Hydrothermal Alteration of Glass from Underground Nuclear Tests: Formation and Transport of Pu-clay Colloids at the Nevada National Security Site

Zavarin, M.¹  Zhao, P.¹  Joseph, C.¹  Begg, J.¹  Boggs, M.¹  Dai, Z.¹  Kersting, A. B.¹ 

[1]Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

关键词: SMECTITE;    LEACHING;    CONCENTRATION RATIO;    COLLOIDS;    GROUND WATER;    BOROSILICATE GLASS;    UNDERGROUND;    ABUNDANCE;    PLUTONIUM;    NEVADA TEST SITE;    HYDROTHERMAL ALTERATION;    PLUTONIUM 239;    PLUTONIUM 240;    TRITIUM;    RADIONUCLIDE MIGRATION;    RADIOACTIVE WASTES;    URANIUM ISOTOPES;    CAVITIES;    HAZARDS;    MINERALOGY;    WELLS;    YIELDS;    UNDERGROUND EXPLOSIONS;    NUCLEAR EXPLOSIONS;    LIMITING VALUES;    TEMPERATURE RANGE 0273-0400 K;    TEMPERATURE RANGE 0400-1000 K;   

DOI  :  10.2172/1251034
RP-ID  :  LLNL--TR-671402
PID  :  OSTI ID: 1251034
Others  :  TRN: US1601343

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来源: SciTech Connect

【 摘 要 】

The testing of nuclear weapons at the Nevada National Security Site (NNSS), formerly the Nevada Test Site (NTS), has led to the deposition of substantial quantities of plutonium into the environment. Approximately 2.8 metric tons (3.1??10⁴ TBq) of Pu were deposited in the NNSS subsurface as a result of underground nuclear testing. While ³H is the most abundant anthropogenic radionuclide deposited in the NNSS subsurface (4.7??10⁶ TBq), plutonium is the most abundant from a molar standpoint. The only radioactive elements in greater molar abundance are the naturally occurring K, Th, and U isotopes. ²³⁹Pu and ²⁴⁰Pu represent the majority of alpha-emitting Pu isotopes. The extreme temperatures associated with underground nuclear tests and the refractory nature of Pu results in most of the Pu (98%) being sequestered in melted rock, referred to as nuclear melt glass (Iaea, 1998). As a result, Pu release to groundwater is controlled, in large part, by the leaching (or dissolution) of nuclear melt glass over time. The factors affecting glass dissolution rates have been studied extensively. The dissolution of Pu-containing borosilicate nuclear waste glasses at 90??C has been shown to lead to the formation of dioctahedral smectite colloids. Colloid-facilitated transport of Pu at the NNSS has been observed. Recent groundwater samples collected from a number of contaminated wells have yielded a wide range of Pu concentrations from 0.00022 to 2.0 Bq/L. While Pu concentrations tend to fall below the Maximum Contaminant Level (MCL) established by the Environmental Protection Agency (EPA) for drinking water (0.56 Bq/L), we do not yet understand what factors limit the Pu concentration or its transport behavior. To quantify the upper limit of Pu concentrations produced as a result of melt glass dissolution and determine the nature of colloids and Pu associations, we performed a 3 year nuclear melt glass dissolution experiment across a range of temperatures (25-200 ?��C) that represent hydrothermal conditions representative of the underground nuclear test cavities (when groundwater has re-saturated the nuclear melt glass and glass dissolution occurs). Colloid loads and Pu concentrations were monitored along with the mineralogy of both the colloids and the secondary mineral phases. The intent was to establish an upper limit for Pu concentrations at the NNSS, provide context regarding the Pu concentrations observed at the NNSS to date and the Pu concentrations that may be observed in the future. The results provide a conceptual model for the risks posed by Pu migration at the NNSS.