Yellow cake is the final product of the milling of Natural radioactive element with the atomic number 92. The n... ore. It contains about 80 % of Natural radioactive element with the atomic number 92. The n... oxide and is usually represented by the formula U3O8. The yellow cake produced by most modern mills is actually brown or black, not yellow; the name comes from the colour and texture of the concentrates produced by early mining operations due to impurities from ammonium diuranate (NH4)2U2O7.