Cesium isotope ratios as indicators of nuclear power plant operations

There are multiple paths by which radioactive cesium can reach the effluent from reactor operations. The radioactive 135Cs/137Cs ratios are controlled by these paths. In an effort to better understand the origin of this radiation, these 135Cs/137Cs ratios in effluents from three power reactor sites have been measured in offsite samples. These ratios are different from global fallout by up to six fold and as such cannot have a significant component from this source. A cesium ratio for a sample collected outside of the plant boundary provides integration over the operating life of the reactor. A sample collected inside the plant at any given time can be much different from this lifetime ratio. The measured cesium ratios vary significantly for the three reactors and indicate that the multiple paths have widely varying levels of contributions. There are too many ways these isotopes can fractionate to be useful for quantitative evaluations of operating parameters in an offsite sample, although it may be possible to obtain limited qualitative information for an onsite sample.

► Iodine and xenon isotopes in the particular decay chain are precursors to cesium isotopes and are more likely to leak from a compromised fuel rod than cesium.
► The 135 decay chain has much longer lived iodine and xenon isotopes than the 137 decay chain.
► 135Xe has a very large neutron-capture cross section.
► If best operating principles were followed, very little cesium would be released, and that small amount should be higher in 135Cs than 137Cs.