Modeling nuclear energy's future role in decarbonized energy systems.

Increased attention has been focused on the potential role of nuclear energy in future electricity markets and energy systems as stakeholders target rapid and deep decarbonization and reductions in fossil fuel use. This paper examines models of electric sector planning and broader energy systems optimization to understand the prospective roles of nuclear energy and other technologies. In this perspective, we survey modeling challenges in this environment, illustrate opportunities to propagate best practices, and highlight insights from the deep decarbonization literature on the range of visions for nuclear energy's role. Nuclear energy deployment is highest with combinations of stringent emissions policies, nuclear cost reductions, and constraints on the deployment of other technologies, which underscores model dimensions related to these areas. New modeling capabilities are needed to adequately address emerging issues, including representing characteristics and applications of nuclear energy in systems models, and to ensure the relevance of models for policy and planning as deeper decarbonization is explored.

Isolation and characterization of four gram-positive nickel-tolerant microorganisms from contaminated sediments.

Microbial communities from riparian sediments contaminated with high levels of Ni and U were examined for metal-tolerant microorganisms. Isolation of four aerobic Ni-tolerant, Gram-positive heterotrophic bacteria indicated selection pressure from Ni. These isolates were identified as Arthrobacter oxydans NR-1, Streptomyces galbus NR-2, Streptomyces aureofaciens NR-3, and Kitasatospora cystarginea NR-4 based on partial 16S rDNA sequences. A functional gene microarray containing gene probes for functions associated with biogeochemical cycling, metal homeostasis, and organic contaminant degradation showed little overlap among the four isolates. Fifteen of the genes were detected in all four isolates with only two of these related to metal resistance, specifically to tellurium. Each of the four isolates also displayed resistance to at least one of six antibiotics tested, with resistance to kanamycin, gentamycin, and ciprofloxacin observed in at least two of the isolates. Further characterization of S. aureofaciens NR-3 and K. cystarginea NR-4 demonstrated that both isolates expressed Ni tolerance constitutively. In addition, both were able to grow in higher concentrations of Ni at pH 6 as compared with pH 7 (42.6 and 8.5 mM Ni at pH 6 and 7, respectively). Tolerance to Cd, Co, and Zn was also examined in these two isolates; a similar pH-dependent metal tolerance was observed when grown with Co and Zn. Neither isolate was tolerant to Cd. These findings suggest that Ni is exerting a selection pressure at this site for metal-resistant actinomycetes.

Reduction of nickel and uranium toxicity and enhanced trichloroethylene degradation to Burkholderia vietnamiensis PR1301 with hydroxyapatite amendment.

The use of hydroxyapatite (HA) to sequester metals at mixed waste sites may reduce metal toxicity and facilitate microbial degradation of cocontaminant organics. The constitutive trichloroethylene (TCE) degrader, Burkholderia vietnamiensis PR1301, grew at 34.1 and 1.7 mM Ni at pH 5 and 7, respectively, with 0.01 g mL(-1) HA compared to 17 and 0.85 mM Ni without HA. PR1 grew at 4.2 mM U at pH 5 and 7 with 0.01 g mL(-1) HA compared to 1.1 mM U without HA. A similar decrease in the toxicity of Ni and U in combination was observed with HA. The ability of PR1 to degrade TCE at 0.85, 1.7, and 3.4 mM Ni and at 0.42 and 1.1 mM U was examined. The presence of TCE resulted in a decreased tolerance of PR1 to Ni and U; however, HA facilitated TCE degradation in the presence of Ni and U, effectively doubling the metal concentrations at which TCE degradation proceeded. These studies suggest that metal sequestration via HA amendments may offer a feasible approach to reducing metal toxicity to microorganisms at mixed waste sites, thereby enhancing the degradation of cocontaminant organics.

Characterzation of colloidal and humic-bound Ni and U in the "dissolved" fraction of contaminated sediment extracts.

