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Radioactive materials

Spent nuclear fuel

Fresh nuclear fuel consists of uraniumoxide, packed in thin tubes of Zircalloy. The tubes are bundled into fuel elements. The uranium has been enriched in the fissile isotope uranium-235. Fresh nuclear fuel is weakly radioactive due to the radioactivity of the uranium atoms. During the fission process highly radioactive fission products and actinides are formed. When the fissile content of the nuclear fuel decreases below a certain level the fission process cannot sustain itself and the fuel is removed from the reactor as spent nuclear fuel. Spent fuel is extremely radioactive and generates much heat, due to the radioactive decay of its contents. The material has to be cooled in spent fuel pools for many years to prevent melting and consequently the release of the contents into the environment. More than 90% of the man-made radioactivity generated during fission is contained in the spent fuel elements.

Fission products

Fission products are the atoms resulting from the fission of uranium atoms. Nearly all elements of the Periodic System are represented in the mixture. A substantial fraction of the fission products are highly radioactive, with half lifes varying from seconds to millions of years. During the first four centuries after removal from the reactor, the radioactivity of spent sfuel is mainly determined by the fission products.

Actinides and minor actinides

Actinides are radionuclides formed from uranium atoms by neutron capture and have a higher atom number (= number of protons in the nucleus) than uranium. First neptunium atoms are formed, which quickly decay to plutonium atoms. By repeated neutron capture the plutonium atoms are transmutated into still heavier atoms, These elements, including plutonium, are often called the actinides and do not occur in nature. The minor actinides are the radionuclides beyond plutonium, such as americium, curium and californium. All actinides are highly radioactive and most of them emit dangerous alpha rays and gamma rays. A number of the minor actinides exhibit spontaneous fission, causing neutron radiation and complications in an operating reactor. One of the americium isotopes has a critical mass of some 7 grams under certain conditions. The half-lifes of the minor actinides vary from a few decades to millions of years.

Activation products

In addition to fission products and actinides a third category of radioactive atoms are generated in an operating nuclear reactor: activation products. By neutron irradiation non-radioactive construction materials become radioactive, often with very long half-lifes.

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raddecayloglogv3

Figure 10-1. Radioactive decay of spent nuclear fuel over time

The specific radioactivity, in gigabecquerel per kilogram (GBq/kg), of spent fuel. Note that both axes have logarithmic scales. Each scale division denotes a factor ten. With a linear time scale the horizontal axis would be about 100 kilometers long. On the horizontal axis a reverse historic timescale is indicated, to give an idea of the time frames involved. The green line indicates the natural radioactivity of the human body (143 Bq/kg). The orange line indicates the lethal amount of radioactivity in the human body. Nuclear fuel from the new types of nuclear reactors has considerably higher burnup than the fuel this diagram is based on and so its specific radioactivity is considerably higher. The contributions of tritium and carbon-14 and of activation products are not included in these curves.