Short-lived Fission Products

Most fission products vanish rather rapidly

The vast majority of the radioactive fission products are short-lived or medium-lived. For radioactive waste management, a radionuclide is said short- lived when its period is less than 5 years and medium-lived when it is between 5 and 100 years. Such timescales seem very long, but they are actually short compared to the period of some fission products or actinides, which are counted in thousands of years. A period of 30 years, such as that of cesium-137, appears rather long when it comes to handle the radioactive contamination resulting from a nuclear accident. Instead, a physicist, who often has to deal with radioactive nuclei living less than a second, would ses periods of a few hours or days as excessively long.

A double cascade...
The products found in the spent fuel from reactors are no longer the fresh fragments from fission. In this example, a heavy fragment of xenon-143 through a series of intermediate nuclei, whose period is a fraction of a second to 14 days. After one minute, the fragment was transformed into lanthanum-143. After a few months, all kernels have reached the neodymium-143, a stable core of the chain terminus. The light fragment of bromine-90 is changing rapidly at first, but the waterfall is still pending with the fourth element, strontium-90 whose period is around 30 years. It will take about 10 times that long for the radioactivity of the fragment has completely disappeared. Strontium-90 is one of the main fission products found in the average life reactors.

These radioactive life times are to be compared with the residence time of the nuclear fuel in the reactorswhich is usually three years. When the period is very short - for example a few days - much of the radioactive nuclei have reached stability when the fuel is discharged from the core. Will be present at a given time only freshly produced such nuclei . On the contrary, if the half-life is of a few years, the radioactive nuclei will accumulate because they do not have had time generally to disappear.

Examples of very short-lived fission products are xenon-135 (period 8 hours) and iodine-131 (period of 8 days). The accumulation of xenon-135 after a reactor shutdown becomes a poison for fission reactions and prevents its immediate restart. Iodine-131 is one of the fresh fission products that were released during the Chernobyl accident. This highly mobile and volatile element was the main radioactive hazard immediately after the accident, since it is estimated that around 10% of fission products present in the reactor have been dispersed in the atmosphere.

Heat from a container of vitrified waste
Containers of vitrified waste from the nuclear industry are too recent to have had time to age. They still emit excessive heat caused mainly by the decay of fission products contained therein. At the age of 5 years, the short-lived fission products contribute about 90% of the heat release and the minor actinides (americium and curium) for only 11%. At age 35 the heat of 2350 watts fell to 700 watts, two products of fission - Cesium-137 and strontium-90 - alone emit 84% of this heat.

Fission products such as cesium-137 (30-year period) and strontium-90 (28-year period) have "intermediate" iife times. They live long, but not extremely long. They are troublesome, becaus if they much less active than iodine-131 for instance, they disappear slowly. They represent the main source of contaminations due to the Chernobyl accident and former tests of atomic bombs decades after these events.

Fission products are responsible for virtually all the radioactivity of spent fuel just removed from a reactor core: 99.5% of beta and gamma activity. This activity is very high, estimated to be about 2.5 million curies ( 7 times 10 to the power 16 becquerels) per ton of nuclear spent fuel.

Before handling spent fuel assemblies, it is necessary for radioprotection to store these assemblies inside pools by letting their radioactivity decrease a few years : first in a pool close to the reactor and , in the case of a reprocessing, inside the interim storage pool of the reproceessing plant (la Hague in France) where they are transfered. The storage pools are also designed to evacuate the heat generated by the radioactive decays.

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