answer - Nuclear Fission and Fusion Solutions 1 a Nuclear...

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Nuclear Fission and Fusion - Solutions 1. a) Nuclear fusion occurs when two nuclei, of small mass, fuse together to form a single nucleus. Very high speeds are needed to overcome Coulombic repulsion between nuclei as they approach each other. Nuclear fission occurs when a large unstable nucleus splits into smaller nuclei with the emission of neutrons and energy. This is induced by neutron bombardment. b) In both reactions the binding energy per nucleon of the products is higher than the reactants. This means that mass is converted into energy in the reaction. …………………………………………………………………………………………………………………………… 2. The capture of neutrons is easier if they are slow moving. When this neutron is absorbed, this creates an imbalance in the proton-neutron ratio in the nuclei causing fission. If neutrons are moving too quickly they must be deflected or the nucleus can be shattered. …………………………………………………………………………………………………………………………… 3. i) X = 1 1 H ii) Y = ) 3(n 1 0 reaction fission a is This energy ) 3(n Kr Sn n U ii) reaction fusion a is This energy He H H i) 1 0 101 42 132 50 1 0 235 92 3 2 1 1 2 1 …………………………………………………………………………………………………………………………… 4. a) Moderator material is used to slow down neutrons. b) When particles of comparable mass collide, momentum and energy transfer readily occurs. Therefore, after a few collisions, more energy is lost and neutrons slow down to speeds that enable them to be captured by larger nuclei. If a larger atom is used as moderator material it is more likely that the neutron will bounce off without any loss of speed [collide elastically] and make them unable to initiate fission.
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5. a) A chain reaction is a self sustaining reaction. This involves the release of neutrons during fission which in turn trigger further fission reactions to occur. b) The material surrounding the fission reaction must be pure enough and massive enough to make the reaction self sustaining. c) The critical mass is the amount of fissionable material required to make the fission reaction self sustaining. d) In order to get two positive nuclei to fuse together they must get very close to each other. At small distances nuclear forces to come into play and electrostatic forces need to be overcome. The particles must be travelling very fast to overcome the repulsive effects of each other’s positive nature. This corresponds to very high temperatures, which translates into very high kinetic energies.
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