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Although reducing conservatism would alleviate unnecessary constraints in processing, storage, transportation, and disposal of nuclear materials, excessively conservative approaches are still utilized in many safety analyses. Criticality safety limits are put in place to reduce the likelihood of having a nuclear criticality accident to a value that is deemed incredible but often utilize parameters that are conservative to the point of becoming incredible themselves. The analyses that determine criticality limits are supposed to be based on credible instead of incredible events and circumstance, highlighting the need to be able to distinguish between what is in the realm of possibility and what is not. This paper provides a quantitative approach for reducing unrealistically conservative parameters by recalculating limiting factors in a state that deviates from the worst-case scenario and assigning probability distributions to these systematic deviations. This provides a technical basis for replacing excessively conservative values with something that is both objective and reasonably bounding, which may be systematically utilized in any criticality safety analyses. The assumption of "perfect sphericity" in the TRUPACT-II package's fissile contents model was used as an example case to demonstrate the proposed approach for replacing qualitative reductions to conservatism with quantitative reductions. Through a series of Monte Carlo calculations and statistical analyses, it was shown that conservative deviations from sphericity will provide lower keff values, where the magnitude and impact of this deviation is system specific. The statistical significance from applying probabilistic conservatism will be dependent on the chosen κ value and integration limit for the exponential distribution, as it varies the degree of conservatism applied to any parameter of interest. This approach is not limited to geometric assumptions and may be applied to a variety of conservative parameters. In an effort to move toward a standard method for reducing conservatism, this objective approach may be used in lieu of or in conjunction with subjective methods for relaxing constraints. [ABSTRACT FROM AUTHOR]

This study investigates the impact of hybrid nanoparticles on the temperature of nuclear reactor coolant, with a focus on graphene nanoplatelet (GNP)‐based hybrid nanoparticles. Sixteen different hybrid nanofluids were analyzed, and their performance was compared with a standard water‐based coolant. The criticality values were obtained through MCNP modeling, revealing that higher nanoparticle ratios led to increased criticality, with the highest value of 1.3239 observed in GNP‐Fe3O4 + Al2O3 nanofluids (0.05 wt%) and the lowest value of 1.2935 in GNP–Fe3O4 + SiO2 nanofluids (0.001 wt%). Temperature variations showed that increasing nanoparticle concentrations resulted in slightly higher temperatures, with a maximum of 611.97 K for 0.05 vol.% GNP nanoparticles. Additionally, the departure from nucleate boiling ratio values were consistently above the safety threshold of 2.08, with the lowest value of 3.657 for GNP–Fe3O4 + SiO2 nanofluids (0.05 vol.%). These findings suggest that hybrid nanofluids, particularly those with higher nanoparticle ratios, can enhance the thermal performance and safety margins of nuclear reactor coolants, offering a promising avenue for future research and application. [ABSTRACT FROM AUTHOR]

A new nuclear data library for the advanced lattice physics code CASMO5 has been prepared based on JENDL-5. In JENDL-5, the range of data, such as the number of nuclides, has been dramatically expanded for general-purpose applications. At the same time, many essential nuclides for conventional light water reactor (LWR) analysis have also been modified based on state-of-the-art evaluations. The new JENDL-5–based CASMO5 library was prepared by replacing as much of the nuclear data of the current CASMO5 ENDF/B-VII.1–based library as possible with JENDL-5. This study performs initial verification and validation of the new library under typical LWR conditions. Verifications were performed based on the Organisation for Economic Co-operation and Development/Nuclear Energy Agency burnup credit criticality safety benchmark phase III-C, and the calculated kinf and fuel compositions of the boiling water reactor fuel assembly were compared with reported benchmark results. Comparison with the MCNP6.2 result was also performed using the same benchmark model. In addition, a tank-type critical assembly critical experiment and Takahama-3 postirradiation experiment were used for validation. The results indicate that the new library performs well and is comparable to the ENDF/B-VII.1–based library in predictions of reactivity and fuel compositions for typical LWR systems. [ABSTRACT FROM AUTHOR]

