From An Introduction to Nuclear Waste Immobilisation
11.1. Introduction
Long-lived waste contains significant levels of radionuclides with half-lives greater than 30.2 years. Figure 11.1 illustrates the decay of radionuclides in HLW indicating that with time the activity of the long-lived radionuclides becomes dominant.
Figure 11.1: Activity of radionuclides in HLW from the reprocessing of SNF with time after reprocessing.
Radioactive wastes from fuel fabrication, nuclear reactor operation and decommissioning can also contain long-lived radionuclides. Among the most important long-lived radionuclides in radioactive waste are 14C, 99Tc, 129I, 238,239,240,242pu and 237Np.
11.2. Carbon-14
14C is a naturally occurring radionuclide. Before man-made disruptions of the normal level of 14C by burning of fossil fuels and atmospheric testing of nuclear weapons this radionuclide was produced in nature only by cosmic rays impinging on the atmosphere. Almost all 14C in nature is produced by the reaction:
| (11.1) |  |
Nuclear explosions generate 14C through the same reactions. Reactor fission supplies large numbers of neutrons, which induce neutron activation of reactor materials such as construction materials, coolant, moderator, fuel, gases and impurities. The inventory of natural 14C on the Earth is estimated as 44.2 t with an additional 1.3 t resulting from nuclear weapons testing. As a naturally occurring radionuclide 14C can be used for historical dating to determine the age of certain archaeological artefacts of biological origin up to about 50 000 years old. The ratio of stable 12C to 14C in the air and in all living species at...
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