Calorie is a unit of energy and a gram of Uranium consists 20 billion calories, it is said that. But if we understand the science behind It, then we it known that Natural Uranium is made from two isotopes- Uranium-238 and Uranium -235 whose compositions are 98.3% and 0.7% respectively.
Uranium -238 produces 413 million calories while alpha decay which much less than 20 billion calories. At the same Uranium-235 produces same energy as of Uranium -238.Therefore, 20 billion calories is very higher for 1 gram of Uranium.
Now, if we talk in terms of fission reaction of both the Uranium. In this reaction,Uranium-235 is only fissile not Uranium-238. In the fission reaction, Uranium-235 will produce about 20 billion calories. But to get this result of fission chain reaction, critical mass of Uranium -235 will be needed which is more than one gram.
Therefore, a gram of Uranium consists 20 billion calories can not be concluded if we go through science.