The dissolved phase of environmental aqueous samples is generally defined by filtration at 0.2 microm or even 0.45 microm. However, it is also acknowledged that colloids <0.2 microm suspended in the aqueous phase can be important for determining contaminant availability and mobility. We have used flow field-flow fractionation (FI FFF) and size exclusion chromatography (SEC) coupled to UV-absorbance (UVA) and inductively coupled plasma mass spectrometry (ICP-MS) to study the dissolved organic matter (DOM) and colloidal binding of U and Ni in water extracts of sediments collected from a contaminated area of the Savannah River Site, a U.S. Department of Energy former nuclear materials production and processing facility, near Aiken, SC. High-performance SEC-UVA-ICP-MS was well-suited to the separation of DOM overthe molecular weight (MW) range of approximately 200-7000 Da. The ICP-MS element specific data indicated that a significant fraction of U was associated with DOM. Uranium exhibited a bimodal distribution and the other fraction was greater than the exclusion limit for the column and coeluted with Al. Flow FFF was used to size this fraction as colloidal with an approximate effective spherical diameter of 0.09-0.12 microm. Element specific ICP-MS data confirmed that U and Al were associated with the colloidal phase. High-field FI FFF was also applicable to sizing DOM but resolution was poorer than SEC. The results of this study suggest that "dissolved" U at this site is predominantly either complexed by DOM or bound to a colloidal fraction while Ni is predominately present as labile complexes or the free cation and, therefore, potentially bioavailable.

Effect of pH on the toxicity of nickel and other divalent metals to Burkholderia cepacia PR1(301).

Nickel (Ni) is a common cocontaminant at many waste sites where the soils and sediments often are acidic, thereby influencing metal availability. Growth of Burkholderia cepacia PR1(301) was not affected at 3.41 mM Ni at pH 5, but was inhibited by 73.2% at pH 6 and inhibited completely at pH 7 compared to growth without Ni. This pH effect was not observed in the Ni-resistant strains, Ralstonia metallidurans CH34 and 31A. Predicted Ni speciation did not explain the observed toxicity trends. Sorption of Ni to PR1 increased with increasing pH (1.49, 1.12, and 3.88 mg Ni/g dry weight at pH 5, 6, and 7, respectively), but was low at all three pH values, and most likely does not explain the observed pH effect. Growth inhibition of PRI with increasing pH also was observed for other divalent cations, with growth observed at 4.24 mM Co, 2.22 mM Cd, and 3.82 mM Zn at pH 5 and 6, but totally inhibited at pH 7. These studies suggest that, at circumneutral pH, PRI would be considered sensitive to Ni and other divalent cations, in spite of the ability to grow in higher concentrations at lower pH values.

Partitioning and availability of uranium and nickel in contaminated riparian sediments.

The effects of iron oxides and organic matter on the partitioning and chemical lability of U and Ni were examined for contaminated riparian sediments from the U.S. Department of Energy's Savannah River Site. In sequential extractions of four sediments that ranged from 12.7 to 82.2 g kg(-1) in organic carbon, U was found almost exclusively in moderately labile fractions (93% in acid-soluble + organically bound). Nickel was distributed across all operationally defined fractions, including substantial amounts in the very labile fractions (4-15% in water-soluble + exchangeable), noncrystalline and crystalline iron oxides (38-49%), and in the nonlabile residual fraction (25-34%). Aqueous U concentrations in 1:1 sediment-water extracts were highly correlated to dissolved organic carbon (DOC) (R2 = 0.96; p < 0.0001) and ranged from 29 to 410 microg L(-1). Aqueous concentrations of Ni exceeded U by two to three orders of magnitude (124-2227 microg L(-1)) but were not correlated with DOC (R2 = 0.04; p = 0.53). Partitioning and solubility trends suggest that Ni availability is controlled primarily by iron-oxide phases, whereas U availability is dominated by naturally occurring organic carbon. Discrete mineral phases were also identified as nonlabile reservoirs of anthropogenic metals. In spite of comparably high sediment concentrations, Ni appears to be significantly more available than U in riparian sediments and therefore warrants greater consideration in terms of environmental consequences (i.e., transport, biological uptake, and toxicity).