Impact of Approximations in Operating History Data on Spent Fuel Properties with Serpent 2

In this work, the effect of averaging operating history parameters such as power history, boron concentration and coolant density and temperature on spent nuclear fuel properties was investigated. The examined properties were assembly activity, decay heat, photon emission rate, spontaneous fission rate and the concentration of some mobile nuclides and fissile nuclides. Calculations were performed on two similar VVER-440 fuel assemblies irradiated in different positions of the core using Serpent 2. Averaging power history over the entire irradiation history had a significant effect on assembly activity, decay heat and photon emission rate overestimating these properties approximately 70 % right after irradiation. However, the effect quickly died out and after 10 years of cooling the effect was less than 1 %. If the last cycle (3rd cycle) was modelled accurately and the power density of only the first two cycles were averaged, the differences remained always below 1 %. The effect of operating history approximations on spontaneous fission rate and the nuclide concentrations was much smaller reamaining mostly below 1.5 %. The sensitivity of nuclide concentrations to approximations in individual operating history parameters was dependent on the nuclide in question and no trend applying to all studied nuclides could be observed.

Stabilized Supralinear Network: Model of Layer 2/3 of the Primary Visual Cortex

Electrophysiological recording in the primary visual cortex (V1) of mammals have revealed a number of complex interactions between the center and surround. Understanding the underlying circuit mechanisms is crucial to understanding fundamental brain computations. In this paper we address the following phenomena that have been observed in V1 of animals with orientation maps: 1) surround suppression that is accompanied by a decrease in the excitatory and inhibitory currents that the cell receives as the stimulus size increases beyond the cell’s summation field; 2) surround tuning to the center orientation, in which the strongest suppression arises when the surround orientation matches that of the center stimulus; and 3) feature-specific suppression, in which a surround stimulus of a given orientation specifically suppresses that orientation’s component of the response to a center plaid stimulus. We show that a stabilized supralinear network that has biologically plausible connectivity and synaptic efficacies that depend on cortical distance and orientation difference between neurons can consistently reproduce all the above phenomena. We explain the mechanism behind each result, and argue that feature-specific suppression and surround tuning to the center orientation are independent phenomena. Specifically, if we remove some aspects of the connectivity from the model it will still produce feature-specific suppression but not surround tuning to the center orientation. We also show that in the model the activity decay time constant is similar to the cortical activity decay time constant reported in mouse V1. Our model indicates that if the surround activates neurons that fall within the reach of the horizontal projections in V1, the above mentioned phenomena can be generated by V1 alone without the need of cortico-cortical feedback. Finally, we show that these results hold both in networks with rate-based units and with conductance-based spiking units. This demonstrates that the stabilized supra-linear network mechanism can be achieved in the more biological context of spiking networks.

Management of hospital radioactive liquid waste: treatment proposal for radioimmunoassay wastes

<abstract> <p>Radioactive liquid wastes are produced at hospitals from diagnostic and therapeutic applications of radionuclides. The most usual management of these wastes is temporary storage at the hospital for radioactivity decay and, then, discharge into sewage if not other pollutants are present in waste, always after authorization of the corresponding institution. In some cases, radioactive wastes have other hazards, such as chemical or biological ones, which can be more dangerous than radiological hazard, and do not allow direct discharge into sewage in spite of decaying activity below the clearance level. Therefore, these wastes have to be treated and condition before discharge in spite of activity decay below discharge limit. This is the case of liquid wastes from radioimmunoassay (RIA), a laboratory technique that allows to determine human substances in very low concentrations (below 10^-12 g/mL), like hormones, using ¹²⁵I as radionuclide. This study summarizes the usual management of radioactive liquid wastes from hospitals, including conventional and recent treatments applied. Furthermore, based on experimental results obtained with real RIA wastes, this work exposes a proposal of treatment with ultrafiltration and reverse osmosis membranes, and determines the most suitable application of this treatment according to radiological and operational considerations.</p> </abstract>

The three-stage rock failure dynamics of the Drus (Mont Blanc massif, France) since the June 2005 large event

