Allison Macfarlane

U‐Pb and 40Ar/39Ar geochronology of the Symvolon granodiorite: Implications for the thermal and structural evolution of the Rhodope metamorphic core complex, northeastern Greece

North Aegean continental lithosphere was thickened by southwest vergent thrusting and continental subduction within the Alpine collisional orogen but has subsequently been greatly extended on a northeast-southwest axis in the back arc of the Hellenic subduction zone. Crosscutting relationships with two granodiorite bodies emplaced at ∼31–33 Ma, the Xanthi and eastern Vrondou plutons, constrain a pre-mid-Oligocene origin of Alpine convergent structures in northeastern Greece. Post-Alpine thinning of the north Aegean nappe pile began in earliest Miocene time and has been accommodated by a succession of distinct structural systems. The earliest of these, the “Symvolon shear zone”, appears to represent a midcrustal, coaxial rupture of the Falakron marble series, a carbonate platform >5000 m thick that was subducted northeastward beneath high-grade rocks of the Rhodope metamorphic province in late Alpine time. Zircon and titanite U-Pb dates and hornblende 40Ar/39Ar dates obtained in this study constrain the intrusion and incipient mylonitization of the Symvolon or “Kavala” granodiorite within the Symvolon shear zone at ∼21–22 Ma. Following its emplacement, the Symvolon body resided at temperatures between 300°C and 500°C for 5–7 m.y., during which coaxial deformation may have continued within a widening Symvolon rupture. The Strymon Valley detachment, a regionally south-west dipping low-angle normal fault, succeeded the Symvolon shear zone in middle Miocene time. Southwestward displacement of the Serbo-Macedonian gneiss complex by as much as 80 km in the hanging wall of this detachment facilitated the unroofing of the Rhodope metamorphic core complex, including the Falakron marble series and several Tertiary plutons, in its footwall. Biotite and K-feldspar 40Ar/39Ar dates from 11.1 ± 0.2 Ma to 15.5 ± 0.3 Ma yielded by Symvolon granodiorite samples document the cooling of the Rhodope core complex below 150°C–300°C during its southwestward-progressive exhumation in the footwall of the Strymon Valley detachment.

Chronology of tectonic events in the crystalline core of the Himalaya, langtang National Park, central Nepal

The Main Central Thrust (MCT) is an important intracontinental subduction zone that accommodated a significant proportion of shortening between India and Asia during Tertiary Himalayan orogenesis. Argon 40/argon 39 geochronology indicates at least two distinct periods of thrust movement on the MCT in the Langtang National Park region of central Nepal. Ductile deformation and associated mylonitization characterizing the earlier event are constrained by 40Ar/39Ar dating of muscovites to have occurred sometime before 5.8 Ma. A later period of brittle deformation, resulting in the juxtaposition of rocks of different lithology within the MCT zone, occurred at approximately 2.3 Ma, based on 40Ar/39Ar dating of neoblastic muscovites from the brittle fault zones. The hanging wall of the MCT contains amphibolite to upper amphibolite grade gneisses and small leucogranite bodies assigned to the Greater Himalayan sequence. Argon 40/argon 39 cooling ages of muscovite and biotite from the gneisses range from 4.6 to 9.7 Ma. These dates contrast with previously obtained 16–21 Ma U-Pb monazite and zircon ages for metasedimentary rocks from the same structural levels [Parrish et al., 1992], indicating relatively slow cooling over the early to middle Miocene interval for much of the MCT hanging wall. However, one 19.3 Ma biotite 40Ar/39Ar cooling age for a sample from the uppermost portion of the hanging wall is only slightly younger than U-Pb monazite ages for nearby anatexites, possibly suggesting rapid cooling of the uppermost Greater Himalayan sequence by tectonic denudation associated with the structurally higher South Tibetan detachment system. The general consistency of 40Ar/39Ar ages throughout the 11-km-thick Greater Himalayan sequence suggests rapid cooling in late Miocene time, probably due to an increase in erosion rate related to ramping on the structurally lower Main Boundary Thrust. An alternative possibility would be massive hydrothermal resetting in late Miocene time of the entire sequence.

