umm... In fusion reactors, two of the three naturaly occuring isotopes are used. Tritium, which is Hydrogen with two neutrons, and Deuterium, which has 1 neutron. They don't use the "plain old kind". go look up Deuterium-Tritium Fusion on google.
Also, isotopes of elements are quite common. Carbon-12 is the common one, but carbon-14 is found in nature and is used for "carbon dating". Potassium isotopes are found in your body.
A denser metal does NOT mean it is stronger. Iron is mixed with carbon to make steel, one of the most common building materials in skyscrapers. Titanium is a light metal, and is very strong and resistant to heat and cold. Mercury is a very dense natural metal, and it is liquid. So much for strong dense metals.
Also, a titanium-carbon blend is not an isotope. It is an alloy called Titanium Carbide. Go do a Wikipedia study time before trying to argue for "Dense" Bithium ore. \
EDIT: By the way, nuclear power plants use Deuterium water (D2O) to abosrb neutrons from the uranium reactor rods, and to cool the reactor.
Oops...Double posted