . These have many safety features, high efficiency and a high power density suitable for vehicles . Notably, they have no high pressures or flammable components in the core . The prototype was the MSRE, which also used the Thorium fuel cycle . As a breeder reactor type, it reprocesses the spent fuel, extracting both Uranium and transuranics, leaving only 0.1% of transuranic waste compared to conventional once - through uranium - fueled light water reactors currently in use . A separate issue is the radioactive fission products, which are not reprocessable and need to be disposed of as with conventional reactors . </Dd> <Dt> Aqueous Homogeneous Reactor (AHR) </Dt>

Why is u-235 used in nuclear power plants instead of smaller simpler radioisotopes