Nuclear fusion is the splitting of atomic nuclei into smaller fragments; nuclear fission is the opposite of this process. true false a thermonuclear chain reaction is considered to be a: self-sustaining reaction in which the products cause new reactions fusion reaction because new material is combined many times common phenomenon for all unstable uranium atoms proton creation process that initiates other uncontrollable reactions daughter atoms x and y are formed from the fission of atom z. which of the following statements is true about x and y? the combined atomic mass of x and y will be greater than the mass of atom z. the atomic masses of elements x and y will always equal the exact atomic mass of z. high temperatures are required to make atoms x and y. atoms x and y will have a smaller combined atomic mass than atom z. which condition is necessary for fusion to occur? unstable isotopes high temperatures heavy isotopes neutrons two fissionable substances commonly used in nuclear reactors and nuclear weapons include uranium-235 and plutonium-239. true false