Final answer:
Uranium hexafluoride gas spreads through the tube in the gaseous diffusion enrichment plant because the slightly lighter 235UF6 molecules diffuse through the barriers faster than the heavier 238UF6 molecules.
Step-by-step explanation:
In the gaseous diffusion enrichment plant where U-235 fuel is prepared, UF6 (uranium hexafluoride) gas at low pressure moves through barriers that have holes just barely large enough for UF6 to pass through. The slightly lighter 235UF6 molecules diffuse through the barrier slightly faster than the heavier 238UF6 molecules. This process is repeated through hundreds of barriers, gradually increasing the concentration of 235UF6 to the level needed by the nuclear reactor.
The basis for this process is Graham's law of diffusion. According to Graham's law, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass. Since 235UF6 has a lower molar mass than 238UF6, it diffuses through the barriers faster, leading to the separation of the isotopes.