Towards a shared method to classify contaminated territories in the case of an accidental nuclear event: the PRIME project

The analysis of the management of the accidentally radioactive contaminated areas such as those around Chernobyl nuclear power plant highlights the fact that the current spatial classification methods hardly help in recovering proper use of the contaminated territory. The cause is mainly to be searched for in the traditional construction of risks assessment methods; these methods rest on criteria defined by institutional experts, which are not applicable in practise because they are not shared by all the stakeholders involved in the management of the contaminated territories. Opposite such top-down tentative management, local efforts supported by Non-Governmental Organizations to restore life in the contaminated area seem to be more fruitful but very time and resources consuming and limited to the specific areas where they are experimented. The aim of the PRIME project, in progress at the French Institute for Radioprotection and Nuclear Safety, is to mix the advantages of both approaches in building a multicriteria decision tool based on the territorial specificities. The criteria of the method are chosen and weighted with representatives of the territory’s stakeholders (decision makers, local actors and experts) to warrant that all the points of view are taken into account and to enable the risk managers to choose the appropriate strategy in case of an accident involving radioactive substances. The area chosen for the pilot study is a 50 km radius territory around the nuclear sites of Tricastin-Pierrelatte in the lower valley of Rhône (France). One of the exploration questions of the PRIME project is whether a multicriteria method may be an appropriate tool to treat the data and make them visible and accessible for all the stakeholders

Activated sampling in complex materials at finite temperature: the properly-obeying-probability activation-relaxation technique

While the dynamics of many complex systems is dominated by activated events, there are very few simulation methods that take advantage of this fact. Most of these procedures are restricted to relatively simple systems or, as with the activation-relaxation technique (ART), sample the conformation space efficiently at the cost of a correct thermodynamical description. We present here an extension of ART, the properly-obeying-probability ART (POP-ART), that obeys detailed balance and samples correctly the thermodynamic ensemble. Testing POP-ART on two model systems, a vacancy and an interstitial in crystalline silicon, we show that this method recovers the proper thermodynamical weights associated with the various accessible states and is significantly faster than MD in the diffusion of a vacancy below 700 K.Comment: 10 pages, 3 figure

Efficient tight-binding Monte Carlo structural sampling of complex materials

While recent work towards the development of tight-binding and ab-initio algorithms has focused on molecular dynamics, Monte Carlo methods can often lead to better results with relatively little effort. We present here a multi-step Monte Carlo algorithm that makes use of the possibility of quickly evaluating local energies. For the thermalization of a 1000-atom configuration of {\it a}-Si, this algorithm gains about an order of magnitude in speed over standard molecular dynamics. The algorithm can easily be ported to a wide range of materials and can be dynamically optimized for a maximum efficiency.Comment: 5 pages including 3 postscript figure

Traveling through potential energy landscapes of disordered materials: the activation-relaxation technique

A detailed description of the activation-relaxation technique (ART) is presented. This method defines events in the configurational energy landscape of disordered materials, such as a-Si, glasses and polymers, in a two-step process: first, a configuration is activated from a local minimum to a nearby saddle-point; next, the configuration is relaxed to a new minimum; this allows for jumps over energy barriers much higher than what can be reached with standard techniques. Such events can serve as basic steps in equilibrium and kinetic Monte Carlo schemes.Comment: 7 pages, 2 postscript figure

Population Size, Sex and Purifying Selection: Comparative Genomics of Two Sister Taxa of the Wild Yeast Saccharomyces paradoxus

This study uses population genomic data to estimate demographic and selection parameters in two sister lineages of the wild yeast Saccharomyces paradoxus and compare their evolution. We first estimate nucleotide and recombinational diversities in each of the two lineages to infer their population size and frequency of sex and then analyze the rate of mutation accumulation since divergence from their inferred common ancestor to estimate the generation time and efficacy of selection. We find that one of the lineages has significantly higher silent nucleotide diversity and lower linkage disequilibrium, indicating a larger population with more frequent sexual generations. The same lineage also shows shorter generation time and higher efficacy of purifying selection, the latter consistent with the finding of larger population size and more frequent sex. Similar analyses are also performed on the ancestries of individual strains within lineages and we find significant differences between strains implying variation in rates of mitotic cell divisions. Our sample includes some strains originating in the Chernobyl nuclear-accident exclusion zone, which has been subjected to high levels of radiation for nearly 30 years now. We find no evidence, however, for increased rates of mutation. Finally, there is a positive correlation between rates of mutation accumulation and length of growing period, as measured by latitude of the place of origin of strains. Our study illustrates the power of genomic analyses in estimating population and life history parameters and testing predictions based on population genetic theory

Self-organization with equilibration: a model for the intermediate phase in rigidity percolation

Recent experimental results for covalent glasses suggest the existence of an intermediate phase attributed to the self-organization of the glass network resulting from the tendency to minimize its internal stress. However, the exact nature of this experimentally measured phase remains unclear. We modify a previously proposed model of self-organization by generating a uniform sampling of stress-free networks. In our model, studied on a diluted triangular lattice, an unusual intermediate phase appears, in which both rigid and floppy networks have a chance to occur, a result also observed in a related model on a Bethe lattice by Barre et al. [Phys. Rev. Lett. 94, 208701 (2005)]. Our results for the bond-configurational entropy of self-organized networks, which turns out to be only about 2% lower than that of random networks, suggest that a self-organized intermediate phase could be common in systems near the rigidity percolation threshold.Comment: 9 pages, 6 figure

Self-organized criticality in the intermediate phase of rigidity percolation

Experimental results for covalent glasses have highlighted the existence of a new self-organized phase due to the tendency of glass networks to minimize internal stress. Recently, we have shown that an equilibrated self-organized two-dimensional lattice-based model also possesses an intermediate phase in which a percolating rigid cluster exists with a probability between zero and one, depending on the average coordination of the network. In this paper, we study the properties of this intermediate phase in more detail. We find that microscopic perturbations, such as the addition or removal of a single bond, can affect the rigidity of macroscopic regions of the network, in particular, creating or destroying percolation. This, together with a power-law distribution of rigid cluster sizes, suggests that the system is maintained in a critical state on the rigid/floppy boundary throughout the intermediate phase, a behavior similar to self-organized criticality, but, remarkably, in a thermodynamically equilibrated state. The distinction between percolating and non-percolating networks appears physically meaningless, even though the percolating cluster, when it exists, takes up a finite fraction of the network. We point out both similarities and differences between the intermediate phase and the critical point of ordinary percolation models without self-organization. Our results are consistent with an interpretation of recent experiments on the pressure dependence of Raman frequencies in chalcogenide glasses in terms of network homogeneity.Comment: 20 pages, 18 figure