1,379 research outputs found
A flood vulnerability index for coastal cities and its use in assessing climate change impacts
Worldwide, there is a need to enhance our understanding of vulnerability and to develop methodologies and tools to assess vulnerability. One of the most important goals of assessing coastal flood vulnerability, in particular, is to create a readily understandable link between the theoretical concepts of flood vulnerability and the day-to-day decision-making process and to encapsulate this link in an easily accessible tool. This article focuses on developing a Coastal City Flood Vulnerability Index (CCFVI) based on exposure, susceptibility and resilience to coastal flooding. It is applied to nine cities around the world, each with different kinds of exposure. With the aid of this index, it is demonstrated which cities are most vulnerable to coastal flooding with regard to the system's components, that is, hydro-geological, socio-economic and politico-administrative. The index gives a number from 0 to 1, indicating comparatively low or high coastal flood vulnerability, which shows which cities are most in need of further, more detailed investigation for decision-makers. Once its use to compare the vulnerability of a range of cities under current conditions has been demonstrated, it is used to study the impact of climate change on the vulnerability of these cities over a longer timescale. The results show that CCFVI provides a means of obtaining a broad overview of flood vulnerability and the effect of possible adaptation options. This, in turn, will allow for the direction of resources to more in-depth investigation of the most promising strategies
Transformation and consumption of high-quality meat: the case of Chianina meat in Umbria, Italy.
Neutron activation as an independent indicator of expected total yield in the production of 82Sr and 68Ge with 66 MeV protons
Introduction
A method based on neutron activation is being developed to assist in resolving discrepancies between the expected yield and actual yield of radionuclides produced with the vertical-beam target station (VBTS) at iThemba LABS.
The VBTS is routinely employed for multi-Ci batch productions of the radionuclide pairs 22Na/68Ga and 82Sr/68Ga using standardized natMg/natGa and natRb/natGa tandem targets, respectively [1]. The metal-clad target discs are bombarded with a primary beam of 66 MeV protons at an intensity of nominally 250 µA. The encapsulation materials are either Nb (for Mg and Ga) or stainless steel (for Rb) which serve to contain the molten target materials during bombardment and act as a barrier to the high-velocity cooling water which surrounds the targets in a 4π geometry. The natRb/natGa targets are typically bombarded according to a two-week cycle while natMg/natGa targets are bombarded on an ad-hoc basis, depending on a somewhat unpredictable 22Na demand.
A too-large deviation between expected yield and actual yield has at times plagued this programme. These deviations can manifest both as an apparent loss or an apparent gain (relative to the expected yield) by up to about 15% in either direction. The resulting uncertainty of up to 30% (in the worst case) from one production batch to another can be costly and is unacceptable in a large-scale production regimen. This phenomenon is believed to be brought about by two types of problems:
(1) Production losses, e.g. during the radio-chemical separation process or incomplete recovery of activated target material during the decapsulation step.
(2) Incorrect values obtained for the accumulated proton charge.
A problem of type (1) will always result in a loss of yield. A problem of type (2) can manifest as an apparent loss or gain. In an effort to get a handle on this second type of problem, neutron activation of suitable material samples, embedded in a target holder, is being investigated as an independent indicator of the total yield. For this purpose, samples of Co, Mn, Ni and Zn were activated during production runs and Co was found to be the most appropriate. Preliminary results will be presented after first discussing why the determination of the accumulated pro-ton charge is a problem with the VBTS.
Materials and Methods
The VBTS consists of a central region in which a target holder is located during bombardment as well as two half-cylindrical radiation shields which completely surround the target. The shields can be moved away from the central region on dedicated rails, e.g. when repairs or maintenance is required. FIGURE 1 shows the VBTS with the shields moved to the “open” position. As some components of the station are located below the vault floor, with the target position near floor level, it proved difficult to electrically isolate the VBTS as was done for the two horizontal-beam target stations at iThemba LABS [1]. The VBTS does not act as a Faraday cup like the other target stations. Instead, the beam current and accumulated charge is measured by means of a calibrated capacitive probe [1,2].
There appears to be a variation in the response of the capacitive probe, sensitive to the beam microstructure, in particular a dependence on the beam packet length. This problem is not yet fully resolved.
FIGURE 2 (a) shows the beamstop of a VBTS target holder with several Co samples mounted on the outside as well as one each of Ni, Mn and Zn. The samples are small “tablets” with a 10 mm diameter and 1 mm thickness. The reactions of interest are 59Co(n,γ)60Co, 59Co(n,3n)57Co, nat-Ni(n,X)60Co, natNi(n,X)57Co, natZn(n,X)65Zn and 55Mn(n,2n)54Mn. The relevant half-lives are 60Co(5.271 a), 57Co(271.8 d), 65Zn(244.3 d) and 54Mn(312.2 d). The half-life should be long compared to the two-week cycle in order to reduce the dependence on the exact beam history, which is very fragmented over any production period. In this respect, 60Co is considered to be particularly attractive as its long half-life of more than 5 years leads to a negligible effect by the beam history.
Note that the tandem targets, shown in FIGURE 2 (b), are mounted just upstream of the beamstop – in fact, the targets and beamstop form a single unit before being fitted into the target holder.
At the end of bombardment, all samples were assayed for their characteristic Îł-emissions using standard off-line Îł-ray spectrometry with an HPGe detector connected to a Genie 2000 MCA. Calculations of the neutron fluence density in the central sample volume on the beamstop were also performed using the Monte Carlo radiation transport code MCNPX. For these calculations, the entire VBTS, a Rb/Ga target and the vault walls were included in the model.
Results and Conclusion
All samples activated significantly – copious amounts of 60Co were detected in the Co discs after a two-week run.
