Radionuclide Partitioning in an Underground Nuclear Test Cavity

Abstract

In 2004, a borehole was drilled into the 1983 Chancellor underground nuclear test cavity to investigate the distribution of radionuclides within the cavity. Sidewall core samples were collected from a range of depths within the re-entry hole and two sidetrack holes. Upon completion of drilling, casing was installed and a submersible pump was used to collect groundwater samples. Test debris and groundwater samples were analyzed for a variety of radionuclides including the fission products 99 Tc, 125 Sb, 129 I, 137 Cs, and 155 Eu, the activation products 60 Co, 152 Eu, and 154 Eu, and the actinides U, Pu, and Am. In addition, the physical and bulk chemical properties of the test debris were characterized using Scanning Electron Microscopy (SEM) and Electron Microprobe measurements. Analytical results were used to evaluate the partitioning of radionuclides between the melt glass, rubble, and groundwater phases in the Chancellor test cavity. Three comparative approaches were used to calculate partitioning values, though each method could not be applied to every nuclide. These approaches are based on: (1) the average Area 19 inventory from Bowen et al. (2001); (2) melt glass, rubble, and groundwater mass estimates from Zhao et al. (2008); and (3) fission product mass yield data from England and Rider (1994). The U and Pu analyses of the test debris are classified and partitioning estimates for these elements were calculated directly from the classified Miller et al. (2002) inventory for the Chancellor test. The partitioning results from this study were compared to partitioning data that were previously published by the IAEA (1998). Predictions of radionuclide distributions from the two studies are in agreement for a majority of the nuclides under consideration. Substantial differences were noted in the partitioning values for 99 Tc, 125 Sb, 129 I, and uranium. These differences are attributable to two factors: chemical volatility effects that occur during the initial plasma condensation, and groundwater remobilization that occurs over a much longer time frame.

Chancellor Water Colloids: Characterization and Radionuclide Associated Transport

“…Two 55-gallon drums of water were collected from borehole U-19adPS#1a soon after it was drilled into the Chancellor nuclear test cavity in 2004. Chancellor water was used for this study because it has relatively high colloid concentrations and very high 239/240 Pu concentrations compared to other NNSS cavity waters, and most of the Pu is associated with the colloids (Reimus et al, 2006, Appendix C;Rose et al, 2011). It is the only cavity water we are aware of in which the alpha activity is dominated by 239/240 Pu rather than by uranium isotopes.…”

“…We chose to focus on colloids from the Chancellor nuclear test cavity at the NNSS because this cavity water has unusually high concentrations of 137 Cs and 239/240 Pu compared to other NNSS cavity waters, and both of these radionuclides were already known to be quite strongly associated with colloids in the water (Reimus et al, 2006;Rose et al, 2011). The high radionuclide concentrations in the Chancellor cavity water may be related to a hydrologic isolation of the cavity that is also inferred from high residual temperatures and more reducing redox conditions than is typical of cavity waters at the NNSS (Rose et al, 2011).…”

“…We chose to focus on colloids from the Chancellor nuclear test cavity at the NNSS because this cavity water has unusually high concentrations of 137 Cs and 239/240 Pu compared to other NNSS cavity waters, and both of these radionuclides were already known to be quite strongly associated with colloids in the water (Reimus et al, 2006;Rose et al, 2011). The high radionuclide concentrations in the Chancellor cavity water may be related to a hydrologic isolation of the cavity that is also inferred from high residual temperatures and more reducing redox conditions than is typical of cavity waters at the NNSS (Rose et al, 2011). While we recognize that such hydrologic conditions would tend to limit colloid and radionuclide transport out of the cavity, thus reducing the potential relevance of experimental results obtained using Chancellor water, there appears to be nothing unique about the properties of Chancellor water colloids relative to other NNSS cavity waters, and it is quite possible that similar radionuclide concentrations and temperatures existed in more hydrologically-connected cavities at early times after nuclear detonation.…”

Appendix C: Hydrologic Source Term Screening and Distribution

“…These concepts are discussed more thoroughly in IAEA (1998a, b, c), the original YF HST reports (Carle et al, 2008;McNab;2008;Pawloski, et al, 2008;and Tompson, et al, 2008), in the more recent studies of Rose et al (2011) and Tompson et al (2011), and in Section C.3.…”

“…Partitioning was based on IAEA (1998a) recommendations and adjusted, when appropriate, based on direct evidence from the NNSS. The direct evidence came primarily from two sources: The Rainier Mesa HST analysis (Tompson, et al, 2011) and the summary of radionuclide partitioning based on sampling efforts at the Chancellor site (Rose et al, 2011). Uncertainty in the glass partitioning is not well known.…”