Nuclear proliferation: Time to bury plutonium

Recycling plutonium is dangerous and costly. Britain should take the lead on direct disposal, say Frank von Hippel, Rodney Ewing, Richard Garwin and Allison Macfarlane.

It’s 2050: Do you know where your nuclear waste is?:

In light of Japan’s nuclear disaster, a major lesson can be learned related to the back end of the fuel cycle: Planning is necessary for the safe and secure management of spent nuclear fuel and nuclear waste. But the topic of storing waste continues to be subject to last-minute solutions, as the experiences of a number of countries besides Japan show. Countries with nuclear power programs need a medium-term strategy for spent fuel storage prior to the long-term plan for spent fuel or high-level waste disposal. Though difficult, the disposal of high-level nuclear waste is possible, and a clear strategy to develop a repository combines both technical and societal criteria in a phased approach. After Fukushima, it is now imperative to redefine what makes a successful nuclear power program—from cradle to grave. Nuclear waste management must be designed from the beginning; otherwise, the public in many countries will reject nuclear as an energy choice.

The problem of used nuclear fuel: lessons for interim solutions from a comparative cost analysis

Abstract An acceptable long-term solution for used (spent) fuel from nuclear power reactors has evaded all countries engaged in the civilian nuclear fuel cycle. Furthermore, many countries are trying to develop interim storage solutions that address the shortage of storage in the spent fuel cooling pools at reactors. The United States has a particularly acute problem due to its adherence to an open fuel cycle and its large number of reactors. Two main options are available to address the spent fuel problem: dry storage on-site at reactors and centralized storage at a facility away from reactors. Key to deciding which option makes better policy sense is the comparative economics of the two options. This paper provides one of the few comprehensive comparisons of costs for the two alternatives and discusses implications for other schemes and possible alternative solutions to the spent fuel problem for the United States.

Stuck on a Solution

It is still not clear whether the planned is even a viable site. nuclear waste repository at Yucca Mountain And the waste keeps piling up.

Response by the Authors to the NRC Review of “Reducing the Hazards from Stored Spent Power-Reactor Fuel in the United States”

Our study addressed the hazards associated with the dense-packed storage of spent fuel in pools next to U.S. nuclear-power reactors. Our concern was the possibility of a spent-fuel fire resulting in case of a sudden loss of coolant or cooling. We considered ways in which this hazard might be mitigated—especially by reducing the packing density.1 The summary in the conclusion of the NRC’s critique of our article states that

Burning waste or playing with fire? Waste management considerations for non-traditional reactors

ABSTRACT Nuclear energy-producing nations are almost universally experiencing delays in the commissioning of the geologic repositories needed for the long-term isolation of spent fuel and other high-level wastes from the human environment. Despite these problems, expert panels have repeatedly determined that geologic disposal is necessary, regardless of whether advanced reactors to support a “closed” nuclear fuel cycle become available. Still, advanced reactor developers are receiving substantial funding on the pretense that extraordinary waste management benefits can be reaped through adoption of these technologies. Here, the authors describe why molten salt reactors and sodium-cooled fast reactors – due to the unusual chemical compositions of their fuels – will actually exacerbate spent fuel storage and disposal issues. Before these reactors are licensed, policymakers must determine the implications of metal- and salt-based fuels vis a vis the Nuclear Waste Policy Act and the Continued Storage Rule.

The Authors Respond to Allan Benjamin's Comments

As the multiple references to it in our article attest, we have learned a great deal from the pioneering work of Allan Benjamin et al, Spent Fuel Heatup Following Loss of Water During Storage (NUREG/CR-0649; SAND77-1371 R-3, 1979). Indeed, many of our conclusions and recommendations essentially echo those made in that report 24 years ago, but never implemented because the probability of an accidental loss of water was estimated to be too low to justify action. Benjamin argues that we should have estimated the probability that sabotage or terrorist attack might cause a loss of water. Indeed, he seems to suggest that the probability can be calculated with some precision with methods that his company offers. While we believe that systematic analysis is useful in identifying vulnerabilities, we are skeptical about the predictive value of probabilistic calculations—especially for malevolent acts. We respond more briefly to Benjamin’s other comments below: