Physical basis of nuclear power. Safety measures in using nuclear energy
The specific binding energy of medium nuclei is about 1 MeV per nucleon higher than that of uranium, so when a heavy nucleus splits into two fragments, energy is released: one ²³⁵U fission gives ≈ 200 MeV, most of it as kinetic energy of the fragments. Fission was identified in 1938–39 by O. Hahn and F. Strassmann; a ²³⁵U nucleus that absorbs a neutron splits into two fragments (for example Ba and Kr) and 2–3 neutrons. If these neutrons split other nuclei, a chain reaction results. The neutron multiplication factor k is the ratio of the number of neutrons in one generation to that in the previous one: for k < 1 the reaction dies out, for k = 1 it proceeds at constant power (critical state, normal reactor operation), for k > 1 the power rises quickly. Natural uranium has only ≈ 0.7 % ²³⁵U, so power-reactor fuel is enriched to ≈ 3–5 %. A reactor has fuel rods; a moderator (water, graphite) to slow fast neutrons; control rods of boron or cadmium that absorb neutrons (inserting them lowers k); and a coolant that carries heat to a steam generator which drives a turbine. Protection: thick concrete and steel containment, several barriers, backup cooling systems; nuclear waste is stored long-term and strictly accounted. The Chernobyl (1986) and Fukushima (2011) accidents showed how important a safety culture and multi-barrier protection are.
In your notebook make a table of neutron generations: starting with 1 neutron, for k = 1, k = 0.5 and k = 2 write the number of neutrons over 5 generations and say which case matches which operating mode. A calculation exercise.