Unraveling Environmental Contribution to Fallout Formation
Open Access
- Author:
- Jacomb-hood, Timothy Wykeham
- Area of Honors:
- Nuclear Engineering
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Igor Jovanovic, Thesis Supervisor
Seungjin Kim, Thesis Honors Advisor - Keywords:
- Nuclear
Fallout
Radiochemistry
Geochemistry - Abstract:
- The mechanism of fallout formation remains a poorly-understood phenomenon. Fallout, produced during nuclear detonations, is a mixture of device materials, radioactivity released during fission, and the near surface environment. In this study, the elemental and isotopic composition of fallout is examined to better constrain the environmental contribution to fallout. Nine surface soil samples and six glassy fallout beads, collected from the National Nuclear Security Site (NNSS), were analyzed for major and trace element concentration, mineralogy, and uranium and thorium isotopic composition. In the Buckboard Mesa the major minerals were found to be quartz, orthoclase, and sanidine. The major elements found in oxide form were silicon, aluminum, and iron. In the Northern Yucca Flat the major minerals were found to be quartz, calcite, calcite magnesian, and dolomite. The major elements found, in oxide form, were silicon, calcium, aluminum, and iron. In the Frenchman Flat the major minerals were found to be quartz, calcite magnesian, and dolomite. The major elements found, in oxide form, were silicon, calcium, and aluminum. The uranium found in the soil samples is predominantly natural uranium (0.712 -0.771 atom % 235U), but two samples showed elevated levels of 235U (1.44 and 1.60 atom %). The bulk uranium content of the soil was very similar in all locations ranging from 0.521 to 2.980 μg/g with an average of 2.26 μg/g. The bulk thorium content of the soil is highly dependent on location, in the Buckboard Mesa there is ~22.8 μg/g, in the Northern Yucca Flats there is ~2.1 μg/g, and in the Frenchman Flats there is ~12.0 μg/g. The thorium isotopic composition (232Th/230Th) of the soil also shows locational dependence, in the Buckboard Mesa it is 8.835 × 10^4 to 1.1349 × 10^5, in the Northern Yucca Flats it is 2.00 × 105 to 2.153× 10^5, and in the Frenchman Flats it is 4.917 × 10^5 to 5.460 × 10^5. The glassy fallout beads are highly enriched in 235U (79.0-84.2 atom % 235U) and 236U (0.385-0.410 atom % 236U). The thorium isotopic composition (232Th/230Th) of the glassy fallout beads is 2.330 × 10^5 to 2.733 × 10^5. The beads display remarkable macro-scale isotopic homogeneity. The observed uranium isotopic composition of the glassy fallout beads can be explained by simple two-component mixing between the soil and a device end-member with a nominal isotopic composition of 92 atom % 235U. Age dating of the glassy fallout beads was attempted using a 234U to 230Th decay measurement. Ages of 163 – 213 years were calculated, which proves there is fractionation between the uranium and thorium.