When nuclear fission occurs ...

A. it must be initiated by bombardment with neutrons.

B. two nuclei combine to produce a heavier nucleus.

C. it is a spontaneous reaction.

D. the chain reaction that results cannot be controlled.

Nuclear fission is a process in which a heavy nucleus is broken down into lighter nuclei.

Within a nucleus, the subatomic particles are held together by very strong forces of attraction. Neutrons which have the mass of a proton but no nuclear charges, is used to initiate fission reactions.

For example: The fission of U-235 into Barium (Ba-144) and Krypton (Kr - 89) is initiated by bombardment with neutron

₉₂U²³⁵ + ₀n¹ ⇒ ₉₂U²³⁶

₉₂U²³⁶ ⇒ ₅₆Ba¹⁴⁴ + ₃₆Kr⁸⁹ + 3₀n¹

Ans) A