What’s wrong with Fission Products and what about Thorium?

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Fission Product Yield of Uranium-235 and Uranium-233

I suggested to our nuclear regulator (ARPANSA), that Figure 1 on their web site might give the wrong impression about the very high radioactivity of certain fission products. It might incorrectly suggest that most fission products quickly decay to stable isotopes.

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In their example (Figure 1), the impact of a single neutron induces one Uranium-235 atom to split into a smaller fragment with 36 protons and mass 92 (Krypton) and another larger fragment with 56 protons and mass 141 (Barium) and in the process release enormous energy. In their example, 3 neutrons are also emitted which induce the bomb-like nuclear chain reaction  in the surrounding Uranium atoms. It can be seen from the graph of Fission Product Yield that most fission produces masses that are approximately 2/5 and 3/5 the mass of the parent nucleus but only 6% of fission produces fragments of mass 92 and 141, each of which decays relatively quickly to a stable isotope.

I suggested another slightly less likely but far more representative example in Figure 2 where fission produces fragments of masses 90 and 143 which remain highly radioactive for centuries. In this example, there is rapid beta decay of Krypton-90 to Rubidium-90 into Strontium-90. At this point in the decay chain about 6% of fission has produced a radioisotope with a half-life of 28.79 years that is high level nuclear waste.

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Strontium-90 together with Cesium-137 represent a major storage problem that is presently unsolved and will still remain with us for hundreds of years after nuclear is discontinued. In the case of Activated Products such as Uranium-233 and Plutonium-239, once they are created in nuclear reactors, they remain until they are either used further as nuclear fuel or deposited as waste in storage for thousands of years. At present, most high level waste is stored adjacent to reactors awaiting reprocessing.

In the Thorium Cycle (Uranium-233 fission), it can be seen from the Fission Product Yield graph, that both Strontium-90 and Iodine-131 are produced most abundantly. Because of the critical bio-activity of both Iodine (Thyroid hormone) and Strontium (mimics Calcium),  these radio-isotopes in particular, were quickly identified in babies following initial nuclear tests as potential indicators of the adverse impacts of radioactivity on humans.

In the 1950s to 1970s there were good inverse correlations between these radioisotopes in babies teeth and birth weights of babies. Atmospheric nuclear tests were banned quickly but nuclear accidents and underground nuclear tests are continuing today.

And don’t forget where all the medical isotopes end up.

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