Mechanics of a Meltdown: For all the potential hazards presented by nuclear power plants, there have been very few incidents that have caused human or environmental harm. However, those that have happened have often had disastrous consequences, for example, the accidents at Chernobyl in Ukraine and, just last year, at Fukushima Daiichi in Japan. Burns et al. (p. 1184 ; see the cover) review the state of knowledge on the chemical and physical processes following the nuclear reactor accident and how these results may inform decision-making during future events. Because a large portion of prior research has focused on radionuclide transport following leaks from nuclear waste repositories, and not active reactors, experiments at the more extreme conditions experienced in major nuclear core-melt accidents may have more predictive value.

Petrographic and geochemical analyses of travertine-depositing hot springs at Angel Terrace, Mammoth Hot Springs, Yellowstone National Park, have been used to define five depositional facies along the spring drainage system. Spring waters are expelled in the vent facies at 71 to 73°C and precipitate mounded travertine composed of aragonite needle botryoids. The apron and channel facies (43-72°C) is floored by hollow tubes composed of aragonite needle botryoids that encrust sulfide-oxidizing Aquificales bacteria. The travertine of the pond facies (30-62°C) varies in composition from aragonite needle shrubs formed at higher temperatures to ridged networks of calcite and aragonite at lower temperatures. Calcite "ice sheets", calcified bubbles, and aggregates of aragonite needles ("fuzzy dumbbells") precipitate at the air-water interface and settle to pond floors. The proximal-slope facies (28-54°C), which forms the margins of terracette pools, is composed of arcuate aragonite needle shrubs that create small microterracettes on the steep slope face. Finally, the distal-slope facies (28-30°C) is composed of calcite spherules and calcite "feather" crystals. Despite the presence of abundant microbial mat communities and their observed role in providing substrates for mineralization, the compositions of spring-water and travertine predominantly reflect abiotic physical and chemical processes. Vigorous CO2 degassing causes a +2 unit increase in spring water pH, as well as Rayleigh-type covariations between the concentration of dissolved inorganic carbon and corresponding δ13C. Travertine δ13C and δ18O are nearly equivalent to aragonite and calcite equilibrium values calculated from spring water in the higher-temperature (∼50-73°C) depositional facies. Conversely, travertine precipitating in the lower-temperature (<∼50°C) depositional facies exhibits δ13C and δ18O values that are as much as 4‰ less than predicted equilibrium values. This isotopic shift may record microbial respiration as well as downstream transport of travertine crystals. Despite the production of H2S and the abundance of sulfide-oxidizing microbes, preliminary δ34S data do not uniquely define the microbial metabolic pathways present in the spring system. This suggests that the high extent of CO2 degassing and large open-system solute reservoir in these thermal systems overwhelm biological controls on travertine crystal chemistry.

While the d⁰ transition-metal POMs of Group V (V⁵⁺, Nb⁵⁺, Ta⁵⁺) and Group VI (Mo⁶⁺, W⁶⁺) have been known for more than a century, the actinyl peroxide POMs, specifically those built of uranyl triperoxide or uranyl dihydroxidediperoxide polyhedra, were only realized within the last decade. While virtually every metal on the Periodic Table can form discrete clusters of some type, the actinyls are the only—in addition to the transition-metal POMs– whose chemistry is dictated by the prevalence of the ‘yl’ oxygen ligand. Thus this emerging structural, solution, and computational chemistry of actinide POMs warrants comparison to the mature chemistry of transition-metal POMs. This assessment between the transition-metal POMs and actinyl POMs (uranyl peroxide POMs, specifically) has provided much insight to the similarities and differences between these two chemistries. We further break down the comparison between the alkaline POMs of Nb and Ta; and the acidic POMs of V, Mo and W. This more indepth literature review and discussion reveals that while an initial evaluation suggests the actinyl POMs are more akin to the alkaline transition-metal POMs, they actually share characteristics unique to the acidic POMs as well. This tutorial review is meant to provide fodder for deriving new POM chemistries of both the familiar transition-metals and the emerging actinides, as well as fostering communication and collaboration between the two scientific communities.