There is disagreement over how much Werner Heisenberg knew about reactors and the bomb 1939–1945. A critical paper, declassified and returned to the Germans in 1971, has been inadequately understood in the controversy. Heisenberg calculated the critical radius for a spherical reactor with a tamper but the same formula applies whether spherical reactor or spherical bomb because his result is independent of the reactor's neutron source at criticality (a bomb has no external neutron source). He didn't emphasize the source-independence of the critical radius but instead misinterpreted it. Under captivity at Farm Hall in England in 1945 with eight other leading German nuclear researchers and Max v. Laue, Heisenberg recalled his 1939 theory from memory and made the same critical radius calculation for a bomb with a tamper that Serber had made in 1943 at Los Alamos. Heisenberg's 1945 formula for a bomb is identical to his 1939 critical radius formula derived for a reactor. His 1945 predictions for the numerical values of the critical radius are close to Serber's, except that Heisenberg only knew the fast fission cross section within two bounds, 0.5 and 2.5 barn. Serber's fast fission cross section was in the middle at 1.5 barn. This places Heisenberg's 1939 and 1945 theory and calculations, finally, in the correct historic context. Editor's Note: Historians debate whether Heisenberg was aware, before the launching of the American Manhattan project, of the possible characteristics of a U-235 bomb. This paper presents convincing evidence that in 1939 Heisenberg had already performed the calculation that was presented in 1943 by Serber to the Los Alamos group that predicted a critical radius. This paper will be of interest to those who teach about the development of the atomic bomb. [ABSTRACT FROM AUTHOR]

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Different deterministic solution methods are being used to solve the criticality problem of the reflected systems with different anisotropic behaviours for both fission neutrons and neutrons undergoing scattering. In the present study, the criticality problem is simulated using the developed Monte Carlo algorithm. Different angular distributions, such as Inönü, Anlı–Güngör and tetra-anisotropic kernels are taken into account. For each anisotropic kernel, the sampling method of the direction cosine of either newborn fission neutrons or scattered neutrons is presented by extracting the required probability distribution function (PDF). The simulation starts with an initial guess and following the presented procedure, the criticality thickness of the system is estimated with a certain convergence criterion. Different variance reduction methods, such as implicit capture, Russian-roulette and splitting, are also implemented to get more precise results with a reasonable computational time cost. The validity of the presented method is verified by comparing with the results of different deterministic methods. Finally, using an exact scattering function, the three considered kernels are compared with each other. It is seen that the tetra-anisotropic kernel is the best among the considered kernels. [ABSTRACT FROM AUTHOR]

In this paper, the author investigates particular disconjugate even‐order linear difference equations with two terms and classify them based on the properties of their recessive solutions at plus and minus infinity. The main theorem described states that the studied equation is (k−p+1)$(k-p+1)$‐critical whenever a specific second‐order linear difference equation is p$p$‐critical. In the proof, the author derived closed‐form solutions for the studied equation wherein the solutions of the said second‐order equation appear. Furthermore, the solutions were organized, in order to determine recessive solutions, into a linear chain by sequence ordering that compares the solutions at ±∞$\pm \infty$. [ABSTRACT FROM AUTHOR]

The adaptation underlying many realistic processes plays a pivotal role in shaping the collective dynamics of diverse systems. Here, we untangle the generic conditions for synchronization transitions in a system of coupled phase oscillators incorporating the adaptive scheme encoded by the feedback between the coupling and the order parameter via a power-law function with different weights. We mathematically argue that, in the subcritical and supercritical correlation scenarios, there exists no critical adaptive fraction for synchronization transitions converting from the first (second)-order to the second (first)-order. In contrast to the synchronization transitions previously deemed, the explosive and continuous phase transitions take place in the corresponding regions as long as the adaptive fraction is nonzero, respectively. Nevertheless, we uncover that, at the critical correlation, the routes toward synchronization depend crucially on the relative adaptive weights. In particular, we unveil that the emergence of a range of interrelated scaling behaviors of the order parameter near criticality, manifesting the subcritical and supercritical bifurcations, are responsible for various observed phase transitions. Our work, thus, provides profound insights for understanding the dynamical nature of phase transitions, and for better controlling and manipulating synchronization transitions in networked systems with adaptation. [ABSTRACT FROM AUTHOR]

Burnable absorbers play an important role in nuclear reactor safety and exploring their effect on the reactor core behavior is an important issue in the design and operation of the reactor core. The aim of the present work was to discover the effect of burnable absorbers on criticality and reactivity coefficient of VVER-1000 assembly. The calculations were conducted with a 3-D Monte Carlo transport code MCNP6 and ENDF/B-VII.1 nuclear library. The Gd2O3 content varying from 0 wt.% to 8 wt.% was considered to complete the inter-comparison analysis between the criticality and the reactivity coefficients for three UO2+Gd2O3 fuel configurations. At the beginning of the cycle, there is a significant difference between the criticality (kinf) of assembly with and without Gd2O3, however, at the middle of the cycle those differences become very small and almost the same at the end of the cycle. The Dopp ler temperature coefficient values are always sufficiently negative and demonstrate a more negative trend with increasing gadolinium concentrations and fuel burnup. At beginning of the cycle, the moderator temperature coefficient value increases negatively as gadolinium concentration increases but, at middle of the cycle this trend does not occur. The fuel composition is predicted to be the reason behind this situation. At the end of cycle, there is no clear trend in the moderator temperature coefficient values with respect to Gd2O3 concentration. The absorbing effect of Gd2O3 appears to have diminished significantly. Overall, this research provides insights into the influence of the burnable absorbers on the neutronic parameters of the VVER-1000 assembly and its contribution to reactor safety. [ABSTRACT FROM AUTHOR]

Since the compositions and properties of the fuel debris are uncertain, critical experiments are required to validate calculation codes and nuclear data used for the safety evaluation. For this purpose, the Japan Atomic Energy Agency (JAEA) has been modifying a critical assembly called "STACY." The first criticality of the modified STACY is scheduled for spring 2024. This paper reports the consideration results of the core configurations of the modified STACY for the first criticality and inspections. The specifications of these core configurations were determined in advance, we tried to make them with a simplified computational model that considers the reactivity effect around the core. At the first criticality, two types of grid plates with different neutron moderation conditions (their hole spacings are 1.50 cm and 1.27 cm) were prepared. On the other hand, there is a limitation on the number of available UO 2 fuel rods. The core configurations for the first criticality satisfying these experimental constraints were designed by computational analysis. A cylindrical core configuration with a 1.50 cm pitch grid plate close to the optimum moderation condition needs 253 fuel rods to reach criticality. As to the 1.27 cm grid plate, we considered core configurations with 2.54 cm intervals by using doubled pitches of the grid plate. It will need 213 fuel rods for the criticality to be reached. In addition, the experimental core configuration was considered with steel/concrete simulant rods to simulate fuel debris conditions. This paper shows six core configurations with different conditions, and all of them satisfy the regulatory requests. [ABSTRACT FROM AUTHOR]

A Nuclear Accident Dosimetry program is a federal requirement for all facilities that have the potential to have a criticality accident. Personnel Nuclear Accident Dosimeter (PNAD) theory and analytical procedures are driven by various scientific needs and interacting regulations. A brief history of the status of USA Department of Energy (DOE) nuclear accident dosimetry regulations, recommendations, and performance testing criteria are given. Then, the history of the Lawrence Livermore National Laboratory (LLNL) PNAD is explored, including changes in the physical dosimeter and adjustments of the analysis method through the last four decades. Finally, the performance of LLNL's PNAD at criticality accident intercomparison training exercises since 2009 is explored. In general, reported neutron doses have been within or close to DOE-STD-1098 performance criteria while reported gamma doses have been outside of DOE-STD-1098 performance criteria. Reported total absorbed doses have varied in meeting ANSI/HPS N13.3 and ANSI/HPS N13.3 (R2019) performance criteria. Dosimetry staff retirement and turnover have left historical knowledge gaps, yet provided opportunities within the NAD program at LLNL. This review paper serves as an overview of the history and status of the NAD program. Brief technical, procedural and programmatic recommendations to improve LLNL's NAD program are given. Technical recommendations include investigating orientation factors through modeling or empirical experimentation, investigating gamma dosimetry methods for high-dose scenarios, and exploring other dosimetric methods for simpler, quicker NAD analysis. Procedural recommendations include better documentation of conversion factor (activity-to-fluence and fluence-to-dose) derivations and spectrum uses, and updated analysis spreadsheets or simple Graphic User Interfaces for dose calculations. Programmatic recommendations include formalized training for NAD analysts, and having multiple SMEs trained on the NAD program. • Requirements for Nuclear Accident Dosimetry programs are reviewed. • Provides an overview of the Lawrence Livermore National Laboratory nuclear accident. • Discussion of technical, procedural and programmatic improvements to the NAD program. [ABSTRACT FROM AUTHOR]

• Periodic wavy reactivity could have dominated the Windscale Works criticality accident. • The uneven arrangement of fluids causes oscillations in neutron leakage and absorption. • Emulsion might not be a significant mechanism for reactivity change. • OpenFOAM CFD solver multiphaseInterFoam and MC code Serpent were used. The Windscale Works criticality accident in 1970 resulted from mixing an aqueous solution with an organic solvent with different plutonium concentrations and densities. Although this accident has been studied using improved computer capabilities in recent years, a precise criticality scenario has not yet been identified. This study aims to clarify a possible criticality scenario of the accident—the time variation of reactivity and its mechanism. The accident was simulated by combining the multiphase computational fluid dynamics solver of OpenFOAM and the delta-tracking-based Monte Carlo neutron transport code Serpent2. Consequently, the periodic uneven arrangement of fluids might have caused oscillations in neutron leakage and absorption, resulting in periodic wavy reactivity changes. Furthermore, the emulsion, which was thought to be the primary cause, might not be the dominant mechanism for reactivity change, although it contributed to the criticality of the accident. [ABSTRACT FROM AUTHOR]

The nuclear criticality safety analysis is crucial for the nuclear safety of spent fuel reprocessing plants. Thus, a set of numerical tools with high accuracy and efficiency to predict the criticality and simulate the hypothetical accidents in the reprocessing procedure is of great importance. Among them, this paper focus on the analysis of the storage tank which is used for dissolution and storage of the spent fuel. According to the characteristics of spent fuel solution system, a parallel 3D critical safety analysis tool for fissile solution system, Hydra-TD, is developed. Generally, it contains the cross-section generation model, the three-dimensional space-time neutron kinetics model, and the R-Z two-dimensional heat conduction and radiolysis gas simulation model. The verifications based on the experiments of the SILENE and TRACY facility are conducted. The results show good accuracy of the prediction of the first fission power peak, multiplication time and total fission energy, indicating the reliability of these numerical models. [ABSTRACT FROM AUTHOR]

The safe and secure management of civil separated plutonium is a UK government and NDA priority. One potential solution to address this considers the manufacture of a modified version of mixed oxide (MOX) fuel, comprising PuO 2 dispersed within a UO 2 matrix and doped with a suitable neutron absorbing element to maintain criticality control. As an initial step to understand whether an industrially-relevant, proven MOX fuel fabrication process could offer a potential route to the production of a Pu-disposition matrix based on MOX, a series of Gd-doped UO 2 pellets were prepared by Orano at the CDA workshop of the MELOX facility in France. Characterisation was performed to quantify the density, morphology (grain size and porosity), Gd distribution and Gd incorporation mechanism. It was found that the materials produced were highly reproducible and similar in density and morphology, irrespective of the variables investigated, and similar to unirradiated UOX and MOX fuel. Gd was distributed in a similar manner to the distribution of PuO 2 in unirradiated MIMAS (MIcronisation of a MASter Blend) MOX fuel and evidence for the existence of a solid solution between Gd 2 O 3 and UO 2 was ascertained, which could be viewed as favourable from a GDF post-closure criticality control perspective. The source of the powder had the greatest effect on the final characteristics of the Pu-disposition MOX pellets, due to sintering reactivity; however, these differences were minor. These results are a promising step towards the full-scale manufacture of ceramics suitable for the immobilisation and disposition of separated PuO 2 in a GDF, should policy dictate. [ABSTRACT FROM AUTHOR]

The present study focuses on the influence of neutron scattering cross section and angular distribution when employing the Thermal Scattering Law (TSL) on criticality calculation benchmarks, in comparison with the Free Gas Model (FGM). The benchmarks sensitive to beryllium oxide (HMT-027) and graphite (HMT-026) as well as some simplified Pressurized Water Reactor (PWR) benchmarks are interpreted using the OpenMC code to assess the influence of TSL. The results indicate that the influence of TSL mainly attributes to both the total scattering cross section and the secondary angular distribution characterized by the average cosine of the scattering angle. The HMT-027 and HMT-026 series benchmarks respectively show the more significant influence of one of these two effects. For the simplified PWR benchmark, employing the TSL of H in H 2 O weakens the neutron moderation effect, which results in a reduction of around 100 pcm in the calculated k eff , while the combined influence of cross section and secondary angular distribution is negligible for the TSL of UO 2 fuel. • The impact of TSL on criticality calculations is systematically investigated. • Both scattering cross section and angular distribution of TSL influence k eff. • The impact of TSL for BeO on k eff can be up to 1700 pcm for the HMT-027 benchmarks. • The integral impact of TSL for UO 2 is negligible in typical PWR. • TSL for H 2 O weakens neutron moderation and burnup-dependently reduces k eff. [ABSTRACT FROM AUTHOR]

North Korea has released rare photos of its nuclear program, revealing its upgraded bombmaking capacity. The photos show North Korean leader Kim Jong Un touring a centrifuge hall and a reactor that may be producing plutonium and tritium for atomic weapons. Experts estimate that North Korea has enough fissile material for up to 90 warheads and has assembled about 50 nuclear weapons. The photos provide valuable insights into North Korea's nuclear efforts, including the use of centrifuges to enrich uranium. The revelation of these advancements raises concerns about the growing threat posed by North Korea's nuclear arsenal. [Extracted from the article]

At each site of a supercritical Galton-Watson tree place a parking spot which can accommodate one car. Initially, an independent and identically distributed number of cars arrive at each vertex. Cars proceed towards the root in discrete time and park in the first available spot they arrive at. Let X be the total number of cars that arrive at the root. Goldschmidt and Przykucki proved that X undergoes a phase transition from being finite to infinite almost surely as the mean number of cars arriving to each vertex increases. We show that EX and P(X = 0) are discontinuous at the critical threshold, describe the growth rate of EX above criticality, and prove that X stochastically increases as the initial car arrival distribution becomes less concentrated. We also provide a new characterization of the threshold with a generating function condition satisfied by the time of first arrival at the root. For the simple case that either 0 or 2 cars arrive at each vertex of a d-ary tree, we give improved bounds on the critical threshold and also prove that the location of the phase transition depends on more than just the mean number of cars arriving to each vertex. [ABSTRACT FROM AUTHOR]

Nuclear energy has been one of the sustainable energy sources, but after the Fukushima Daiichi nuclear accident, large-scale light water reactors are losing price competitiveness due to the rising costs to meet elevated safety standards. On the other hand, small modular reactors (SMRs) have been developed by various teams and are expected to provide not only electricity but also heat for small communities, chemical plants, factories, mines, and hydrogen production. Since 2017, a multipurpose very small modular reactor (vSMR), namely, Mobile-Very-small reactor for Local Utility in X-mark (MoveluXTM), has been studied at Toshiba Energy Systems and Solutions Corporation as a feasible distributed energy source. The main concept to MoveluX is a heat pipe–cooled calcium hydride–moderated core to simplify the reactor system while increasing inherent safety and nuclear security. Portable vSMRs are useful for remote places; therefore, criticality safety during their transport is essential for vSMRs to gain popularity. In a previous paper, we discuss positive temperature reactivity coefficients of the hydride-moderated core and its control method. The phenomenon is caused by thermal-neutron spectrum shifts at increased temperatures. In the current paper, we show that a positive temperature reactivity coefficient can be utilized to maintain subcriticality during transport. The reactor core requires preheating to achieve criticality, which means the core does not become critical even though safety rods have been extracted in the low-temperature range. The positive reactivity in the low-temperature range establishes inherent criticality safety during transport of the reactor system. [ABSTRACT FROM AUTHOR]

Design Thinking has gained recognition as an acclaimed process for generating innovative, human centred solutions at a social and business level. It has also gained notoriety amongst many designers, who claim that its success as an exported element of the design process has resulted in its commodification, and led to it becoming a diluted series of processes that lack criticality. This article describes the findings from a conversation session held at DRS2018 which posed the question: Have we reached peak design thinking? Participants were asked to identify with a range of positions on the topic and were then given three questions to provide a constructive debate. The findings point to a lack of a clear distinction between design and Design Thinking, a lack of consensus as to whether a designer is required in the process and also a lack of agreement as to its benefits. In order to prevent a continued backlash against Design Thinking, the findings point to the need for a framework that can outline the clear distinction between design and Design Thinking, whether the designers skills are required and the context and scale of a project that would require design or Design Thinking or both. [ABSTRACT FROM AUTHOR]

Recently, Das et al. [J. Chem. Phys. 125, 024506 (2006)] established that computer simulations of critical dynamics in a binary Lennard-Jones mixture are consistent with the predicted Stokes-Einstein behavior of the asymptotic decay rate of the order-parameter fluctuations near criticality. Here, we show that the noncritical or “background” contributions to the computed diffusion coefficient are also in agreement with both theory and experiment, thus further validating the feasibility of molecular dynamics simulations for studying dynamic critical behavior. [ABSTRACT FROM AUTHOR]

To elucidate the accuracy of benchmarks of criticality at the Kyoto University Critical Assembly (KUCA), uncertainty analysis is conducted for manufacturing tolerances in highly enriched uranium (HEU) plates and modeling of core configurations in addition to nuclear data. For evaluation of eigenvalue bias, eigenvalue calculations are conducted using MCNP6.1 and SCALE6.2/KENO-VI together with ENDF/B-VII.1. The modeling of reference core configurations and material properties with average values is validated through a comparison between calculated and measured results. The uncertainty induced by nuclear data is evaluated with SCALE6.2/TSUNAMI-3D together with ENDF/B-VII.1 for sensitivity calculations and 56groupcov.7.1 for the covariance matrix. In the breakdown of the uncertainty induced by nuclear data, the impact of 235U shows significant dominance, about 900 pcm in hard and soft spectrum cores. Furthermore, uncertainty evaluation by manufacturing tolerances in HEU plates and reproducibility of control rod positions demonstrates that the impact of variation on measured reactivity is minor. Through experimental analyses, the index of accuracy in benchmark experiments of criticality is conducive to the reliability of benchmarks at KUCA. [ABSTRACT FROM AUTHOR]

Highlights • New multigroup Monte Carlo code specifically designed for TRIGA reactor core design. • Complement other MC codes for functionalities that are less optimal. • Burnup calculations based on homogenized group cross sections. • Improved simulation performance in k e f f and fission source convergence. Abstract This paper outlines the details of the development of TRIMON (TRI GA Mon te Carlo Code), a next-generation reactor code that integrates diffusion-theory-type group cross sections into Monte Carlo method for TRIGA reactors. Such hybrid combination speeds up stochastic simulations via homogenization of complex local core regions. TRIGA reactors are currently installed in 24 different countries, thus, a robust core management code must be developed for their safety analysis. TRIMON includes important features that improve certain functionalities that are less optimal in most state-of-the-art Monte Carlo codes such as direct integration of local fuel burnup in core calculation, sophisticated reactor core design considerations and simulation time improvements in complex core configurations. TRIMON also eliminates the complicated jargon related to the core geometry and tally specifications imposed by most general multi-purpose Monte Carlo codes during the process of defining TRIGA core problems. [ABSTRACT FROM AUTHOR]

Analyzes the emergence of criticality in the single-particle orientational relaxation near the isotropic-nematic (IN) phase transition by molecular dynamics simulations. Details of the simulation; Molecular dynamics simulation of Lennard-Jones ellipsoids; Implication for isotropic-nematic transition.

Investigates the heat capacity of the restricted primitive model near criticality. Use of the Monte Carlo technique in studying the phase transition of the model; Absence of indication of Isong-type divergence of the heat capacity; Occurrence of Ising behavior close to critical point in some electrolyte solution.

