Why is the efficiency of nuclear power plants so low and what is the energy content?

Because they use the same old technology that an oil or coal fired plant uses (creating steam and using that to turn a steam turbine and electric generator), just the fuel used to create heat is different. Plus, nuclear is more inefficient than oil or coal because they need to have an additional heat loop process between the reactor and the steam turbine to isolate the radioactive water circulating in the core from the nonradioactive water circulating through the steam turbine and condenser. The energy content of uranium and plutonium : lots and lots of heat energy. Radium: hardly any heat energy at all.