LJ-- Atomic weight is grams per mole, not the inverse as you put forward.

J-- I did say that I am not a mathematician nor a nuclear physicist,
The following should be correct then.

Uranium atomic weight is about 238 grams per mole
Lead atomic weight is about 206 grams per mole
Difference in atomic weights per mole is about 32 grams
When one mole of Uranium 238 decays into lead, about 32 grams of matter has been converted to energy. (Otherwise something 'mysterious' has happened to that mass.)
One ton of lead 206 contains about 4854 moles, if one mole has lost 32 grams then 32*4854 = 155328 grams of mass is lost from each ton of uranium.
155328*9.e18 = 1.397952e24. One ton of uranium releases 1.397952e24 Joules of energy during its conversion to lead.
1.397952e24*476000 = 6.65425152e29 Joules of energy will be released from the stored uranium hexafluoride in its total life time of 4.5 billion years..
6.65425152e29 / 4500000000 = 1.47872256e20 is the total yearly energy release from the amount of uranium hexafluoride that is stored in the US.
According to this calculation 16184000 tons of uranium will heat up the Earth's atmosphere one degree in one year.

LJ -- Uranium does decay into lead, after several intermediate steps, but it ends up as lead 206 and it takes 4.5 billion years.
In the decay process, alpha particles (2 neutrons and 2 protons) and beta particles (electrons) are given off. These particles have significant mass, especially the alphas, and account for most of the change in mass between uranium and lead. Not a lot is actually converted into energy, certainly far less than the 31 grams you conjecture, which means that the rest of your argument is fatally flawed.

J -- Like I said, I'm not an expert in this area, perhaps you have some other information that I would find interesting, such as the weight of the 'spent' uranium that comes out of the reactors. I would like to know how the weight of the uranium changes as it is 'spent'. Perhaps you can also tell me what happens to those alpha and beta particles, and how do they 'loose' their energy. I am also interested in knowing how the gamma 'photos' loose their energy.

LJ -- I realize I'm wasting my time here, but hey it's late in the day...

J -- Thank you for your time, I appreciate any and all the information that you are willing and able to give.

Johannes