The neutron fluence density for the case of a 250 µA, 66 MeV proton beam on a natRb/natGa tandem target is shown in FIGURE 3. The dominance of low-energy neutrons is evident, which is in part due to the large amount of paraffin-wax shielding material in close proximity to the target. While reactions such as the (n,2n) and (n,3n) would be sensitive to the more energetic part of the neutron spectrum, the (n,γ) capture reaction benefits from the large low-energy component. This explains the copious amounts of 60Co formed. It was therefore decided to only retain the central Co sample for subsequent bombardments, as shown in FIGURE 4.
The first results are shown in TABLE 1. The accumulated charge as obtained from the capacitive probe (Q), the specific 60Co activity (A) at the end of bombardment (EOB), and their ratio (A/Q) are presented in the table, together with the deviation of individual ratios relative to their average for the case of the Mg/Ga tandem tar-gets only. Note that all samples were counted until the statistical uncertainties were negligible. Any systematic uncertainties are ignored at this stage as they are considered to remain the same from one batch production to another.
For the sake of argument, the average value of the ratio is taken as the expected value. A positive deviation of the A/Q value is then indicative of a too-small value of the accumulated charge obtained from the capacitive probe, leading to a corresponding overproduction. Likewise, a negative value is indicative of a too-large value of the accumulated charge, leading to a corresponding underproduction.
It is certainly true that the data in TABLE 1 are currently very limited. It is envisaged, however, that with time the growing database of values will assist in reducing the uncertainty in determining the accumulated charge and reduce the discrepancies between predicted and actual yields significantly. TABLE 1 illuminates the underlying problem satisfactorily. The four Mg/Ga tandem target bombardments, on identical targetry, were performed successively. The neutron activation correlates well the with actual yields, pointing directly to the current integration as the main source of error.
The method already proves to be useful. An indication of an over or underprediction can be obtained prior to the target processing by recovering and measuring the Co disc. This in-formation can be used to make a decision concerning the present batch production and/or the subsequent one. One can either add beam to the present production target and/or in-crease/reduce the total beam on the subsequent production target to compensate for an expected overproduction or shortfall.
In conclusion, we would like to stress that the capacitive probes show great promise and that better understanding and/or possibly some development of their signal processing algorithm may improve their ability to measure the accumulated charge to the desired accuracy. Segmented capacitive probes used at iThemba LABS and elsewhere for beam position measurement [1,3] are not affected by beam microstructure as only the ratios of the signal strengths on the different sectors are important. In this case, changes in response affect all sec-tors equally and the ratios are unaffected
Determining the moment-curvature diagram of aluminium cross-sections in fire; a numerical code and practical examples
The use of aluminium as a construction material has been increasing since its first invention. At present, limited knowledge of the behaviour of aluminium beams in fire gives rise to excessively high insulation demands, decreasing its competitiveness. To study the behaviour of (statically undetermined) bending members in fire, it is required to know the moment-curvature diagram. For aluminium and stainless steel at both room and elevated temperatures, and for steel at elevated temperatures, this curve cannot be calculated analytically, but needs to be obtained numerically. A versatile numerical code is presented, capable of calculating the moment-curvature diagram for arbitrary geometries along any axis with arbitrary temperature and initial strain distributions
Het Melkveemodel
Voor het nabootsen van de opbouw van een veestapel is bij het PR het melkveemodel ontwikkeld. Met dit model is het mogelijk om een veestapel samen te stellen waarbij rekening wordt gehouden met verschillen in leeftijd en kalfdatum en met uitstoot van dieren tijdens de lactatie. Het model geeft aan hoeveel van welke dieren op een bepaald moment in de veestapel voorkomen. Het melkveemodel geeft alleen de opbouw van de veestapel. In combinatie met het koemodel en het jongveemodel (modellen die voeding en productie per dier berekenen) kunnen kengetallen voor voeropname en productie van een veestapel worden berekend
Waardevol - Indicatoren voor Valorisatie
Het rapport Waardevol – Indicatoren voor valorisatie biedt kennisinstellingen, wetenschapsfinanciers en kennisgebruikers een gemeenschappelijk kader voor valorisatie. De methode werkt met valorisatiekaarten waarmee betrokken organisaties systematisch kunnen monitoren hoe groot de valorisatie-inspanningen zijn, wat de opbrengsten zijn en welke mogelijkheden voor verbetering er zijn. Het gebruik van valorisatiekaarten zal helpen om het inzicht te vergroten in wat publieke kennisinstellingen, wetenschapsfinanciers en bedrijfsleven nu al doen aan valorisatie.
Dynamic behaviour of AA 2024 under blast loading : experiments and simulations
The dynamic behaviour of AA2024-T3 is investigated. Dynamic tensile tests using a servo-hydraulic and a light weight shock testing machine (LSM) have been performed. The servo-hydraulic test machine proves to be more reliable and reaches higher strain rates. Neither test revealed any strain rate effect of AA2024-T3. Two types of fracture tests were carried out to determine the dynamic crack propagation behaviour of this alloy, using prestressed plates and pressurized barrels, both with the help of explosives. The prestressed plates proved to be not suitable, whereas the barrel tests were quite reliable, allowing to measure the crack speeds. Computer simulations with a user defined, rate dependent cohesive zone model were in agreement with experiments, capturing the rate toughening effect
Non-parametric Bayesian drift estimation for stochastic differential equations
We consider non-parametric Bayesian estimation of the drift coefficient of a
one-dimensional stochastic differential equation from discrete-time
observations on the solution of this equation. Under suitable regularity
conditions that are weaker than those previosly suggested in the literature, we
establish posterior consistency in this context. Furthermore, we show that
posterior consistency extends to the multidimensional setting as well, which,
to the best of our knowledge, is a new result in this setting.Comment: 27 page
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