a) Thermal Nuclear Power Systems rely on specific neutron interactions within the components in the nuclear core, including various types of absorption and scattering events, in order to maintain a neutron flux and continue operations. Components include fuel casings and fuel, moderators, coolants, controls, and shielding. Considering this please answer the following questions: (i) Describe: the 4 main categories of neutron interaction in the nuclear core in normal operation, the main components in the core within which these interactions occur, and identify how the probabilities of these interactions are expressed in core performance calculations. (ii) Describe for these main neutron interaction types one strong dependence on neutron mean energy and one strong dependence on the interacting material's atomic mass. (iii) Describe for each of the 4 main neutron interaction types how they positively and negatively influence neutron flux and the criticality of the reactor and how this influences material selection for of each of the core components. (iv) Describe and illustrate with a sketch the arrangement of components and the main material types used in the nuclear core of the Fukushima reactor. (v) Describe and illustrate with a sketch the arrangement of components and materials used in the nuclear core of the Chernobyl reactor. (vi) The different coolant and moderator materials in the Chernobyl and Fukushima cores meant that when the major disaster incidents occurred that caused core temperature to rise and the cooling water to have an increase in the steam fraction (increase in steam 'voids') there were very different effects on the nuclear reaction rate because of positive vs. negative 'void coefficients' respectively, explain why the same increase in steam voids in the coolant had different effect in each case due to other design decisions and material selections. (vii) Both the Chernobyl and Fukushima reactors had issues with hydrogen explosions after their cores had overheated, describe the components and conditions for reaction that led to the hydrogen generation in the core. (viii) Consider the situation in a nuclear reactor where a thermal neutron is captured by Boron forming He(4) and Li(7) nuclei where the Li nuclei is formed at 0.96MeV above its ground state and both the He and Li atoms have kinetic energy associated with the loss of mass in the reaction (i.e. 10B + 1n -> 4He + 7Li*0.96MeV). Describe the component where this may occur in the reactor. Describe what would subsequently happen to the kinetic and potential energy of the resulting atoms. (ix) Explain in terms of neutron interactions how fuel enrichment influences criticality. Explain three neutron interaction parameters important for assessment of moderator performance and how these can be combined into a moderator effectiveness number. Give 2 examples of different fuel and moderator combinations in currently operating western pressurised water reactors and explain the trade-offs influencing these selections. (x) Explain the two separate terms 'fast' and 'breeder' in terms of nuclear reactors. Give 2 examples of fast breeding fuel from different source materials. Give 2 advantages of fast breeder reactors, 1 disadvantage, and 1 area of concern.