233U/236U signature allows to distinguish environmental emissions of civil nuclear industry from weapons fallout

Isotopic ratios of radioactive releases into the environment are useful signatures for contamination source assessment. Uranium is known to behave conservatively in sea water so that a ratio of uranium trace isotopes may serve as a superior oceanographic tracer. Here we present data on the atomic ²³³U/²³⁶U ratio analyzed in representative environmental samples finding ratios of (0.1–3.7)⋅10⁻². The ratios detected in compartments of the environment affected by releases of nuclear power production or by weapons fallout differ by one order of magnitude. Significant amounts of ²³³U were only released in nuclear weapons fallout, either produced by fast neutron reactions or directly by ²³³U-fueled devices. This makes the ²³³U/²³⁶U ratio a promising new fingerprint for radioactive emissions. Our findings indicate a higher release of ²³³U by nuclear weapons tests before the maximum of global fallout in 1963, setting constraints on the design of the nuclear weapons employed.