So I ran the numbers. U-235 decays into Pb-207, which means about 12% of its mass is radiated away in alpha decay. Which sounds like a fuckton.
Also, it’ll mean that that chunk of lead will be a touch heavier, at 13.2 lbs
The Maths:
U-235 decays into Pb-207. To three significant digits, 207/235 = 0.881, equivalent to 88.1%, meaning 11.9% is radiated away.
88.1% of 15 lbs = 13.2 lbs.
So it would be more accurate to say that 13.2 lbs would be a minimum for the lump’s mass.