Physics · Ch 8 — Atomic and Nuclear Physics
Nuclear Fission
Nuclear Fission
In 1939, the German scientists Otto Hahn and Fritz Strassmann discovered that when a uranium nucleus is bombarded with a neutron, it can break apart into two smaller nuclei of roughly comparable mass, releasing a large amount of energy in the process - a phenomenon called nuclear fission. The fission process is always accompanied by the release of additional free neutrons, and the energy released per fission event is many orders of magnitude greater than the energy released in any ordinary chemical reaction.
The fission reaction. Uranium-235 can undergo fission in roughly 90 different ways; two representative, commonly cited product combinations are
When the slow (thermal) neutron is absorbed, the mass number increases by one and the nucleus enters a short-lived excited state , which lasts no longer than about s before splitting into two daughter nuclei plus, on average, about 2.5 free neutrons. Working through equation (8.45)'s mass bookkeeping (reactant mass 236.054398 u versus product mass 235.829095 u) gives a mass defect of 0.225303 u, corresponding to an energy release of about 200 MeV per fission event (this energy first appears as kinetic energy of the fragments and neutrons, and is subsequently transferred to the surrounding material as heat).
Chain reaction. Since each fission releases about three additional neutrons, and each of those can go on to trigger fission in another uranium-235 nucleus, the number of fission events (and neutrons) can grow rapidly, roughly in geometric progression - this self-sustaining, multiplying process is called a chain reaction. If left completely unchecked, this is an uncontrolled chain reaction, releasing its entire stored energy in a tiny fraction of a second - the principle behind the atomic bomb (as tragically demonstrated when the USA dropped atom bombs on Hiroshima and Nagasaki in August 1946, killing large numbers of people and devastating both cities, with side effects persisting for residents even today). If instead the reaction rate is deliberately damped so that, on average, exactly one neutron from each fission goes on to trigger the next fission, the result is a controlled chain reaction, which can be sustained steadily and harvested for useful energy - this is exactly what happens inside a nuclear reactor. …
What this figure shows. This figure diagrams the fission sequence step by step: an incoming neutron labelled 'n' is captured by a heavy nucleus, forming a short-lived excited compound nucleus, which is then shown splitting apart into two medium-mass 'daughter nuclei' fragments while simultaneously releasing two or three fresh neutrons. The four-panel layout (neutron capture, excited nucleus, fission, daughter nuclei plus neutrons) makes the whole process, which happens in roughl …
What this figure shows. This figure shows one incoming neutron causing fission in a nucleus, which produces fragment nuclei (examples shown include xenon-138, iodine-138/135, barium-141, strontium-95, krypton-92, yttrium-95 and niobium-98 across two illustrated fission events) along with several fresh neutrons, and then traces how those released neutrons go on to strike further uranium-235 nuclei, each triggering its own fission and releasing yet more neutrons. The picture visually captures the geometric, branching growth of a chain reaction - the number of fission events and neutrons multiplying rapidly generation after generation - which is exactly the effect that must be damped to …
What this figure shows. This two-part figure shows the complete layout of a power-generating nuclear reactor: a reactor core containing the uranium fuel containers and moderator, surrounded by protective shielding, with control rods that can be inserted to different depths; a primary coolant loop (hot liquid, often water or liquid sodium) driven by pumps that carries heat away from the core through a heat exchanger; a secondary loop in which the transferred heat converts water to steam; and the steam then driving a turbine connected to an electric generator, with a condenser and further pumps completing the cycle. Together the two panels show both the simplified block-diagram view of the energy flow and the more detailed schematic showing exactly how the containment vessel, core, coolant, steam turbine and generator are physically connected in an actual power reactor …