NUCLEAR PROCESSES Fission & Fusion

F ISSION VS. F USION

 In nuclear fusion, two nuclei combine to form a more massive nucleus.  Fusion means the merging together of different elements to make a new element.  In nuclear fission, a massive nucleus splits into two lighter nuclei.  Fission means breaking something up into parts.

M ASS E NERGY E QUIVALENCE In the nucleus the strong interaction binds the nucleons tightly together. When nucleons are bound together by the strong interaction, their energy is reduced — they go into a low- energy state. The binding energy of a nucleus is the energy required to break up the nucleus into its individual protons and neutrons.

M ASS E NERGY E QUIVALENCE

M ASS D EFECT  The law of conservation of mass says that mass is never created or destroyed.  It is surprising that accurate measurements show that a bit of mass disappears when nuclei form from their individual nucleons:  the mass of a nucleus is less than the sum of the masses of the individual protons and neutrons that make up the nucleus.  This missing mass is called the mass defect

L AW OF C ONSERVATION OF M ASS -E NERGY  Mass can be transformed into energy and energy can be transformed into mass.  This explains the mass defect: when nucleons are bound together, their energy is reduced, so their mass is also reduced.  The binding energy of a nucleus is the mass-energy equivalent of its mass defect.

M ASS E NERGY E QUIVALENCE Determine the mass-energy equivalent of 1.0 kg of gasoline.

N UCLEAR R EACTIONS

N UCLEAR F ISSION Fission is the reaction in which a massive nucleus splits into two or more lighter nuclei. 1. Spontaneous Fission: Spontaneous fission occurs when an unstable isotope splits into two or more smaller nuclei without any external interaction.  It is only seen in nuclei with atomic mass numbers above 230 (elements near thorium).  Induced Fission: a nucleus absorbs a neutron, forming a highly unstable isotope that breaks up almost instantly into two lighter nuclei

N UCLEAR F ISSION Thorium-230 decays to polonium-218 by three alpha decays. Write the equations for the reactions. Use a periodic table.

N UCLEAR F ISSION

N UCLEAR F USION The dominant fusion reaction in stars the size of our Sun or smaller is the proton-proton chain.

N UCLEAR F USION Another form of fusion is neutron absorption.