Since the end of the Little Ice Age, the west face of the Drus (Mont Blanc massif, France) has been affected by a retrogressive erosion dynamic marked by large rockfall events. From the 1950s onwards, the rock failure frequency gradually increased until the large rockfall event (292,680 m3) of June 2005, which made the Bonatti Pillar disappear. Aiming to characterize the rock failure activity following this major event, which may be related to permafrost warming, the granitic rock face was scanned each autumn between October 2005 and September 2016 using medium- and long-range terrestrial laser scanners. All the point clouds were successively compared to establish a rockfall source inventory and determine a volume-frequency relationship. Eleven years of monitoring revealed a phase of rock failure activity decay until September 2008, a destabilization phase between September 2008 and November 2011, and a new phase of rock failure activity decay from November 2011 to September 2016. The destabilization phase was marked by three major rockfall events covering a total volume of 61,494 m3, resulting in the progressive collapse of a new pillar located in the northern part of the June 2005 rockfall scar. In the same way as for the Bonatti Pillar, rock failure instability propagated upward with increasing volumes. In addition to these major events, 304 rockfall sources ranging from 0.002 to 476 m3 were detected between 2005 and 2016. The temporal evolution of rock failure activity reveals that after a major event, the number of rockfall sources and the eroded volume both follow a rapid decrease. The rock failure activity is characterized by an exponential decay during the period following the major event and by a power-law decay for the eroded volume. The power law describing the distribution of the source volumes detected between 2005 and 2016 indicates an exponent of 0.48 and an average rock failure activity larger of more than six events larger than 1 m3 per year. Over the 1905–2016 period, a total of 426,611 m3 of rock collapsed from the Drus west face, indicating a very high rock wall retreat rate of 14.4 mm year−1 over a surface of 266,700 m2. Averaged over a time window of 1000 years, the long-term retreat rate derived from the frequency density integration of rock failure volumes is 2.9 mm year−1. Despite difficulty in accessing and monitoring the site, our study demonstrates that long-term surveys of high-elevation rock faces are possible and provide valuable information that helps improve our understanding of landscape evolution in mountainous settings subject to permafrost warming.

Engineering bunched Pt-Ni alloy nanocages for efficient oxygen reduction in practical fuel cells

Development of efficient and robust electrocatalysts is critical for practical fuel cells. We report one-dimensional bunched platinum-nickel (Pt-Ni) alloy nanocages with a Pt-skin structure for the oxygen reduction reaction that display high mass activity (3.52 amperes per milligram platinum) and specific activity (5.16 milliamperes per square centimeter platinum), or nearly 17 and 14 times higher as compared with a commercial platinum on carbon (Pt/C) catalyst. The catalyst exhibits high stability with negligible activity decay after 50,000 cycles. Both the experimental results and theoretical calculations reveal the existence of fewer strongly bonded platinum-oxygen (Pt-O) sites induced by the strain and ligand effects. Moreover, the fuel cell assembled by this catalyst delivers a current density of 1.5 amperes per square centimeter at 0.6 volts and can operate steadily for at least 180 hours.

Subunit exchange enhances information retention by CaMKII in dendritic spines

Molecular bistables are strong candidates for long-term information storage, for example, in synaptic plasticity. CaMKII is a highly expressed synaptic protein which has been proposed to form a molecular bistable switch capable of maintaining its state for years despite protein turnover and stochastic noise. It has recently been shown that CaMKII holoenzymes exchange subunits among themselves. Here we used computational methods to analyze the effect of subunit exchange on the CaMKII pathway in the presence of diffusion in two different microenvironments, the Post Synaptic Density (PSD) and spine cytosol. We show that in the PSD, subunit exchange leads to coordinated switching and prolongs state stability of the fraction of CaMKII that is present in clusters; and underlies spreading of activation among the remaining CaMKII that is uniformly distributed. Subunit exchange increases the robustness of the CaMKII switch measured as range of bistability both with respect to protein phosphatase 1 (PP1) levels and protein turnover rates. In the phosphatase-rich spine cytosol, subunit exchange leads to slower decay of activity following calcium stimuli. We find that subunit exchange can explain two time-courses of CaMKII activity decay observed in recent experiments monitoring endogenous activity of CaMKII in the spine. Overall, CaMKII exhibits multiple timescales of activity in the synapse and subunit exchange enhances the information retention ability of CaMKII by improving the stability of its switching in the PSD, and by slowing the decay of its activity in the spine cytosol. The existence of diverse timescales in the synapse has important theoretical implications for memory storage in networks.Significance StatementDespite everyday forgetfulness, we can recall some memories years after they were formed. How are we able to protect some memories for so long? Previous work has shown that the abundant brain protein Calcium/calmodulin dependent protein Kinase II (CaMKII) can form a very stable binary switch which can store information for years. Building on this work, we analyzed the implications of a recently discovered phenomenon of subunit exchange on the state switching properties of CaMKII. In subunit exchange fragments of one CaMKII molecule detatch and exchange with another. We discovered that this improves the information retention ability of CaMKII both in the context where it stores information for long times, and also where it integrates information over the timescale of minutes.

Response repetition biases in human perceptual decisions are explained by activity decay in competitive attractor models

Animals and humans have a tendency to repeat recent choices, a phenomenon known as choice hysteresis. The mechanism for this choice bias remains unclear. Using an established, biophysically informed model of a competitive attractor network for decision making, we found that decaying tail activity from the previous trial caused choice hysteresis, especially during difficult trials, and accurately predicted human perceptual choices. In the model, choice variability could be directionally altered through amplification or dampening of post-trial activity decay through simulated depolarizing or hyperpolarizing network stimulation. An analogous intervention using transcranial direct current stimulation (tDCS) over left dorsolateral prefrontal cortex (dlPFC) yielded a close match between model predictions and experimental results: net soma depolarizing currents increased choice hysteresis, while hyperpolarizing currents suppressed it. Residual activity in competitive attractor networks within dlPFC may thus give rise to biases in perceptual choices, which can be directionally controlled through non-invasive brain stimulation.