Question

Yellow cake, an item of smuggling across borders is ____________________.

Answer 1

Yellow cake is a common name for a substance from which an element used in making an atomic bomb is extracted. Yellow cake can vary in proportion and colour from yellow to orange to dark green.

Complete answer:

Uranium oxide, commonly known as yellow cake, is the substance that is smuggled across borders because it contains U - 235 isotope used in making nuclear weapons and due to nuclear disarmament treaties not every country can procure it legally. That is why it is smuggled.
Yellow cake is in fact a refined form of uranium ore which is extracted from Earth’s crust. After it has been taken out of the Earth’s crust, it is purified by both physical and chemical methods to give a coarse powder, whose colour depends upon its composition and is often, but not always, yellow.
Progressive enrichment of this ore in Uranium gives two major isotopes of Uranium, viz., U - 238 (greater than 99 % abundance) and U - 235 ($\sim 0.72\%$ abundance). It is U - 235 isotope which is used in making nuclear weapons while U - 238 is used in nuclear power stations. Uranium enrichment procedures are carried out until a minimum mass of a Uranium isotope (called, critical mass) is obtained that can sustain a nuclear reaction. U - 235 Enrichment is directly linked to a nuclear weapon program and that is why they are strictly monitored by IAEA (International Atomic Energy Agency) whose work is to promote peaceful use of nuclear technology.

Note: Yellow cake is NOT sulphur although it is yellow in colour. Also, Uranium oxide is not always yellow in colour and its colour depends on the amount and composition of impurities present in it.