In a PERT network, the importance of each activity needs to be assessed in order to identify those that warrant more attention from the project management. For this purpose, two measures are frequently used: criticality index; and uncertainty importance measure. While the former is used to identify which activity's expected duration to decrease in order to decrease the expected project completion time, the latter can be used to identify those that deserve more attention in reducing the magnitude of the uncertainty (variability) in the project completion time (T). This paper defines an uncertainty-importance measure of an activity, or of a pair of activities, and develops a method for evaluating the defined measure under the assumption that the durations of activities are independently and symmetrically distributed. The method utilizes Taguchi tolerance design technique with modifications. First, PERT networks are classified into two types: Type A with a dominantly longer path than the others, and Type B without such a path. For a Type A network, T can be closely approximated by the sum of the duration of activities on the dominantly longer path, and the contribution of the uncertainty in each activity duration to the uncertainty in T can be analytically determined with ease. For a Type B network, however, such an analytic approach is not feasible. In addition, activity durations may have curvature and interaction effects on T. We therefore employ an appropriate experimental strategy for Type B networks, and calculate the so called contribution ratios which evaluate the main and/or interaction effects of activity durations on the variability of T. Contribution ratios are used to estimate the defined uncertainty importance measure of an activity or of a pair of activities. The proposed method is easy to use and, in particular, computationally efficient for large-sized PERT networks as compared to other existing methods which require Monte Carlo simulation. [ABSTRACT FROM AUTHOR]

The article offers a critical look on the prospects and production of nuclear fuel. According to the author, most of the significant uranium-producing nations have slew virtually all security restrictions on information relating to nuclear raw materials since 1955. For him, it is important that resources of nuclear fuels be considered from a world point of view. All nations planning or considering atomic power development need access to the maximum information on the availability and cost of the nuclear fuel.

We review some ideas about the quantum physics of black hole information storage and processing in terms of a general phenomenon of quantum criticality. [ABSTRACT FROM AUTHOR]

The hexagonal ZrNiAl-type (space group: P 2m) and the tetragonal Mo2FeB2-type (space group: P4/mbm) structures, which are formed in the same Yb-based alloys and exhibit physical properties related to valence-fluctuation, can be regarded as approximants of a hypothetical dodecagonal quasicrystal. Using Pd-Sn-Yb system as an example, a model of quasicrystal structure has been constructed, of which five-dimensional crystal (space group: P12/mmm, a DD = 5.66 Å and c = 3.72 Å) consists of four types of acceptance regions located at the following crystallographic sites; Yb [0 0 0 0 0], Pd[1/3 0 1/3 0 1/2], Pd[1/3 1/3 1/3 1/3 0] and Sn[1/2 00 1/2 1/2]. In the three-dimensional space, the quasicrystal is composed of three types of columns, of which c-projections correspond to a square, an equilateral triangle and a three-fold hexagon. They are fragments of two known crystals, the hexagonal α-YbPdSn and the tetragonal Yb2Pd2Sn structures. The model of the hypothetical quasicrystal may be applicable as a platform to treat in a unified manner the heavy fermion properties in the two types of Yb-based crystals. [ABSTRACT FROM AUTHOR]

We present a source convergence acceleration method for Monte Carlo criticality calculations. The method gradually increases the neutron population size over the successive inactive as well as active criticality cycles. This helps to iterate the fission source faster at the beginning of the simulation where the source may contain large errors coming from the initial cycle; and, as the neutron population size grows over the cycles, the bias in the source gets reduced. Unlike previously suggested acceleration methods that aim at optimisation of the neutron population size, the new method does not have any significant computing overhead, and moreover it can be easily implemented into existing Monte Carlo criticality codes. The effectiveness of the method is demonstrated on a number of PWR full-core criticality calculations using a modified SERPENT 2 code. [ABSTRACT FROM AUTHOR]

Recent theoretical and experimental studies have shown that ferromagnetic quantum criticality is always avoided in clean systems. Two possibilities have been identified. In the first scenario, the ferromagnetic transition becomes of the first order at a tricritical point before being suppressed. A wing structure phase diagram is observed indicating the possibility of a new type of quantum critical point under magnetic field. In a second scenario, a transition to a modulated magnetic phase occurs. Our recent studies on the compound LaCrGe 3 illustrate a third scenario where not only a new magnetic phase occurs, but also a change of order of the transition at a tricritical point leading to a wing-structure phase diagram. Careful experimental study of the phase diagram near the tricritical point also illustrates new rules near this type of point. [ABSTRACT FROM AUTHOR]