Abstract
With recent developments of analytical instruments and measurement techniques able to detect ultralow amount of 236U (T1/2=2.342×107 y, α-decay) in the environment. U-236 is being recognized as a potential tracer not only for monitoring of uranium contamination release from nuclear cycle, but also for geochemical dynamics studies.
For accurate determination of 236U in the environment at low level, the 236U input from global fallout has to be considered. However, information on 236U background has not been clearly defined. In current study, 236U along with 239+240Pu and 137Cs, were measured to estimate their isotope ratio, concentration and inventory in soil samples from forest in Ishikawa, Japan. This area has not been affected by local or regional 236U releases from nuclear facilities or the Chernobyl accident. The activity of 137Cs, 239+240Pu, 238U and 236U/238U atom ratio in the soil (0-10, 0-20 and 0-30 cm) were measured with γ-ray and α- spectrometries and accelerator mass spectrometry (AMS).
The 236U/238U isotopic ratio and 236U concentration ranged from 1.85×10-8 to 1.09×10-7 and 8.92×108 to 3.76×109 atoms/g dry soil, respectively. The average 236U/239+240Pu ratio (atoms/Bq) was (1.56±0.10)×1011. That translates into the 236U/239Pu atom ratio of 0.235 using the global fallout ratio for 240Pu/239Pu of 0.176. This is within the range of 0.05 - 0.5 reported for the locations with high 239Pu and low 238U soil concentrations. No significant correlation was found between 236U and 137Cs concentrations in measured soils as opposite to 239+240Pu.
Our results indicate that surface soil has been contaminated with 236U from globalfallout of 108 - 109 atoms/g (1013 atoms/m2 in soil from this area). The total surface soil deposition of 236U due to global fallout was estimated as ca. 900 kg.
The study was undertaken to characterize the Black Rain area of Hiroshima atomic bomb. The distributions of 236U, 239+240Pu and 137Cs isotopes were measured in more than 60 surface soil samples from Hiroshima. The results of this study will be presented and discussed in the presentation.
