Reactor accidents in perspective.

Abstract

When uranium fuel undergoes fission in a nuclear reactor, the heavy uranium nucleus divides into two non-equal parts called “fission products”. These fission products are formed with considerable kinetic energy and loose this energy by collision (slowing down) with other uranium atoms in the fuel lattice. These collisions generate the heat which is removed from the central “core” of the reactor by a coolant material (usually gas or water) which is then used to raise steam to rotate turbines, thus producing electricity. The fission products formed are statistically distributed amongst a range of elements, from zinc through strontium and iodine to barium and cerium. For each of these elements there are formed a varying number of radioisotopes having radioactive half-lives from fractions of a second to tens of years and occasionally more. The actual quantity of each of these radioisotopes that will be present in a nuclear reactor at any point in time is determined by the yield of that radioisotope per fission event, the power level of the reactor (i.e. the fission rate) and the length of time for which the fuel has been irradiated. The fuel cycle in a modern reactor will normally have the uranium fuel in the core for several years. During this time, long-lived fission products such as caesium 137, with a half-life of 30 years, build up linearly with fuel irradiation.