Spatial Effects in Low Neutron Source Start-up and Associated Stochastic Phenomena

This work concerns the calculation of the neutron source strength necessary to start up a nuclear reactor such that the likelihood of an undesirable stochastic transient is reduced to a specified value (e.g. 10−8). We extend our earlier point model work on low source calculations to include the spatial variation of the neutron source. Results for the source multiplier for a given safety factor are obtained for slab, cylindrical and spherical systems. The spatial term in the Pál-Bell equation is dealt with by Chebyshev-Gauss-Lobatto collocation methods and this enables an extrapolation distance to be included, thereby simulating a reflector. Results are given for a range of system sizes, and corresponding source multipliers for safe source determination are obtained. The saddlepoint method is used to invert the generating function. In addition to the low source calculations, we have also tested the collocation method on the survival probability in a sphere which demonstrates excellent convergence. We also comment on the usefulness of the Gamma pdf for spatially dependent problems. For clarity of presentation, some of the detailed mathematical work is relegated to Appendices

The energy dependent Pál–Bell equation and the influence of the neutron energy on the survival probability in a supercritical medium

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CALLISTO-SPK: A Stochastic Point Kinetics Code for Performing Low Source Nuclear Power Plant Start-up and Power Ascension Calculations

This paper presents the theory and application of a code called CALLISTO which is used for performing NPP start-up and power ascension calculations. The CALLISTO code is designed to calculate various values relating to the neutron population of a nuclear system which contains a low number of neutrons. These variables include the moments of the PDF of the neutron population, the maturity time and the source multiplier. The code itself is based upon the mathematics presented in another paper and utilises representations of the neutron population which are independent of both space and angle but allows for the specification of an arbitrary number of energy groups. Five examples of the use of the code are presented. Comparison is performed against results found in the literature and the degree of agreement is discussed. In general the agreement is found to be good and, where it is not, plausible explanations for discrepancies are presented. The final two cases presented examine the effect of the number of neutron groups included and finds that, for the systems simulated, there is no significant difference in the key results of the code

Mandibular myiasis: A case report

Myiasis is a condition characterised by the infestation of live vertebrates with dipterous larvae. Myiasis is derived from the Greek word mya, meaning fly. Dipterans, (order Diptera), are two-winged insects known as true flies. Myiasis tends to occur in cutaneous, necrotic lesions of patients who have weak immune systems, who inhabit unhygienic environments. Maggot Debridement Therapy is an uncommon treatment of debriding necrotic tissue, with some utility. A case of mandibular myiasis involving a malignant mandibular lesion that was colonised by maggots is discussed

THE PROSPECT, PROMISES AND HINDRANCES OF STATIN BASE MOLECULES: LOOK BACK TO LOOK FORWARD

This review narrates the importance of the statin-based molecules and their inherent challenges during their administration. The chronological appearance of the statin, their source and the journey with time so to evolve as one of the successful cholesterol-lowering agents to prevent the morbidity and mortality especially related to coronary heart disease have been illustrated along with their recent utilities in neurodegenerative diseases. The statins, because of their respective physicochemical characters pose several challenges in regards to their effective administration to the patients. One of the major issues related their poor bioavailability is their aqueous solubility. The different approaches for the enhancement of solubility and hence bioavailability have been discussed systematically. This review finally suggests the importance of more related research in regards to their successful administration so to have greater realization of therapeutics efficiency.Keywords: Statin-based molecules, Poor solubility, Solubility enhancemen

Wetting-induced volumetric collapse of UO2 powder beds and the consequence on transient nuclear criticality excursions

Mathematical and computational models are proposed to simulate wetting-induced volumetric collapse of fissile powder beds. Slumping, nuclear thermal hydraulics, radiolytic gas, and steam production models are coupled with point neutron kinetics to investigate transient nuclear criticality excursions in two 5-wt% enriched UO2 fissile powder beds with varying levels of wetting-induced volumetric collapse. The two beds are distinguished by their mean powder particle size of 30 μm and 100 μm. For the UO2 powder beds modelled, the re-distribution of UO2 powder and moderator due to slumping introduced a negative reactivity into the system. This increased the amount of time taken for a delayed critical state to be reached once infiltration began, and also reduced the total fission energy generated over the course of the simulated transient. The total fission energy generated ranged from 42 MJ to 48 MJ 100 seconds after the initial nuclear criticality excursion was observed for the 30 μm sized UO2 powder bed. The fission energy of the larger sized powder bed (100 μm), varied from 42 MJ to 57 MJ. Larger discrepancies between the slumped and un-slumped initial peak power are predicted. Peak powers varied from 29.2 MW to 106 MW for the smaller-sized powder particles, whereas for larger particles, the peak powers varied from 255 MW to 501 MW

Graphitization behavior of iodine-treated Bombyx mori silk fibroin fiber

The original publication is available at www.springerlink.comArticleJOURNAL OF MATERIALS SCIENCE. 44(16):4235-4240 (2009)journal articl

Transient nuclear criticality excursion analysis of highly dispersed particulate three-phase fluidised systems

The aim of this study was to perform sensitivity analysis, investigating how different fluidisation and sedimentation characteristics of three-phase wetted UO2 powder beds, may affect a transient nuclear criticality excursion initiated through the addition of water into a fissile powder bed. This type of postulated nuclear criticality accident scenario may occur in nuclear fuel fabrication facilities when a fire is fought through the use of water, supplied via an automatic or manual fire-suppression system. A similar scenario may also develop as a result of water leaks or flooding of the process area housing UO2 powder. The article introduces a model for gas-bubble induced fluidisation of a UO2 powder bed and examines how this phenomenon may affect the neutron kinetic response of a three-phase fluidised fissile powder system. Empirical analysis has shown that fissile suspensions form agglomerated structures when suspended in water, at agglomerate sizes that range from 18 to 40 . Simulation results indicate that both the critical gas velocity and rate of fluidisation may significantly affect transient nuclear criticality excursion dynamics. The re-distribution of fissile mass into a highly dispersed suspension generally reduces the reactivity of the system, however, depending on the H/U ratio, a positive reactivity may be added to the system. Low Péclet numbers in the suspension suggest that gas-bubble induced motion of the suspension causes a highly dispersive flow field. An oscillatory power response is predicted for low critical gas velocities where the reactivity of the system is predominantly governed by the re-distribution of fissile mass within the system. The frequency of these oscillations is greater for a higher hindered settling rate of powder particles. At a higher critical gas velocity, the transient nuclear criticality excursion is governed by the voidage reactivity feedback, making the response quite independent of fluidisation. In all cases, large volumes of UO2 powder may leave the domain due to overflowing of the suspension. Transient nuclear criticality excursions in UO2 powder beds with a low critical gas velocity are terminated once the bed becomes fully saturated