Radioactivity of Yellowcake

Yellowcake Source: License: Public Domain

Yellowcake, chemically U3O8, is a uranium concentrate powder recovered from leach solutions. Yellowcake is an intermediate step in the processing of uranium ores and in the nuclear fuel production. Typically, yellowcake can be obtained through the conventional milling and chemical processing of uranium ore forming a coarse powder. Yellowcake is a highly concentrated uranium powder and around 75% of material is made up of uranium, or 750 kg of uranium oxide per tonne.

Radioactivity of Yellowcake

Since yellowcake is a highly concentrated uranium powder, it must be somehow radioactive. Due to the fact, the uranium in yellowcake is almost exclusively (>99%) uranium 238 with long half-life, its specific activity is very low. 238U decays via alpha decay to 234Th with half-life of ~4.5×109 years. 238U occasionally decays by spontaneous fission with probability of 0.000055%. Its specific activity is very low ~3.4×10-7 Ci/g. Due to chemical leaching, yellowcake contains almost no descendants of radioactive cascades, that have been in radioactive equilibrium for millions of years. These radioactive descendants are were separated and are disposed in uranium tailings. Therefore, pure uranium-238 is weakly radioactive (proportional to its long half-life), but a uranium ore is about 13 times more radioactive than the pure uranium-238 metal because of its daughter isotopes (e.g. radon, radium etc.) it contains. Yellowcake is as radiologically harmless as natural potassium-carrying minerals or thorium-oxide mantles used in paraffin fuel lanterns.

Radioactivity - BecquerelThe following figure illustrates the amount of material necessary for 37 GBq of radioactivity. It is obvious, that the longer the half-life, the greater the quantity of radionuclide needed to produce the same activity. Of course, the longer lived substance will remain radioactive for a much longer time. As can be seen, the amount of material necessary for 37 GBq of radioactivity can vary from an amount too small to be seen (0.00088 gram of cobalt-60), through 1 gram of radium-226, to almost three tons of uranium-238.

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See above:

Nuclear Fuel Cycle