Been through Nuclear Prototype school, been certified as a nuclear reactor Operator AND Nuclear reactor techician by the NRC.
A lot of good theory mentioned above. Remember C^2 is a really big number, so any easily measured amount of mass converted directly to energy would be, well, hard to measure, since it would be really REALLY hot, melting your tiny little scale.
in basic nuclear reaction, you have an unstable element (uranium for example, not iron) which will decay on its own, releasing heat and nutrons (and decay daughters, which further decay, ususally faster, adding heat and other radiation, but we will ignore that for now). You have a material to absorb the neutrons that are produced, and reaction stays slow. as you remove the dampening material the neutrons begin to collide with the uranium, making the half life thatis normally around 700 million years to become moot, releasing more and more neutrons.
This energy release is primarily from the kenetic energy of the fission fragments.
You could just as well, and perhaps more interestingly ask, since photosynthisis is transfering light energy into chemical energy, how come we can not reliably and repeatably measure the increase in weight in a closed system where only light is added (though I believe it has been done, within an expected range) . (weight of plant tissue is acounted for by water and carbon dioxide it absorbs) Again, E-mc^2 is a nice tool, but hard to meausre weight loss with energy use in a real world setting.
Even metabolising glucose and transfering energy from ATP to ADP in a human body uses mass, thoretically, but the effoct is insignificant compared to the amount of water, and even the amount of salt you may lose during such a test.
So, to wrap it up, do we create and destory mass, sure. press the "I believe" button if you like. And if you are a theoretical physicist, you may even crunch the numbers. But when you realize that the amounts discussed are less than the significant digits used in most calcuations, most tend to disregard that minor effect.
We do not run out of Uranium because it loses mass, we run out of uranium (at a level that promotes being critical in the same configuration as it being safe to use when start reactor for first time) because it decays into decay daughters.
Like asking about the energy/mass relationship of E=MC^2 relationship of gasoline in a car engine. You can not weigh your gas, and crunch the numbers and figure one gallon of gas will take you 1,000,000,000,000 miles. virtually all the mass leaves the tailpipe. I hope that makes it clear, without ruining any attempt and being somewhat humorous.
