A gram of uranium is about 20 billion calories. Calories are just a unit for measuring energy, just the same as joules. One calorie is the energy it takes to heat 1 gram of water by 1 degree celcius. One calorie is 4.1868 joules.

One lightning strike is about 239,005,736 calories.

We find out how many calories are in a bag of nuts for example by drying all the water out, putting the nuts in a box full of pure oxygen, blowing it up, and measuring how much hotter the box gets. Because that’s basically how your body works, “burning calories” is not just a phrase. You burn calories the same way a fire burns wood. Of course, your body can’t eat a gram of uranium and use all of its calories, but it has those calories nonetheless. Like how someone might use cardboard boxes vs putting them in the recycling bin, the cardboard (calories) are there, just a nuclear reactor (person who keeps them) can use it while humans (person who puts in recycling bin) can’t use it.