The global U-236 inventory

The uranium isotope 236 U (half-life 2.310 7 a) is produced via thermal neutron capture on 235 U. Natural production results from neutrons present from (a,n) reactions on lighter nuclides, spontaneous fission of 238 U, induced fission of 235 U and at the earth`s surface from the cosmic rays. The global equilibrium inventory is only ~ 30 kg of 236 U resulting in a very low 236 U/U isotopic ratio between 110 -14 and 510 -14 for typical crustal rocks and similar numbers for uranium released during rock weathering and dissolved in river water. Accelerator Mass Spectrometry (AMS) is the only detection technique suitable for the low natural isotopic abundance of 236 U. Today due to the nuclear industry a total anthropogenic inventory of 106 kg 236 U is estimated. Significant releases to the environment can be expected, but the environmental distribution of anthropogenic 236 U is not well assessed yet. 236 U/U ratios from 10-6 up to 10-3 have been measured in soils from the close vicinity of the Chernobyl power plant, near uranium reprocessing factories they range between 10-6 and 1.410 -4 . Another important contribution to anthropogenic 236 U was created in above-ground nuclear explosions. Despite the total amount was probably smaller, 100 % release into the environment can be expected, partially via global fallout. Whereas the global distribution of plutonium isotopes from the same sources is fairly well known and has found successful applications, 236 U was not yet traced in global fallout due to detection difficulties. In the frame work of the proposed project the distribution of 236 U will be assessed for representative sites in our environment. Our main object of study will be creeks and rivers, which are expected to average over their whole feed area. We will follow the water cycle starting from alpine affluents to the Danube, and the Danube itself. The water samples will be backed by measurements on soil samples from the alpine area, where we already measured anthropogenic 236 U. Marine samples will be collected from the Black Sea, the Mediterranean Sea and also from the Atlantic Ocean. This will allow us to estimate the extent of 236 U dissolution and transport from the fallout region to the rivers and subsequently to the oceans. The challenge of this project is twofold: the measurement of the very low natural 236 U/U ratio and the chemical processing of large water samples. 236 U measurements have been developed at the AMS facility at VERA since 2002. In various environmental samples natural isotopic ratios, which are not accessible for any other method were found. Uranium separation from large water samples (25 L) will require new chemical procedures as conventional methods can not be directly scaled to this sample size. We plan to use ionic liquids for purification or specific extraction of uranium.

The uranium isotope 236 U (half-life 2.310 7 a) is produced via thermal neutron capture on 235 U. Natural production results from neutrons present from (a,n) reactions on lighter nuclides, spontaneous fission of 238 U, induced fission of 235 U and at the earth`s surface from the cosmic rays. The global equilibrium inventory is only ~ 30 kg of 236 U resulting in a very low 236 U/U isotopic ratio between 110 -14 and 510 -14 for typical crustal rocks and similar numbers for uranium released during rock weathering and dissolved in river water. Accelerator Mass Spectrometry (AMS) is the only detection technique suitable for the low natural isotopic abundance of 236 U. Today due to the nuclear industry a total anthropogenic inventory of 106 kg 236 U is estimated. Significant releases to the environment can be expected, but the environmental distribution of anthropogenic 236 U is not well assessed yet. 236 U/U ratios from 10-6 up to 10-3 have been measured in soils from the close vicinity of the Chernobyl power plant, near uranium reprocessing factories they range between 10-6 and 1.410 -4 . Another important contribution to anthropogenic 236 U was created in above-ground nuclear explosions. Despite the total amount was probably smaller, 100 % release into the environment can be expected, partially via global fallout. Whereas the global distribution of plutonium isotopes from the same sources is fairly well known and has found successful applications, 236 U was not yet traced in global fallout due to detection difficulties. In the frame work of the proposed project the distribution of 236 U will be assessed for representative sites in our environment. Our main object of study will be creeks and rivers, which are expected to average over their whole feed area. We will follow the water cycle starting from alpine affluents to the Danube, and the Danube itself. The water samples will be backed by measurements on soil samples from the alpine area, where we already measured anthropogenic 236 U. Marine samples will be collected from the Black Sea, the Mediterranean Sea and also from the Atlantic Ocean. This will allow us to estimate the extent of 236 U dissolution and transport from the fallout region to the rivers and subsequently to the oceans. The challenge of this project is twofold: the measurement of the very low natural 236 U/U ratio and the chemical processing of large water samples. 236 U measurements have been developed at the AMS facility at VERA since 2002. In various environmental samples natural isotopic ratios, which are not accessible for any other method were found. Uranium separation from large water samples (25 L) will require new chemical procedures as conventional methods can not be directly scaled to this sample size. We plan to use ionic liquids for purification or specific extraction of uranium.