Kinetic modelling of the dispersion of plutonium in the eastern Irish Sea: two approaches

A new model to simulate the dispersion of plutonium in the eastern Irish Sea is presented. The model solves the 3D hydrodynamic equations using normalized σ coordinates in the vertical simultaneously with the suspended matter equation. Pu can be present in three phases: water, suspended matter and bottom sediments. Reduction and oxidation reactions are also included in the model, in terms of reaction rates, since Pu can be present in the marine environment in principally two different oxidation states. Two kinetic models are presented to describe the transfers of radionuclides between the liquid and solid phases: a one-step model consisting of a single reversible reaction and a two-step model consisting of two consecutive reversible reactions. It has been found that both models can properly simulate the contamination of the waters and sediments from the eastern Irish Sea due to the releases from the BNFL Sellafield nuclear fuel processing plant, since the outputs from both models are very similar and in agreement with observations. Also, both models can simulate the speciation of Pu between the reduced and oxidized forms. However, if the dominant source of radionuclides to the water column is redissolution from a contaminated sediment, a process that is actually occurring in the Irish Sea, it has been found that a two-step model must be used. Indeed, a one-step model predicts the redissolution from the sediment to be unrealistically rapid.     

Liner costs and pricing polices: a reconsideration of regulatory issues

A number of shipping analysts have adopted as a convenient approximations, the assumption that linear costa are homogeneous per unit of cargo. The paper points to recent evidence suggesting that considerable differences exist in the estimates of implicit marginal cost for various types of cargo, and re-evaluates the regulatory arguments which are based on the assumption of homogeneity of costs.     

Interval constraints for the treatment of the partial data problem in gravity model calibration

The problem of indeterminacy, in the solution parameters of the Evans-Kirby calibration model, is discussed. A proposal is made to alleviate this problem, in the case of missing data, by introducing interval constraints.     

A Model Reduction Procedure for the Dynamic Analysis of Rail Vehicles Subjected to Linear Creep Forces

The set of differential equations governing the motion of an unrestrained coned wheelset travelling on a tangent section of track and acted upon by creep forces arising from the contact between wheel and rail are, in the terminology of numerical analysis, extremely "stiff". This stiffness can be attributed to the existence of two negative real eigenvalues in the solution of the eigenproblem associated with the linearized equations of motion. Compared with the two complex conjugate eigenvalues that complete this solution, the real eigenvalues have large magnitudes and necessitate that relatively. small timesteps be used in order to obtain an accurate numerical integration of the full set of equations of motion. However, by truncating the set of left and right eigenvectors to eliminate these real eigenvalues in a modal analysis of the wheelset, it was found that their contribution to the overall dynamic response is negligible. This same modal truncation approach was then applied to the sub-structured equations of motion for a simple rail vehicle system consisting of two wheelsets connected to a main body by linear springs and dampers. Essentially, the physical degrees of freedom for each wheelset substructure were replaced by a single complex coordinate obtained from the previous normal modes analysis. Using this model reduction procedure, accurate numerical results for the motion of the rail vehicle were generated several times faster than the results obtained by numerically integrating the full set of differential equations directly.     

The econometric testing of investment incentives: A cointegrated approach

Thie paper re-examines the link between shipping investment and investment incentives, first explored in this journal by Marlow, and subsequently developed by mcWilliams et al. in the July-September issue. Examination of the ststistical properties of the data reveals that some of McWilliams' results may be flawed. A modified version of the Marlow model is estimated. checking for statistical and economic consistency, using cointegration, checking for statistical and economic consistency, using cointegration techniques. The estimated model resolves marlow's original challenge, but does not appear to be as robust as could be wished.     

Vertical Load-Deflection Behaviour of a Pneumatic Tire Subjected to Slip and Camber Angles

In this paper the vertical load-deflection behaviour of a pneumatic tire has been studied theoretically. A simple mathematical model which is especially suitable for a tire applied by a side force has been developed. Researches carried out in the past show that the tire vertical stiffness varies neither proportionally nor symmetrically as the slip angle of a cambered tire is changed. This effect can be explained by the theory developed here.

The model predictions have been verified using experimental results obtained from literature. Moreover, tire cornering characteristic curves obtained under different test conditions, i. e. during increasing of the slip angle the vertical load is kept constant or not, have been discussed through a simulation example. This study shows that the characteristic curves vary rather considerably under the different conditions.     

Operating differential subsidies: problems and alternatives for the United States

In this paper a proposal for a Competitive Rate of Return (CRR) based operating-subsidy system is made as an alternative to the existing US foreign-cost parity ODS system. Such an alternative, if adopted, could be successful in meaningfully relating US government maritime subsidy programmes to the legitimate financial needs of the industry. From the operator's viewpoint, the risk of drastic revenue fluctuations is removed, yet adequate opportunities are provided for firms with above average performance to be rewarded for their efficiency in higher than average profits. similarly, poorly run firms have an opportunity to lose money, in spite of government subsidies.

From the government's viewpoint, the CRR programme guarantees the provision of essential shipping services at the lowest possible cost. Because of substantial incentives to the operator on both the revenue and cost side of his profit equation, it is more likely that long-run government operating subsidies can be reduced under CRR than under the present system.

Although the CRR system has been described for use in determining subsidies for both bulk and general cargo carriers, it has been recommended that this system be considered first for implementation in the case of bulk vessels, because of the newness of the bulk subsidy programme. In addition, the 1970 Act gives MarAd the flexibility to initiate such a programme for bulk vessels without the necessity of new legislation.